Structure
Epithelial tissue covers the whole surface of the body. It is made up of cells closely packed and ranged in one or more layers. This tissue is specialised to form the covering or lining of all internal and external body surfaces. Epithelial tissue that occurs on surfaces on the interior of the body is known as endothelium. Epithelial cells are packed tightly together, with almost no intercellular spaces and only a small amount of intercellular substance. Epithelial tissue, regardless of the type, is usually separated from the underlying tissue by a thin sheet of connective tissue; basement membrane. The basement membrane provides structural support for the epithelium and also binds it to neighbouring structures.Types of Epithelial Tissue
Epithelial tissue can be divided into two groups depending on the number of layers of which it is composes. Epithelial tissue which is only one cell thick is known as simple epithelium. If it is two or more cells thick such as the skin, it is known as stratified epithelium.Simple epithelium Simple epithelium can be subdivided according to the shape and function of its cells.
- Squamous
(pavement) epithelium.
Squamous cells have the appearance
of thin, flat plates. The
shape of the nucleus usually
corresponds to the cell form
and help to identify the type of
epithelium. Squamous cells, for
example, tend to have horizontall flattened,
elliptical nuclei because of the thin
flattened form of the cell. They
form the lining of cavities such as
the mouth, blood vessels, heart
and lungs and make up the
outer layers of the skin.
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Simple
sqaumous epithelium
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- Simple
Cuboidal Epithelium. As
their name implies, cuboidal cells
are roughly square or cuboidal in
shape. Each cell has a spherical
nucleus in the centre. Cuboidal
epithelium is found in glands
and in the lining of the kidney
tubules as well as in the ducts
of the glands. They also
constitute the germinal epithelium
which produces the egg cells in
the female ovary and the sperm
cells in the male testes.
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Simple
cuboidal epithelium
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- Simple
Columnar Epithelium Columnar
epithelial cells occur in one or
more layers. The cells are elongated
and column-shaped. The nuclei
are elongated and are usually
located near the base of the cells.
Columnar epithelium forms the lining
of the stomach and intestines.
Some columnar cells are specialised
for sensory reception such as in
the nose, ears and the taste buds
of the tongue. Goblet cells
(unicellular glands) are found
between the columnar epithelial
cells of the duodenum. They secrete
mucus or slime, a lubricating
substance which keeps the surface
smooth.
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Simple
columnar epithelium
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- Ciliated
Columnar Epithelium
These are simple
columnar epithelial cells,
but in addition, they posses fine
hair-like outgrowths, cilia on their free
surfaces. These cilia are
capable of rapid,
rhythmic, wavelike beatings
in a certain direction. This
movement of the cilia in a
certain direction causes the
mucus, which is secreted by
the goblet cells, to move
(flow or stream) in that
direction. Ciliated
epithelium is usually found
in the air passages like the
nose. It is also found in
the uterus and Fallopian
tubes of females. The movement of
the cilia propel the ovum to
the uterus.
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Ciliated
columnar epithelium
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- Glandular
Epithelium Columnar
epithelium with goblet
cells is called glandular
epithelium. Some parts of
the glandular epithelium
consist of such a large number
of goblet cells that
there are only a few normal epithelial
cells left. Columnar and
cuboidal epithelial cells
often become specialised
as gland cells which are
capable of synthesising
and secreting certain
substances such as enzymes,
hormones, milk, mucus, sweat,
wax and saliva. Unicellular
glands consist of single,
isolated glandular cells such
as the goblet cells.
Sometimes a portion of the
epithelial tissue becomes
invaginated and a multicellular
gland is formed.
Multicellular glands are composed
of clusters of cells. Most
glands are multicellular
including the the salivary glands.
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- Stratified
Epithelium. Where body
linings have to withstand wear and tear, the
epithelia are composed of several layers
of cells and are then called compound or
stratified epithelium. The top cells
are flat and scaly and it may or may
not be keratinised (i.e. containing a
tough, resistant protein called keratin). The
mammalian skin is an example of dry,
keratinised, stratified epithelium. The
lining of the mouth cavity is an
example of an unkeratinisied, stratified
epithelium.
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Stratified
epithelium
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Functions of Epithelial Tissue
- Protection Epithelial cells from
the skin protect underlying tissue from
mechanical injury, harmful chemicals,
invading bacteria and from excessive loss of
water.
- Sensation Sensory stimuli penetrate
specialised epithelial cells. Specialised epithelial tissue
containing sensory nerve endings is found in
the skin, eyes, ears, nose and on the tongue.
- Secretion In glands, epithelial
tissue is specialised to secrete specific
chemical substances such as enzymes,
hormones and lubricating fluids.
- Absorption Certain epithelial cells
lining the small intestine absorb
nutrients from the digestion of food.
- Excretion Epithelial tissues in the
kidney excrete waste products from the
body and reabsorb needed materials from the
urine. Sweat is also excreted from
the body by epithelial cells in the sweat
glands.
- Diffusion Simple epithelium promotes
the diffusion of gases, liquids and nutrients.
Because they form such a thin lining, they
are ideal for the diffusion of gases (eg. walls of capillaries and lungs).
- Cleaning Ciliated epithelium
assists in removing dust particles and
foreign bodies which have entered the air
passages.
- Reduces Friction The smooth,
tightly-interlocking, epithelial cells that
line the entire circulatory system reduce
friction between the blood and the walls of
the blood vessels.