Epithelial tissue: Epithelial tissue is the simplest animal tissue which composed of one or more layer of cells covering external and internal body organs. There are two types of epithelial tissues:

1. Simple Epithelial tissue: It is composed of single layer of cells.

2. Stratified epithelial tissue: It is composed of many layers of cells.

Depending of the shape of the cells, epithelial tissue is of following types:

(a) Squamous epithelial tissue: The cells of squamous epithelium tissue are extremely thin and flat and form a delicate lining. It is found in the alveoli of lungs and blood vessels and protects the underlying parts from germs, harmful chemicals and injury.

(b) Cuboidal epithelial tissue: It is composed of a single layer of cube-like cells. It is found in kidney tubules and salivary glands. It provides mechanical support to organs and helps in absorption and excretion.

(c) Columnar epithelial tissue: It is composed of a single layer of tall and slender cells. Their nuclei are located at the base. Free surface may have microvilli. It is generaly found in the lining of stomach and intestine and help in secretion and absorption.

(d) Ciliated epithelial tissue: It has finger-like projections called cilia on the outer suface of columnar epithelial. These cilia can move, and their movement pushes the mucus forward to clear it. It is found in respiractory tract.

(e) Glandular epithelial tissue: Some of the columnar or cuboidal epithelial cells get specialised for secretion and are called glandular epithelium. It is found in salivary, gastric, intestine and sweat glands.