Question

How are epithelial tissues classified ?

Answer 1

Epithelial tissues

1. The epithelial tissue is a type of simple tissue which forms the covering or outermost layer in the animal body.
2. Epithelium covers most organs and cavities within the body and carries out the function of protection, secretion, and absorption.

Classification

The epithelial tissues are classified into three types based on the following factors :

1. Arrangement of layers
2. Shape of cell
3. Specific functions:

i.Based on the arrangement of layers, the epithelial tissues are classified into two types

a.Simple epithelium

When the epithelial tissues are made up of a single layer of cells, then it is called the simple epithelium.

b.Stratified or compound epithelium.

The epithelial tissues are made up of more than one layer of cells. Then it is called stratified or compound epithelium.

ii. Based on the shape of the cells, the epithelial tissues are classified into three types

a.Squamous

Cells are flattened and fit like the tiles of the floor.

b.Columnar epithelium

1. The cells of the columnar epithelium are taller and look like a pillar.
2. The nucleus of the cell is located at the base.
3. Sometimes the free ends have a brush-like border or hair-like projections on the outer surface of epithelial cells.
4. These are called cilia.
5. This type of columnar cell is called the ciliated columnar epithelium.

Cuboidal epithelium

1. Cells are cube-shaped and are squarish in cross-section.
2. The free surface appears hexagonal.

iii. Based on the specific functions, the epithelial tissues are classified into four types

Sensory epithelium :

1. Sensory epithelium possesses sensory hairs on its free surface.
2. These cells help to pick up the external stimuli with the help of their hair.

The sensory epithelium is seen in taste buds and in the olfactory epithelium.

Germinal epithelium :

1. The germinal epithelium consists of cuboidal cells. It occurs in gonads and helps in the formation of gametes.

Ciliated epithelium :

1. It may be columnar or cuboidal epithelium in which the cells possess cilia on their free surface.
2. It occurs in sperm ducts, uriniferous tubules, the lining of the respiratory tract, and oviducts.
3. The cilia help to push the ovum in the oviduct and in the respiratory tract, and the mucus helps to trap dust particles and microbes.

Glandular epithelium :

1. The epithelial cells also have additional specialization as gland cells, which can secrete substances.
2. Sometimes a part of the epithelial tissue folds inward to form a multicellular gland. This is the glandular epithelium.

These glandular epithelial cells secrete chemicals such as

1. Enzymes (digestive glands)
2. Hormones (endocrine glands)
3. Sweat (sweat glands)
4. Oils (oil glands).