epilthelial tissue lecture
TRANSCRIPT
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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