Animal Tissues (Classification)
I. Epithelial Tissues:
An epithelial tissue covers a surface or lines a cavity in the body.
There are two types of epithelial tissues:
(a) Simple Epithelial Tissue:
These tissues are one-celled in thickness and are of the following types:
(i) Squamous Epithelium:
Cells are pentagonal with a distinct nucleus. This tissue forms the superficial layer of the skin.
(ii) Cubical Epithelium:
Cells of this tissue are cubical with a large and oval nucleus. It lines the ducts of various glands as the bile-duct of the liver.
(iii) Columnar Epithelium:
Cells of this tissue are conical with rounded granular nucleus. It lines the intestine of the alimentary canal, especially the parts which secrete digestive juice or absorb digested food.
(iv) Striated Epithelium:
Cells are columnar with free border.
(v) Ciliated Epithelium:
The cells are columnar and their surface is covered by fine processes of protoplasm known as cilia. It lines the buccal cavity in frog and the bronchi in rabbit.
(b) Stratified Epithelial Tissue:
It is a compound tissue and many layered in thickness. It is composed of a basal layer called the Malpighian layer. This tissue is found in the skin of mammals and conjunctiva of the eye.
II. Connective Tissues:
The cells of this tissue are known as corpuscles and secrete a ground substance – the matrix. This tissue may be divided into the following types:
(i) Areolar Tissue:
This tissue occurs in all parts of the body and binds together other tissues. It connects the skin with the muscles, the muscles with one another and the muscles with the bones.
(ii) Adipose Tissue:
This is a fibrous tissue and stores fat in the body.
(iii) Cartilagenous Tissue:
It is a supporting tissue having tough elastic matrix with numerous cavities each of which encloses a cartilage cell, e.g., cartilage of trachea, ear and nose.
(iv) Bone Tissue:
It is also a supporting tissue and forms the greater part of the skeleton.
III. Blood Tissues:
Blood tissue is a type of connective tissue in which the ground substance is the fluid plasma. The blood consists of plasma and the corpuscles. The plasma is composed of water in which mineral salts, sugar and proteins like albumin, globulin and fibrinogen are dissolved. The blood corpuscles in the plasma are red blood cells (R.B.C.), white blood cells (W.B.C.) and blood platelets.
IV. Muscular Tissues:
It forms the muscle of the body. It is of three types:
(i) Smooth Muscles:
The cells of smooth muscles are long, spindle shaped with a large oval nucleus. It is found in the walls of intestine, stomach, lungs and urinary bladder.
(ii) Striated Muscles:
These consist of large bundles of elongated cells or fibres covered by transparent elastic membranes called sarcolemma. They are found in the muscles of the limbs.
(iii) Cardiac Muscles:
They occur in the muscular tissues of the heart. The cell is spindle-shaped and uninucleated.
V. Nervous Tissues:
These tissues are found in the nervous system and are concerned with the reception, conduction and co-ordination of stimuli. They consist of nerve cells, fibres and the associated connective tissues. The nerve cells are found in the grey matter and ganglia whereas the nerve fibres are found in the white matter.