Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

I. Organization of the

Human Body

4. Histology: The Study of

Tissues

Companies, 2004

In some ways, the human bodyis like

a complexmachine such asa car. Both

consistof many parts, which are made

of materials consistentwith their spe-

cialized functions. For example, the win-

dows of a car are made of transparent

glass, the tiresare made of synthetic rubber

reinforced with a varietyof ﬁbers, the engine is

made of a varietyof metal parts, and the hoses that

move water, air, and gasoline are made ofsyntheticrubber or plastic. All parts of

an automobile cannotbe made of a single type ofmaterial. Metal capable of with-

standing the heatof the engine cannot be used for windows or tires. Similarly,

the manyparts of the human body are made of collections of specialized cells

and the materials surrounding them. Muscle cells, which contractto produce

movementsof the body, are structurally different and have different functions

than those ofepithelial cells, which protect, secrete, or absorb. Also, cellsin the

retina ofthe eye, specialized to detect light and allow us to see, do not contract

like muscle cellsor exhibit the functions of epithelial cells.

The structure and function oftissues are so closely related thatyou should

be able to predictthe function of a tissue when given itsstructure, and vice versa.

Knowledge of tissue structure and function isimportant in understanding the

structure and function of organs, organ systems, and the complete organism.

Thischapter begins with brief discussionsof tissues and histology (105) and the

development of embryonic tissue (105) and then describes the structural and

functionalcharacteristics of the major tissue types: epithelial tissue (105), con-

nective tissue(117), classiﬁcation of connective tissue(119), muscle tissue (128),

and nervous tissue (129). In addition, the chapter provides an explanation of

membranes(132), inﬂammation (133), and tissue repair (135).

Histology:

The Study of

Tissues

Colorized SEM of simple columnarepithelial

cells, with cilia, of the uterine tube.

CHAPTER

Part 1 Organization ofthe Human Body

Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

I. Organization of the

Human Body

4. Histology: The Study of

Tissues

Companies, 2004

Chapter 4 Histology: The Study of Tissues 105

Tissues and Histology

Objectives

■ List the characteristics used to classifytissues into one of

the fourmajor tissue types.

■ Deﬁne histology and explain its importance in assessing

health.

Tissues(tish⬘u¯z) are collections ofsimilar cells and the sub-

stances surrounding them. Specialized cells and the extracellular

matrix surrounding them form all the different tissue types found

at the tissue level of organization. The classiﬁcation of tissue

types is based on the structure of the cells; the composition of the

noncellular substances surrounding cells,called the extracellular

matrix; and the functions of the cells. The four primary tissue

types, which include all tissues,and from which all organs of the

body are formed,are

1. epithelial tissue;

2. connective tissue;

3. muscle tissue;

4. nervous tissue.

Epithelial and connective tissues are the most diverse in form.

The different types ofepithelial and connective tissues are classiﬁed

by structure, including cell shape,relationship of cells to one an-

other,and the material making up the extracellular matrix. In con-

trast,muscle and nervous tissues are classiﬁed mainly by function.

The tissues ofthe body are interdependent. For example, mus-

cle tissue cannot produce movement unless it receives oxygen car-

ried by red blood cells,and new bone tissue cannot be formed unless

epithelial tissue absorbs calcium and other nutrients from the diges-

tive tract.Also, all tissues in the body die if cancer or some other dis-

ease destroys the tissues ofvital organs such as the liver or kidneys.

Histology (his-tol⬘o¯-je¯) is the microscopic study of tissues.

Much information about the health of a person can be gained by

examining tissues.A biopsy (bı¯⬘op-se¯) is the process ofremoving

tissue samples from patients surgically or with a needle for diag-

nostic purposes. Examining tissue samples from individuals with

various disorders can distinguish the speciﬁc disease.For example,

some red blood cells have an abnormal shape in people suffering

from sickle-cell disease,and red blood cells are smaller than nor-

mal in people with iron-deﬁciency anemia.White blood cells have

an abnormal structure in people who have leukemia,and the white

blood cell number can be greatly increased in people who have in-

fections.Epithelial cells from respiratory passages have an abnor-

mal structure in people with chronic bronchitis and in people with

lung cancer.Tissue samples can be sent to a laboratory and results

are reported after tissue preparation and examination. In some

cases tissues can be removed surgically,prepared quickly, and re-

sults reported while the patient is still anesthetized.The appropri-

ate surgical procedure is based to a large degree on the results.For

example,the amount of tissue removed as part of breast or other

types of cancer surgery can be determined by the results.

An autopsy(aw⬘top-se¯) is an examination ofthe organs of a

dead body to determine the cause ofdeath or to study the changes

caused by a disease.Microscopic examination of tissue is often part

ofan autopsy.

1. Name the four primary tissue types, and list three

characteristicsused to classify them. How doesthe

classiﬁcation of epithelial and connective tissue differfrom

the classiﬁcation of muscle and nervoustissue?

2. Deﬁne histology. Explain how microscopic examination of

cellsby biopsy or autopsy can diagnose some diseases.

Embryonic Tissue

Objective

■ Name and describe the derivatives of the three embryonic

germ layers.

Approximately 13 or 14 days after fertilization,the cells that

give rise to a new individual, called embryonic stem cells,form a

slightly elongated disk consisting oftwo layers called ectoderm and

endoderm.Cells of the ectoderm then migrate between the two lay-

ers to form a third layer called mesoderm.Ectoderm, mesoderm,

and endoderm are called germ layers because the beginning of all

adult structures can be traced back to one ofthem (see chapter 29).

Theendoderm (en⬘do¯-derm),the inner layer,for ms the lin-

ing of the digestive tract and its derivatives. The mesoderm

(mez⬘o¯-derm), the middle layer, forms tissues such as muscle,

bone, and blood vessels. The ectoderm (ek⬘to¯-derm),the outer

layer,forms the skin, and a portion of the ectoderm, called neu-

roectoderm (noor-o¯-ek⬘to¯-derm), becomes the nervous system

(see chapter 13).Groups of cells that break away from the neuroec-

toderm during development, called neural crest cells,give rise to

parts of the peripheral nerves (see chapters 11, 12, and 14), skin

pigment (see chapter 5),and many tissues of the face.

3. What adult structures are derived from endoderm,

mesoderm, ectoderm, neuroectoderm, and neural crest

cells?

Epithelial Tissue

Objectives

■ List the features that characterize epithelium.

■ Describe the characteristics that are used to classify

epithelia.

■ Describe the relationship between the structures of the

differenttypes of epithelia and their functions.

■ Deﬁne the term gland, and describe the two major

categoriesof glands.

Epithelium (ep-i-the¯⬘le¯-u˘m; pl., epithelia, ep-i-the¯⬘le¯-a˘)

or epithelial tissue can be thought of as a protective covering of

surfaces, both outside and inside the body.Characteristics com-

mon to most types of epithelium are (ﬁgure 4.1):

1. Epithelium consists almost entirely ofcells, with very little

extracellular material between them.

2. Epithelium covers surfaces ofthe body and forms glands

that are derived developmentally from body surfaces.The

body surfaces include the outside surface ofthe body, the

lining ofthe digestive tract, the vessels, and the linings of

many body cavities.

Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

I. Organization of the

Human Body

4. Histology: The Study of

Tissues

Companies, 2004

3. Most epithelial tissues have one free,or apical (ap⬘i-k˘al),

surfacenot attached to other cells; a lateral surface,

attached to other epithelial cells;and a basal surface. The

basal surface ofmost epithelial tissues is attached to a

basement membrane.The basement membrane is a

specialized type of extracellular material that is secreted by

the epithelial cells and by connective tissue cells.It is like the

adhesive on the underside ofScotch tape. It helps attach the

epithelial cells to the underlying tissues,and it plays an

important role in supporting and guiding cell migration

during tissue repair.A few epithelia, such as in lymphatic

capillaries and liver sinusoids,do not have basement

membranes,and some epithelial tissues (e.g., in some

endocrine glands) do not have a free surface or a basal

surface with a basement membrane.

4. Specialized cell contacts,such as tight junctions and

desmosomes,bind adjacent epithelial cells together.

5. Blood vessels do not penetrate the basement membrane to

reach the epithelium;thus all gases and nutrients carried in

the blood must reach the epithelium by diffusing across the

basement membrane from blood vessels in the underlying

Part1 Organization ofthe Human Body106

connective tissue.In epithelia with many layers of cells, the

most metabolically active cells are close to the basement

membrane.

6. Epithelial cells retain the ability to undergo mitosis and

therefore are able to replace damaged cells with new

epithelial cells.Undifferentiated cells (stem cells)

continuously divide and produce new cells.In some types

ofepithelia, such as in the skin and in the digestive tract,

cells that are lost or die are continuously replaced by

newcells.

Functionsof Epithelia

Major functions ofepithelia include:

1. Protecting underlying structures.Examples include the skin

and the epithelium ofthe oral cavit y,which protects the

underlying structures from abrasion.

2. Acting as barriers.Epithelium prevents the movement of

many substances through the epithelial layer.For example,

the skin acts as a barrier to water and prevents water loss

from the body.The skin is also a barrier that prevents the

Epithelial cells with

little extracellular

materials between

the cells

Nucleus

Free surface

Capillary

Basement

membrane

Surface view

Cross-sectional view

Lung

Pleura

LM 640x

Figure 4.1

Characteristicsof Epithelium

Surface and cross-sectionalviews of epithelium illustrate the following characteristics: little extracellular materialbetween cells, a free surface, a basement

membrane attaching epithelialcells to underlying tissues. Capillariesin connective tissue do not penetrate the basement membrane. Nutrients, oxygen, and waste

productsmust diffuse across the basement membrane between the capillaries and the epithelialcells.

Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

I. Organization of the

Human Body

4. Histology: The Study of

Tissues

Companies, 2004

Clinical Focus MicroscopicImaging

We see objectsbecause light either passes

through them or is reﬂected off them and

enters our eyes (see chapter 15). We are

limited, however, in whatwe can see with

the unaided eye. Withoutthe aid of magni-

fying lenses, the smallestobjects we can re-

solve, or identify as separate objects, are

approximately100 ␮m, or 0.1 mm, in diam-

eter, which isapproximately the size of a

ﬁne pencildot. Resolution is a measure of

the abilityto distinguish detail in small ob-

jects, and a microscope can be used to re-

solve structures less than 100 ␮m in

diameter.

Two basic types ofmicroscopes have

been developed: light microscopes and

electron microscopes. As their names im-

ply, lightmicroscopes use light to produce

an image, and electron microscopes use

beamsof electrons. Light microscopesusu-

allyuse transmitted light, which is light that

passesthrough the object being examined,

but some light microscopes are equipped

to use reﬂected light. Glasslenses are used

in light microscopes to magnify images,

and imagescan either be observed directly

bylooking into the microscope, or the light

from the images can be used to expose

photographic ﬁlm to make a photomicro-

graph of the images. Video cameras are

also used to record images. The resolution

oflight microscopes is limited by the wave-

length of light, the lower limit of which is

approximately0.1 ␮m—about the size of a

smallbacterium.

A biopsyis the process of removing tis-

sue from living patients for diagnostic ex-

amination. For example, changesin tissue

structure allow pathologists to identify tu-

mors and to distinguish between non-

cancerous (benign) and cancerous

(malignant) tumors. Light microscopy is

used on a regular basisto examine biopsy

specimens. Light microscopy is used

instead of electron microscopy because

lesstime and effort are required to prepare

materials for examination, and the resolu-

tion is adequate to diagnose most condi-

tionsthat cause changesin tissue structure.

Because imagesare usually produced

using transmitted light, tissues to be ex-

amined mustbe cut very thinly to allow the

light to pass through them. Sectionsare

routinelycut between 1 and 20 ␮m thick to

make them thin enough for light mi-

croscopy. To cutsuch thin sections, the tis-

sue must be fixed or frozen, which is a

process that preserves the tissue and

makesit more rigid. Fixed tissues are then

embedded in some material, such aswax

or plastic, that makes the tissue rigid

enough for cutting into sections. Frozen

sections, which can be prepared rapidly,

are rigid enough for sectioning, buttissue

embedded in wax or plastic can be cut

much thinner, which makes the image

seen through the microscope clearer. Be-

cause mosttissues are colorlessand trans-

parent when thinly sectioned, the tissue

mustbe colored with a stain or dye so that

the structuraldetails can be seen. As a re-

sult, the colors seen in color photomicro-

graphsare not the true colors of the tissue

but instead are the colors of the stains

used. The color ofthe stain can also pro-

vide speciﬁcinformation about the tissue,

because special stains color onlycertain

structures.

To see objectsmuch smaller than a cell,

such ascell organelles, an electron micro-

scope,which has a limit of resolution of ap-

proximately0.1 nm, must be used; 0.1 nm is

aboutthe size of some molecules. In objects

viewed through an electron microscope, a

beam ofelectrons either is passed through

objectsusing a transmission electron micro-

scope (TEM) or isreﬂected off the surface of

objects using a scanning electron micro-

scope (SEM). The electron beam isfocused

with electromagnets. For both processes,

the specimen mustbe ﬁxed, and for TEM the

specimen mustbe embedded in plastic and

thinlysectioned (0.01–0.15 ␮m thick). Care

must be taken when examining specimens

in an electron microscope because a fo-

cused electron beam can cause most tis-

sues to quickly disintegrate. Furthermore,

the electron beam is notvisible to the hu-

man eye; thus it must be directed onto a

ﬂuorescentor photographic plate on which

the electron beam isconverted into a visible

image. Because the electron beam doesnot

transmit color information, electron micro-

graphsare black and white unless color en-

hancementhas been added using computer

technology.

The magniﬁcation abilityof SEM is not

asgreat as that of TEM; however, depth of

focus of SEM is much greater and allows

for the production of a clearer three-

dimensionalimage of the tissue structure.

Chapter 4 Histology: The Study of Tissues 107

entry of many toxic molecules and microorganisms into

thebody.

3. Permitting the passage ofsubstances. Epithelium allows the

movement ofmany substances through the epithelial layer.

For example,oxygen and carbon dioxide are exchanged

between the air and blood by diffusion through the

epithelium in the lungs.

4. Secreting substances.Examples include the sweat glands,

mucous glands,and the enzyme-secreting portion of the

pancreas.

5. Absorbing substances.The cell membranes of certain

epithelial tissues contain carrier molecules (see chapter 3)

that regulate the absorption ofmaterials.

Classiﬁcation ofEpithelium

The major types of epithelia and their distributions are illustrated

in ﬁgure 4.2. Epithelium is classiﬁed primarily according to the

number of cell layers and the shape of the superﬁcial cells.There

are three major types of epithelium based on the number of cell

layers in each type.

1. Simple epitheliumconsists of a single layer of cells, with

each cell extending from the basement membrane to the

free surface.

2. Stratiﬁed epitheliumconsists of more than one layer of

cells,only one of which is attached to the basement

membrane.

Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

I. Organization of the

Human Body

4. Histology: The Study of

Tissues

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Part1 Organization ofthe Human Body108

Simple

cuboidal

epithelial

cell

Basement

membrane

Nucleus

Free surface

Kidney

(b)Simple cuboidal epithelium

Location: Kidney tubules, glands and their ducts, choroid plexus of the brain, lining of terminal bronchioles

of the lungs, and surface of the ovaries.

Structure: Single layer of cube-shaped cells; some cells have microvilli (kidney tubules) or cilia (terminal

bronchioles of the lungs).

Function: Active transport and facilitated diffusion result in secretion and absorption by cells of the kidney tubules;

secretion by cells of glands and choroid plexus; movement of particles embedded in mucus out of the terminal

bronchioles by ciliated cells.

LM 640x

Figure 4.2

Typesof Epithelium

Simple

squamous

epithelial

cell

Basement

membrane

Nucleus

Free surface

Lung alveoli

(a) Simple squamous epithelium

Location: Lining of blood and lymphatic vessels (endothelium) and small ducts, alveoli of the lungs,

loop of Henle in kidney tubules, lining of serous membranes (mesothelium), and inner surface of

the eardrum.

Structure: Single layer of flat, often hexagonal cells. The nuclei appear as bumps when viewed as

a cross section because the cells are so flat.

Function: Diffusion, filtration, some protection against friction, secretion, and absorption.

LM 640x

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4. Histology: The Study of

Tissues

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Chapter 4 Histology: The Study of Tissues 109

Simple

columnar

epithelial

cell

Basement

membrane

Nucleus

Free surface

Lining of

stomach and

intestines

Location: Glands and some ducts, bronchioles of lungs, auditory tubes, uterus, uterine tubes, stomach,

intestines, gallbladder, bile ducts, and ventricles of the brain.

Structure: Single layer of tall, narrow cells. Some cells have cilia (bronchioles of lungs, auditory tubes,

uterine tubes, and uterus) or microvilli (intestines).

Function: Movement of particles out of the bronchioles of the lungs by ciliated cells; partially responsible

for the movement of the oocyte through the uterine tubes by ciliated cells. Secretion by cells of the

glands, the stomach, and the intestine. Absorption by cells of the intestine.

Goblet cell

containing mucus

LM 640x

Moist stratified

squamous

epithelial cell

Basement

membrane

Nuclei

Free surface

Skin

Cornea

Mouth

Esophagus

(d)Stratified squamous epithelium

Location:Moist–mouth, throat, larynx, esophagus, anus, vagina, inferior urethra, and cornea.

Keratinized–skin.

Structure: Multiple layers of cells that are cuboidal in the basal layer and progressively flattened toward

the surface. The epithelium can be moist or keratinized. In moist stratified squamous epithelium the

surface cells retain a nucleus and cytoplasm. In keratinized stratified epithelium, the cytoplasm of

cells at the surface is replaced by keratin, and the cells are dead.

Function: Protection against abrasion and infection.

LM 286x

Figure 4.2

(continued)

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I. Organization of the

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4. Histology: The Study of

Tissues

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Part1 Organization ofthe Human Body110

Stratified

columnar

epithelial

cell

Basement

membrane

Nucleus

Free surface

Larynx

(f) Stratified columnar epithelium

Location: Mammary gland duct, larynx, and a portion of the male urethra.

Structure: Multiple layers of cells, with tall, thin cells resting on layers of more

cuboidal cells. The cells are ciliated in the larynx.

Function: Protection and secretion.

LM 413x

Figure 4.2

(continued)

Stratified

cuboidal

epithelial

cell

Basement

membrane

Nucleus

Free surface

Parotid gland

duct

Submandibular

gland duct

Sublingual

gland duct

(e) Stratified cuboidal epithelium

Location: Sweat gland ducts, ovarian follicular cells, and salivary gland ducts.

Structure: Multiple layers of somewhat cube-shaped cells.

Function: Secretion, absorption, and protection against infection.

LM 413x

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4. Histology: The Study of

Tissues

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Chapter 4 Histology: The Study of Tissues 111

Pseudostratified

columnar

epithelial cell

Basement

membrane

Nucleus

Free surface

Cilia

Goblet cell

containing mucus

Trachea

Bronchus

(g)Pseudostratified columnar epithelium

Location: Lining of nasal cavity, nasal sinuses, auditory tubes, pharynx, trachea, and bronchi

of lungs.

Structure: Single layer of cells; some cells are tall and thin and reach the free surface,

and others do not; the nuclei of these cells are at different levels and appear stratified;

the cells are almost always ciliated and are associated with goblet cells that secrete mucus

onto the free surface.

Function: Synthesize and secrete mucus onto the free surface and move mucus (or fluid)

that contains foreign particles over the surface of the free surface and from passages.

LM 413x

Tissue stretched

Basement

membrane

Nucleus

Free surface

Transitional

epithelial cell

Basement

membrane

Nucleus

Free surface

Transitional

epithelial

cell

Ureter

Urinary bladder

Urethra

Location: Lining of urinary bladder, ureters, and superior urethra.

Structure: Stratified cells that appear cuboidal when the organ or tube is not stretched and squamous when

the organ or tube is stretched by fluid.

Function: Accommodates fluctuations in the volume of fluid in an organ or tube; protection against the caustic

effects of urine.

(h)Transitional epithelium

Tissue not stretched

LM 413x

Figure 4.2

(continued)

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4. Histology: The Study of

Tissues

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Part1 Organization ofthe Human Body112

Table 4.1

Number of Layers

or Category Shape of Cells

Classification of Epithelium

Simple (single layer of cells) Squamous

Cuboidal

Columnar

Stratified (more than one Squamous

layer of cells) Moist

Keratinized

Cuboidal (very rare)

Columnar (very rare)

Pseudostratified Columnar

(modification of simple

epithelium)

Transitional (modification of Roughly cuboidal to columnar

stratified epithelium) when not stretched and

squamouslike when stretched

3. Pseudostratiﬁed columnar epithelium(ﬁgure 4.2g) is a

special type of simple epithelium.The preﬁx pseudo- means

false,so this type of epithelium appears to be stratiﬁed but is

not.It consists of one layer of cells, with all the cells attached

to the basement membrane.There is an appearance of two

or more layers ofcells because some of the cells are tall and

reach the free surface.Pseudostratiﬁed columnar epithelium

is found lining some ofthe respiratory passages, such as the

nasal cavity,trachea, and bronchi. Pseudostratiﬁed columnar

epithelium secretes mucus,which covers its surface, and cilia

located on the free surface move the mucus and the debris

that accumulates in it over the surfaces ofthe respiratory

passages and toward the exterior ofthe body.

There are three types ofepithelium based on the shape of the

epithelial cells.

1. Squamous(skwa¯⬘mu˘s; ﬂat) cells are ﬂat or scalelike.

2. Cuboidal(cubelike) cells are cube-shaped; about as wide as

they are tall.

3. Columnar(tall and thin, similar to a column) cells are

taller than they are wide.

In most cases an epithelium is given two names,such as simple

squamous,stratiﬁed squamous, simple columnar, or pseudostrati-

fied columnar.The ﬁrst name indicates the number of layers,and the

second indicates the shape ofthe cells (table 4.1) at the free surface.

Stratiﬁed squamous epithelium can be classiﬁed further as

either moist or keratinized,according to the condition of the out-

ermost layer ofcells. Moist stratiﬁed squamous epithelium (ﬁg-

ure 4.2d), found in areas such as the mouth,esophagus, rectum,

and vagina, consists of living cells in the deepest and outermost

layers.A layer of ﬂuid covers the outermost layers of cells, which

makes them moist.In contrast, keratinized (ker⬘˘a-ti-nizd)strati-

ﬁed squamous epithelium,found in the skin (see chapter 5), con-

sists of living cells in the deepest layers, and the outer layers are

composed of dead cells containing the protein keratin.The dead,

keratinized cells give the tissue a durable,moisture-resistant, dry

character.

A unique type of stratiﬁed epithelium called transitional

epithelium (ﬁgure 4.2h) lines the urinary bladder, ureters, and

pelvis ofthe kidney including the major and minor calyces (kal⬘-

i-s¯ez).These are structures where considerable expansion can oc-

cur. The shape of the cells and the number of cell layers vary,

depending on whether the transitional epithelium is stretched or

not.The surface cells and the underlying cells are roughly cuboidal

or columnar when the epithelium is not stretched, and they be-

come more ﬂattened or squamouslike when the epithelium is

stretched.Also, the number of layers of epithelial cells decreases in

response to stretch.As the epithelium is stretched, the epithelial

cells have the ability to shift on one another so that the number of

layers decreases from ﬁve or six to two or three.

4. List six characteristics common to most types of

epithelium. Deﬁne free (apical), lateral, and basal surfaces

of epithelial cells.

5. What is the basement membrane and what are its

functions? Whymust metabolically active epithelial cells be

close to the basementmembrane?

6. List six major functions of epithelia.

7. Describe simple, stratiﬁed, and pseudostratiﬁed epithelia.

Distinguish between squamous, cuboidal, and columnar

epithelial cells.

8. How do moist stratiﬁed squamous epithelium and

keratinized stratiﬁed squamousepithelium differ? Where is

each type found?

9. Describe the change in shape and number of layers that

occursin cells of transitional epithelium. Where is

transitional epithelium found?

FunctionalCharacteristics

Epithelial tissues have many functions (table 4.2),including form-

ing a barrier between a free surface and the underlying tissues and

secreting, transporting, and absorbing selected molecules. The

type and arrangement of organelles within each cell (see chapter

3),the shape of cells, and the organization of cells within each ep-

ithelial type reﬂect these functions. Accordingly,structural spe-

cializations of epithelial cells are consistent with the functions

they perform.

Cell Layersand Cell Shapes

Simple epithelium,with its sing le layer of cells,covers surfaces in

organs and functions to control diffusion of gases (lungs), ﬁlter

blood (kidneys), secrete cellular products (glands), or absorb

nutrients (intestines). The selective movement of materials

through epithelium would be hindered by a stratiﬁed epithelium,

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Chapter 4 Histology: The Study of Tissues 113

which is found in areas where protection is a major function.The

multiple layers ofcells in stratiﬁed epithelium are well adapted for

a protective role because,as the outer cells are damaged, they are

replaced by cells from deeper layers and a continuous barrier ofep-

ithelial cells is maintained in the tissue. Stratiﬁed squamous ep-

ithelium is found in areas of the body where abrasion can occur,

such as the skin,mouth, throat, esophagus, anus, and vagina.

Differing functions are also reﬂected in cell shape.Cells that

allow substances to diffuse through them and that ﬁlter are nor-

mally ﬂat and thin. For example, simple squamous epithelium

forms blood and lymphatic capillaries,the alveoli (air sacs) of the

lungs,and parts of the kidne y tubules.Cells that secrete or absorb

are usually cuboidal or columnar.They have greater cytoplasmic

volume compared to that of squamous epithelial cells; this cyto-

plasmic volume results from the presence oforganelles responsible

for the tissues’functions. For example, pseudostratiﬁed columnar

epithelium,which secretes large amounts of mucus, lines the respi-

ratory tract (see chapter 23) and contains large goblet cells,which

are specialized columnar epithelial cells.The goblet cells contain

abundant organelles responsible for the synthesis and secretion of

mucus,such as ribosomes, endoplasmic reticulum, Golgi appara-

tuses,and secretory vesicles ﬁlled with mucus.

PREDICT

Explain the consequencesof having (a) moiststratiﬁed epithelium

rather than simple columnar epithelium lining the digestive tract, (b)

moiststratiﬁed squamous epithelium rather than keratinized stratiﬁed

squamousepithelium in the skin, and (c) simple columnar epithelium

rather than moiststratiﬁed squamous epithelium lining the mouth.

Cell Surfaces

The free surfaces ofepithelia can be smooth, contain microvilli, be

ciliated, or be folded. Smooth surfaces reduce friction.Simple

squamous epithelium with a smooth surface forms the covering of

serous membranes. The lining of blood vessels is a simple squa-

mous epithelium that reduces friction as blood ﬂows through the

vessels (see chapter 21).

Microvilli and cilia were described in chapter 3.Microvilli

are nonmotile and contain microﬁlaments.They greatly increase

surface area and are found in cells that absorb or secrete,such as

the lining of the small intestine (see chapter 24). Stereocilia are

elongated microvilli. They are found where absorption is an im-

portant function,and are found in places such as in the epithelium

ofthe epididymis. Cilia are motile and contain microtubules. They

move materials across the surface of the cell. Simple ciliated

cuboidal, simple ciliated columnar,and pseudostratiﬁed ciliated

columnar epithelia are in the respiratory tract (see chapter 23),

where cilia move mucus that contains foreign particles like dust

out ofthe respiratory passages.

Transitional epithelium has a rather unusual plasma mem-

brane specialization: More rigid sections of membrane are sepa-

rated by very ﬂexible regions in which the plasma membrane is

folded. When transitional epithelium is stretched,the folded re-

gions ofthe plasma membrane can unfold. Transitional epithelium

is specialized to expand.It is found in the urinary bladder, ureters,

kidney pelvis,and calyces of the kidney.

Cell Connections

Lateral and basilar surfaces have structures that serve to hold cells

to one another or to the basement membrane (ﬁgure 4.3).These

structures do three things:(1) they mechanically bind the cells to-

gether,(2) they help form a permeability barrier, and (3) they pro-

vide a mechanism for intercellular communication.Epithelial cells

secrete glycoproteins that attach the cells to the basement mem-

brane and to one another.This relatively weak binding between

cells is reinforced by desmosomes (dez⬘mo¯-so¯mz), disk-shaped

structures with especially adhesive glycoproteins that bind cells to

one another and intermediate ﬁlaments that extend into the cyto-

plasm of the cells. Many desmosomes are found in epithelia that

are subjected to stress,such as the stratiﬁed squamous epithelium

of the skin. Hemidesmosomes, similar to one-half of a desmo-

some,attach epithelial cells to the basement membrane.

Free surface

Actin

filaments

Plaque

Basement membrane

Intermediate

filaments

Channel

Desmosome

Gap junction

Tight

junction

Hemidesmosome

Zonula

occludens

Zonula

adherens

Figure 4.3

CellConnections

Desmosomesanchor cells to one another and hemidesmosomes anchor cells

to the basementmembrane. Tight junctions consistof a zonula occludens and

zonula adherens. Gap junctionsallow adjacent cellsto communicate with each

other. Few cellshave all ofthese different connections.

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Part1 Organization ofthe Human Body114

cells, and active transport, cotransport, and facilitated diffusion

move most nutrients through the epithelial cells ofthe intestine.

A gap junction is a small specialized contact region be-

tween cells containing protein channels that aid intercellular

communication by allowing ions and small molecules to pass

from one cell to another (see ﬁgure 4.3).The exact function of

gap junctions in epithelium is not entirely clear,but they are im-

portant in coordinating the function ofcardiac and smooth mus-

cle tissues.Because ions can pass through the gap junctions from

one cell to the next,electric sig nals can pass from cell to cell to

coordinate the contraction of cardiac and smooth muscle cells.

Thus electric signals that originate in one cell of the heart can

spread from cell to cell and cause the entire heart to contract.The

gap junctions between cardiac muscle cells are found in special-

ized cell-to-cell connections called intercalated disks.Gap junc-

tions between ciliated epithelial cells may function to coordinate

the movements ofthe cilia.

10. What kind of functions would a single layer of epithelial

cellsbe expected to perform? A stratiﬁed layer?

11. In locations in which diffusion or ﬁltration is occurring,

whatshape would you expect the epithelial cells to be?

Tight junctionshold cells together and form a permeability

barrier (see ﬁgure 4.3). They consist of a zonula adherens and a

zonula occludens,which are found in close association with each

other.The zonula adherens (zo¯⬘nu¯-la˘,zon⬘u¯-la˘ ad-he¯r⬘enz) is lo-

cated between the plasma membranes ofadjacent cells and acts like

a weak glue that holds cells together.The zonulae adherens are best

developed in simple epithelial tissues;they for m a girdle of adhe-

sive glycoprotein around the lateral surface ofeach cell and bind

adjacent cells together.These connections are not as strong as

desmosomes.

The zonula occludens (o¯-klood⬘enz) forms a permeability

barrier.It is formed by plasma membranes of adjacent cells that join

one another in a jigsaw fashion to form a tight seal (see ﬁgure 4.3).

Near the free surface of simple epithelial cells,the zonulae occlu-

dens form a ring that completely surrounds each cell and binds ad-

jacent cells together.The zonulae occludens prevent the passage of

materials between cells.For example, in the stomach and in the uri-

nary bladder chemicals cannot pass between cells.Thus water and

other substances must pass through the epithelial cells,which can

actively regulate what is absorbed or secreted.Zonulae occludens

are found in areas where a layer ofsimple epithelium forms a per-

meability barrier.For example, water can diffuse through epithelial

Table 4.2

Simple Squamous Simple Cuboidal Simple Columnar

Function Epithelium Epithelium Epithelium

Diffusion Blood and lymph capillaries,

alveoli of lungs, thin segment

of loop of Henle

Filtration Bowman s capsule of kidney

Secretion or absorption Mesothelium (serous fluid) Choroid plexus (produces cerebro- Stomach, small intestine,

spinal fluid), part of kidney tubule, large intestine, uterus,

many glands and their ducts many glands

Protection (against friction Endothelium (e.g., epithelium

and abrasion) of blood vessels)

Mesothelium (e.g., epithelium

of body cavities)

Movement of mucus Terminal bronchioles of lungs Bronchioles of lungs,

(ciliated) auditory tubes,

uterine tubes, uterus

Capable of great stretching

Miscellaneous Lines the inner part of the Surface of ovary, inside lining of Bile duct, gallbladder,

eardrum, smallest ducts of eye (pigmented epithelium of ependyma (lining of

glands retina), ducts of glands brain ventricles and

central canal of spinal

cord), ducts of glands

Function and Location of Epithelial Tissue

’

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Chapter 4 Histology: The Study of Tissues 115

12. Why are cuboidal or columnar cells found where secretion

orabsorption is occurring?

13. What is the function of an epithelial free surface that is

smooth, hascilia, has microvilli, or is folded? Give an

example of epithelium in which each surface type isfound.

14. Name the ways in which epithelial cells are bound to one

anotherand to the basement membrane.

15. In addition to holding cells together, name an additional

function of tightjunctions. What is the general function of

gap junctions?

Glands

Glandsare secretory organs. Most glands are composed primarily

of epithelium, with a supporting network of connective tissue.

These glands develop from an infolding or outfolding of epithe-

lium in the embryo.If the gland maintains an open contact with

the epithelium from which it developed,a duct is present. Glands

with ducts are called exocrine(ek⬘so¯-krin) glands,and their ducts

are lined with epithelium.Alternatively,some glands become sepa-

rated from the epithelium of their origin. Glands that have no

ducts are called endocrine(en⬘do¯-krin) glands.Endocrine glands

have extensive blood vessels in the connective tissue ofthe glands.

The cellular products of endocrine glands, which are called hor-

mones(ho¯r⬘mo¯nz),are secreted into the bloodstream and are car-

ried throughout the body.Some of the endocrine glands, such as

the adrenal gland,form from non-epithelial tissue.

Most exocrine glands are composed of many cells and are

called multicellular glands, but some exocrine glands are com-

posed ofa single cell and are called unicellular glands (ﬁgure 4.4a).

Gobletcells (see ﬁgure 4.2c) of the respiratory system are unicel-

lular glands that secrete mucus.Multicellular glands can be classi-

fied further according to the structure oftheir ducts (ﬁgure 4.4b–i).

Glands that have ducts with few branches are called simple, and

glands with ducts that branch repeatedly are called compound.

Further classiﬁcation is based on whether the ducts end in tubules

(small tubes) or saclike structures called acini(as⬘i-nı¯;grapes,sug-

gesting a cluster of grapes or small sacs) or alveoli (al-ve¯⬘o¯-lı¯;a

hollow sac).Tubular glands can be classiﬁed as straight or coiled.

Most tubular glands are simple and straight,simple and coiled, or

compound and coiled.Acinar glands can be simple or compound.

Exocrine glands can also be classiﬁed according to how

products leave the cell.Merocrine (mer⬘o¯-krin) glands, such as

water-producing sweat glands and the exocrine portion of the

Skin (epidermis), cornea,

mouth and throat,

epiglottis, larynx,

esophagus, anus,

vagina

Larynx, nasal cavity,

paranasal sinus,

nasopharynx, auditory

tube, trachea, bronchi

of lungs

Urinary bladder,

ureter, upper

part of urethra

Lower part of urethra, Sweat gland ducts Part of male urethra, Part of male urethra,

sebaceous gland duct epididymis, ductus salivary gland duct

deferens, mammary

gland duct

Stratified Stratified Stratified Pseudostratified Transitional

Squamous Epithelium Cuboidal Epithelium Columnar Epithelium Columnar Epithelium Epithelium

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pancreas,secrete products with no loss of actual cellular material

(ﬁgure 4.5a).Secretions are either actively transported or packaged

in vesicles and then released by the process ofexocytosis at the free

surface ofthe cell. Apocrine (ap⬘o¯-krin) glands,such as the milk-

producing mammary glands, discharge fragments of the gland

cells in the secretion (ﬁgure 4.5b).Products are retained within the

cell,and large portions of the cell are pinched off to become part of

the secretion. Holocrine (hol⬘o¯-krin) glands, such as sebaceous

(oil) glands of the skin,shed entire cells (ﬁgure 4.5c). Products ac-

cumulate in the cytoplasm of each epithelial cell,the cell ruptures

and dies,and the entire cell becomes part of the secretion.

Part1 Organization ofthe Human Body116

Endocrine glands are so variable in their structure that

they are not classiﬁed easily.They are described in chapters 17

and 18.

16. Define the term gland. Distinguish between exocrine and

endocrine glands. Describe the classiﬁcation scheme for

multicellularexocrine glands on the basis of their duct

systems.

17. Describe three different ways in which exocrine glands

release theirsecretions. Give an example for each

method.

(a) Unicellular

(goblet cells in large and small

intestine and respiratory passages)

(b) Simple straight tubular

(glands in stomach

and colon)

(f) Simple branched acinar

(sebaceous glands of skin)

(g) Compound tubular

(mucous glands of duodenum)

(glands in lower portion

of stomach)

(d) Simple coiled tubular

(lower portion of stomach

and small intestine)

(e) Simple acinar

(sebaceous glands

of skin)

(h) Compound acinar

(mammary glands)

(i) Compound tubuloacinar

(pancreas)

Single gland cell

in epithelium

Figure 4.4

Structure ofExocrine Glands

The namesof exocrine glands are based on the shapes of their secretory units and their ducts.

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Chapter 4 Histology: The Study of Tissues 117

Connective Tissue

Objectives

■ List the functions of connective tissue.

■ List and describe the cells found in connective tissue.

■ Name the major large molecules of the connective tissue

matrix, and explain theirfunctions in the matrix.

Connective tissue is abundant,and it makes up part of every

organ in the body.The major structural characteristic that distin-

guishes connective tissue from the other three tissue types is that it

consists ofcells separated from each other by abundant extracellu-

lar matrix.Connective tissue structure is diverse, and it performs a

variety of important functions.

Functionsof Connective Tissue

Connective tissues perform the following major categories of

functions:

1. Enclosing and separating.Sheets of connective tissues form

capsules around organs such as the liver and kidneys.

Connective tissue also forms layers that separate tissues and

organs.For example, connective tissues separate muscles,

arteries,veins, and nerves from one another.

2. Connecting tissues to one another.For example, tendons are

strong cables,or bands, of connective tissue that attach

muscles to bone,and ligaments are connective tissue bands

that hold bones together.

3. Supporting and moving.Bones of the skeletal system provide

rigid support for the body,and the semirigid cartilage

supports structures such as the nose,ears, and surfaces of

joints.Joints between bones allow one part of the body to

move relative to other parts.

4. Storing.Adipose tissue (fat) stores high-energy molecules,

and bones store minerals such as calcium and phosphate.

5. Cushioning and insulating.Adipose tissue cushions and

protects the tissue it surrounds and provides an insulating

layer beneath the skin that helps conserve heat.

6. Transporting.Blood transports substances throughout the

body,such as gases, nutrients, enzymes, hormones,and cells

ofthe immune system.

7. Protecting.Cells of the immune system and blood provide

protection against toxins and tissue injury,as well as from

microorganisms.Bones protect underlying structures from

injury.

18. What is the major characteristic that distinguishes

connective tissue from othertissues?

19. List the functions of connective tissue, and give an example

of a connectve tissue thatperforms each function.

Cellsof Connective Tissue

The specialized cells ofthe var ious connective tissues produce the

extracellular matrix.The names of the cells end with sufﬁxes that

identify the cell functions as blasts,cytes, or clasts. Blasts create the

matrix,cytes maintain it, and clasts break it down for remodeling.

For example,ﬁbroblasts are cells that form ﬁbrous connective tis-

sue and ﬁbrocytes maintain it. Chondroblasts form cartilage

(chondro- refers to cartilage) and chondrocytes maintain it.

Secretion in duct

Vesicle releasing

contents into duct

Vesicle containing

secretory products

Secretory products

stored in the cell

Replacement

cell

Cell shed into

the duct

Pinched-off portion

of cell in the secretion

Dying cell releases

secretory products

(a) Merocrine gland

Cells of the gland produce vesicles that

contain secretory products, and the vesicles

empty their contents into the duct through

exocytosis.

(b) Apocrine gland

Secretory products are stored in the cell near

the lumen of the duct. A portion of the cell

near the duct that contains the secretory

products is actually pinched off the cell and

joins the secretion.

Secretory products are stored in the

cells of the gland. Entire cells are shed

by the gland and become part of the

secretion. The lost cells are replaced

by other cells deeper in the gland.

Figure 4.5

Exocrine Glandsand Secretion Types

Exocrine glandsare classiﬁed according to the type ofsecretion.

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Osteoblastsform bone (osteo- means bone), osteocytes maintain

it,and osteoclasts break it down (see chapter 6).

Adipose(ad⬘i-po¯s;fat), or fat, cells, also called adipocytes

(ad⬘i-po¯ -sı¯tz),contain large amounts oflipid. The lipid pushes the

rest ofthe cell contents to the periphery, so that each cell appears to

contain a large,centrally located lipid droplet with a thin layer of

cytoplasm around it.Adipose cells are rare in some connective tis-

sue types such as cartilage, are abundant in others such as loose

connective tissue,or are predominant such as in adipose tissue.

Mast cells are commonly found beneath membranes in

loose connective tissue and along small blood vessels of organs.

They contain chemicals such as heparin,histamine, and proteolytic

enzymes.These substances are released in response to injury such

as trauma and infection and play important roles in inﬂammation.

White blood cells continuously move from blood vessels

into connective tissues.The rate of movement increases dramati-

cally in response to injury or infection.In addition, accumulations

of lymphocytes, a type of white blood cell, are common in some

connective tissues,such as in the connective tissue beneath the ep-

ithelial lining ofcertain par ts of the digestive system.

Macrophages are found in some connective tissue types.

They are derived from monocytes, a white blood cell type.

Macrophages are either ﬁxedand do not move through the con-

nective tissue in which they are found or are wandering

macrophagesand move by ameboid movement through the con-

nective tissue. Macrophages phagocytize foreign or injured cells,

and they play a major role in providing protection against

infections.

Undifferentiated mesenchymal cells, sometimes called

stem cells,are embryonic cells that persist in adult connective tis-

sue.They have the potential to differentiate to form adult cell types

such as ﬁbroblasts or smooth muscle cells in response to injury.

Extracellular Matrix

The extracellular matrix ofconnective tissue has three major com-

ponents: (1) protein ﬁbers, (2) ground substance consisting of

nonﬁbrous protein and other molecules,and (3) ﬂuid. The struc-

ture ofthe matrix gives connective tissue types most of their func-

tional characteristics,such as the ability of bones and cartilage to

bear weight,of tendons and ligaments to withstand tension, and

of dermis of the skin to withstand punctures, abrasions, and

other abuses.

Protein Fibersof the Matrix

Three types of protein ﬁbers—collagen, reticular, and elastic

ﬁbers—help form connective tissue.

Collagen(kol⬘la˘-jen)ﬁbersconsist of collagen,which is the

most common protein in the body.Collagen accounts for one-

fourth to one-third of the total body protein, which is approxi-

mately 6% of the total body weight. Each collagen molecule

resembles a microscopic rope consisting of three polypeptide

chains coiled together.Collagen is very strong and ﬂexible but

quite inelastic. There are at least 15 different types of collagen,

many ofwhich are speciﬁc to certain tissues. Collagen ﬁbers differ

in the types of amino acids that make up the polypeptide chains.

Of the 15 types of collagen, 6 types are most common. Bone,

Part1 Organization ofthe Human Body118

dentin,and cementum contain mainly type I collagen, cartilage is

mainly type II collagen, and reticular ﬁbers are mainly type III

collagen.

Reticular(re-tik⬘u¯-la˘r ; netlike) ﬁbers are actually very ﬁne

collagen ﬁbers and therefore are not a chemically distinct category

ofﬁbers. They are very short, thin ﬁbers that branch to form a net-

work and appear different microscopically from other collagen

ﬁbers.Reticular ﬁbers are not as strong as most collagen ﬁbers, but

networks ofreticular ﬁbers ﬁll space between tissues and organs.

Elastic ﬁberscontain a protein called elastin (e˘-las⬘tin).As

the name suggests,this protein is elastic with the ability to return to

its original shape after being distended or compressed.Elastin gives

the tissue in which it is found an elastic quality.Elastin molecules

look like tiny coiled springs,and individual molecules are cross-

linked to produce a large,interwoven meshwork of springlike mol-

ecules that extend through the entire tissue.

OtherMatrix Molecules

Two types of large, nonﬁbrous molecules called hyaluronic acid

and proteoglycans are part ofthe extracellular matrix. These mol-

ecules constitute most ofthe ground substance ofthe matr ix,the

“shapeless”background against which the collagen ﬁbers are seen

through the microscope.The molecules themselves, however, are

not shapeless but are highly structured. Hyaluronic (hı¯⬘a˘-loo-

ron⬘ik; glassy appearance) acid is a long, unbranched polysaccha-

ride chain composed ofrepeating disaccharide units. It gives a very

slippery quality to the ﬂuids that contain it;for that reason, it is a

good lubricant for joint cavities (see chapter 8).Hyaluronic acid is

also found in large quantities in connective tissue and is the major

component of the vitreous humor of the eye (see chapter 15). A

proteoglycan(pro¯⬘te¯-o¯-glı¯⬘kan; formed from proteins and poly-

saccharides) is a large molecule that consists ofnumerous polysac-

charides, called glycosaminoglycans (glı¯⬘k¯os-am-i-n¯o-glı¯⬘kan)

each attached at one end to a common protein core.These proteo-

glycan monomers resemble minute pine tree branches.The pro-

tein core is the branch of the tree, and the proteoglycans are the

needles.The protein cores of proteoglycan monomers can attach to

a molecule of hyaluronic acid to form a proteoglycan aggregate.

The aggregate resembles a complete pine tree,with hyaluronic acid

represented by the tree trunk and the proteoglycan monomers

forming the limbs. Proteoglycans trap large quantities of water,

which gives them the capacity to return to their original shape

when compressed or deformed.There are several different types of

glycosaminoglycans,and their abundance varies with each connec-

tive tissue type.

Several adhesive moleculesare found in ground substance.

These adhesive molecules hold the proteoglycan aggregates to-

gether and to structures such as the plasma membranes.A speciﬁc

adhesive molecule type predominates in certain types of ground

substance.For example, chondronectin is in the ground substance

ofcartilage, osteonectin is in the ground substance of bone, and ﬁ-

bronectinis in the ground substance of ﬁbrous connective tissues.

20. Explain the difference between connective tissue cells that

are termed blast, cyte, orclast cells.

21. Describe and give the functions of the cells of connective

tissue.

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22. What three components are found in the extracellular matrix

of connective tissue? Contrastthe structure and

characteristicsof collagen ﬁbers, reticular ﬁbers, and

elastin ﬁbers.

23. Describe the structure and function of hyaluronic acid and

proteoglycan aggregates. Whatis the function of adhesive

molecules?

Classiﬁcation ofConnective Tissue

Objective

■ List the major categories of connective tissue, and describe

the characteristicsof each.

Connective tissue types blend into one another,and the tran-

sition points cannot be deﬁned precisely.As a result, the classiﬁca-

tion scheme for connective tissues is somewhat arbitrary.

Classiﬁcation schemes for connective tissue are inﬂuenced by (1)

protein ﬁbers and the arrangement ofprotein ﬁbers in the extracel-

lular matrix,(2) protein ﬁbers and ground substance in the extracel-

lular matrix,and (3) a ﬂuid extracellular matrix. The classiﬁcation of

connective tissues used here is presented in Table 4.3.

The two major categories ofconnective tissue are embryonic

and adult connective tissues.

Embryonic Connective Tissue

Embryonic connective tissue is called mesenchyme(mez⬘en-kı¯m).

It is made up ofirregularly shaped ﬁbroblasts surrounded by abun-

dant semiﬂuid extracellular matrix in which delicate collagenous

ﬁbers are distributed (ﬁgure 4.6a).It forms in the embr yo during

the third and fourth weeks of development from mesoderm and

neural crest cells (see chapter 29), and all adult connective tissue

types develop from it.By 8 weeks of development most of the mes-

enchyme has become specialized to form types ofconnective tissue

seen in adults as well as muscle,blood vessels, and other tissues. The

major source ofremaining embryonic connective tissue in the new-

born is found in the umbilical cord,where it is called mucous con-

nective tissue or Wharton’s jelly (ﬁgure 4.6b). The structure of

mucous connective tissue is similar to mesenchyme.

Adult Connective Tissue

Adult connective tissue consists ofsix types: loose, dense, connec-

tive tissue with special properties,cartilage, bone, and blood.

Loose Connective Tissue

Loose connective tissue(ﬁgure 4.7a) which is sometimes referred

to as areolar (a˘-re¯⬘o¯-la˘r; area) tissue, consists of protein ﬁbers

that form a lacy network with numerous ﬂuid-ﬁlled spaces.Areo-

lar tissue is the “loose packing”material of most organs and other

tissues,and attaches the skin to underlying tissues. It contains col-

lagen,reticular, and elastic ﬁbers and a variety of cells. For exam-

ple, ﬁbroblasts produce the ﬁbrous matrix,macrophages move

through the tissue engulﬁng bacteria and cell debris, mast cells

contain chemicals that help mediate inﬂammation,and lympho-

cytes are involved in immunity.The loose packing of areolar tissue

is often associated with other connective tissue types such as retic-

ular tissue and fat (adipose tissue).

Dense Connective Tissue

Protein ﬁbers ofdense connective tissue form thick bundles and

ﬁll nearly all of the extracellular space.Most of the cells of devel-

oping dense connective tissue are spindle-shaped ﬁbroblasts.Once

the ﬁbroblasts become completely surrounded by matrix,they are

ﬁbrocytes.Dense connective tissue can be subdivided into two ma-

jor groups:regular and irregular.

Dense regular connective tissuehas protein ﬁbers in the ex-

tracellular matrix that are oriented predominantly in one direction.

Dense regular collagenous connective tissue (ﬁgure 4.7b) has

abundant collagen ﬁbers.The collagen ﬁbers give this tissue a white

appearance. Dense regular collagenous connective tissue forms

structures such as tendons, which connect muscles to bones (see

chapter 11),and most ligaments, which connect bones to bones (see

chapter 8). The collagen ﬁbers of dense connective tissue resist

stretching and give the tissue considerable strength in the direction

ofthe ﬁber orientation. Tendons and most ligaments consist almost

entirely of thick bundles of densely packed parallel collagen ﬁbers

with the orientation of the collagen ﬁbers in one direction which

makes the tendons and ligaments very strong cable-like structures.

The general structure of tendons and ligaments is similar,

but major differences between them exist.The differences include

the following: (1) collagen ﬁbers of ligaments are often less com-

pact,(2) some ﬁbers of many ligaments are not parallel, and (3) lig-

aments usually are more ﬂattened than tendons and form sheets or

bands oftissues.

Dense regular elastic connective tissue(ﬁgure 4.7c) consists

of parallel bundles of collagen ﬁbers and abundant elastic ﬁbers.

The elastin in elastic ligaments gives them a slightly yellow color.

Dense regular elastic connective tissue forms some elastic liga-

ments, such as those in the vocal folds and the nuchal(noo⬘ka˘l;

back of the neck) ligament, which lies along the posterior of the

neck and helps hold the head upright.When elastic ligaments are

stretched,they tend to shorten to their original length, much like

an elastic band.

Table 4.3 Classification of Connective Tissue

A. Embryonic connective tissue

1. Mesenchyme

2. Mucous

B. Adult connective tissue

1. Loose

2. Dense

a. Irregularly arranged

1. Collagenous

2. Elastic

b. Regularly arranged

1. Collagenous

2. Elastic

3. Special properties

a. Adipose

b. Reticular

4. Cartilage

5. Bone

6. Hemopoietic tissue and blood

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PREDICT

Explain the advantagesof having elastic ligamentsthat extend from

vertebra to vertebra in the vertebralcolumn and why it would be a

disadvantage iftendons, which connectskeletal muscles to bone,

were elastic.

Dense irregular connective tissue contains protein ﬁbers

arranged as a meshwork ofrandomly oriented ﬁbers. Alternatively,

the ﬁbers within a given layer ofdense ir regular connective tissue

can be oriented in one direction whereas the ﬁbers ofadjacent lay-

Part1 Organization ofthe Human Body120

ers are oriented at nearly right angles to that layer.Dense irregular

connective tissue forms sheets of connective tissue that have

strength in many directions,but less strength in any single direc-

tion than does regular connective tissue.

PREDICT

Scarsconsist of dense irregular connective tissue made ofcollagen

ﬁbers. Vitamin Cis required for collagen synthesis. Predictthe effect

ofscurvy, which is a nutritional disease caused byvitamin C

deﬁciency, on wound healing.

Intercellular

matrix

Nuclei of

mesenchyme

cells

Location: Mesenchyme is the embryonic tissue from which connective

tissues, as well as other tissues, arise.

(a)Mesenchyme

Structure: The mesenchymal cells are irregularly shaped. The extracellular

matrix is abundant and contains scattered reticular fibers.

LM 200x

Intercellular

matrix

Nuclei of

mucous

connective

tissue cells

Umbilical

cord

(b) Mucous connective tissue

Location: Umbilical cord of newborn.

Structure: Mucous tissue is mesenchymal tissue that

remains unspecialized. The cells are irregularly shaped.

The extracellular matrix is abundant and contains

scattered reticular fibers.

LM 200x

Figure 4.6

EmbryonicConnective Tissue

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Dense irregular collagenous connective tissue (ﬁgure

4.7d) forms most of the dermis of the skin, which is the tough, in-

ner portion ofthe skin (see chapter 5) and of the connective tissue

capsules that surround organs such as the kidney and spleen.

Dense irregular elastic connective tissue (ﬁgure 4.7e) is

found in the wall of elastic arteries. In addition to collagen ﬁbers,

oriented in many directions,there are abundant elastic ﬁbers in the

layers ofthis tissue.

Loose connective

tissue

Skin

(a) Loose, or areolar, connective tissue

Location: Widely distributed throughout the body; substance on which epithelial basement membranes

rest; packing between glands, muscles, and nerves. Attaches the skin to underlying tissues.

Structure: Cells (e.g., fibroblasts, macrophages, and lymphocytes) within a fine network of mostly

collagen fibers. Often merges with denser connective tissue.

Function: Loose packing, support, and nourishment for the structures with which it is associated.

Muscle

Fat

Elastic

fiber

Collagen

fiber

Nucleus

LM 400x

Tendon

(b) Dense regular collagenous connective tissue

Location: Tendons (attach muscle to bone) and ligaments (attach bones to each other).

Structure: Matrix composed of collagen fibers running in somewhat the same direction.

Function: Ability to withstand great pulling forces exerted in the direction of fiber orientation, great

tensile strength, and stretch resistance.

Nucleus of

fibroblast

Collagen

fibers

LM 165x

Figure 4.7

Typesof Connective Tissue

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Part1 Organization ofthe Human Body122

Vocal folds

(true vocal cords)

Nucleus of

fibroblast

Elastin

fibers

Vestibular fold

(false vocal cord)

Base of tongue

Location: Ligaments between the vertebrae and along the dorsal aspect of the neck (nucha) and in the

vocal cords.

Structure: Matrix composed of regularly arranged collagen fibers and elastin fibers.

Function: Capable of stretching and recoiling like a rubber band with strength in the direction of fiber

orientation.

LM 100x

Loose connective

tissue

Epidermis

(d) Dense irregular collagenous connective tissue

Location: Sheaths; most of the dermis of the skin; organ capsules and septa; outer covering of body tubes.

Structure: Matrix composed of collagen fibers that run in all directions or in alternating planes of fibers

oriented in a somewhat single direction.

Function: Tensile strength capable of withstanding stretching in all directions.

Muscle

Fat

Dermis

Skin

Dense irregular

collagenous

connective tissue

of dermis

Epidermis

LM 100x

Figure 4.7

(continued)

24. List the two types of embryonic connective tissue. To what

doesmesenchyme give rise in the adult?

25. Describe the ﬁber arrangement in loose (areolar)

connective tissue. Whatare the functions of this tissue type?

26. Structurally and functionally, what is the difference between

dense regularconnective tissue and dense irregular

connective tissue?

27. Name the two kinds of dense regular connective tissue, and

give an example of each. Do the same fordense irregular

connective tissue.

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(e)Dense irregular elastic connective tissue

Location: Elastic arteries.

Structure: Matrix composed of bundles and sheets of collagenous and elastin fibers oriented in

multiple directions.

Function: Capable of strength with stretching and recoil in several directions.

Aorta

Dense irregular

elastic connective

tissue

LM 265x

(f) Adipose tissue

Location: Predominantly in subcutaneous areas, mesenteries, renal pelvis, around kidneys, attached to

the surface of the colon, mammary glands, and in loose connective tissue that penetrates into spaces and

crevices.

Structure: Little extracellular matrix surrounding cells. The adipocytes, or fat cells, are so full of lipid that

the cytoplasm is pushed to the periphery of the cell.

Function: Packing material, thermal insulator, energy storage, and protection of organs against injury from

being bumped or jarred.

Adipose

tissue

Mammary

gland

Adipocytes

or fat cells

Nucleus

LM 100x

Figure 4.7

(continued)

Connective Tissue with Special Properties

Adipose tissue and reticular tissue are connective tissues with spe-

cial properties.Adipose tissue(ﬁgure 4.7f ) consists of adipocytes,

or fat cells,which contain large amounts of lipid. Unlike other con-

nective tissue types,adipose tissue is composed of large cells and a

small amount of extracellular matrix that consists of loosely

arranged collagen and reticular ﬁbers with some scattered elastic

ﬁbers. Blood vessels form a network in the extracellular matrix.

The fat cells are usually arranged in clusters or lobules separated

from one another by loose connective tissue.Adipose tissue func-

tions as an insulator,a protective tissue, and a site of energy stor-

age.Lipids take up less space per calorie than either carbohydrates

or proteins and therefore are well adapted for energy storage.

Adipose tissue exists in both yellow (white) and brown

forms. Yellow adiposetissue is by far the most abundant. Yellow

adipose tissue appears white at birth,but it turns yellow with age

because ofthe accumulation of pigments such as carotene,a plant

pigment that humans can metabolize as a source of vitamin A.

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Storage,insulation, and protection are functions of yellow adipose

tissue.Brown adipose tissue is found only in speciﬁc areas of the

body such as the axillae (armpits),neck, and near the kidneys. The

brown color results from the cytochrome pigments in its numer-

ous mitochondria and its abundant blood supply.Although brown

fat is much more prevalent in babies than in adults,it is difﬁcult to

distinguish brown fat from yellow fat in babies because the color

difference between them is not great. Brown fat is specialized to

generate heat as a result ofoxidative metabolism of lipid molecules

in mitochondria and can play a signiﬁcant role in body tempera-

ture regulation in newborn babies.

Reticular tissue forms the framework of lymphatic tissue

(ﬁgure 4.7g),such as in the spleen and lymph nodes, as well as in

bone marrow and the liver.It is characterized by a network ofretic-

ular ﬁbers and reticular cells.Reticular cells produce the reticular

ﬁbers and remain closely attached to them.The spaces between the

reticular ﬁbers can contain a wide variety of other cells, such as

dendritic cells, which look very much like reticular cells but are

cells of the immune system, macrophages, and blood cells (see

chapter 22).

28. What feature of the extracellular matrix distinguishes

adipose tissue from otherconnective tissue types? What is

an adipocyte?

29. List the functions of adipose tissue. Name the two types

ofadipose tissue. Which one is important in generating

heat?

30. What is the function of reticular tissue?

Where isit found?

Cartilage

Cartilage (kar⬘ti-lij) is composed of cartilage cells, or chondro-

cytes (kon⬘dro¯ -sı¯tz),located in spaces called lacunae (la˘-koo⬘ne¯)

within an extensive and relatively rigid matrix.Next to bone, carti-

lage is the ﬁrmest structure in the body.The matrix contains pro-

tein ﬁbers, ground substance, and ﬂuid.The protein ﬁbers are

collagen ﬁbers or,in some cases, collagen and elastic ﬁbers. The

ground substance consists ofproteoglycans and other organic mol-

ecules. Most of the proteoglycans in the matrix form aggregates

with hyaluronic acid.Within the cartilage matrix, proteoglycan ag-

gregates function as minute sponges capable of trapping large

quantities of water.This tr apped water allows cartilage to spring

back after being compressed.The collagen ﬁbers give cartilage con-

siderable strength.

The surface of nearly all cartilage is surrounded by a layer of

dense irregular connective tissue called the perichondrium(per-i-

kon⬘dre¯-u˘ m).The str ucture of the perichondrium is described in

more detail in chapter 6.Cartilage cells arise from the perichondrium

and secrete cartilage matrix.Once completely surrounded by matrix

the cartilage cells are called chondrocytesand the spaces in which they

are located are called lacunae.Cartilage has no blood vessels or nerves

except those ofthe perichondrium; it therefore heals very slowly after

an injury because the cells and nutrients necessary for tissue repair

cannot reach the damaged area easily.

There are three types ofcartilage.

1. Hyaline(hı¯⬘a˘ -lin) cartilage has large amounts of both

collagen ﬁbers and proteoglycans (ﬁgure 4.7h).Collagen

ﬁbers are evenly dispersed throughout the ground

Part1 Organization ofthe Human Body124

Tonsils

(g) Reticular tissue

Location: Within the lymph nodes, spleen, and bone marrow.

Structure: Fine network of reticular fibers irregularly arranged.

Function: Provides a superstructure for the lymphatic and hemopoietic tissues.

Spleen

Peyer’s patches

in intestinal wall

Thymus

Bone marrow

Appendix

Lymph

node

Nucleus of

lymphocyte

Reticular

fibers

LM 100x

Figure 4.7

(continued)

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Bone

Hyaline

cartilage

Chondrocyte

in a lacuna

Nucleus

Matrix

(h)Hyaline cartilage

Location: Growing long bones, cartilage rings of the respiratory system, costal cartilage of ribs, nasal

cartilage, articulating surface of bones, and the embryonic skeleton.

Structure: Collagen fibers are small and evenly dispersed in the matrix, making the matrix appear transparent.

The cartilage cells, or chondrocytes, are found in spaces, or lacunae, within the firm but flexible matrix.

Function: Allows growth of long bones. Provides rigidity with some flexibility in the trachea, bronchi, ribs, and

nose. Forms rugged, smooth, yet somewhat flexible articulating surfaces. Forms the embryonic skeleton.

LM 240x

Figure 4.7

(continued)

substance,and in joints, hyaline cartilage has a very smooth

surface.Specimens appear to have a glassy, translucent

matrix when viewed through a microscope.Hyaline

cartilage is found in areas in which strong support and

some ﬂexibility are needed,such as in the rib cage and the

cartilage within the trachea and bronchi (see chapter 23).It

also covers the surfaces ofbones that move smoothly

against each other in joints.Hyaline cartilage forms most of

the skeleton before it is replaced by bone in the embryo,and

it is involved in growth that increases the length ofbones

(see chapter 6).

2. Fibrocartilagehas more collagen ﬁbers than proteoglycans

(ﬁgure 4.7i).Compared to hyaline cartilage, ﬁbrocartilage

has much thicker bundles ofcollagen ﬁbers dispersed

through its matrix.Fibrocartilage is slightly compressible

and very tough.It is found in areas of the body where a

great deal ofpressure is applied to joints, such as the knee,

the jaw,and between vertebrae.

3. Elastic cartilage has elastic ﬁbers in addition to collagen

and proteoglycans (ﬁgure 4.7j).The numerous elastic ﬁbers

are dispersed throughout the matrix ofelastic car tilage.It is

found in areas,such as the external ears, that have rigid but

elastic properties.

PREDICT

One ofseveral changes caused byrheumatoid arthritis in joints is the

replacementof hyaline cartilage with dense irregular collagenous

connective tissue. Predictthe effect of replacing hyaline cartilage with

ﬁbrousconnective tissue.

Bone

Bone is a hard connective tissue that consists of living cells and

mineralized matrix.Bone matrix has an organic and an inorganic

portion. The organic portion consists of protein ﬁbers,pr imarily

collagen,and other organic molecules. The mineral, or inorganic,

portion consists of specialized crystals called hydroxyapatite

(hı¯-drok⬘se¯ -ap-a˘-tı¯t), which contain calcium and phosphate. The

strength and rigidity ofthe mineralized matr ix allow bones to sup-

port and protect other tissues and organs ofthe body. Bone cells,or

osteocytes (os⬘te¯-o¯-sı¯tz), are located within holes in the matrix,

which are called lacunae and are similar to the lacunae ofcartilage.

Two types ofbone exist.

1. Cancellous(kan⬘se˘-lu˘ s), or spongy,bone has spaces

between trabeculae(tra˘-bek⬘u¯-le¯; beams),or plates, of

bone and therefore resembles a sponge (ﬁgure 4.7k).

2. Compact bone is more solid with almost no space between

many thin layers,or lamellae (la˘-mel⬘¯e; pl., la˘-mel⬘a˘; sing.)

ofbone (ﬁgure 4.7l).

Bone, unlike cartilage, has a rich blood supply.For this reason,

bone can repair itselfmuch more readily than can cartilage. Bone is

described more fully in chapter 6.

HemopoieticTissue and Blood

Blood is unusual among the connective tissues because the ma-

trix between the cells is liquid (ﬁgure 4.7m). Like many other

connective tissues blood has abundant extracellular matrix.The

cells of most other connective tissues are more or less stationary

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within a relatively rigid matrix, but blood cells are free to move

within a ﬂuid matrix.Some blood cells leave the bloodstream and

wander through other tissues.The liquid matrix of blood allows

it to ﬂow rapidly through the body,carrying food, oxygen, waste

products, and other materials. The matrix of blood is also un-

usual in that most ofit is produced by cells contained in other tis-

sues rather than by blood cells.Blood is discussed more fully in

chapter 19.Hemopoietic (he¯⬘mo¯ -poy-et⬘ik) tissue forms blood

Part1 Organization ofthe Human Body126

cells. Most of the hemopoietic tissue is found in bone marrow

(mar⬘o¯) (ﬁgure 4.7n), which is the soft connective tissue in the

cavities of bones. Two types ofbone marrow exist: yellow mar-

rowand red marrow (see chapter 6).Yellow marrow consists of

yellow adipose tissue, and red marrow consists of hemopoietic

tissue surrounded by a framework of reticular ﬁbers. Hemopoi-

etic tissue produces red and white blood cells and is described in

detail in chapter 19.

(i) Fibrocartilage

Location: Intervertebral disks, symphysis pubis, articular disks (e.g., knee and temporomandibular [jaw]

joints).

Structure: Collagenous fibers similar to those in hyaline cartilage. The fibers are more numerous than

in other cartilages and are arranged in thick bundles.

Function: Somewhat flexible and capable of withstanding considerable pressure. Connects structures

subjected to great pressure.

Intervertebral

disk

Chondrocyte

in lacuna

Nucleus

Collagen fibers

in matrix

LM 240x

Chondrocytes

in lacunae

Nucleus

Elastic fibers

in matrix

(j) Elastic car tilage

Location: External ear, epiglottis, and auditory tubes.

Structure: Similar to hyaline cartilage, but matrix also contains elastin fibers.

Function: Provides rigidity with even more flexibility than hyaline cartilage because elastic fibers return

to their original shape after being stretched.

LM 240x

Figure 4.7

(continued)

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Chapter 4 Histology: The Study of Tissues 127

31. Describe the cells and matrix of cartilage. What are

lacunae? Whatis the perichondrium? Why does cartilage

heal slowly?

32. How do hyaline cartilage, ﬁbrocartilage, and elastic

cartilage differin structure and function? Give an example

of each.

33. Describe the cells and matrix of bone. Differentiate between

cancellousbone and compact bone.

34. What characteristic separates blood from the other

connective tissues?

35. Describe the function of hemopoietic tissue. Explain the

difference between red marrowand yellow marrow.

Bone

trabecula

Bone marrow

Osteocyte

nucleus

Osteoblast

nuclei

Matrix

Irregular bone (sphenoid)

from the skull

(k)Cancellous bone

Location: In the interior of the bones of the skull, vertebrae, sternum, and pelvis;

also found in the ends of the long bones.

Structure: Latticelike network of scaffolding characterized by trabeculae with large

spaces between them filled with hemopoietic tissue. The osteocytes, or bone cells,

are located within lacunae in the trabeculae.

Function: Acts as a scaffolding to provide strength and support without the greater

weight of compact bone.

LM 240x

(l) Compact bone

Location: Outer portions of all bones and the shafts of long bones.

Structure: Hard, bony matrix predominates. Many osteocytes (not seen in this bone preparation)

are located within lacunae that are distributed in a circular fashion around the central canals. Small

passageways connect adjacent lacunae.

Function: Provides great strength and support. Forms a solid outer shell on bones that keeps them

from being easily broken or punctured.

Lacuna

Central

canal

Matrix

organized

into lamellae

Bone

LM 240x

Figure 4.7

(continued)

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Part1 Organization ofthe Human Body128

Muscle contraction is accomplished by the interaction ofcontrac-

tile proteins,which are described in chapter 9. Muscles contract to

move the entire body,to pump blood through the heart and blood

vessels,and to decrease the size of hollow organs, such as the stom-

ach and urinary bladder.The three types of muscle tissue are skeletal,

cardiac,and smooth muscle. The types of muscle tissue are grouped

according to both structure and function (table 4.4).Muscle tissue

grouped according to structure is either striated (strı¯⬘a¯t-e˘d), in

which microscopic bands or striations can be seen in muscle cells,

Cancellous bone

with red marrow

Cells destined

to become red

blood cells

Nuclei

Fat

(n)Bone marrow

Location: Within marrow cavities of bone. Two types: yellow marrow (mostly adipose tissue) in the

shafts of long bones; and red marrow (hemopoietic or blood-forming tissue) in the ends of long bones

and in short, flat, and irregularly shaped bones.

Structure: Reticular framework with numerous blood-forming cells (red marrow).

Function: Production of new blood cells (red marrow); lipid storage (yellow marrow).

LM 600x

Figure 4.7

(continued)

Muscle Tissue

Objectives

■ List the main characteristics of muscle tissue.

■ Name the types of muscle tissue, and list their major

characteristics.

The main characteristic of muscle tissue is that it contracts

or shortens with force,and therefore is responsible for movement.

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Chapter 4 Histology: The Study of Tissues 129

ornonst riated. When classiﬁed according to function, a muscle

is voluntary, meaning that it is usually consciously controlled, or

involuntary, meaning that it is not normally consciously con-

trolled. Thus the three muscle types are striated voluntary, or

skeletal muscle (ﬁgure 4.8a); striated involuntary, or cardiac

muscle (ﬁgure 4.8b); and nonstriated involuntary, or smooth

muscle(ﬁgure 4.8c).

For most people,the term muscle means skeletal muscle (see

chapter 9), which constitutes the meat ofanimals and represents a

large portion ofthe total weight of the human body.Skeletal muscle,

as the name implies, attaches to the skeleton and,by contracting,

causes the major body movements.Cardiac muscle is the muscle of

the heart (see chapter 20), and contraction of cardiac muscle is re-

sponsible for pumping blood.Smooth muscle is widespread through-

out the body and is responsible for a wide range offunctions, such as

movements in the digestive,urinary, and reproductive systems.

36. Functionally, what is unique about muscle tissue? Contrast

the structure of skeletal, cardiac, and smooth muscle cells.

Which of the muscle typesis under voluntary control? What

tasksdoes each type perform?

Nervous Tissue

Objective

■ Describe the characteristics of nervous tissue.

Table 4.4

Features Skeletal Muscle Cardiac Muscle Smooth Muscle

Comparison of Muscle Types

Location Attached to bones Heart Walls of hollow organs, blood

vessels, eyes, glands, and skin

Cell shape Very long, cylindrical cells (1–40 mm Cylindrical cells that branch Spindle-shaped cells (15–200 µm

in length and may extend the (100–500 µm in length; in length; 5–10 µm in

entire length of the muscle; 100–200 µm in diameter) diameter)

10–100µm in diameter)

Nucleus Multinucleated, peripherally located Single, centrally located Single, centrally located

Striations Yes Ye s No

Control Voluntary Involuntary Involuntary

Ability to contract

spontaneously No Yes Yes

Function Body movement Contraction provides the major force Movement of food through the

for moving blood through the digestive tract, emptying of the

blood vessels urinary bladder, regulation of

blood vessel diameter, change in

pupil size, contraction of many

gland ducts, movement of hair,

and many more functions

Special features Branching fibers, intercalated disks Gap junctions

join the cells to each other

(gap junctions)

The fourth and ﬁnal class of tissue is nervous tissue. It is

found in the brain,spinal cord, and nerves, and is characterized by

the ability to conduct electric signals called action potentials. It

consists ofneurons, which are responsible for this conductive abil-

ity,and support cells called neuroglia.

Neurons,or nerve cells (ﬁgure 4.9), are the actual conduct-

ing cells ofnervous tissue. Just as an electrical wiring system trans-

ports electricity throughout a house, neurons transport electric

signals throughout the body.They are composed of three major

parts: cell body,dendrites, and axon. The cell body contains the

nucleus and is the site of general cell functions. Dendrites and

axons are two types of nerve cell processes, both consisting of

projections of cytoplasm surrounded by membrane. Dendrites

(den⬘drı¯tz) usually receive action potentials and conduct them to-

ward the cell body.They are much shorter than axons and usually

taper to a ﬁne tip.Axons (ak⬘sonz) usually conduct action poten-

tials away from the cell body.They can be much longer than den-

drites,and they have a constant diameter along their entire length.

Neurons that possess several dendrites and one axon are

calledmultipolar neurons (ﬁgure 4.9a). Neurons that possess a sin-

gle dendrite and an axon are called bipolar neurons.Some very spe-

cialized neurons,called unipolar neurons (ﬁgure 4.9b), have only

one axon and no dendrites.Within each subgroup are many shapes

and sizes ofneurons, especially in the brain and the spinal cord.

Neuroglia(noo-rog⬘le¯-a˘;ner ve glue) are the support cells of

the brain, spinal cord, and peripheral nerves (ﬁgure 4.10).The

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term neuroglia originally referred only to the support cells of the

central nervous system,but it is now also applied to cells in the pe-

ripheral nervous system. Neuroglia nourish,protect, and insulate

neurons. Neurons and neuroglial cells are described in greater

detail in chapter 11.

Part1 Organization ofthe Human Body130

37. Functionally, what is unique about nervous tissue?

38. Deﬁne and list the functions of the cell body, dendrites, and

axon of a neuron. Differentiate between multipolar, bipolar,

and unipolarneurons.

39. What is the general function of neuroglia?

Muscle

(a)Skeletal muscle

Location: Attaches to bone.

Structure: Skeletal muscle cells or fibers appear striated (banded). Cells are large, long, and cylindrical,

with many nuclei located at the periphery.

Function: Movement of the body; under voluntary control.

Nucleus (near periphery

of cell)

Striations

Skeletal

muscle

fiber

LM 800x

Nucleus (central)

Striations

Intercalated disks

(special junctions

between cells)

Cardiac

muscle cell

(b)

Cardiac muscle

Location:

Structure: Cardiac muscle cells are cylindrical and striated and have a single, centrally located nucleus.

They are branched and connected to one another by intercalated disks.

Function:

Cardiac muscle is in the heart.

Pumps the blood; under involuntary control.

LM 800x

Figure 4.8

Typesof Muscle Tissue

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Nucleus

Smooth

muscle cell

Wall of stomach

Wall of colon

Wall of small

intestine

Location: Smooth muscle is in hollow organs such as the stomach and intestine.

Structure: Smooth muscle cells are tapered at each end, are not striated, and have

a single nucleus.

Function: Regulates the size of organs, forces fluid through tubes, controls

the amount of light entering the eye, and produces “goose flesh” in the skin; under

involuntary control.

(c)Smooth muscle

LM 800x

Figure 4.8

(continued)

Cell body

Brain

Spinal

cord

Spinal

nerves

Nucleus

Dendrite

Nuclei of

neuroglia cells

Neuroglia cells

Axon

Location: Neurons are located in the brain, spinal cord, and ganglia.

(a)Multipolar neuron

Structure: The neuron consists of dendrites, a cell body, and a long axon. Neuroglia, or support

cells, surround the neurons.

Function: Neurons transmit information in the form of action potentials, store "information," and

in some way integrate and evaluate data. Neuroglia support, protect, and form specialized sheaths

around axons.

LM 240x

Figure 4.9

Typesof Neurons

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Membranes

Objective

■ List the functional and structural characteristicsof mucous,

serous, and synovial membranes.

A membrane is a thin sheet or layer of tissue that covers a

structure or lines a cavity.Most membranes are formed from ep-

ithelium and the connective tissue on which it rests.The three ma-

jor categories of internal membranes are mucous membranes,

serous membranes,and synovial membranes.

Amucous (mu¯⬘ku˘s)membrane consists of epithelial cells,

their basement membrane,a thick layer of loose connective tissue

called the lamina propria(lam⬘i-na˘pro¯⬘pre¯-a˘),and, sometimes, a

Part1 Organization ofthe Human Body132

layer ofsmooth muscle cells. Mucous membranes line cavities and

canals that open to the outside of the body,such as the digestive,

respiratory, excretory,and reproductive passages (ﬁgure 4.11).

Many,but not all,mucous membranes contain goblet cells or mul-

ticellular mucous glands,which secrete a viscous substance called

mucus(mu¯⬘ku˘s).The functions of the mucous membranes vary,

depending on their location, and include protection,absorption,

and secretion.

Aserous (ser⬘u˘s)membraneconsists of three components:

a layer of simple squamous epithelium called mesothelium

(mez-o¯ -the¯⬘le¯-u˘ m), its basement membrane,and a delicate layer

ofloose connective tissue. Serous membranes line cavities such as

the pericardial,pleural, and peritoneal cavities that do not open to

the exterior (see ﬁgure 4.11).Serous membranes do not contain

glands but are moistened by a small amount ofﬂuid, called serous

ﬂuid,produced by the serous membranes. The serous ﬂuid lubri-

cates the serous membranes and makes their surfaces slippery.

Serous membranes protect the internal organs from friction,help

hold them in place,and act as selectively permeable barriers that

prevent the accumulation of large amounts of ﬂuid within the

serous cavities.

Synovial(si-no¯⬘ve¯-a˘l)membranesconsist of modiﬁed con-

nective tissue cells either intermixed with part of the dense con-

nective tissue ofthe joint capsule or separated from the capsule by

areolar or adipose tissue.Synovial membranes line freely movable

joints (see chapter 8) (see ﬁgure 4.11).They produce a ﬂuid rich in

hyaluronic acid,which makes the joint ﬂuid very slippery, thereby

facilitating smooth movement within the joint.

40. Compare mucous, serous, and synovial membranes

according to the type of cavitythey line and their

secretions.

Cell body

Nuclei of

neuroglia

Nucleus

Branches of axon

(not visible in

photomicrograph)

(b) Unipolar neuron

Location: Cell bodies are located in ganglia outside of the brain and spinal cord.

Structure: The neuron consists of a cell body with one axon.

Function: Conducts action potentials from the periphery to the brain or spinal cord.

LM 240x

Figure 4.9

(continued)

Nucleus

Neuron

cell bodies

Nuclei of

neuroglia

LM 240x

Figure 4.10

Neuroglia

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Chapter 4 Histology: The Study of Tissues 133

Inﬂammation

Objective

■ Describe the process of inﬂammation, and explain why

inﬂammation isprotective to the body.

The inﬂammatory response occurs when tissues are dam-

aged (ﬁgure 4.12) or in association with an immune response.Al-

though many possible agents cause injury, such as microorgan-

isms,cold, heat, radiant energy, chemicals, electricity,or mechani-

cal trauma,the inﬂammatory response to all causes is similar. The

inﬂammatory response mobilizes the body’s defenses,isolates and

destroys microorganisms and other injurious agents,and removes

foreign materials and damaged cells so that tissue repair can

proceed.The details of the inﬂammatory response are presented in

chapter 22.

Inﬂammation produces ﬁve major manifestations:redness,

heat, swelling, pain, and disturbance of function.Althoug h un-

pleasant,these processes usually beneﬁt recovery, and each of the

symptoms can be understood in terms ofevents that occur during

the inﬂammatory response.

After a person is injured,chemical substances called media-

tors of inﬂammation are released or activated in the tissues and

the adjacent blood vessels. The mediators include histamine,

kinins, prostaglandins, leukotrienes,and others. Some mediators

induce dilation ofblood vessels and produce the symptoms of red-

ness and heat.Dilation of blood vessels is beneﬁcial because it in-

creases the speed with which white blood cells and other

substances important for ﬁghting infections and repairing the in-

jury arrive at the site of injury.

Mediators of inﬂammation also stimulate pain receptors

and increase the permeability of blood vessels. The increased

permeability allows the movement of materials such as clotting

proteins and white blood cells out ofthe blood vessels and into

the tissue,where they can deal directly with the injur y.As pro-

teins from the blood move into the tissue,they change the os-

motic relationship between the blood and the tissue. Water

follows the proteins by osmosis,and the tissue swells, produc-

ing edema (e-de¯⬘ma˘). Edema increases the pressure in the

tissue, which can also stimulate neurons and cause the sensa-

tionof pain.

Clotting proteins found in blood diffuse into the interstitial

spaces and form a clot.Clotting of blood also occurs in the more

severely injured blood vessels.The effect of clotting is to isolate the

injurious agent and to separate it from the remainder ofthe body.

Foreign particles and microorganisms present at the site ofinjur y

are “walled off”from tissues by the clotting process. Pain, limita-

tion ofmovement resulting from edema, and tissue destruction all

contribute to the disturbance offunction. This disturbance can be

valuable because it warns the person to protect the injured struc-

ture from further damage.Sometimes the inﬂammatory response

lasts longer or is more intense than is desirable,and drugs are used

to suppress the symptoms.Antihistamines block the effects of his-

tamine,aspirin prevents the synthesis of prostaglandins, and corti-

sone reduces the release ofseveral mediators of inﬂammation. On

the other hand, the inﬂammatory response by itself may not be

enough to combat the effects of injury or ﬁght off an infection.

Medical intervention such as administering antibiotics may be

required.

41. What is the function of the inﬂammatory response? Name

ﬁve manifestationsof the inﬂammatory response, and

explain howeach is produced.

(a) Mucous membranes

(b) Serous membranes

Respiratory

Digestive

Pleural

Peritoneal

Figure 4.11

Membranes

(a) Mucousmembranes line cavities that open to the outside and often

contain mucousglands, which secrete mucus. (b) Serous membranes line

cavitiesthat do not open to the exterior, and do not contain glands, butdo

secrete serousﬂuid. (c) Synovial membranes line cavities that surround

synovialjoints.

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Part1 Organization ofthe Human Body134

Blood

vessel

Bacteria

introduced

Splinter

Epidermis

Dermis

Neutrophil

phagocytizing

bacteria

Bacteria

proliferating

Neutrophil

migrating through

blood vessel wall

1. A splinter in the skin causes damage and introduces

bacteria. Mediators of inflammation are released from

injured tissues including damaged blood vessels.

Some blood vessels are ruptured causing bleeding.

Mediators of inflammation cause other blood vessels

(capillaries) to begin dilating, causing the skin to

become red. Mediators of inflammation also cause

capillary permeability to increase, and fluid leaves the

capillaries causing swelling (arrows).

2. White blood cells (e.g. neutrophils and macrophages)

leave the dilated blood vessels and move to the site of

bacterial infection, where they begin to phagocytize

bacteria and other debris.

ProcessFigure 4.12

Inﬂammation

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Chapter 4 Histology: The Study of Tissues 135

PREDICT

In some injuries, tissuesare so severely damaged that areasexist

where cellsare killed and blood vessels are destroyed. For injuries

such asthese, where do the signs of inﬂammation such asredness,

heat, edema, and pain occur?

ChronicInﬂammation

When the agentresponsible for an injury is not removed or ifsome

interference occurswith the process of healing, the inﬂammatory

response persistsand is called chronicinﬂammation. For example, an

infection ofthe lung can result in a brief period of inﬂammation followed

byrepair, but a prolonged infection causes chronicinﬂammation, which

resultsin tissue destruction and permanent damage to the lung. Also,

chronicinﬂammation of the stomach or small intestine mayresult in an

ulcer. Prolonged infections, prolonged exposure to irritantssuch assilica

in the lung, or abnormalimmune responses can result in chronic

inﬂammation. White blood cellsinvade areas ofchronic inﬂammation,

and ultimatelyhealthy tissues are destroyed and replaced bya ﬁbrous

connective tissue, which isan important cause of the lossof organ

function. Chronicinﬂammation of the lungs, the liver, the kidney, or

other vitalorgans can lead to death.

Tissue Repair

Objective

■ Describe the major events involved in tissue repair.

Tissue repairis the substitution of viable cells for dead cells,

and it can occur by regeneration or replacement.In regeneration

(re¯ ⬘jen-er-a¯⬘shu˘ n),the new cells are the same ty pe as those that were

destroyed,and normal function is usually restored.In replacement,

a new type oftissue develops that eventually causes scar production

and the loss ofsome tissue function. Most wounds heal through re-

generation and replacement;which process dominates depends on

the tissues involved and the nature and extent ofthe wound.

Cells are classiﬁed into three groups called labile, stable,or

permanent cells,according to their ability to regenerate.Labile cells,

including cells of the skin, mucous membranes, and hemopoietic

and lymphatic tissues,continue to divide throughout life. Damage to

these cells can be repaired completely by regeneration.Stable cells,

such as connective tissues and glands,including the liver, pancreas,

and endocrine glands,do not divide after growth ceases; but they do

retain the ability to divide and are capable of regeneration in re-

sponse to injury.Permanent cells have very limited ability to repli-

cate,and, if killed, they are usually replaced by a different type of cell.

Neurons ﬁt into this category,although neurons are able to recover

from damage.If the cell body of a neuron is not destroyed,most neu-

rons can replace a damaged axon or dendrite;but if the neuron cell

body is destroyed,the remainder of the neuron dies. Evidence indi-

cates that some undifferentiated cells ofthe central ner vous system

can undergo mitosis and form functional neurons,although the de-

gree to which mitosis occurs and its functional signiﬁcance is not

clear.Undifferentiated cells of skeletal and cardiac muscle also have

very limited ability to regenerate in response to injury,although in-

dividual skeletal and cardiac muscle cells can repair themselves.In

contrast,smooth muscle readily regenerates following injury.

Skin repair is a good example ofwound repair (ﬁgure 4.13).

The basic pattern of the repair is the same for other tissues,espe-

cially ones covered by epithelium.If the edges of the wound are

close together such as in a surgical incision,the wound heals by a

process called primary union, or primary intention. If the edges

are not close together,or if extensive loss of tissue has occurred,the

process is called secondary union,or secondary intention.

In primary union, the wound ﬁlls with blood, and a clot

forms (see chapter 19). The clot contains a threadlike protein,

ﬁbrin (fı¯⬘brin),that binds the edges of the wound together. The

surface ofthe clot dries to form a scab, which seals the wound and

helps prevent infection.An inﬂammatory response induces vasodi-

lation and brings increased numbers ofblood cells and other sub-

stances to the area.Blood vessel permeability increases, resulting in

edema.Fibrin and blood cells move into the wounded tissues be-

cause of the increased vascular permeability.Fibrin acts to isolate

and wall offmicroorganisms and other foreign matter. Some of the

white blood cells that move into the tissue are phagocytic cells

calledneutrophils (noo⬘tro¯ -ﬁlz; ﬁgure 4.13b). They ingest bacte-

ria,thus helping to ﬁght infection, and they also ingest tissue debris

and clear the area for repair.Neutrophils are killed in this process and

can accumulate as a mixture ofdead cells and ﬂuid called pus (pu˘s).

Fibroblasts from surrounding connective tissue migrate into

the clot and produce collagen and other extracellular matrix com-

ponents. Capillaries grow from blood vessels at the edge of the

wound and revascularize the area,and ﬁbrin in the clot is broken

down and removed.The result is the replacement of the clot by a

delicate connective tissue,called gr anulation tissue, which con-

sists of ﬁbroblasts, collagen, and capillaries. A large amount of

granulation tissue sometimes persists as a scar(skar), which at ﬁrst

is bright red because ofvascular ization of the tissue. Later,the scar

blanches and becomes white,as collagen accumulates and the vas-

cular channels are compressed.

Repair by secondary unionproceeds in a fashion similar to

healing by primary union,but some differences exist. Because the

wound edges are far apart, the clot may not close the gap com-

pletely,and it takes the epithelial cells much longer to regenerate

and cover the wound.With increased tissue damage, the degree of

the inﬂammatory response is greater,more cell debris exists for the

phagocytes to remove,and the r isk of infection is greater.Much

more granulation tissue forms,and wound contraction occurs as

a result of the contraction of ﬁbroblasts in the granulation tissue.

Wound contraction leads to disﬁguring and debilitating scars.

Thus,it is advisable to suture a large wound so that it can heal by

primary rather than secondary union. Healing is faster,the risk of

infection is lowered,and the degree of scarring is reduced.

42. Deﬁne tissue repair. Differentiate between tissue repair that

occursby regeneration and by replacement.

43. Compare labile cells, stable cells, and permanent cells. Give

examplesof each type. What is the signiﬁcance of these cell

typesto tissue repair?

44. Describe the process of wound repair. Contrasthealing by

primaryunion and secondary union.

45. What is pus? Describe granulation tissue. How does

granulation tissue contribute to scarsand wound

contraction?

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Tissues and Aging

Objective

■ Describe age-related changes at the tissue level.

Age-related changes are well documented.For example, re-

duced visual acuity and reduced smell,taste, and touch sensation

have been documented.A clear decline in many types of athletic

performance can be measured after approximately age 30–35.

Part1 Organization ofthe Human Body136

Ultimately there is a substantial decrease in the number of neu-

rons and muscle cells.The functional capacity of systems such as

the respiratory and cardiovascular systems declines. The rate of

healing and scarring are very different in the elderly than in the

very young and major changes in the structural characteristics of

the skin develop.Characteristic alterations in brain function also

develop in the elderly.All of these changes result in the differences

between young,middle-age, and older people.

Epidermis

Blood

vessel

Dermis

Blood

clot

Macrophages

migrating to

wound site

Scab

Fibroblasts migrating

to wound site

New epidermis

growing into wound

Freshly healed

epidermis

Epidermis

Subcutaneous

fat

Granulation tissue

being replaced

with dermis

Granulation tissue

(fibroblasts proliferating)

New

epidermis

Scab

1. Fresh wound cuts through the epithelium (epidermis)

and underlying connective tissue (dermis), and a clot

forms.

3. Approximately 2 weeks after the injury, the epithelium

has grown completely into the wound, and granulation

tissue has formed.

4. Approximately 1 month after the injury, the wound has

completely closed, the scab has been sloughed, and the

granulation tissue is being replaced with dermis.

2. Approximately 1 week after the injury, a scab is present,

and epithelium (new epidermis) is growing into the wound.

Subcutaneous

fat

ProcessFigure 4.13

Tissue Repair

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4. Histology: The Study of

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Clinical Focus Cancer Tissue

Cancer(kan⬘ser) is a malignant, spreading

tumor and the illnessthat results from such

a tumor. A tumor(too⬘mo˘r) isany swelling,

although modern usage has limited the

term to swellings that involve neoplastic

tissue. Oncology(ong-kol⬘o¯-je¯; the study of

tumors) isthe study of tumors and their as-

sociated problems. Neoplasm(ne¯⬘o¯-plazm)

meansnew growth and refers to abnormal

tissue growth resulting in unusually rapid

cellular proliferation that continues after

normalgrowth of the tissue has stopped or

slowed considerably. A neoplasm can be

eithermalignant (ma˘ -lig⬘na˘nt; with malice

or intent to cause harm), able to spread

and become worse, or benign (be¯-nı¯n⬘;

kind), notinclined to spread and not likely

to become worse. Although benign tumors

are usuallyless dangerous than malignant

tumors, they can cause problems. As a

benign tumor enlarges, it can compress

surrounding tissuesand impair their func-

tions. In some cases (e.g., brain tumors),

the resultcan be death.

Malignant tumorscan spread by local

growth and expansion or by metastasis

(me˘-tas⬘ta˘-sis, meaning moving to another

place), which resultsfrom tumor cells sepa-

rating from the main neoplasm and being

carried bythe lymphatic or circulatory sys-

tem to a new site, where a second neo-

plasm forms. A carcinoma(kar-si-no¯⬘ma˘) is

a malignantneoplasm derived from epithe-

lialtissue. A sarcoma (sar-ko¯ ⬘ma˘) isa ma-

lignant neoplasm derived from connective

tissue.

Malignant neoplasmslack the normal

growth control that is exhibited by most

other adulttissues, and in many ways they

resemble embryonictissue. Rapid growth is

one characteristicof embryonic tissue, but

asthe tissue begins to reach its adult size

and function, itslows or stops growing com-

pletely. This cessation of growth is con-

trolled at the individual celllevel. Cancer

results when a cell or group of cells, for

some reason, breaks awayfrom that con-

trol. This breaking loose involves the ge-

netic machinery and can be induced by

viruses, environmental toxins, and other

causes. The illnessassociated with cancer

usuallyoccurs as the tumor invadesand de-

stroys the healthy surrounding tissues,

eliminating their functions.

Cancer therapy concentratesprimar-

ily on trying to confine and then kill the

malignantcells. This goal is accomplished

currentlyby killing the tissue with x raysor

lasers, by removing the tumor surgically,

or by treating the patient with drugsthat

kill rapidly dividing cells or reduce the

blood supply to the tumor. The major

problem with currenttherapy is that some

cancerscannot be removed completely by

surgery or killed completelyby x rays and

laser therapy. These treatmentscan also

kill normal tissue adjacentto the tumor.

Manydrugs used in cancer therapy kill not

only cancer tissue butalso other rapidly

growing tissues, such as bone marrow,

where new blood cellsare produced, and

the lining of the intestinal tract. Loss of

these tissuescan result in anemia, caused

bythe lack of red blood cells, and nausea,

caused bythe loss of the intestinal lining.

A newer class of drugs eliminates these

unwanted side effects. These drugs pre-

vent blood vessel development, thus de-

priving the cancer tissue of a blood

supply, rather than attacking dividing

cells. Other normaltissues, in which cells

divide rapidly, have well-established

blood vessels and are, therefore, not af-

fected bythese drugs.

Promising anticancer therapiesare be-

ing developed in which cellsresponsible for

immune responses can be stimulated to

recognize tumor cellsand destroy them. A

major advantage in such anticancer treat-

ments isthat the cells of the immune sys-

tem can speciﬁcallyattack the tumor cells

and notother, healthy tissues.

Chapter 4 Histology: The Study of Tissues 137

At the tissue level,age-related changes affect cells and the ex-

tracellular materials produced by them. In general, cells divide

more slowly in older than in younger people.Collagen ﬁbers be-

come more irregular in structure,even though they may increase in

number.As a consequence,connective tissues with abundant colla-

gen,such as tendons and ligaments, become less ﬂexible and more

fragile. Elastic ﬁbers fragment, bind to calcium ions,and become

less elastic.Consequently, elastic connective tissues, such as elastic

ligaments,become less elastic.

Changes in the structure ofelastic and collagen ﬁbers of ar te-

rial walls cause them to become less elastic.Atherosclerosis results as

plaques form in the walls of blood vessels, which contain collagen

ﬁbers,lipids, and calcium deposits (see chapter 21). These changes

result in reduced blood supply to tissues and increased susceptibil-

ity to blockage and rupture.The rate of red blood cell synthesis de-

clines in the elderly as well.Reduced ﬂexibility and elasticity of con-

nective tissue is responsible for increased wrinkling ofskin as well as

the increased tendency for bones to break in older people.

Injuries in the very young heal more rapidly and more com-

pletely than in older people.A fracture in the femur of an infant is

likely to heal quickly and eventually leave no evidence ofthe frac-

ture in the bone.A similar fracture in an adult heals more slowly

and a scar,seen in x rays of the bone, is likely to persist through-

out life.

46. Describe the age-related changes that occur in cells such

asnerve cells, muscle cells, and cells of hemopoietic

tissues.

47. Describe the age-related changes in tissues with abundant

collagen and elasticﬁbers.

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Part1 Organization ofthe Human Body138

Tissuesand Histology

(p. 105)

1. Tissues are collections of similar cells and the substances

surrounding them.

2. The four primary tissue types are epithelial, connective,muscle, and

nervous tissues.

3. Histology is the microscopic study of tissues.

EmbryonicTissue

(p. 105)

All four ofthe primary tissue types are derived from each of the three germ

layers (mesoderm,ectoderm, and endoderm).

EpithelialTissue

(p. 105)

1. Epithelium consists of cells with little extracellular matrix, it covers

surfaces,it has a basement membrane, and it does not have blood

vessels.

2. The basement membrane is secreted by the epithelial cells and

attaches the epithelium to the underlying tissues.

Classiﬁcation ofEpithelium

1. Simple epithelium has a single layer of cells,stratiﬁed epithelium

has two or more layers,and pseudostratiﬁed epithelium has a single

layer that appears stratiﬁed.

2. Cells can be squamous (ﬂat),cuboidal, or columnar.

3. Stratiﬁed squamous epithelium can be moist or keratinized.

4. Transitional epithelium is stratiﬁed,with cells that can change shape

from cuboidal to ﬂattened.

FunctionalCharacteristics

1. Simple epithelium is usually involved in diffusion,ﬁltration,

secretion,or absorption. Stratiﬁed epithelium serves a protective

role.Squamous cells function in diffusion and ﬁltration. Cuboidal

or columnar cells,with a larger cell volume that contains many

organelles,secrete or absorb.

2. A smooth free surface reduces friction (mesothelium and

endothelium),microvilli increase absorption (intestines), and cilia

move materials across the free surface (respiratory tract and uterine

tubes).Transitional epithelium has a folded surface that allows the

cell to change shape,and the number of cells making up the

epithelial layers changes.

3. Cells are bound together mechanically by glycoproteins,

desmosomes,and the zonulae adherens and to the basement

membrane by hemidesmosomes.The zonulae occludens and

zonulae adherens form a permeability barrier or tight junction,and

gap junctions allow intercellular communication.

Glands

1. Glands are organs that secrete.Exocrine glands secrete through

ducts,and endocrine glands release hormones that are absorbed

directly into the blood.

2. Glands are classiﬁed as unicellular or multicellular.Goblet cells are

unicellular glands.Multicellular exocrine glands have ducts,which are

simple or compound (branched).The ducts can be tubular or end in

small sacs (acini or alveoli).Tubular glands can be straight or coiled.

3. Glands are classiﬁed according to their mode ofsecretion.

Merocrine glands (pancreas) secrete substances as they are

produced,apocrine glands (mammary glands) accumulate

secretions that are released when a portion ofthe cell pinches off,

and holocrine glands (sebaceous glands) accumulate secretions that

are released when the cell ruptures and dies.

Connective Tissue

(p. 117)

Connective tissue is distinguished by its extracellular matrix.

Cellsof Connective Tissue

1. The extracellular matrix results from the activity of specialized

connective tissue cells;in general, blast cells form the matrix, cyte

cells maintain it,and clast cells break it down. Fibroblasts form

protein ﬁbers ofmany connective tissues, osteoblasts form bone,

and chondroblasts form cartilage.

2. Adipose (fat) cells,mast cells, white blood cells, macrophages, and

mesenchymal cells (stem cells) are commonly found in connective

tissue.

Extracellular Matrix

The extracellular matrix of connective tissue has protein ﬁbers,ground

substance,and ﬂuid as major components.

Protein Fibersof the Matrix

1. Collagen ﬁbers structurally resemble ropes.They are strong and

ﬂexible but resist stretching.

2. Reticular ﬁbers are ﬁne collagen ﬁbers that form a branching

network that supports other cells and tissues.

3. Elastin ﬁbers have a structure similar to a spring.After being

stretched they tend to return to their original shape.

Other MatrixMolecules

1. Hyaluronic acid makes ﬂuids slippery.

2. Proteoglycan aggregates trap water,which gives tissues the capacity

to return to their original shape when compressed or deformed.

3. Adhesive molecules hold proteoglycans together and to plasma

membranes.

Classiﬁcation ofConnective Tissue

(p. 119)

Connective tissue is classiﬁed according to the type of protein and the

proportions ofprotein, ground substance, and ﬂuid in the matrix.

EmbryonicConnective Tissue

Mesenchyme arises early,consists ofir regularly shaped cells and abundant

matrix,and gives rise to adult connective tissue.

AdultConnective Tissue

1. Loose Connective Tissue

• Loose (areolar) connective tissue has many different cell types and

a random arrangement ofprotein ﬁbers with space between the

ﬁbers.This tissue ﬁlls spaces around the organs and attaches the

skin to underlying tissues.

2. Dense Connective Tissue

• Dense regular connective tissue is composed of ﬁbers arranged in

one direction,which provides strength in a direction parallel to the

ﬁber orientation.Two types of dense regular connective tissue

exist:collagenous (tendons and most ligaments) and elastic

(ligaments ofvertebrae).

• Dense irregular connective tissue has ﬁbers organized in many

directions,which produces strength in different directions. Two

types ofdense irregular connective tissue exist: collagenous

(capsules oforgans and dermis of skin) and elastic (large ar teries).

SUMMARY

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Chapter 4 Histology: The Study of Tissues 139

3. Connective Tissue with Special Properties

• Adipose tissue has fat cells (adipocytes) ﬁlled with lipid and very

little extracellular matrix (a few reticular ﬁbers).

Adipose tissue functions as energy storage,insulation, and

protection.

Adipose tissue can be yellow (white) or brown.Brown fat is

specialized for generating heat.

• Reticular tissue is a network of reticular ﬁbers and forms the

framework oflymphoid tissue, bone marrow, and the liver.

• Hemopoietic tissue, or red bone marrow,is the site of blood cell

formation,and yellow bone marrow is a site of fat storage.

4. Cartilage

• Cartilage has a relatively rigid matrix composed of protein ﬁbers

and proteoglycan aggregates.The major cell type is the

chondrocyte,which is located within lacunae.

Hyaline cartilage has evenly dispersed collagen ﬁbers that

provide rigidity with some ﬂexibility.Examples include the

costal cartilage,the covering over the ends of bones in joints,

the growing portion oflong bones, and the embryonic

skeleton.

Fibrocartilage has collagen ﬁbers arranged in thick bundles,it

can withstand great pressure,and it is found between

vertebrae,in the jaw, and in the knee.

Elastic cartilage is similar to hyaline cartilage,but it has elastin

ﬁbers.It is more ﬂexible than hyaline cartilage. It is found in

the external ear.

5. Bone

Bone cells,or osteocytes, are located in lacunae that are surrounded

by a mineralized matrix (hydroxyapatite) that makes bone very hard.

Cancellous bone has spaces between bony trabeculae,and compact

bone is more solid.

6. HemopoieticTissue and Blood

• Blood cells are suspended in a ﬂuid matrix.

• Hemopoietic tissue forms blood cells.

Muscle Tissue

(p. 128)

1. Muscle tissue has the ability to contract.

2. Skeletal (striated voluntary) muscle attaches to bone and is

responsible for body movement.Skeletal muscle cells are long,

cylindrically shaped cells with many peripherally located nuclei.

3. Cardiac (striated involuntary) muscle cells are cylindrical,

branching cells with a single,central nucleus.Cardiac muscle is found

in the heart and is responsible for pumping blood through the

circulatory system.

4. Smooth (nonstriated involuntary) muscle forms the walls of hollow

organs,the iris of the eye, and other structures. Its cells are spindle-

shaped with a single,central nucleus.

NervousTissue

(p. 129)

1. Nervous tissue has the ability to conduct electric impulses and is

composed ofneurons (conductive cells) and neuroglia (support cells).

2. Neurons have cell processes called dendrites and axons.The

dendrites can receive electric impulses,and the axons can conduct

them.Neurons can be multipolar (several dendrites and an axon),

bipolar (one dendrite and one axon),or unipolar (one axon).

Membranes

(p. 132)

1. Mucous membranes consist ofepithelial cells, their basement

membrane,the lamina propria, and, sometimes, smooth muscle

cells;they line cavities that open to the outside and often contain

mucous glands,which secrete mucus.

2. Serous membranes line cavities that do not open to the exterior,do

not contain glands,but do secrete serous ﬂuid.

3. Synovial membranes are formed by connective tissue and line joint

cavities.

Inﬂammation

(p. 133)

1. The function of the inﬂammatory response is to isolate injurious

agents from the rest ofthe body and to attack and destroy the

injurious agent.

2. The inﬂammatory response produces ﬁve symptoms:redness, heat,

swelling,pain, and disturbance of function.

Tissue Repair

(p. 135)

1. Tissue repair is the substitution of viable cells for dead ones.Tissue

repair occurs by regeneration or replacement.

• Labile cells divide throughout life and can undergo regeneration.

• Stable cells do not ordinarily divide after growth is complete but

can regenerate ifnecessary.

• Permanent cells cannot replicate.If killed, permanent tissue is

repaired by replacement.

2. Tissue repair by primary union occurs when the edges of the wound

are close together.Secondary union occurs when the edges are far

apart.

Tissuesand Aging

(p. 136)

1. Age-related changes in tissues result from reduced rates ofcell

division and changes in the extracellular ﬁbers.

2. Collagen ﬁbers become less ﬂexible and have reduced strength.

3. Elastic ﬁbers become fragmented and less elastic.

1. Given these characteristics:

1. capable of contraction

2. covers free body surfaces

3. lacks blood vessels

4. composes various glands

5. anchored to connective tissue by a basement membrane

Which ofthese are characteristics of epithelial tissue?

a. 1,2,3

b. 2,3,5

c. 3,4,5

d. 1,2,3,4

e. 2,3,4,5

2. Which of these embryonic germ layers gives rise to muscle,bone,

and blood vessels?

a. ectoderm

b. endoderm

c. mesoderm

3. A tissue that covers a surface,is one cell layer thick, and is composed

ofﬂat cells is

a. simple squamous epithelium.

b. simple cuboidal epithelium.

c. simple columnar epithelium.

d. stratiﬁed squamous epithelium.

e. transitional epithelium.

REVIEW AND COMPREHENSION

Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

I. Organization of the

Human Body

4. Histology: The Study of

Tissues

Companies, 2004

4. Epithelium composed of two or more layers of cells with only the

deepest layer in contact with the basement membrane is known as

a. stratiﬁed epithelium.

b. simple epithelium.

c. pseudostratiﬁed epithelium.

d. columnar epithelium.

e. cuboidal epithelium.

5. Stratiﬁed epithelium is usually found in areas of the body where the

principal activity is

a. ﬁltration.

b. protection.

c. absorption.

d. diffusion.

e. secretion.

6. Which of these characteristics do not describe moist stratiﬁed

squamous epithelium?

a. many layers of cells

b. surface cells are ﬂat

c. surface cells are living

d. found in the skin

e. outer layers covered by ﬂuid

7. In parts of the body such as the urinary bladder,where

considerable expansion occurs,one can expect to ﬁnd which type

ofepithelium?

a. cuboidal

b. pseudostratiﬁed

c. transitional

d. squamous

e. columnar

8. A tissue that contains cells with these characteristics:

1. covers a surface

2. one layer ofcells

3. cells are ﬂat

Performs which ofthe following functions?

a. phagocytosis

b. active transport

c. secretion of many complex lipids and proteins

d. is adapted to allow certain substances to diffuse across it

e. protection from abrasion

9. Epithelial cells with microvilli are most likely found

a. lining blood vessels.

b. lining the lungs.

c. in serous membranes.

d. lining the digestive tract.

e. in the skin.

10. Pseudostratiﬁed ciliated columnar epithelium can be found lining

the

a. digestive tract.

b. trachea.

c. thyroid gland.

d. kidney tubules.

e. urinary bladder.

11. A type of cell connection whose only function is to prevent the cells

from coming apart is the

a. desmosome.

b. gap junction.

c. tight junction.

12. Those glands that lose their connection with epithelium during

embryonic development and secrete their cellular products into the

bloodstream are called glands.

a. apocrine

b. endocrine

c. exocrine

d. holocrine

e. merocrine

Part1 Organization ofthe Human Body140

13. Glands that accumulate secretions and release them only when the

individual secretory cells rupture and die are called

glands.

a. apocrine

b. holocrine

c. merocrine

14. A gland has a duct that branches repeatedly, and the ducts end in

saclike structures.This describes a gland.

a. simple tubular

b. compound tubular

c. simple coiled tubular

d. simple acinar

e. compound acinar

15. The ﬁbers in dense connective tissue are produced by

a. ﬁbroblasts.

b. adipocytes.

c. osteoblasts.

d. osteoclasts.

e. macrophages.

16. Mesenchymal cells

a. form embryonic connective tissue.

b. give rise to all adult connective tissues.

c. in adults produce new connective tissue cells in response to

injury.

d. all ofthe above

17. A tissue with a large number of collagen ﬁbers organized parallel to

each other would most likely be found in

a. a muscle.

b. a tendon.

c. adipose tissue.

d. a bone.

e. cartilage.

18. Extremely delicate ﬁbers that make up the framework for organs

such as the liver,spleen, and lymph nodes are

a. elastic ﬁbers.

b. reticular ﬁbers.

c. microvilli.

d. cilia.

e. collagen ﬁbers.

19. In which of these locations would dense irregular elastic connective

tissue be found?

a. ligaments

b. nuchal ligament

c. dermis of skin

d. large arteries

e. adipose tissue

20. Which of these is not true of adipose tissue?

a. site of energy storage

b. a type of connective tissue

c. acts as a protective cushion

d. brown adipose is found only in babies

e. functions as a heat insulator

21. Which of these types of connective tissue has the smallest amount of

extracellular matrix?

a. adipose

b. bone

c. cartilage

d. loose connective tissue

e. blood

22. Given these characteristics:

1. cells located in lacunae

2. proteoglycans in ground substance

3. no collagen ﬁbers present

4. perichondrium on surface

5. heals rapidly after injury

Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

I. Organization of the

Human Body

4. Histology: The Study of

Tissues

Companies, 2004

Chapter 4 Histology: The Study of Tissues 141

Which ofthese characteristics apply to cartilage?

a. 1,2,3

b. 1,2,4

c. 2,4,5

d. 1,2,4,5

e. 2,3,4,5

23. Fibrocartilage is found

a. in the cartilage of the trachea.

b. in the rib cage.

c. in the external ear.

d. on the surface ofbones in moveable joints.

e. between vertebrae.

24. A tissue in which cells are located in lacunae surrounded by a hard

matrix ofhydroxyapatite is

a. hyaline cartilage.

b. bone.

c. nervous tissue.

d. dense regular collagenous connective tissue.

e. ﬁbrocartilage.

25. Which of these characteristics apply to smooth muscle?

a. striated, involuntary

b. striated,voluntary

c. unstriated, involuntary

d. unstriated,voluntary

26. Which of these statements about nervous tissue is not true?

a. Neurons have cytoplasmic extensions called axons.

b. Electric signals (action potentials) are conducted along axons.

c. Bipolar neurons have two axons.

d. Neurons are nourished and protected by neuroglia.

e. Dendrites receive electric signals and conduct them toward the

cell body.

27. Linings of the digestive, respiratory, excretory,and reproductive

passages are composed of

a. serous membranes.

b. mucous membranes.

c. mesothelium.

d. synovial membranes.

e. endothelium.

28. Chemical mediators of inﬂammation

a. cause blood vessels to constrict.

b. decrease the permeability ofblood vessels.

c. initiate processes that lead to edema.

d. help to prevent clotting.

e. decrease pain.

29. Which of these types of cells are labile?

a. neurons

b. skin

c. liver

d. pancreas

30. Permanent cells

a. divide and replace damaged cells in replacement tissue repair.

b. form granulation tissue.

c. are responsible for removing scar tissue.

d. are usually replaced by a different cell type ifthey are destroyed.

e. are replaced during regeneration tissue repair.

Answers in Appendix F

1. Given the observation that a tissue has more than one layer ofcells

lining a free surface,(1) list the possible tissue types that exhibit

those characteristics,and (2) explain what additional observations

need to be made to identify the tissue as a speciﬁc tissue type.

2. A patient suffered from kidney failure a few days after he was

exposed to a toxic chemical.A biopsy of his kidney indicated that

many ofthe thousands of epithelium-lined tubules that make up the

kidney had lost the layer ofsimple cuboidal epithelial cells that

normally line them,although the basement membranes appeared to

be mostly intact.Predict how likely this person is to fully recover.

3. Compare the cell shapes and surface specializations ofan epithelium

that functions to resist abrasion to those ofan epithelium that

functions to carry out absorption ofmaterials.

4. Tell how to distinguish between a gland that produces a merocrine

secretion and a gland that produces a holocrine secretion.Assume

that you have the ability to chemically analyze the composition of

secretions.

5. Indicate whether the following statement is appropriate or not:“If a

tissue is capable ofcontracting, is under involuntary control, and

has mononucleated cells,it is smooth muscle.”Explain your answer.

6. Antihistamines block the effect of a chemical mediator of

inﬂammation called histamine,which is released during the

inﬂammatory response.What effect does administering

antihistamines have on the inﬂammatory response,and is use of an

antihistamine beneﬁcial?

Answers in Appendix G

CRITICAL THINKING

1. a. Secretion of mucus and digestive enzymes and the absorption of

nutrients normally occur in the digestive tract.Simple columnar

epithelial cells contain organelles that are specialized to carry out

nutrient absorption and secretion ofmucus and digestive

enzymes.Stratiﬁed squamous epithelium is not specialized to

either absorb or secrete,and the layers of epithelial cells reduce

the ability ofnutrient molecules to pass through them and,

therefore,to be absorbed. The ability of digestive enzymes to

pass through the layers ofepithelial cells, and therefore be

secreted,is also reduced.

b. Keratinized stratiﬁed epithelium forms a tough layer that is a

barrier to the movement ofwater. Replacing the epithelium of

skin with moist stratiﬁed squamous epithelium increases the loss

ofwater across the skin because water can diffuse through moist

stratiﬁed squamous epithelium,and it is more delicate and

provides less protection than keratinized stratiﬁed squamous

epithelium.

c. The stratiﬁed squamous epithelium that lines the mouth provides

protection.Replacement of it with simple columnar epithelium

makes the lining ofthe mouth much more susceptible to damage

because the single layer ofepithelial cells is easier to damage.

ANSWERS TO PREDICT QUESTIONS

Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

I. Organization of the

Human Body

4. Histology: The Study of

Tissues

Companies, 2004

2. Elastic ligaments attached to the vertebrae help the vertebral column

return to its normal upright position after it is ﬂexed.The elastic

ligaments act much like elastic bands.Tendons attach muscles to

bones.When muscles contract, they pull on the tendons,which in

turn pull on bones.Because they are not elastic, when the muscle

pulls on the tendon,all of the force is applied to the bone, causing it to

move.If tendons were elastic, when the muscle contracted,the tendon

would stretch,and not all of the tension would be applied to the bone.

3. Collagen synthesis is required for scar formation.If collagen

synthesis does not occur because ofa lack of vitamin C or if collagen

synthesis is slowed,wound healing does not occur or is slower than

normal.One might expect that the density of collagen ﬁbers in a scar

is reduced and the scar is not as durable as a normal scar.

Part1 Organization ofthe Human Body142

4. Hyaline cartilage provides a smooth surface so that bones in joints

can move easily.When the smooth surface provided by hyaline

cartilage is replaced by dense ﬁbrous connective tissue,the smooth

surface is replaced by a less smooth surface,and the movement of

bones in joints is much more difﬁcult.The increased friction helps

to increase inﬂammation and pain that occurs in the joints of

people who have rheumatoid arthritis.

5. In severely damaged tissue in which cells are killed and blood vessels

are destroyed,the usual symptoms of inﬂammation cannot occur.

Surrounding these areas ofsevere tissue damage, however,where

blood vessels are still intact and cells are still living,the classic signs

ofinﬂammation do develop. The signs of inﬂammation therefore

appear around the periphery ofseverely injured tissues.

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