COVERING EPITHELIUM.ppt and its characteristics

This ppt. explaine types, characterstics and functions of epithelium

1. TISSUE
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2. 
Organs can be divided into parenchyma,
which is composed of the cells responsible
for the main functions typical of the organ,
and stroma, which is the supporting tissue.

Except in the brain and spinal cord, the stroma is Except in the brain and spinal cord, the stroma is
made of connective tissuemade of connective tissue.
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Tissue concept and classificationTissue concept and classification
oTissue may be defined as aggregates or groups of cells
organized to perform one or more functions.
oThe concept of tissues provides a basis for understanding
and recognizing the many cell types within the body and
how they interrelate.
o
Despite variations in general appearance, structural
organization and physiologic properties of the various
body organs, the cell aggregations that make them up are
reduced to and classified under four tissue types.
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These basic or fundamental tissues are:
I.Epithelial tissue – which covers body surfaces, lines body
cavities and forms glands.
II.Connective tissue - which underlies or surrounds and supports
the other three basic tissues both structurally and functionally.
III. Muscular tissue - which is made up of contractile cells and is
responsible for movement of the body and its parts.
IV. Nervous tissue - which gathers, transmits and integrates
information from outside and inside the body to control the
activities of the body and its parts.
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6. COVERING EPITHELIUM
EPITHELIAL TISSUE
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7. Epithelium
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Epithelial tissues are composed of closely aggregated
polyhedral cells with very little extracellular substance.
These cells have strong adhesion and form cellular sheets
that cover the surface of the body and line its cavities. Or
Epithelia also form most of the body’s glands.
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8. Special characteristics of epitheliaSpecial characteristics of epithelia
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
Epithelial tissues have many characteristics that distinguish
them from other tissue types.
1. Cellularity.
Epithelia are composed almost entirely of cells.
These cells are separated by a minimal amount of
extracellular material, mainly projections of their integral
membrane proteins into the narrow spaces between the cells.
2. Specialized contacts.
Adjacent epithelial cells are directly joined at many points by
special cell junctions.
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9. Special characteristics of epithelia
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3. Polarity.
All epithelia have a free upper (apical) surface and a lower
(basal) surface.
They exhibit polarity, a term meaning that the cell regions
near the apical surface differ from those near the basal
surface.
The basal surface of an epithelium lies on a thin sheet called a
basal lamina, which is part of a basement membrane.
 The free apical surface abuts the open space of a cavity,
tubule, gland, or hollow organ.
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10. Special characteristics of epithelia
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4. Support by connective tissue.
All epithelial sheets in the body are supported by an
underlying layer of connective tissue.
5. Avascular but innervated.
Whereas most tissues in the body are vascular (contain blood
vessels), epithelium is avascular (a-vas ku-lar) meaning it lacks
blood vessels.
Epithelial cells receive their nutrients from capillaries in the
underlying connective tissue.
Although blood vessels do not penetrate epithelial sheets,
nerve endings do; that is, epithelium is innervated.
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11. Special characteristics of epithelia
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6. Regeneration.

Epithelial tissue has a high regenerative capacity.

Some epithelia are exposed to friction, and their
surface cells rub off.

Others are destroyed by hostile substances in the
external environment such as bacteria, acids, and
smoke.

As long as epithelial cells receive adequate nutrition,
they can replace lost cells quickly by cell division.
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In some locations cells are found aggregated in close
apposition to one another but lack a free surface, such cell
aggregates are called epitheloid tissues.
Epitheloid cells include: the interstitial cells of Leydig in the
testis, the Luteal cells of the ovary, the parenchyma of the
adrenal gland and the epithelioreticular cells of the
thymus.
 Epitheloid patterns are also found in pathologic responses
to injury and in many tumors.
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Covering and lining epithelia form a continuous sheet –
like cellular investment that separates the underlying or
adjacent connective tissue from the external environment
and from the environment of internal cavities, both closed
cavities and those lead to the exterior of the body.
This epithelial investment serves as a selective barrier
capable of facilitating or inhibiting the passage of specific
substance between the exterior (as well as body cavities)
and the underlying connective tissue compartment.
A given epithelium may serve one or more other functions,
depending on the activity of the cell types present.
Functions of epithelium
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It can be an almost impermeable barrier, as in the
epidermis or urinary bladder.
It can be secretory, as in the stomach.
It can be both secretory and absorptive, as in intestine.
It can provide a transport system through motile cilia on its
surface to move particles and mucus, as in the trachea and
bronchi.
It can serve to receive sensory stimuli, as in the taste buds
of the tongue of the retina of the eye.
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15. FUNCTIONS OF EPITHELIUM
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
Owing to the strategic location of epithelium at the border
between the internal and external environments, the
functions of epithelium are many and varied, but can be
conveniently divided into two major categories: protective protective
or metabolic. or metabolic.
1. Protective functions 1. Protective functions of epithelium include protection
against :
mechanical damage
loss of fluids (desiccation) - waterproofing
 invasion of foreign bodies
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16. FUNCTIONS OF EPITHELIUM
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2. Metabolic functions of epithelium include : 2. Metabolic functions of epithelium include :
Exchange of metabolites.
All the substances entering or leaving the body must pass
through epithelium and are under its control.
The ion-transporting epithelium may become highly specialized
for absorption or excretion.
The glandular secretions of the body by glands (exocrine and
endocrine) are mainly a function of specialized epithelium.
Some epithelia are modified for sensory reception including
recognition of sensory stimuli such as pain or as
chemoreceptor (such as taste buds).
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17. MORPHOLOGICAL CLASSIFICATION OF EPITHELIAMORPHOLOGICAL CLASSIFICATION OF EPITHELIA
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
Epithelia are described according to the number of layers they
possess and the appearance of the cells at the border
adjacent to the external environment.
Simple epithelia : are composed of a single layer of epithelial cells.
 Stratified epithelia : are composed of more than one layer of
epithelial cells.

The histological appearance describing the various epithelia is
that seen in vertical sections.

It is customary when drawing epithelia to show the free surface
directed to the top of the page, whereas the basal surface is
depicted in the direction of the bottom of the page.
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
The individual cells that compose an epithelium are
described as:
I. Squamous - where the width of the cell is
greater than its height.
II. Cuboidal – where the width, depth and
height are approximately the same.
III. Columnar - where the height of the cell
appreciably exceeds the width (the term low columnar
is used where cell’s height only slightly exceeds its other
dimensions).
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If all the cells in the epithelium consist of a single cell type, the
epithelium is described as being homogeneous.
If the epithelium has more than one cell type, the epithelium is
described as being heterogeneous.
If the apical edge of the epithelium has cilia, the epithelium is
described as being ciliated.
Epithelial cells that have large numbers of microvilli on the
apical or luminal surface (such as the columnar absorptive cells
of the small intestine) are described as possessing a brush
border.
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The cells in many of exocrine glands have a more or less
pyramidal shape, with their apices directed toward the lumen.
These cells are classified as either cuboidal or columnar,
depending on their height rather than on their width at the
base of the cell.
In a stratified epithelium, the shape and height of the cells
usually vary from layer to layer, but only the shape of the cells
forming the surface layer is utilized in classifying the
epithelium.
Pseudostratified epithelium and transitional epithelium are
special classification of epithelium.
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
Pseudostratified epithelium –
 has the appearance of being stratified.
Some of the cells do not reach the free surface, however all rest on
the basement membrane.

Thus it is actually a simple epithelium.

Transitional epithelium – is a name applied to the epithelium
lining the pelvis of the kidney, the ureters, the urinary bladder and
part of the urethra.

Transitional epithelium is a specialized form of stratified epithelium
that is capable of being distended without becoming leaky and of
returning to its original configuration after being stretched out of
shape. It is found lining the urinary tract.
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Epithelium may be derived from ectoderm, mesoderm or
endoderm although in the past it was thought that true
epithelia were only of ectodermal or endodermal origin.
Two types of epithelium derived from mesoderm were not
considered to be true epithelium, i.e. the lining of blood and
lymphatic vessels (endothelium) and the linings of the serous
body cavities (mesothelium).
By both morphological and functional criteria, such distinction
has little practical value; nevertheless, the terms endothelium
and mesothelium are still used to describe these types of
epithelium.
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36. Special Cytological Features of EpitheliaSpecial Cytological Features of Epithelia
Basement membrane-Basement membrane-
 is the term originally given to a layer of variable thickness at
the basal surfaces of epithelia.
Epithelia are separated/attached from the underlying
connective tissue by an extracellular supporting layer called
the basement membrane.
The basement membrane is composed of two sublayers.
The basal lamina (about 100 nm thick) consists of fine protein
filaments embedded in an amorphous matrix, which is
produced by the epithelial cells.
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37. BM
The reticular lamina consists of reticular fibres embedded in
ground substance.
The components of the reticular lamina are synthesised by
cells of the connective tissue underlying the epithelium.
In addition to its function as support of the epithelium, the
basal lamina acts as a selectively permeable filter between
epithelium and connective tissue.
Unless special stains are used, the basement membrane is
rarely visible using light microscopy.
 A prominent structure referred to as basement membrane is
observed with hematoxylin and eosin stain, in a few locations
such as the trachea sometimes urinary bladder and ureters.
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Basal lamina –
is the structural attachment site.
Examination of the site of epithelial basement
membrane with electron microscope reveals a discrete
layer of electron-dense matrix, 50-100 nm thick
between the epithelium and adjacent connective tissue.
Between the basal lamina and the cell is relatively clear
or electron-lucent area called the lamina lucida (lamina
rara).
 It contains fine filaments that join the plasma membrane
to the basal lamina.
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Function of the basal laminaFunction of the basal lamina
I.Structural attachment
II. Compartmentalization: connective tissue from
epithelia, nerve and muscle tissues.
III. Filtration: well characterized in the kidney.
IV. Polarity induction: specific properties attributable to the
basal membrane surface as apposed to the apical and
lateral membrane surfaces are induced by the presence
of the basal lamina.

Multiple roles of the basal lamina and basement
membrane are not fully elucidated.
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Cell surface modification/ Specialisations of the apical
surface
Surface modifications that are dealt with in relation to
epithelia are:
1. Microvilli –
are finger-like cytoplasmic projections on the apical
surface of most epithelial cells, they provide an
enormous increase in the free surface area of the cell.
The number and the shape of the microvilli of a given
cell type correlate with its absorptive capacity.
Those cells whose principal function is the transport of
fluid and absorption of metabolites exhibit many closely
packed tall microvilli.
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Cells in which fluid transport across the epithelium are less
active exhibit smaller, more irregular microvilli.
The fluid-transporting epithelia e.g. those of the intestine
and kidney tubule, a distinctive border of vertical striation
at the apical surface of the cell is detectable with light
microscope.
These surface structure was given the name striated
border for the intestinal absorptive cell and the brush
border for the kidney tubule cell.
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2. Stereocilia –
microvilli of unusual length.
They are limited to the epididymis of the male
reproductive system and to the sensory (hair) cells of the
ear.
Stereocilia of the ducts epididymis are extremely long
processes that extend from the apical surface of the cell.
They lack notable internal structural features.
Stereocilia of the sensory epithelium of the ear are
uniform in diameter and possess an internal filamentous
structure.
They serve as a receptor device rather than an
absorptive structure.
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3. Cilia – 3. Cilia –
are motile, cytoplasmic processes that move fluid and
particles along epithelial surface.
Cilia possess an internal structure that provides movement
for it.
In most ciliated epithelia, as in trachea, bronchi or oviduct,
there are as many as several hundred cilia to a cell, all
arranged in orderly rows.
In tracheobronchial tree, the cilia play a cleansing role by
sweeping mucus and trapped particulate material toward
the oropharynx where it is swallowed along with saliva
and thus eliminated
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
In oviduct, cilia help sweep ova and fluid along the tract
toward the uterus.

In some epithelia, only a single cilium per cell may be
present e.g. the epithelial cells of the rete testis in the male
reproductive tract and the vestibular hair cells of the ear.

Here the single cilium is regarded as having a sensory role. Here the single cilium is regarded as having a sensory role.
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
Atypical epithelia such as the adrenal cortical cells, the
Leydig cells of the testis and the lutein cells of the
ovary (all of which are endocrine components) lack a
free surface.
 These secretory cells are referred to as epitheloid.

Endothelium and mesothelium differ from epithelia derived
from other two germ layers, in that they have no continuity
or communication with the exterior of the body.
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
Epithelial cells in numerous locations, particularly
sweat glands, mammary glands, salivary glands and
the iris of the eye, may take on the characteristics of
smooth muscle cells (myoepithelial cells).
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