Anatomy (Lec 6) Epithelial tissue 2021-2022.pdf

Anatomy (Lec 6) Epithelial tissue 2021-2022.pdf

1. STRUCTURE OF CELL
Pharm D 1
st
Prof
Bahauddin Zakariya University Multan
Waseem Ashraf

2. Recommended Books:
•Fundamentals of Anatomy & Physiology. Martini F, Nath JL, Bartholomew EF. 10th Edition
•Principles of Anatomy and Physiology. Tortora GJ, Derrickson B. 13th Edition

3. Body tissues can be classified into four basic types according to their structure and function:
1.Epithelialtissuescoverbodysurfacesandlineholloworgans,bodycavities,andducts;theyalsoformsglands.This
tissueallowsthebodytointeractwithbothitsinternalandexternalenvironments.
2.Connectivetissuesprotectandsupportthebodyanditsorgans.Varioustypesofconnectivetissuesbindorgans
together,storeenergyreservesasfat,andhelpprovidethebodywithimmunitytodisease-causingorganisms.
3.Musculartissuesarecomposedofcellsspecializedforcontractionandgenerationofforce.Intheprocess,muscular
tissuesgenerateheatthatwarmsthebody.
4.Nervoustissuedetectschangesinavarietyofconditionsinsideandoutsidethebodyandrespondsbygenerating
electricalsignalscallednerveactionpotentials(nerveimpulses)thatactivatemuscularcontractionsandglandular
secretions.
TYPES OF TISSUES

4. Anepithelialtissue(ep-i-THE¯-le¯-al)orepithelium(pluralisepithelia)consistsofcellsarrangedincontinuoussheets,in
eithersingleormultiplelayers.Becausethecellsarecloselypackedandareheldtightlytogetherbymanycelljunctions,there
islittleintercellularspacebetweenadjacentplasmamembranes.Epithelialtissuesformcoveringsandliningsthroughoutthe
body.Epitheliacoverinternalorexternalsurfaces.Theyarerarelycoveredbyanothertissue,sotheyalwayshaveafree
surface.
Glandsare structures that produce fluid secretions. They are either attached to or derived from epithelia.
Epithelia have several important characteristics:
1.Cellularity. Epithelia are made almost entirely of cells bound closely together by interconnections known as cell junctions.
In other tissue types, the cells are often widely separated by extracellular materials.
2.Polarity. An epithelium has an exposed surface, which faces the exterior of the body or an internal space, and a base, which
is attached to underlying tissues. The term polarity refers to the presence of structural and functional differences between
the exposed and attached surfaces. An epithelium consisting of a single layer of cells has an exposed apical surface and an
attached basal surface. The two surfaces differ in plasma membrane structure and function.
EPITHELIAL TISSUES

5. 3.Attachment. The base of an epithelium is bound to a thin, noncellular basement membrane. The basement membrane is a
complex structure produced by the basal surface of the epithelium and the underlying connective tissue.
4.Avascularity.Epithelia are avascular (a.-VAS-ku. -lar; a-, without + vas, vessel), which means that they lack blood
vessels. Epithelial cells get nutrients by diffusion or absorption across either the exposed or the attached epithelial surface.
5.Regeneration.Epithelial cells that are damaged or lost at the exposed surface are continuously replaced through stem cell
divisions in the epithelium. Regeneration is a characteristic of other tissues as well, but the rates of cell division and
replacement are typically much higher in epithelia than in other tissues
EPITHELIAL TISSUES
Thebasementmembraneisathinextracellularlayerthatcommonlyconsistsoftwo
layers,thebasallaminaandreticularlamina.
Thebasallamina(laminathinlayer)iscloserto—andsecretedby—theepithelialcells.It
containsproteinssuchaslamininandcollagen,aswellasglycoproteinsandproteoglycans.
Thereticularlaminaisclosertotheunderlyingconnectivetissueandcontainsproteins
suchascollagenproducedbyconnectivetissuecellscalledfibroblasts.

6. Epitheliaperformfouressentialfunctions:
1.ProvidePhysicalProtection.Epitheliaprotectexposedandinternalsurfacesfromabrasion,dehydration,anddestruction
bychemicalorbiologicalagents.
2.ControlPermeability.Anysubstancethatentersorleavesyourbodymustcrossanepithelium.Someepitheliaare
relativelyimpermeable.Othersareeasilycrossedbycompoundsaslargeasproteins.Manyepitheliacontainthemolecular
“machinery”neededforselectiveabsorptionorsecretion.Theepithelialbarriercanalsoberegulatedandmodifiedin
responsetostimuli.Forexample,hormonescanaffectthetransportofionsandnutrientsthroughepithelialcells.Even
physicalstresscanalterthestructureandpropertiesofepithelia.Forexample,callusesformonyourhandswhenyoudo
manuallaborforsometime.
3.ProvideSensation.Mostepitheliaareextremelysensitivetostimulation,becausetheyhavealargesensorynervesupply.
Thesesensorynervescontinuallyprovideinformationabouttheexternalandinternalenvironments.Forexample,thelightest
touchofamosquitowillstimulatesensoryneuronsthattellyouwheretoswat.Aneuroepitheliumisanepitheliumthatis
specializedtoperformaparticularsensoryfunction.Ourneuroepitheliacontainsensorycellsthatprovidethesensationsof
smell,taste,sight,equilibrium,andhearing.
EPITHELIAL TISSUES

7. 4.ProduceSpecializedSecretions.Epithelialcellsthatproducesecretionsarecalledglandcells.Individualglandcellsare
usuallyscatteredamongothercelltypesinanepithelium.Inaglandularepithelium,mostoralloftheepithelialcellsproduce
secretions.Thesecellseitherdischargetheirsecretionsontothesurfaceoftheepithelium(toprovidephysicalprotectionor
temperatureregulation)orreleasethemintothesurroundinginterstitialfluidandblood(toactaschemicalmessengers).
EPITHELIAL TISSUES
Tobeeffectiveasabarrier,anepitheliummustformacompletecoverorlining.Threefactorshelpmaintainthephysical
integrityofanepithelium:
(1)Intercellularconnections
(2)Attachmenttothebasementmembrane
(3)Epithelialmaintenanceandrepair

9. Typesofcoveringandliningepithelialtissuesareclassifiedaccordingtotwocharacteristics:thearrangementofcellsinto
layersandtheshapesofthecells.
Classification of Epithelial Tissues

23. Glandular Epithelium
The function of glandular epithelium is secretion, which is accomplished by glandular cells that often lie in clusters deep
to the covering and lining epithelium. A gland may consist of a single cell or a group of cells that secrete substances into
ducts (tubes), onto a surface, or into the blood. All glands of the body are classified as either endocrine or exocrine.
Endocrine Glands
An
endocrine gland produces endocrine (endo-, inside + krino, to separate) secretions, which are released directly into
the surrounding interstitial fluid. These secretions, also called hormones, then enter the bloodstream for distribution
throughout the body. Hormones regulate or coordinate the activities of various tissues, organs, and organ systems. Examples
of
endocrine glands include the thyroid gland and the pituitary gland. Because their secretions are not released into ducts,
endocrine glands
are often called ductless glands. Endocrine cells may also be part of an epithelial surface, such as the
lining of
the digestive tract, or they may b e found in separate organs, such as the pancreas, thyroid gland,
thymus, and pituitary gland.

25. Exocrineglands
Exocrineglandsproduceexocrine(exo-,outside)secretions,whicharedischargedontoanepithelialsurface.Mostexocrine
secretionsreachthesurfacethroughtubularducts,whichemptyontotheskinsurfaceorontoanepitheliumlininganinternal
passagewaythatcommunicateswiththeexterior.Examplesofexocrinesecretionsareenzymesenteringthedigestivetract,
perspirationontheskin,tearsintheeyes,andmilkproducedbymammaryglands.
Exocrineglandshaveseveraldifferentmethodsofsecretion.Forthisreason,theyareclassifiedbytheirmodeandtypeof
secretion,andbythestructureoftheglandcellsandassociatedducts.
ModesofSecretion.
Aglandularepithelialcellreleasesitssecretionsby
(1)Merocrinesecretion,
(2)Apocrinesecretion,
(3)Holocrinesecretion.
Glandular Epithelium

28. Merocrinesecretion,SecretionsofmerocrineglandsaresynthesizedonribosomesattachedtoroughER;processed,sorted,
andpackagedbytheGolgicomplex;andreleasedfromthecellinsecretoryvesiclesviaexocytosis.Mostexocrineglandsof
thebodyaremerocrineglands.Examplesincludethesalivaryglandsandpancreas.
Apocrinesecretionaccumulatetheirsecretoryproductattheapicalsurfaceofthesecretingcell.Then,thatportionofthecell
pinchesoffbyexocytosisfromtherestofthecelltoreleasethesecretion.Theremainingpartofthecellrepairsitselfand
repeatstheprocess.Electronmicroscopyhasrecentlyconfirmedthatthisisthemechanismofsecretionofmilkfatsinthe
mammaryglands.Recentevidencerevealsthatthesweatglandsoftheskin,namedapocrinesweatglandsafterthismodeof
secretion,actuallyundergomerocrinesecretion.
Holocrinesecretion,bycontrast,destroystheglandcell.Duringholocrinesecretion,theentirecellbecomespackedwith
secretoryvesiclesandthenbursts,releasingthesecretion,butkillingthecell.Furthersecretiondependsonreplacing
destroyedglandcellsbythedivisionofstemcells.Becausethecellrupturesinthismodeofsecretion,thesecretioncontains
largeamountsoflipidsfromtheplasmamembraneandintracellularmembranes.Sebaceousglands,associatedwithhair
follicles,produceanoilyhaircoatingbymeansofholocrinesecretion.
Glandular Epithelium

29. Merocrine Secretion

30. Apocrine Secretion

31. Holocrine Secretion

32. Structural Classification of Exocrine Glands
Exocrineglandsareclassifiedasunicellularormulticellular.
Asthenameimplies,unicellularglandsaresingle-celledglands.Gobletcellsareimportantunicellularexocrineglandsthat
secretemucusdirectlyontotheapicalsurfaceofaliningepithelium.
Thesimplestmulticellularexocrineglandisasecretorysheet,inwhichglandcellsformanepitheliumthatreleases
secretionsintoaninnercompartment.Mucin-secretingcellsthatlinethestomach,forinstance,protectthatorganfromits
ownacidsandenzymesbycontinuouslysecretingmucin.Mostothermulticellularexocrineglandsareinpocketssetback
fromtheepithelialsurface.Theirsecretionstravelthroughoneormoreductstothesurface.Examplesincludethesalivary
glands,whichproducemucinsanddigestiveenzymes.
Threecharacteristicsareusefulindescribingthestructureofmulticellularexocrineglands:
1.TheStructureoftheDuct.Aglandissimpleifithasasingleductthatdoesnotdivideonitswaytotheglandcells.The
glandiscompoundiftheductdividesoneormoretimesonitswaytotheglandcells.
2.TheShapeoftheSecretoryPortionoftheGland.Glandswhoseglandularcellsformtubesaretubular;thetubesmay
bestraightorcoiled.Thosethatformblindpocketsarealveolaroracinar.Glandswhosesecretorycellsformbothtubes
andpocketsarecalledtubulo-alveolarandtubulo-acinar.

33. Structural Classification of Exocrine Glands
3.TheRelationshipbetweentheDuctsandtheGlandularAreas.Aglandisbranchedifseveralsecretoryareas(tubularor
acinar)shareaduct.(“Branched”referstotheglandularareasandnottotheduct.)
Combinationsofthesefeaturesarethecriteriaforthefollowingstructuralclassificationschemeformulticellularexocrine
glands:
I.Simpleglands
A.Simpletubular.Tubularsecretorypartisstraightandattachestoasingleunbranchedduct.Example:glandsinthelarge
intestine.
B.Simplebranchedtubular.Tubularsecretorypartisbranchedandattachestoasingleunbranchedduct.Example:gastric
glands.
C.Simplecoiledtubular.Tubularsecretorypartiscoiledandattachestoasingleunbranchedduct.Example:sweatglands.
D.Simpleacinar.Secretoryportionisroundedandattachestoasingleunbranchedduct.Example:glandsofthepenile
urethra.
E.Simplebranchedacinar.Roundedsecretorypartisbranchedandattachestoasingleunbranchedduct.Example:
sebaceousglands.

34. Structural Classification of Exocrine Glands
II.Compoundglands
A.Compoundtubular.Secretoryportionistubularandattachestoabranchedduct.Example:bulbourethral(Cowper’s)
glands.
B.Compoundacinar.Secretoryportionisroundedandattachestoabranchedduct.Example:mammaryglands.
C.Compoundtubuloacinar.Secretoryportionisbothtubularandroundedandattachestoabranchedduct.Example:acinar
glandsofthepancreas.