Answer:
Epithelial tissue is the thin protective layer of cells
which covers the surface of the body and lines the internal organs. The cells
of this tissue are generally packed close together. The shape of the cells
depends on the location and function of the tissue. Epithelial tissue originates
from the ectoderm. But, epithelial tissue lining the intestine originates from
the endoderm.
Epithelial tissue may be simple, i.e., composed of single
layer of cells or stratified, i.e., made up of several layers of cells. Depending
upon the shape and function of cells epithelial tissues are classified as
(i) Squamous epithelial tissue This tissue is
composed of a single layer of thin and fiat, plate-like cells. The cells fit
closely, like the bricks in a wall, to form a smooth membrane.
It is also known as tessellated and pavement epithelium.
It is found in the outer layer of the skin, and covers internal cavities and
ducts. Tongue, esophagus and the lining of the mouth are made up of squamous
epithelium.
It is also found in blood vessels and alveoli. It protects
the underlying parts of body from mechanical injury, entry of germs, chemicals
and drying. It also forms a selectively permeable surface through which
filtration occurs.
(ii) Cuboidal epithelial tissue
This tissue is composed of cube-like cells that fit closely.
The cells look like squares in section, but the free surface appears hexagonal.
This tissue lines the inside of the kidney tubules (the tubes leading from the
cups of nephrons) thyroid vesicles and in glands like sweat glands, exocrine
pancreas and the salivary glands.
It forms germinal epithelium of gonads (testes and
ovaries). It helps in absorption, excretion and secretion. It also provides
mechanical support,
(iii) Columnar ciliated epithelial tissue This
tissue is generally composed of a single layer of column like cells. The
presence of a conspicuous striated border of microvillus at the free surface
end of each cell increases the surface area of the cell for absorption and secretion,
It is generally found in the inner lining of the alimentary canal.
It also forms the lining of gall bladder and oviducts. The
major functions of this tissue includes secretion (e.g., mucus of goblet cells)
and absorption (e.g., stomach and intestine).
In some parts of the body, columnar epithelium develops
protoplasmic outgrowths called cilia. The constant lashing movements of the
cilia help to move substances.
It is found in the sperm ducts. It also lines the trachea
(wind-pipe), bronchi (lungs), kidney tubules and oviducts (Fallopian tubes).
Ciliated epithelium helps the movement of ova in the fallopian tubes and the
movement of mucus in the respiratory tract.
(iv) Stratified squamous epithelial tissue This
tissue is found in skin and covers the external dry surface of the skin. Cells
of this tissue are arranged in many layers, but the cells forming different
layers of this epithelium are not similar. Deeper layers of the tissue have
cuboidal cells which become polygonal and finally flattened (squamous) towards the
free surface.
The flattened cells of superficial layer may contain a
fibrous protein, the keratin and become dead cells and are called keratinized
stratified squamous epithelium. This epithelium is water proof and highly
resistant to mechanical injury.
(v) Glandular epithelial tissue Epithelial tissue
often acquire additional specialization as gland cells, which can secrete
substances at the epithelial surface.
Sometimes, a portion of epithelial tissue folds in wards
and a multicellular gland is formed. This is called glandular epithelium.
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