2 epithelial tissue

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EPITHELIAL TISSUE


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EPITHELIAL TISSUE - Introduction

n The outer surface of the body and the luminalsurfaces of cavities within the body are lined

by continuous cellular sheets called

Epithelial Membranes orepithelia

n which along with various glands that developfrom them, constitute epithelial tissue


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EPITHELIAL TISSUE - Origin

n Epithelial component of skin arises fromectoderm

n Epithelial lining and glands of the digestivetract are derived from endoderm

n Serous linings of the peritoneal, pleural,pericardial cavities, and also the circulatory

system, are products ofmesoderm


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EPITHELIAL TISSUE - Functions

n Epithelia mediate a wide range of activitiessuch as

q Selective diffusionqAbsorptionq Secretionq Physical protection and containment


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EPITHELIAL TISSUE Common

Characteristicsn Cell Junctionsq Membrane specializations that bind adjacent

epithelial cells

n Basement Membraneq Membrane which separates epithelia from

underlying supporting tissue

n Polarityq Basal surfaceqApical surface


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Classification of epithelium

n Epithelial membranes are classified according to thenumber of constituent cell layers and the cell

shape at the free surface

n Simple epithelium - Single layer of cellsn Stratified epithelium At least 2 layers thickn Pseudostratified epithelium - epithelium is only

one cell thick, but it gives the impression of being

stratified because some of its cells are shorter thanothers and therefore do not reach its free surface


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EPITHELIAL TISSUE Other Names

nEndotheliumq Epithelial tissue lining the blood vesselsq Derived from Mesoderm

n Mesotheliumq Epithelial tissue lining the serous body cavities like

pleura, pericardium, & peritoneum

q Derived from Mesodermn Glandular Epithelium

q Epithelium that is primarily involved in secretion isoften arranged into structures called glands


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SIMPLE EPITHELIA

n Consists of a single layer of cellsn Found at interfaces involved in selective

diffusion, absorption and/or secretion

n Provide little protection against mechanicalabrasion

n Cells range in shape from extremely flattened totall columnar cells

n Exhibit a variety of surface specializations, suchas microvilli and cilia, which facilitate their

specific surface functions


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SIMPLE SQUAMOUS EPITHELIUM


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n Pavement Epitheliumn Consists of a single

layer of flat, scale

shaped cellsn Best suited in areas

where the primary

function is diffusion

q Lungs Gasesq Capillaries - Fluids


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SIMPLE SQUAMOUS EPITHELIUM Silver Method/Neutral Red preparation


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n Common sites:q Alveoli of lungsq Free surface of serous pericardium, of the pleura, & of the

peritoneum collectively called as Mesothelium

q Inside of the heart called as Endocardiumq Inside (luminal side) of the blood vessels called as

Endothelium

q Thin walled tubules in the renal medullaq Some parts of Inner ear


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SIMPLE CUBOIDAL EPITHELIUM


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nConsists of single layer ofcuboidal cells

n In transverse section thecells appear square

n Best suited for areas withexcretory, secretory, &

absorptive functions

q Collecting tubules of thekidney and the smallexcretory ducts of the

salivary glands andpancreas


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n Common sites:q Follicles of thyroid glandq Ducts of many glandsq

Surface of ovary called as Germinal Epitheliumq Choroid plexusq Inner surface of lensq Pigment cell layer of retina


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SIMPLE COLUMNAR EPITHELIUM


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n Consists of single layer of tallcolumnar cells

n Best Suited in places withabsorptive function like Small

Intestine or Places withsecretory function likeStomach & gall bladder

n Seen in linings of minor ductsof many exocrine glands


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SIMPLE COLUMNAR CILIATEDEPITHELIUM


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n Simple secretory columnarepitheliumq Mucous secreting as well as

protective

q No cilia or microvilli

n Simple ciliated columnarepitheliumq Cilia cover the free surface

n Simple columnar epitheliumwith microvilliq Striated borderq Brush border


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Goblet Cells: The apical parts of the secretory epithelium contains mucouscontaining vesicles which are light staining. These cells are called Goblet cells.


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n Common sites:q Simple columnar (without cilia or microvilli)

n Lines the stomach, uterine cervix, & large intestineq Simple ciliated columnar

n Seen in most of respiratory tract, uterus, & uterine tubesn Also seen in efferent ductules of testis, parts of middle ear &

auditory tube, ependyma lining central canal of spinal cord, &ventricles of brain

q Simple columnar with microvillin Striated border

q Small intestinen Brush border

q Gall bladder


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PSEUDOSTRATIFIED COLUMNAREPITHELIUMn All component cells lie in

contact with the basement

membrane

n Nuclei are visible at more thanone level

n Some of its cells are shorterand do not reach its free

surface

n Pseudostratified ciliatedcolumnar epitheliumq With scattered goblet cells,

lines most of the major airways


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PSEUDOSTRATIFIED COLUMNAREPITHELIUM


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PSEUDOSTRATIFIED COLUMNAREPITHELIUM


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PSUEDO-STRATIFIED EPITHELIUM

n Common sites:q Some part of auditory tubeq Ductus deferensq Male urethra (membranous & penile part)

n Ciliated pseudostratified columnar epitheliumq Trachea & in large bronchi


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STRATIFIED EPITHELIA

n Consists of two or more layers of cells

n Mainly perform protective function

n Poorly suited for absorption and secretion


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STRATIFIED SQUAMOUS EPITHELIUM


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STRATIFIED SQUAMOUS EPITHELIUM

n Several layers of cellsn Cells of deepest (or basal) layer rest on basement

membrane & are usually columnar in shape

n As we go to periphery (basal to outer layers) Cellsbecome polyhedral or cuboidal and eventually become

squamous.

n Sub-typesq Non-keratinizedq Keratinized


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STRATIFIED SQUAMOUS NONKERATINIZING EPITHELIUM

n Cells on surface layer hasnuclei & are living

n It lines the wet surfacessubmitted to wear and tear

n Lines the inside of the mouth,the esophagus, the vagina, &the cornea


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STRATIFIED SQUAMOUS NONKERATINIZING EPITHELIUM


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STRATIFIED SQUAMOUSKERATINING EPITHELIUM

n The cells near the free surfacetransform into scales of soft

keratin that remain strongly

adherent to the underlying

layers of living cellsn These cells are dead & lose

their nuclei

n Suitable for protecting surfacesn Seen in epidermal layer of

skin

n Resists water loss and uptake


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STRATIFIED SQUAMOUSKERATINING EPITHELIUM


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SQUAMOUS CELL CARCINOMA


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STRATIFIED CUBOIDAL EPITHELIUM


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TRANSITIONAL EPITHELIUM


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n The microscopic appearanceof Transitional epitheliumchanges as the membranestretches

n In the unstretchedconfiguration usually seen insections, the roundedsuperficial cells bulge out

n Under lateral tension, thesecells are attenuated andsquamous

n Lines almost all ofthe urinarytract.( urinary bladder , ureter )

n Some surface cells of thebladder are multinucleated


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MEMBRANESPECIALIZATIONS

Cell Junctions, Cilia, Microvilli, & Stereocilia


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CELL JUNCTIONS

n Tight junctions or Occluding junctions (Zonulaoccludens)

n Adhering junctionsq Adhesive spots (Desmosomes or macula adherens)q Adhesive belts (Zona adherens)q Adhesive strips (Fascia adherens)q Hemidesmosomes or focal spots

n Gap junctions or Communicating junctions


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Junctional complex


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Tight junctions ( Zonula Occludens)

n Formed by fusion ofopposed cell

membranes

nIt extends completelyaround apical cell

border


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Tight Junctions


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Tight junctions ( Zonula Occludens)

n Transmembrane proteins form the so-called sealing strands that 'stitch' the

membranes together

n Each 'stitch' comprises two molecules ofthe transmembrane protein claudin, one

an integral part of each opposing plasmamembrane, linked tightly together


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TIGHT JUNCTIONS

n Act as barriers that prevent movement of molecules intointercellular spaces

n Seen in between cells of membranes that arespecialized for absorption or secretion

n Prevent leakage of substances absorbed by cells againstconcentration gradient

n Leaky tight junctionsq Incomplete tight junctions


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Fascia Occludens

n Structurally similar but discontinuous strips oftight junctions

n Found between the endothelial cells liningblood vessels, except in the vessels of the

brain where they are of the continuous

(zonula occludens) type


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Desmosomes

n Most common type of cellular junction

n The fibrils seen in the intercellular space (linking the twoplaques) represent CAMs (Cellular adhesion molecules)

n CAMs normally seen areq Integrins (Desmogleins I, II)q Desmoplakinsq Plakoglobin


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Zonula Adherens

n This lies basal to Zonula Occludensn It extends continuously around celln Major site of attachment of microfilaments to

cell membrane. Many of these microfilamentsare arranged circumferentially as marginal

band

n CAMs present are cadherins


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Zonula Adherens

n Differ from Macula adherensq By forming a continuous band passing all around the apical part

of the epithelial cells

q By not having a filaments traverse the gaps between the cells


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Fascia Adherens

n Similar to Zonular adherensn Differ by not going all around the cells (form strips of

attachment)

n Seen in smooth musclesn Intercalated discs of cardiac musclesn Junctions between glial cells & neurons


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i l C l


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Junctional Complex

B b


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Basement membrane

n Matrix structure called a basementmembrane strongly attaches epithelial cells

to the adjacent connective tissue

n Basement membrane is made up off twoadjacent basal lamina or more commonly

basal lamina and reticular lamina.

B b


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Basement membrane

n Basal lamina is made up ofq Type IV collagenq Lamininq Heparan sulphate

n Reticular laminaq Reticular fibers


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EPITHELIAL GLANDS

EPITHELIAL GLANDS


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EPITHELIAL GLANDS

n Are subdivided intoq Exocrine glandsq Endocrine glands

n Exocrine Glandsq Develop as down growths of an epithelial membrane and

secrete onto its surface through their ducts

n Endocrine glandsq Are ductless, they release their secretory product close to

the external surface of thin walled blood vessels. (Product

enters blood stream).


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EPITHELIAL GLANDS


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EPITHELIAL GLANDS

n Goblet cellsq Mucous secreting uni-

cellular exocrine glands

q They assume shape ofgoblet because secretory

vesicles accumulate and

compress neighboring cells

q Nucleus lies in stem likebasal region

qSynthesis of Mucus beginsat rER and glycosylation is

completed in golgi

apparatus

G bl ll


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Goblet cell

E i l d


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Exocrine glands

n Made up ofq Secretory unitq Duct with its supporting connective tissue

n Secretory unitq Group of epithelial cells that release their

secretion into a lumen

nDuctq Epithelially lined tube that conveys the secretion

to epithelial surface

Gl d


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Glands

n Simple glandqA gland with a single unbranched ductq E.g. Sweat gland

n Compound glandqA gland with a branching tree like duct systemq E.g. Pancreas

Si l Gl d


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Simple Gland


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C mp nd rin l nd


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Compound exocrine glands

n Parenchymaq Epithelial component

n Stromaq Supporting connective tissue (Together with blood

vessels and nerve fibres)

n Capsuleq

Gland is enclosed by a fibrous connective tissue


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Depending on the type of secretory


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Depending on the typeofsecretoryproduct

n Serous glandsq Produce watery

secretions

n Mucous glandsq Produce viscid mucus

n Mixed glands

Serous glands Vs Mucous glands


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Serous glands Vs Mucous glands

n Serous glandsq Brightly stainedq Basal part is basophilic

q Large spherical nucleus

n Mucous glandsq Appear paleq Mucus can be stained by

PAS stain

q Nucleus small flattenedagainst basal cell boarder

Serous Mucus and Mixed secretory units


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Serous, Mucus and Mixed secretory units


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Secretory cells of exocrine glands also can


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Secretory cells of exocrine glands also canbe differentiated inton Merocrine glands (eccrine or epicrine)

q Release their secretion by Exocytosisq E.g. Pancreatic acinar cells

n Holocrine glandq Entire cell is sacrificed in producing the secretionq E.g. Sebaceous glands associated with hair follicle

n Apocrine glandq The apical portions are sacrificed during releaseq Eg: Atypical Sweat glands

Endocrine glands


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Endocrine glands

n Instead of releasing their secretions intoducts, they discharge them into the blood

stream

n Secretions of endocrine glands are calledhormones

n They elicit functional changes in target cells

2 epithelial tissue

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