It is composed of one or more layers of cells which cover the external and internal body surface. The cells lie closely connected with no intercellular spaces, on a thin gelatinous membrane and adhere to each other with considerable force. The free surface of the cells may be smooth or may have hair like cytoplasmic extensions such as cilia or microvilli. Blood vessels are absent in epithelial tissue.
Functions: Protection of underlying tissues from injury, microbes and desiccation, Secretion of hormones and enzymes, Sense perception by sensory cells lining sense organs,Respiration,Absorption of digested food and removal of metabolic wastes.
There are four types of epithelial tissues:
1. Squamous epithelium: They are compact, flat plate like cells with no intercellular spaces. They are seen on the lining of skin and cavities of ducts. Whenever it forms a lining as that of blood vessels, it is called by the special name endothelium.
Functions: They protect underling parts from injury, entry of germs, chemicals and drying up.
2. Columnar epithelium: Cells are taller than broad with nucleus towards the base. Certain cells are adapted for secretion. They occur on the intestinal lining, sweat gland, tear gland, salivary gland and kidney tubules.
Function: Gives mechanical strength, concerned with secretions.
3. Cuboidal epithelium: Cube likes cells which are square in section but the free surface appears hexagonal. They occur in kidney tubules, salivaryglands, and inner lining of the cheek.
Functions: Give mechanical strength.
4. Ciliated epithelium: Cuboid or columnar cells with protoplasmic outgrowth called cilia. Occur in lining of trachea of vertebrates, kidney tubules and oviduct.
Functions: Keeps out unwanted particles.