ICHAPTER4

Tissue: The Living Fabric

Chapter Outline and Student Objectives Preview of Selected Key Terms

Epithelial Tissue (pp. 101-112)

1. List several characteristics that typify epithelial tissue.

2. Describe the criteria used to classify epithelia structurally.

3. Name and describe the various types of epithelia, indi-cate their chief function(s) and location(s).

4. Contrast a sheet of covering or lining epithelium withan epithelial membrane. Name and describe the threevarieties of epithelial membranes.

5. Compare endocrine and exocrine glands relative to theirgeneral structure, product(s), and mode of secretion.

6. Describe how multicellular exocrine glands are classi-fied structurally and functionally.

Connective Tissue (pp. 112-123)

7. Describe common characteristics of connective tissue.

8. List the structural elements of connective tissue anddescribe each element.

9. Explain the bases for classification of connective tissue.

10. Describe the types of connective tissue found in thebody, and note their characteristic functions.

Muscle Tissue (pp. 123-125)

11. Compare and contrast the structures and body locationsof the three types of muscle tissue.

Nervous Tissue (p. 125)

12. Note the general characteristics and functions of ner-vous tissue.

Developmental Aspects of Tissues (pp. 127-129)

14. Name the three primary germ layers, and indicate theprimary tissue classes arising from each.

15. Briefly mention tissue changes that occur with age. Citepossible causes of such changes.

100

Tissue Repair (pp. 125-127)

13. Describe the process of tissue repair involved in thenormal healing of a superficial wound.

Tissue (tissu = woven) A group of similar cells (and theirintercellular substance) specialized to perform a specificfunction; primary tissue types of the body are epithelial,connective, muscle, and nervous tissue.

Epithelial (eh"-pih-the'-lÿ-ul) (epi = upon, over; thel = del-icate) Pertaining to a primary tissue that covers the bodysurface, lines its internal cavities, and forms glands.

Gland One or more cells specialized to secrete a particularproduct called a secretion.

Mesenchyme (meh'-zin-kim) Common embryonic tissuefrom which all connective tissues arise.

Matrix Specialized extracellular substance secreted byconnective tissue cells that determines the specialized func-tion of each connective tissue type; typically includes groundsubstance (fluid to hard) and fibers (collagen, elastic, and/or reticular).

Fibroblast/fibrocyte (blast = bud, forming; cyte = cell) Thefibroblast is a young, actively mitotic cell that forms thefibers of connective tissue; in its mature state, it becomes afibrocyte, which maintains the matrix.

Chondroblast/chondrocyte The actively mitotic and maturecell forms, respectively, of cartilage.

Osteoblast/osteocyte The actively mitotic and mature cellforms, respectively, of bone.

Macrophage (ma'-kr6-fÿj") (macro = large; phago = eat) Aprotective cell type common in connective tissue, lym-phatic tissue, and certain body organs that phagocytizes tis-sue cells, bacteria, and other foreign debris.

Germ layers Three cellular layers (ectoderm, mesoderm,and endoderm) that represent the initial specialization ofcells in the embryonic body and from which all body tissuesarise.

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Cells that exist as isolated unice||u]ar (one-cell)organisms, such as amoebas, are rugged individual-

ists. They alone obtain and digest their food, carry out

gas exchange, and perform all the other activities nec-essary to keep themselves alive and healthy. But inthe multicellular human body, cells do not operateindependently or in isolation. Instead, they form tight,interdependent cell communities that function coop-eratively. Individual cells are specialized, with eachtype performing specific functions that help maintainhomeostasis and benefit the body as a whole. The spe-cialization of our cells is obvious: Muscle ceils lookand function quite differently from skin cells, whichin turn are easily distinguished from cells of the brain.Cell specialization allows the various parts of thebody to function in very sophisticated ways, but this

division of labor also has certain hazards. When a

particular group of cells is indispensable, its loss canseverely disable or even destroy the bodyGroups of closely associated cells that are similar

in structure and perform a common function are calledtissues (tissu = woven). There are four primary tissuetypes that interweave to form the "fabric" of the body:epithelial, connective, muscle, and nervous tissue, andeach has several subdivisions or varieties. If we hadto assign a single term to each primary tissue type thatwould best describe its overall role, the terms wouldmost likely be protection (epithelial), support (con-nective), movement (muscle), and control (nervous).However, these terms reflect only a small part of the

total tissue functions.As explained in Chapter 1, tissues are organized

into organs such as the kidneys and the heart. Mostorgans contain several tissue types, and the arrange-ment of the tissues determines the organ's structureand what it is able to do. The study of tissues, or his-tology, complements the study of gross anatomy.Together they provide the structural basis for under-

standing organ physiology.The close correlation between tissue structure and

function makes the study of tissues intriguing, but thevalue of any learning is to increase our ability to per-ceive relationships and see how things "fit" or worktogether. Learning the characteristic patterns of eachtissue type will allow you to predict the function ofan organ once its structure is known, and vice versa.

Chapter 4 Tissue: The Living Fabric 101

covering the walls and organs of the closed ventralbody cavity. Since epithelium forms the boundariesthat mark us off from the outside world, nearly allsubstances received or given off by the body must passthrough epithelium. Glandular epithelium fashions the

glands of the body.Epithelium is highly specialized to accomplish

many functions, including protection, absorption, fil-tration, and secretion. Each of these functions will bedescribed in detail as we consider various types ofepithelium, but briefly, the epithelium of the skin pro-tects underlying tissues from mechanical and chem-ical injury and bacterial invasion; that lining thedigestive tract is specialized to absorb substances; andthat found in the kidneys performs nearly the wholefunctional ,,menu"--absorption, secretion, and filtra-

tion. Secretion is the chief specialty of glands.

Special Characteristics of Epithelium :_

Epithelial (eh"-pih-thÿ'-lÿ-ul) tissue, or epithelium,occurs in the body as (1) covering and lining epithe-lium and (2) glandular epithelium. Covering and lin-ing epithelium is found on free surfaces of the bodysuch as the outer layer of the skin, dipping into andlining the open cavities of the digestive and respira-tory systems, lining blood vessels and the heart, and

Epithelial Tissue

Cellularity. Epithelial tissue is composed almostentirely of cells. In muscle and connective tissues,cells are often widely separated by extracellularmatrix. This is not true of epithelium, where cells

are close together.

Specialized contacts. Epithelial cells fit closetogether to form continuous sheets. Adjacent cellsare bound by lateral contacts, including tight junc-tions and desmosomes (see Chapter 3), which reduceor eliminate the extracellular space between them.

Polarity. Epithelium always has one free (apical)surface--the portion of the epithelium exposed tothe body exterior or the cavity of an internal organ.Some exposed plasma membrane surfaces aresmooth and slick; others exhibit cell surface mod-ifications such as microvilli or cilia. Microvilli,fingerlike extensions of the plasma membrane,increase tremendously the exposed surface area andare common in epithelia that absorb or secrete sub-stances (intestinal lining and kidney tubules). Ciliaprojecting from the epithelial lining of the tracheaand certain other internal tracts propel substances

along the epithelial surface.

4. Avascularity (ÿ,,_vas"-kyooÿlayr'-ih-te). Epithelium

may be well supplied by nerve fibers but is avas-cular (has no blood vessels within it). Epithelialcells receive their nourishment by diffusion of sub-stances from blood vessels in the underlying con-

nective tissue layer.

5. Basement membrane. Epithelium rests on a thinsupporting basal lamina (bÿ'-zul la'-mih-nuh),

Epithelial tissues have many characteristics that dis-tinguish them from other tissue types.

1.

2.

3.

k

102 Unit 1 Organization of the Body

6°

which separates it from the underlying connectivetissue. The basal lamina is a nonliving, adhesivematerial formed largely of glycoproteins secretedby the epithelial cells. The connective tissue cells,just deep to the basal lamina, secrete a similarextracellular material containing fine collagenousor reticular fibers; this layer is called the reticular(rih-tih'-kyoo-ler) lamina. The basal lamina of theepithelium and the reticular lamina of the connec-tive tissue together form the basement membrane.The basement membrane reinforces the epithelialsheet, helping it to resist stretching and tearingforces, and defines the space that may be occupiedby the epithelial cells.

An important characteristic of cancerous epi-thelial cells is their failure to respect this

boundary, which they penetrate to invade the tis-sues beneath. •

Regeneration. Epithelium has a high regenerativecapacity Some epithelia are exposed to friction,and their surface cells tend to abrade off; others aredamaged by hostile substances (bacteria, acids,smoke) in the external environment and die. Aslong as epithelial cells receive adequate nutrition,they are able to replace lost cells rapidly by celldivision.

Classification of Epithelia

The many types of epithelia are identified structurallyaccording to two criteria: the shape of the cells andthe number of cell layers present (Figure 4.1).

All epithelial cells are irregularly polyhedral(many-sided) in cross section, but differ in cell height.On the basis of height, there are three common shapesof epithelial cells. Squamous (skwÿ'-mus) cells areflattened and scalelike (square = scale); cuboidal (kyoo-boy'-dul) cells are approximately as tall as they arewide; and columnar (kuh-lum'-nur) cells are tall andcolumn-shaped. In each case, the shape of the nucleusconforms to that of the cell. The nucleus of a squa-mous cell is thin and flattened; that of a cuboidal cellis spherical; and a columnar cell nucleus is elongatedfrom top to bottom and is usually located close to thecell base. Nuclear shape is an important structuralcharacteristic to keep in mind when you attempt todistinguish epithelial types.

On the basis of cell arrangement (layers), thereare two major varieties of epithelium: simple andstratified. Simple epithelia are composed of a singlelayer of cells. Simple epithelia are typically foundwhere selective absorption and filtration occur andwhere the thinness of the barrier helps to speed theprocess. Stratified epithelia consist of multiple celllayers stacked one on top of the other. They are typi-cally found in high abrasion areas, where protection

is important, such as the skin surface and the liningof the mouth.

The terms denoting shape and arrangement ofepithelial cells are combined to describe epithelia fully,as shown in Figure 4.2. There are four major classes

of simple epithelia: simple squamous, simple cuboi-dal, simple columnar, and a highly modified simpleepithelium called pseudostratified (pseudo -- false).

There are also four major classes of stratified epi-thelia: stratified squamous, stratified cuboidal, strat-ified columnar, and a modified stratified squamousepithelium called transitional epithelium. In terms ofbody distribution and abundance, only stratifiedsquamous and transitional epithelia are significant.Stratified epithelia are named according to the shapeof the cells at the free surface, not according to deepercell types. For example, the surface cells of stratifiedsquamous epithelium are squamous cells, but its basalcells are cuboidal or columnar.

As you read the descriptions of the individualepithelial classes and compare these descriptions withthe illustrations in Figure 4.2, keep in mind that tis-sues are three-dimensional, but that their structure isstudied using stained tissue sections mounted on slidesand viewed through the microscope. Therefore, a cross-sectional view will differ from a longitudinal view ofthe same tissue. Depending on the precise plane ofthe cut made when tissue slides are prepared, thenucleus of a particular cell may or may not be visible,and (frustratingly) the boundaries between epithelialcells are often indistinct.

Simple Epithelia

The simple epithelia are most concerned with absorp-tion, secretion, and filtration. Because they are usuallyvery thin, protection is not one of their "specialties."

Simple Squamous Epithelium. The cells of a simplesquamous epithelium are flattened laterally, and theircytoplasm is sparse (Figure 4.2a). When viewed fromthe surface, the close-fitting cells resemble a tiled floor;when cut perpendicular to their free surface, the cellsresemble fried eggs seen from the side, with their cyto-plasm wisping out from the slightly bulging nucleus.Thin and often permeable, this epithelium is foundwhere filtration or the exchange of substances by dif-fusion is a high priority Capillary walls, through whichexchanges occur between the blood and tissue cells,are composed exclusively of a simple squamous epi-thelium. In the kidneys, simple squamous epitheliumforms part of the filtration membrane; in the lungs, itforms the walls of the air sacs across which gas exchangeoccurs. Mesothelium (meh"-z6-thÿ'-lÿ-um) is the name

given to the simple squamous epithelium lining theventral body cavity and covering its organs.

Chapter 4 Tissue: The Living Fabric 103

(b)

Squamous

Cuboidal

Columnar

Simple

Stratified

Classification of epithelia. (a) Classification onFigure 4.1 cell shape. For each category, a whole cell isthe basis ofshown on the left and a longitudinal section is shoWn on theright. (b) Classification on the basis oi arrangement.

Simple Cuboidal Epithelium. Simple cuboidal epithe-lium consists of a single layer of cubical cells [Figure4.2b]. When viewed microscopically, the sphericalnuclei stain darkly, causing the layer to look like astring of beads. Important functions of simple cuboi-dal epithelium are secretion and absorption• In glands,it forms both the secretory portions and the ducts thatdeliver secretions to their destinations. The simplecuboidal epithelium in the kidney tubules has densemicrovilli, betraying its active role in absorption.

Simple Columnar Epithelium. Simple columnar epi-thelium is seen as a single layer of tall, closely packedcells, aligned like soldiers in a row [Figure 4.2c). Co-lumnar cells are most associated with absorption andsecretion. Cells actively involved in secretion have anelaborate Golgi apparatus and usually abundant roughendoplasmic reticulum. This epithelial type lines thedigestive tract from the stomach to the rectum• Thedigestive tract lining has two distinct modificationsthat reflect its dual function: (17 dense microvilli on

surface of absorptive cells and ÿ2) goblet cells thatthe lubricating mucus• The goblet ceilssecrete a protective,, ,, goblets, of mucus that occupy mostcontain cups, or

of the apical cell volume [see Figure 4.47.Some simple columnar epithelia display cilia on

their free surfaces. This more unusual variety, calledsimple columnar cifiated epithelium, lines the ovi-ducts and limited areas of the respiratory tract.

Pseudostratified Columnar Epithelium. The cells of

pseudostratified [soo,,_dh-stra'-tih-fid) columnar epi-thelium are varied [Figure 4.2d). All of its cells reston the basement membrane, but some are shorter thanothers and, as seen in the figure, may not reach thesurface of the cell layer• Their nuclei vary in shapeand are seen at different levels above the basementmembrane, giving the false impression that severalcell layers are present. This epithelium may contain

goblet cells and is often ciliated, in which case, theepithelium is more precisely called pseudostratifiedcolumnar ciliated epithelium. Both cilia and gobletpseudostratified epithelium thatcells arefound in the m:c:::ÿh-• st of the respiratory tract. The ehnes m.o .._ÿ,loA dust and other aeu ,duced traps mÿ .....

cilia act to propel it away from the lungs.

Stratified Epithelia

Stratified epithelia consist of two or more cell layers.Considerably more durable than the simple epithelia,their mai°r [but not their only) function is protection.

Stratified Squamous Epithelium. Stratified squamousepithelium is the most widespread stratified epithe-lium [Figure 4.2e}. Composed of several layers, it isthick and well suited for its protective role in the body.Its free surface cells are squamous; cells of the deeper

j - __

I;i: 108 Unit 1 Organization of the Bodyu

layers are cuboidal or, less commonly, columnar. Thisepithelium is found in areas subjected to wear andtear, and its surface cells are constantly being rubbedaway and replaced by mitotic division of the cells ofits basal layer. Since epithelium depends on diffusionof nutrients from a deeper connective tissue layer, theepithelial cells farthest from the basement membraneare less viable, and those at the free apical (outer) sur-face are flattened and atrophied.

Stratified sqnamous epithelium covers the tongueand lines the mouth, pharynx, esophagus, anal canal,and vagina. A modified form of this tissue, keratin-ized (keh'-ruh-tin-nizd") stratified squamous epithe-lium, forms the outer layer, or epidermis, of the skin.The surface of the epidermis consists of dead cellsfilled with keratin, a waterproofing protein. The epi-dermis, which provides a tough yet resilient coveringfor the body surface, is discussed in Chapter 5.

terms that indicate their special locations in the bodyand/or denote general functional qualities. Accordingto this scheme, there are endothelium and the morecomplex epithelial membranes.

Endothelium

An endothelium (en-d6-thU-li-um) is a simple epi-thelial sheet composed of a single layer of squamouscells attached to a basement membrane. Endotheliumprovides a slick, friction-reducing lining in all hollowcirculatory system organs--lymphatic vessels, bloodvessels, and the heart (Figure 4.3a). Capillary wallsconsist only of endothelium, which, because of itspermeability and extreme thinness, encourages theexchange of nutrients and wastes across capillary walls.

Transitional Epithelium. Transitional epithelium formsthe lining of urinary organs, which are subjected toconsiderable stretching and varying internal pressure(Figure 4.2h). Cells of its basal layer are cuboidal orcolumnar; those at the free surface vary in appear-ance, depending on the degree of distension of theorgan. When the organ is not stretched, the membraneis many-layered and the superficial cells are roundedand domelike. When the organ is distended with urine,fewer cell layers are obvious, and the surface cellsbecome flattened and squamouslike. This ability oftransitional cells to slide past one another and changetheir shape accommodates the increasing surface areaof a stretching ureter wall as a greater volume of urineflows through the organ; in the bladder, it allows moreurine to be stored. Additionally, transitional epithe-lium appears to have the ability to resist osmotic forcesthat would act to dilute hypertonic urine stored in thebladder.

Classification of epithelia by cell type and arrange-ment allows each epithelium to be described individ-ually and with precision. However, it reveals nothingabout the tissue's body location. In this section, wewill describe the covering and lining epithelia using

Covering and Lining Epithelia

Stratified Columnar Epithelium. True stratified col-umnar epithelium is rare. Its specific locations arelisted in Figure 4.2g. Its free surface cells are colum-nar; those in deeper layers are small and vary in shape.

Stratified Cuboidal Epithelium. Generally formed of onlytwo cell layers, stratified cuboidal epithelium has avery limited distribution in the body (Figure 4.2f). Itis found primarily in the ducts of sweat glands andother large glands.

Epithelial Membranes

There is tremendous variety in the way the term epi-thelial membrane is used. Here, epithelial membraneis defined as a continuous multicellular sheet com-posed of at least two primary tissue types: an epithe-lium bound to a discrete underlying connective tissuelayer. Hence, epithelial membranes can be consideredto be simple organs. Most of the covering and liningepithelia take part in forming one of three commontypes of epithelial membranes: mucous, cutaneous, or

serous.

Mucous Membranes. Mucous membranes, or mucosae(myoo-k6'-sÿ), are epithelial membranes that line bodycavities that are open to the exterior, such as those ofthe digestive, respiratory, and urogenital tracts (Figure4.3b). In all cases, they are "wet," or moist, membranesbathed by secretions or, in the case of the urinarymucosa, urine. Notice that the term mucosa refers tothe location of the epithelial membrane, not its cellcomposition, which varies. However, the majority ofmucosae contain either stratified squamous or simplecolumnar epithelia.

Mucous membranes are often adapted for absorp-tion and secretion. Although many mucosae secretemucus, this is not a requirement. The mucosae of both

the digestive and respiratory tracts secrete copiousamounts of protective lubricating mucus; that of theurinary tract does not.

All mucosae consist of an epithelial sheet directlyunderlain by a lamina propria (la'-mih-nuh pr6'-prÿ-

uh), a layer of loose connective tissue just deep to thebasement membrane. In some, the lamina propria rests

on a third (deeper) layer of smooth muscle cells. Thesevariations will be covered in later chapters dealingwith the appropriate organ systems.

Chapter 4 Tissue: The Living Fabric 109

Heart

(a) Endothelium

Bloodvessel

Endotheliallining

Mucosa ofnasal cavity

Mucosa of mouth

-- Esophagus lining

-- Mucosa oflung bronchi

/

(c) Cutaneoumembrane

(b) Mucous membranes

(d) Serousmembranes

Figure 4.3 Classes of covering andlining epithelial membranes. (a) Endo-thelium lines the lumen of heart, blood

vessels, and lymphatic vessels. (b)Mucous membranes line body cavitiesthat are open to the exterior. (c) Cuta-

Parietalpleura ÿ/

Visceralpleura

q

Parietalperitoneum

Visceralperitoneum

_ÿ_ --sÿ

Parietalpericardium

J

i,

Visceralpericardium

neous membrane, or skin. (d) Serousmembranes line ventral body cavities thatare closed to the exterior.

Cutaneous Membrane. The cutaneous (kyoo-t£'-nÿ-us)membrane is your skin (Figure 4.3c). It is an organconsisting of a keratinized stratified squamous epi-thelium (epidermis) firmly attached to a thick con-nective tissue layer (dermis). Unlike other epithelialmembranes, the cutaneous membrane is exposed tothe air and is a dry membrane. Chapter 5 is devotedto this unique organ.

Serous Membranes. Serous membranes, or serosae(suh-r6'-sÿ), are the moist membranes found in closedventral body cavities (Figure 4.3d). Each serosa con-sists of a parietal layer that lines the cavity wall andthen reflects back as the visceral layer to cover the

outer surface(s) of organs within the cavity Each ofthese layers consists of simple squamous epithelium(mesothelium) resting on a tiny amount of loose con-nective tissue. The mesothelial cells secrete thin, clearserous fluid that lubricates the facing surfaces of theparietal and visceral layers, so that they slide acrosseach other easily This reduction of friction preventsorgans from sticking to the cavity walls and to eachother.

The serosae are named according to site and spe-cific organ associations. For example, the serosa liningthe thoracic wall and covering the lungs is the pleura;that enclosing the heart is the pericardium; and thoseof the abdominopelvic cavity and viscera are theperitoneums.

110 Unit 1 Organization of the Body

Glandular Epithelia

A gland consists of one or more cells that produce andsecrete a particular product. This product, called asecretion, is an aqueous (water-based) fluid, typicallycontaining proteins. Secretion is an active processwhereby glandular cells obtain needed substances fromthe blood and transform them chemically into theirsecretory product, which is then discharged from thecell. Notice that the term secretion can refer to boththe process of secretion formation and release and theproduct of glandular activity.

Glands are classified as endocrine (en'-duh-krin)or exocrine (ek'-suh-krin), depending on their routeof secretion and the general function of their products,and as unicellular or multicellular on the basis of theirstructure. Most multicellular epithelial glands formby invagination of an epithelial sheet and, at least ini-tially, have ducts connecting them to the epithelialsheet.

Multicellular Exocrine Glands. Multicellular exocrineglands have three common structural elements: anepithelium-derived duct, a secretory unit, and, in allbut the simplest glands, supportive connective tissuethat surrounds the secretory unit and supplies it withblood vessels and nerve fibers. Often, the connectivetissue forms a fibrous capsule that extends into thegland proper and divides the gland into lobes.

Multicellular glands can be divided into two majorcategories on the basis of their duct structures. Simpleglands have a single unbranched duct, whereas com.pound glands have a branching or divided duct. Theglands can be further described according to the struc-

Endocrine Glands

Endocrine glands eventually lose their ducts and areoften called ductless glands. They produce regulatorychemicals called hormones, which they secrete directlyinto the extracellular space. The hormones then enterthe blood or lymphatic fluid. Since not all endocrineglands are epithelial derivatives, consideration of theirstructure and function is deferred to Chapter 17. (a)

Exocrine Glands

Exocrine glands are far more numerous than endo-crine glands, and many of their products are familiarones. The multicellular glands secrete their productsthrough a duct onto body surfaces or into body cavi-ties. Exocrine glands are a diverse lot. They includesweat and oil glands, salivary glands, the liver (whichsecretes bile), the pancreas (which synthesizes diges-tive enzymes), mammary glands, mucous glands, and

many others.

Microvilli Secretoryvesiclescontainingmucin

Nucleus --

(b)

Unicellular Exocrine Glands. Unicellular exocrineglands are single cells interposed in an epitheliumbetween cells with other functions. They have no ducts.In humans, all such glands produce mucin (myoo'-sin), a complex glycoprotein that dissolves in water.Once dissolved, mucin forms a slimy coating (mucus)that both protects and lubricates surfaces. Unicellularglands include the goblet cells of the intestinal andrespiratory mucosae (Figure 4.4), as well as mucin-producing cells found in other body regions. Althoughunicellular glands probably outnumber multicellularglands, unicellular glands are the less well-known ofthe two gland types.

Golgiapparatus

h ER

Figure 4.4 Goblet cells, examples of unicellular exocrineglands. (a) Photomicrograph of simple columnar (intestinal)epithelium with goblet cells (approx. 300 x). (b) Diagram of thefine structure of a goblet cell. Notice the large Golgi apparatusthat is involved in packaging the mucin secretion in vesicles.

Chapter 4 Tissue: The Living Fabric 1 1 1

SurfaceepitheliumDuctSecretoryepithelium

[][][]

TUBULAR SECRETORYSTRUCTURE

ALVEOLAR SECRETORYSTRUCTURE

TUBULOALVEOLARSECRETORY STRUCTURE

Compoundductstructure

(duct branches)

(b) SimplecoiledtubularExample:sweat glands

(a) Simple tubularExample: intestinalglands

.... i °t

(c) SimplebranchedtubularExample: stomach(gastric) glands

(f) CompoundtubularExample: Brunner'sglands of smallintestine

Simpleduct structure

(duct doesnot branch)

• le

i

(g) CompoundalveolarExample: pancreas

(h) CompoundtubuloalveolarExample: salivaryglands

Figure 4.5 Type of multicellular exocrine glands. Multicellular glands are classifiedaccording to duct type (simple or compound) and the structure of their secretory units(tubular, alveolar, or tubuloalveolar).

ture of their secretory parts as (1) tubular, with thesecretory cells forming a tube; (2) alveolar (al-vÿ'-uh-ler), or acinar (a'-sih-ner), with the secretory cellsforming small, flasklike sacs (alveolus = small hol-low cavity); and (3) tubuloalveolar, with the secretoryparts having both tubular and alveolar portions. Termsdenoting duct and secretory part structure are com-bined to describe the gland fully (Figure 4.5). Be sureyou understand the type called compound tubulo-alveolar, as it is very common and important.

Since multicellular glands secrete their productsin different ways, they can also be classified function-ally, according to their secretory behavior. Most exo-crine glands are merocrine (mayr'-6-krin) glands,

which secrete their products by exocytosis shortly afterthe products are produced. The secretory cells are notaltered in any way. The pancreas, most sweat glands,and salivary glands belong to this class (Figure 4.6b).

- :_.ÿ _ 1 12 Unit 1 Organization of the Body

(a) Holocrine gland (b) Merocrine gland (c) Apocrine gland

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Figure 4.6 Modes of secretion in exocrine glands. (a) In holocrine glands, theentire secretory cell ruptures, releasing secretions and dead cell fragments• (b) Mero-crine glands secrete their products by exocytosis. (c) In apocrine glands, the apex ofeach secretory cell pinches off and releases its secretions.

Holocrine (hd-luh-krin) glands accumulate theirproducts within them until the secretory cells rup-ture. (They are replaced by the division of underlyingcells.) Since holocrine gland secretions include thesynthesized product plus dead cell fragments (holos= all), you could say that their cells "die for theircause." Sebaceous (oil) glands of the skin are the onlytrue example of holocrine glands (Figure 4.6a).

Apocrine (a'-puh-krin) glands also accumulatetheir products, but in this case, accumulation occursonly at the cell apex (just beneath its free surface).Eventually, the apex of the cell pinches off (apo =from, off) and the secretion is released. The cell repairsits damage and repeats the process again and again.The mammary glands and some sweat glands releasetheir secretions by this mechanism (Figure 4.6c).

Connective tissue does much more than connect

body parts; it has many forms and many functions. Itschief subclasses are connective tissue proper, carti-lage, bone, and blood. Its major functions includebinding, support, protection, insulation, and, as blood,transportation of substances within the body Forexample, cordlike connective tissue structures con-nect muscle to bone (tendons) and bones to bones (lig-aments), and fine, resilient connective tissue invadessoft organs and supports and binds their cells together.Bone and cartilage support and protect body organsby providing hard "underpinnings"; fat cushions,insulates, and protects body organs as well as provid-ing reserve energy fuel.

Common Characteristics ofConnective Tissue

Connective tissue is found everywhere in the body Itis the most abundant and widely distributed of theprimary tissues, but its amount in particular organsvaries greatly. For example, bone and skin are madeup primarily of connective tissue, whereas the braincontains very little.

Connective TissueDespite their multiple and varied functions in the body,connective tissues have certain common propertiesthat set them apart from other primary tissues:

1. Common origin. All connective tissues arise frommesenchyme, an embryonic tissue derived from themesoderm germ layer, and hence have a common kin-

ship (Figure 4.7).