EPITHELIAL TISSUE
An epithelium or epithelial tissue is composed of one or more layers of cells covering the external and internal surfaces of various
body parts. Epithelial tissue also forms glands.
Epithelial membranes are derived from all three germ layers. The epidermis, derived from ectoderm, constitutes the outer portion
of the skin while the inner surface of the digestive tract is lined by an epithelium derived from endoderm and the inner surfaces
of the body cavities are lined by an epithelium derived from mesoderm.
Epithelial tissue usually rests on a thin non-cellular basement membrane composed of outer thin basal lamina consisting
of mucopolysaccharides and glycoproteins and fibrous or reticular lamina consisting of collagen or reticular fibres of the
underlying connective tissue.
These tissues generally lack blood vessels but still have a good power of repair after injury.
They receive useful materials from the blood vessels of the connective tissue across the basement membrane and transfer waste
matter to the blood in the same way.
Specialised Junctions Between Epithelial Cells
These are of five types:
Table : Structure, location and function of different epithelia
Types Structure Location Function
I. Simple Epithelia (Unilayered Epithelia)Simple
squamous epitheliumComposed of a single layer of flat, tile-like
polygonal cells. The nuclei of the cells are
flattened and often lie at the centre of the cells
making a bulge at the cell’s surface. In the blood
vessels and heart, it is called endothelium. In
the coelom, it is called mesothelium.Present in the terminal bronchioles and
alveoli of the lungs, wall of the Bowman’s
capsules and descending limb of loop of
Henle, membranous labyrinth (internal
ear), blood vessels, lymph vessels, heart,
coelomic cavities and rete testis.Protection,
excretion,gas
exchange and
secretion of coelomic
fluid.