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.
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 flat, plate-like cells. The cells fit closely, like the bricks in a wall, to form a smooth membrane. It is also known as tesselated and pavement epithelium. It is found in the outer layer of the skin, and covers internal cavities and ducts. Tongue, oesophagus 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 epithelium: Where absorption and secretion occur, as in the inner lining of the intestine, these tall epithelial cells are present. This columnar epithelial facilitates movement across the epithelial barrier. In the respiratory tract, the columnar epithelial tissue also has cilia, which are hair-like projections on the outer surfaces of epithelial cells. These cilia can move and their movement pushes the mucus forward to clear it. This type of epithelium is thus ciliated columnar epithelium.
(iv) Stratified squamous epithelium: Skin epithelial cells are arranged in many layers to prevent wear and tear. Since, they are arranged in a pattern of layers, the epithelium is called stratified squamous epithelium.
