The group of similar cells which performs same functions is called tissue. There are four types of tissues: epithelial tissues, connective tissues, muscles tissues and nerve tissues.
The sheets of tightly packed cells are called epithelial tissues. Epithelial tissue covers the outside of the body. They also lines organs and cavities within the body. The cells of an epithelium are closely joined. Little material is present between them. The cells are packed together by tight junctions in many epithelia. The epithelium acts as barrier. It protects the body against .mechanical injury, invading microorganisms and fluid loss. So this tight packing is necessary for the epithelium. The free surface of the epithelium is exposed to air or fluid. The cells at the base of the barrier are attached to a basement membrane. The basement membrane is a dense layer of extracellular material.
The epithelial tissue performs following functions:
- Epithelial cells absorb Material like lining of the intestine.
- They are used for transport cf material like kidney.
- They are involved in excretion like sweat glands.
- They are used for protection like skin.
- They contain nerve cells for sensory receptor like taste bud in the tongue The epithelial tissues are classified on the bases of number of cell layers and the shape of the cells on the free surface.
(a) Simple epithelium: A simple epithelium has a single layer of cells.
(b) Stratified epithelium: It has multiple tiers of cells.
(c) Pseudostratified epithelium: It is single-layered, but it appears stratified because the.cells vary in length. The shape of the cells at the free surface of an epithelium may be:
- Cuboidal (like dice).
- Columnar (like bricks on end)
- Squamous (like flat floor tiles)
Combining the features of cell shape and number of layers. we get such terms as simple cuboidal epithelium and stratified squamous epithelium.
(a) Simple squamous epithelium: These consist of a single layer of tightly packed, flattened cells. They have disc shaped central nucleus.
Location: Air sacs of the lungs, kidney glomeruli, lining of heart, blood vessels, and lymphatic vessels.
Function: Allows passage of materials by diffusion and filtration.
(b) Simple cuboidal epithelium: These consist of a single layer of tightly packed, cube-shaped cells.
Location: Kidney tubules, ducts and small glands, and surface of ovary. Function: Secretion and absorption.
(c) Simple columnar epithelium: These consist of a single layer of elongated
Location: Lines digestive tract. gallbladder, and excretory ducts of Some glands.
Fwition: Absorption, enzyme secretion.
(d) Pinto stratified ciliated columnar epithelium: A tuft of cilia is present at the top of each columnar cell, except for goblet cells.
Location: Lines the bronchi, uterine tubes. and some regions of the uterus. Function: Propels mucus or reproductive cells by ciliary action.
(e) Stratified squamous epithelium: These consist of many layers of cells:
Location: L:. .-o the esophagus. mouth, and vagina. Kerarinized variety lines the surface of the skin.
Function: Protects underlying tissues in areas subject to abrasion.