What is the Epithelial Tissue?
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Epithelial Tissue
Epithelial tissue or epithelium (Epi — upon; thele —
nipple) covers both external and internal surfaces of the
animal body.
The epithelial tissue has a free surface, which faces
either a body fluid or the outside environment and
thus, provides a covering or a lining for some part of
the body.
Characteristics
The characteristic features of epithelial tissue are as
follows
(i) The cells are compactly arranged.
(ii) Intercellular spaces are narrow, 20-30 nm wide.
(iii) Adjacent cells are held together by intercellular
junctions.
(iv) The epithelial tissue lies on a thin, non-cellular
basement membrane.
(v) Blood vessels are not present in the epithelial
tissue.
(vi) Materials are exchanged by diffusion between
epithelial cells and the blood vessels of the
connective
tissues across the basement
membrane.
(vii) Nerve endings may penetrate the epithelial
tissues.
Junctions Between Epithelial Cells
The common intercellular junctions may include tight
junctions, gap junctions, desmosomes, intercellular
bridges and interdigitations.
(i) Tight Junctions The plasma membrane in the apical
region of the adjacent epithelial cells become tightly
packed together. These junctions check the flow of
materials between the cells and are called occluding
junctions.
(ii) Adhering Junctions Facilitate the cementing process so as
to keep the neighbouring cells together. They include
desmosomes and hemidesmosomes.
(iii) Desmosomes These are thick and strong junctions. They
serve, anchoring functions.
(iv) Gap Junctions They are fine hydrophilic channels
between adjacent cells formed with the help of protein
cylinders called connexin. They help in chemical
exchange between adjacent cells and hence are called
communicating junctions.
Types of Epithelial Tissues
The epithelial tissues are broadly classified into two groups, i.e.,
simple and compound.
Simple Epithelia
Simple epithelium is made up of a single layer of compactly
arranged cells which rest over a non-cellular basement
membrane. It occurs over moist surfaces where a little wear and
tear occurs by friction. The simple epithelium is generally related
with absorption, secretion, diffusion and movement of
materials.
It is further subdivided into following types
i. Simple
Squamous
Epithelium The squamous
(Squama—scale) is formed of a single layer of closely
fitted, flattened, polygonal cells, which forms bulges on
the cell surface.
The given cells are held together by various types of
junctions, mainly tight junctions. The cells of squamous
epithelium appear as tiles over a floor. They are also
known as pavement epithelium.
The squamous epithelium occurs in the alveoli of the
lungs, Bowman’s capsule, Henle’s loop of uriniferous
tubules, pericardial cavity, abdominal cavity, lining Of
various components of blood vascular system.
Functions Simple squamous epithelium performs the
function of protection, excretion, gas exchange and
secretion of coelomic fluid.
ii. Simple Cuboidal Epithelium It is composed of a
single layer of cube-like cells. The epithelium
overlies on the basement membrane. Nucleus is
rounded and placed centrally. The free surfaces of
the cells may be smooth or bear microvilli. The
microvilli increases the surface area of free ends of
cells by many times.
The simple cuboidal epithelium is commonly
found in the ducts of glands, tubular parts of
nephrons in kidneys, ovaries, seminiferous tubules
of testes, etc.
Functions The main function of this epithelium is
protection, secretion, absorption, excretion and gamete
formation.
iii. Simple Columnar Epithelium It is composed of a
single layer of tall and slender cells. A single oval or
elongated nucleus is situated near the base of the
cell. Some of its cells produce mucus, called goblet
cells.
The simple columnar epithelium occurs in the
lining of stomach, small and large intestine,
digestive glands of stomach, intestine and pancreas,
gall bladder, etc.
The brush border columnar epithelium occurs in
the gall bladder. The mucus secreting goblet cells
are found in the lining layer of stomach, intestine,
respiratory tract, etc.
Functions The simple columnar -epithelium helps in
secretion, absorption and protection to the components of
most glandular epithelia.
v. Simple Ciliated Epithelium If the columnar or
cuboidal cells bear cilia on their free surface they
are called ciliated epithelium. They move particles
or mucus in a specific direction over the
epithelium. The epithelium lies over a basement
membrane. The number of cilia varies in different
cellular forms. In sensory cells of internal ear, a
cilium accompanies number of stereocilia.
This epithelium is of two types, i.e., ciliated columnar and
ciliated cuboidal
(a) Simple Ciliated Columnar Epithelium It possesses
columnar cells that possess cilia over their free surface.
It occurs in respiratory tract, Fallopian tubes, parts of
uterus and cervix, the different tubules of testes, etc.
(b) Simple Ciliated Cuboidal Epithelium It has cuboidal
or cubical cells that bear cilia on their free surface. It
occurs. in many parts of ependyma of nervous system
and parts of uriniferous tubules.
Functions The epithelium maintains a flow of mucus,
liquid or suspended particles constantly in one direction. In
the oviducts, cilia helps in the movement of egg towards the
uterus. In respiratory tract, cilia helps to push the mucus
towards the pharynx. In nephrons of kidney, cilia keep the
urine moving.
In nervous system, cilia of the ventricles of the brain and
central canal of the spinal cord helps in the circulation of
cerebrospinal fluid.
v. Pseudostratified Epithelium The epithelium is
one-cell thick, but appears 2-layered because all the
cells do not reach the free surface. The cells are
attached to the basement membrane, hence they are
called pseudostratified. The mucus secreting goblet
cells also occur in this epithelium.
This epithelium is of two types
(a) Pseudostratified Columnar Epithelium It has
columnar cells without cilia. It lines the large ducts
of certain glands, like parotid salivary glands and
the urethra of human male.
Ciliated
Columnar
(b) Pseudostratified
Epithelium It has columnar cells. The tall cells
bear cilia at the free surfaces and the short cells are
without cilia. The epithelium lines the trachea and
large bronchi. The movements of its cilia push the
mucus laden with dust particles and bacteria
towards the larynx.
Functions The pseudostratified epithelium helps in
protection, movement of secretions from glands, urine and
semen in urethra and mucus loaded with dust particles and
bacteria in trachea.
2. Compound Epithelia
The compound epithelium is made up of more than one
layer of cells. They cover the surfaces where constant
replacement of cells is required due to rapid wear and tear
by friction.
e compound epithelia are of two types, i.e., stratified and
ransitional.
(i) Stratified Compound Epithelia The stratified
epithelia consist of many layers of cells. On the basis
of the shape of the cells present in the superficial
layers, the stratified epithelia are of four types
(a) Stratified Squamous Epithelium The cells in the
basal (deepest) layer are columnar or cuboidal
with oval nuclei. It is called germinal layer. The
cells in this region keep dividing by mitosis to
form new cells.
The stratified squamous epithelium is further
subdivided as two main types, i.e., keratinised
and non-keratinised
• Keratinised Stratified Squamous Epithelium
The cells of the outer few layers replace their
cytoplasm with a hard waterproof protein
called keratin or horn. This is called
keratinisation or cornification. These layers of
flat, dead cells are called stratum corneum or
horny layer.
The heavy deposits of keratin in the dead
superficial cells make the epithelium
impervious to water and highly resistant to
mechanical abrasions.
This epithelium forms the epidermis of the skin
in land vertebrates.
• Non-keratinised Stratified Squamous
Epithelium This epithelium does not have
keratin and is unable to check water loss. It
provides moderate protection against abrasion.
It lines the buccal cavity, pharynx,
oesophagus, canal, lower part of urethra,
vocal cord, vagina, cervix (lower part of
uterus), conjuctiva, cornea of eye and inner
surface of eyelids.
) Stratified Cuboidal Epithelium It has outer
layer of cuboidal cells and basal layer of columnar
cells. It forms the epidermis of fishes and many
urodeles (tailed amphibians like salamanders). It
also lines the sweat gland ducts and larger salivary
and pancreatic ducts.
) Stratified Columnar Ciliated Epithelium Its
outer layer consists of ciliated columnar cells and
basal layer of columnar cells. It lines the larynx
and upper part of the soft palate.
) Stratified Columnar Epithelium It consists Of
columnar cells in both superficial and basal layers•
It covers the epiglottis and lines mammary gland
ducts and parts of urethra.
(ii) Transitional Compound Epithelium The epithelium
consists of more than one layer of cells, but is much
thinner and more stretchable than the stratified
epithelium. It contains cuboidal cells at the base, two
or three layers of large polygonal or pear-shaped cells
in the middle and a superficial layer of large, broad,
rectangular or oval cells.
The transitional epithelium lines the inner surface of
urinary bladder, ureter and renal pelvis. They have
thick membrane with thin regions that fold when the
bladder contracts.
3. Glandular Epithelium
Some of the columnar or cuboidal cells get specialised for
secretion and form the glandular epithelium.
It is of two types
(i) Unicellular Glandular Epithelium It consists Of
isolated glandular epithelial cells called intraepithelial
cells, e.g., goblet cells of the alimentary canal are one
such cell.
(ii) Multicellular Glandular Epithelium It
consists of cluster of epithelial cells called
extra epithelial cells. These cells unite to
make up one gland, e.g., salivary gland.
Gland
A cell, tissue or organ, which secretes some
substance is called a gland. The secretions of glands
may be protein (pancreas), lipids (adrenals),
mixture of carbohydrates and proteins (salivary
gland) or mixture of all the three materials
(mammary glands).
The glands can be classified in different types based
on site of secretion, mode of secretion and
involvement of single or many cells.
i. Based on Site of Secretion
The glands can be exocrine, endocrine or
heterocrine based on the site where the secretion is
released.
(a) Exocrine Glands These glands have ducts to
pour their secretions to their site of action.
They often secrete enzymes and its examples
include salivary glands, intestinal glands,
gastric glands, lacrimal or tear glands.
(b) Endocrine Glands These glands do not have
ducts and pour their secretions directly into
the blood or lymph. These glands are also
called ductless glands and their secretions are
known as hormones. Some examples of
endocrine glands are pituitary, thyroid,
parathyroid, adrenal, etc.
(c) Heterocrine Glands / Myxocrine Glands
These glands are partly exocrine and partly
endocrine in function, e.g., pancreas,
kidneys, stomach, gonads, intestine,
placenta, etc.
ii. Based on Number of Cells
According to the number of cells forming the glands, they are
unicellular and multicellular.
(a) Unicellular Glands The mucus secreting goblet cells of the
alimentary canal are called unicellular glands.
(b) Multicellular Glands These are composed of many cells and
are formed by sinking of the gland into the underlying
connective tissue. The multicellular glands may be simple or
compound glands
• Simple Glands These may be simple tubular glands, e.g.,
crypts of Lieberkuhn, simple coiled tubular glands (sweat
glands) and simple alveolar glands having flask-shaped
secretory units (mucus secreting glands in the skin of frog.)
• Compound Glands These have branch system of ducts.
These may be compound tubular glands (e.g., gastric
glands of stomach, Brunner’s
glands of intestine),
compound alveolar glands (e.g., some sebaceous glands
and salivary glands) and compound tubuloalveolar glands
having both tubular and alveolar secretory units (e.g.,
pancreas, functional mammary glands).
Functions of Epithelial Tissue
The main functions of epithelial tissue are listed below
(i) The epithelial tissue protects the underlying tissues from
mechanical injury, entry of germs, harmful chemicals and
drying up.
(ii) It checks the absorption of harmful or unnecessary materials.
(iii) The epithelium of uriniferous tubules is specialised for urine
excretion.
(iv) The sensory epithelia of sense organs help to receive various
stimuli from the surroundings and convey them to the
brain.
(v) The epithelium of alveoli of the lungs brings about the
exchange of gases between the blood and air.
(vi) The pigmented epithelium of the retina darkens the cavity of
eyeball.
(vii) Epithelium also forms glands that secrete secretions such as
mucus, gastric juice and intestinal juice.
(viii) The germinal epithelium of the ovaries and seminiferous
tubules of the testes produce ova and sperms, respectively.
(ix) Epithelium produces exoskeletal structures like scales,
feathers, hair, nails, claws, horns and hoofs.
(x) Ciliated epithelia (e.g., of respiratory and genital
tracts) serves to conduct the mucus and other fluids in
the ducts they line.
Knowledge Plus
• The term ‘Epithelium’ was coined by Ruysch.
• Transitional epithelium is also known as urothelium when present
in urinary system.
• The pavement epithelium becomes wavy in few regions and is
called