epilthelial tissue lecture

8/2/2019 Epilthelial Tissue Lecture

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EPITHELIAL TISSUE

Emmanuel Twumasi Osei

Dept of Anatomy and Cell BiologyKNUST


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Epithelial Tissue

Epithelia are the cellular tissues that cover and line allbody surfaces, cavities and tubes and extend from thesesurfaces into deeper tissues to form glands.

Functions include Protection, Absorption & Secretion

Generally composed of cells with each cell tightly boundto its neighbours

Lack blood vessels, exhibit polarity are innervated andhave the capacity to regenerate. (Polarity?)


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Epithelial Tissue Contd

Basal surface lies on a thin sheet called Basal Lamina-

part of the basement membrane

Important to note that all epithelial cells have free

apical upper surface and lower basal surface

Although not penetrated by blood vessels, nerve

endings do

Outer surface facing external enviroment specialised

according to fxn and in multilayered epithelia there isan inside out gradation of the specialised features


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Classification of Epithelia

Surface epithelia are classified according to

The thickness: Simple epithelium is 1 cell thick (small intestine,

kidney vascular endothelium)

Compound epithelium is many cells thick (epidermisand esophagus)

Multicellular epithelia often show polarity and are

called stratified (layered) epithelia

Can be pseudostratified epithelia (trachea and vas

deferans)


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Classification ContdClassified by TheShape of Surface Cells:

Cuboidal, Columnar and Squamous (Scally /flattened)

Squamous cell: flattened with height much less thanwidth.

Cuboidal cell: cube shaped, height, width and lengthare similar.

Columnar: A tall cell, Height is greater than width.

Classified by the presence of Surface Specialisation:

Cilia, Microvilli, Stereocilia or Keratin


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Shape Thickness


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Microvilli

Known as Brush or striated border in light

microscopy

Seen as fine striations on free surface of cells which

are parallel to long axis of cells

Has brush like appearance hence name brush border

Seen in EM as tubular projections on apical plasma

membrane Diameter 0.1 micron constant in width

across the body.

Length varies from 1.0 to 3.0 microns but from agiven cell, tends to be of constant length

Large numbers found on one cell


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Microvilli Contd

Microvilli are important as they increase the increase

the surface area (20 40X) of absorptive cells

Microvilli face the compartment that has absorbable

material and is in contact with such material

Normally brush border microvilli as seen in the small

intestine or proximal renal tubule is involved in

absorption but scattered ones seen elsewhere may

not be Brush border contains enzymes that aid in final steps

of digesting proteins, lipids, carbohydrates

immediately before absorption.


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Stereocilia

Extremely long microvilli and less precisely arranged

than microvilli and less widely found

No relationship to true motile cilia

Found in the epididymis, vas deferens and otherparts of the male reproductive system

Project into the lumen and may be involved in

absorption of substances

Hair cells in inner ear have stereocilia and they act as

transducers


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Cilia

Cilia just as in flagella are motile structures

Slender hair-like projections from a cell surface thatmove in a xtic manner

Cilia (5-10 microns) shorter than flagella

100s of cilia may be found on a cell while 1 or 2flagella may be found on one cell

Cilliated cells widely found in the body (Respiratorysystem, uterine tube and male reproductive system)

Sperm are the only human cells with flagella


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Cilia Contd

Cilia beat back and forth in motion whiles flagella

tend to function with an undulating motion

Cilia and flagella possess basal bodies, intracellular

xtures at their base

Each cilium or flagella associates with a single basal

body


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Classification Contd

Classified by different Cell types

In applying this principle a particular Mucous cellknown as the Goblet cell is involved

Constitute a subset of mucus producing cells

Resembles the bowl-like part of a wine glass

Therefore in classifying epithelial cells one shouldfind out if there are any goblet cells present


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Description of Epithelium

Thus an example of a full description of an

epithelium may be something like

Pseudostratified (2) columnar (3) ciliated epithelium

with (4) goblets cells


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Epithelial Tissue

Note: Apical surface free surface, Lateral surface

faces adjacent cell, Basal surface adjacent to thesubstratum usually connective tissue

Epithelia may be secretory, absorptive or protective

An individual epithelial cells may be secretorystructure (e.g the epithelial lining of the stomach

(gastric epithelium), intestinal goblet)

Again a number of secretory epithelial cells may forma homogeneous secretory layer (lining of gastric pits

of stomach)


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Epithelia tissue Contd

Many epithelia located at interfaces btn the body

and the external & internal envts across whichSolvents, Solutes, Metabolic substrates and Gases are

absorbed.

E.g. of absorbing/transporting epithelia: intestinal

epithelium, gall bladder, vascular endothelium andkidney tubule epithelium.

May also be specialised to protect internal and

external envt e.g. epidermis and esophagealepithelium

Structure related to fxn very important


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Junctional Complexes

Areas of contact between epithelia cells have

intercullular junctions called junctional complex

Close arrangement of several cell-cell junctions

usually formed by:

Zonula occludens, Zonula adherens and Desmosome

Usually found in simple epithelia (simple cuboidal

epithelia, simple columnar epithelia)

Found usually on the lateral surface close to free or

luminal surface.


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Junctional Cplx Contd

Desmosomes: Points of mechanical attachment of 1 cell to another

Gap Junctions: seen in EM as close apposition of plasmamembranes of adjacent cells.

The Gap refers to a regular 2 -3nm extracellular space in thejunctional region. Channels provide continuity of one cellscytoplasm with another.

Gap junctions provide avenues for diffusion of substances (e.g ionsand amino acids) from 1 cell to the adjacent cell

Gap junctions may electrically couple cells to allow the spread of

electric potentials through a tissue (e.g in smooth muscle & Cardiacmuscle)

Hence gap junctions have also been called Communicatingjunctions


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Jxnal Cplxes Contd

Tight Junctions/ Zonulae Occludentes/ Occluding Jxns:

A type of junction that encircles a cell, joining that cell toall its immediate neighbouring cells.

Often found in simple columnar or cuboidal epithelia on

the lateral surface of the cell closer to the apical surface

than the basal surface

Point of membrane fusion are called Membrane kissessince fusion of adjacent plasma membrane not even


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Tight Jxn Contd

Tight junctions thought to block or occlude theextracellular space to prevent or retard the flow ofmaterials through the extracellular space from one side ofthe epithelium to the other side

Its effectiveness as a diffuse barrier is a function of thedepth of the junction

Narrow junctions tend to be leaky whiles wide junctionshas more contact points between the connected cells andare more effective

The most complex tight junctions (most rows of ridges andgrooves) found btn the capillary endothelial cells that formthe Blood Brain Barrier and between the Sertoli cells thatform the blood testis barrier. These are essentially absolutebarriers under normal circumstances


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Junctional Complexes Contd

Zonula Adherens: A circumferential, belt-like jxn

usually found deep to a tight junction

Another type of junction, the Hemidesmosome is

closely related to the desmosomes

Found on the basal plasma membrane of epithelial

cells adjacent to basement membrane

Serve as points of mechanical attachment ofepithelial cells to basal lamina.


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Epithelial cell renewal

Generally have a high regenerative ability. Turnover

varies from a few days (small intestine) to approx 1

mth (epidermis).

Dividing cells are often restricted to particular

regions of the epithelium (e.g. basal part of theepidermis and lowest 3rd of intestinal crypts)

Recently divided cells leave the mitotic compartmentand migrate towards the superficial layer (from basal

layer of epidermis to the cornified layer and from

crypt of the intestinal gland to the luminal surface or

villus)


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Epithelial cell renewal Contd

Following stages and events often seen during

Renewal process

Cell division, Migration, Differentiation,

Senescence and Cell loss.


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Epithelial-Connective Tissue Interface

Btn the basal membrane of the epithelium and the

underlying connective tissue is the Basement membrane. A non-cellular layer formed by both the epithelium and

underlying connective tissue

Connective tissue immediately subjacent to the basalepithelial surface

Made up of the Basal lamina, Collagen fibers, Reticular

fibers ( reticulin i.e. solitary thin collagen fibrils)

proteoglycans and glycoproteins

Basal membrane may thicken in some pathologic states

(nephropathies, vasculopathies and auto-immune dx)


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Basal Lamina (Basement lamina)

30 100 nm thick layer (lamina densa)

Separated from basal plasma by a clear electron-lucent zone(lamina lucida).

A filamentous network that contains Type IV Collagen, Laminin andother proteins and glycoproteins

Collagen and Reticular fibers may attach to basal lamina at theconnective tissue surface and serve a anchoring filaments

Provide attachment points for Hemidesmosomes and surface alongwhich cells migrate in cell renewal and wound healing.

Basal lamina is a component of the basement membrane but the 2terms are often used synonymously


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Types of Epithelium

Squamous (Pavement) Epithelium

Flattened interlocking polygonal cells (squames).

Cytoplasm in some places are as little as 0.1 microns

Nucleus usually bulges in overlying space

Type of epithelium lining alveoli of lungs & Renal

Corpuscles, thin segment of nephrotic tubules (loop

of Henle) and as a mesothelium forms the surfaces

of the Pericardial, Pleural and Peritoneal cavities

As the endothelium it lines the blood vascular and

lymphatic channels


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Squamous (Pavement) Epithelium Contd

Squamous epithelium often assoc with passivemovements of water or electrolytes

May also be active in transport evidenced by

pinocytotic vessicles seen in such cells

Presence of Zonula occludens ensure that materials

pass primarily through cells rather than betweenthem


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Cuboidal & Columnar Epithelia

Cylindrical cells set together to form a Palisade-like

layer

Each cell is polygonal in horizontal section, Cuboidal

cells are square in vertical section and Columnar cells

are taller than their diameter Commonly have microvilli to provide large absorptive

area

Large intestine Columnar cells with striated border, Gall bladder columnar cells with brush border

Proximal and distal convoluted tubules of the kidney

cuboidal cells with brush borders)


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Cuboidal & Columnar Epithelia Contd

Ciliated columnar epithelium lines most of the

respiratory tract as far as the respiratory bronchioles,

some of the tympanic cavity and auditory tube and

the uterine tube.

In resp tract surfaces, they are also mucous glands,cilia sweep a layer of viscous fluid and trapped dust

particles from lungs towards the pharynx thus

clearing the resp passage.

In uterine tube cilia action assist ova from peritoneal

cavity to the uterus


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Cuboidal & Columnar Epithelia Contd

Some columnar cells are glandular and their apicalregions contain mucous or protein-carrying vacoules

E.g. the mucin secreting cellsand protease-secreting chiefcells of the gastric epithelium

Mucous pdcing cells interspersed in btn non producingones leads to widening of apices known as Goblet cellscommon in the intestines

Mucus bind water and protect surfaces from drying theycan also slide over each othe easily providing lubrication


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Pseudostratified epithelium

Gives impression that its stratified

But simple columnar epithelium in which

arrangement is distorted

Nuclei lie in dfnt levels in vertical section

Cells are twisted with resp to each other, only partial

profiles of some cells thus appear giving the

impression of more than 1 layer of cells

They assume this when subjected to lateral

compression forces


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Stratified Epitelia

Positioned at surfaces where there is

considerable wear and tear

Chiefxtic is continued replacement of

superficial cells by division of more deep cells


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Stratified Squamous Epithelium

Several layers of cells which vary greatly in shape.

Deepest layer are mostly columnar placed vertically

on basement membrane.

Superficial to this are polyhedral cells which become

more and more compressed as they approach thesurface

Most superficial cells are flattened scales (squames)

which overlap one another and present animbricated appearance


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Stratified Squamous Epithelium Contd

Cells closely bound by Desmosomes

Deepest layers progressively proliferate & undergo

morphological change as they pass to the surface

where they are continually lost by abbrassive wear

and tear

Moist cells though flattened retain their nuclei whiles

dry surfaces have Keratin that effect water-proofingand confer mechanical protection

S ifi d S E i h li C d


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Stratified Squamous Epithelium Contd

2 types termed

Stratified squamous non-keratinized & Stratifiedsquamous keratinized epithelium

Non keratinized mucus membrane of mouth, lower

pharynx, oesophagus, vagina, part of cervix uteri,anal canal epiglottis etc.

Keratinized epidermis of skin, in parts of mouth

(filliform papillae of tongue , hard palate.

Amount of keratin proportional to mechanical stress

of lack of Vit A in diet (induces keratinization in the

cornea)


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Skin (Read on it)

Epithelium on skin Epidermis

Several distinct layers in 2 zones

Deeper Zona germinativa single layer ofColumnar cells

Stratum basale (stratum germinativum orMalpighian layer)

Superficial layer variable thickness ofpolyhedral cells Stratum spinosum


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Stratified Cuboidal and Columnar Epithelia

2 or more layers of cuboidal or columnar cells

Typical of the walls larger ductus of some

exocrine glands

E.g. Pancreas and salivary glands, presumably

affording more strength than a single layer


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Transitional Epithelium

Occurs in the Renal calyces, Renal Pelvis, Ureters, Urinary

bladder, Prostatic urethra in the male and the Proximalpart of the female urethra

Superficial layer flattened & large with depressions ontheir basal surfaces to fit on to the rounded cells of the

second layer (pear shaped)

Its apices then touches the basement membrane

Changes are seen in the cells during distension andcontraction of bladder

Impermeable to water

epilthelial tissue lecture

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