Explain the structure and functions of various types of epithelial tissue.

On the basis of shape of the cells, epithelial tissues are classified into three types :

1. Squamous epithelium — The cells of squamous epithelium are flat, hexagonal or irregular in shape. These cells line the inner walls of the blood vessels, lungs, heart and oesophagus. They also form the epidermis of the skin, where they provide protection and prevent the entry of germs.
   Below diagram shows the structure of Squamous epithelium:

2. Columnar epithelium — The length of the cells of Columnar epithelium appears much more than their width. The cells are placed side by sides like columns. The nucleus is present towards the base of the cell. They are present in the linings of the stomach, intestines and pharynx. They are specialized to absorb nutrients.
   Below diagram shows the structure of Columnar epithelium:

3. Cuboidal epithelium — The cells of Cuboidal epithelium have almost equal height and width. The nucleus is centrally located. They are found in thyroid gland, kidney tubules, salivary gland, pancreas and liver. They are specialized to secrete enzymes and hormones and remove waste (as in kidneys).
   Below diagram shows the structure of Cuboidal epithelium: