Based on morphology, animal tissues can be grouped into four basic types. Multiple tissue types comprise organs and body structures. While all animals can generally be considered to contain the four tissue types, the manifestation of these tissues can differ depending on the type of organism.
The four basic Animal Tissues are:
(i) Epithelial Tissue;
(ii) Connective Tissue;
(iii) Muscular Tissue; and
(iv) Nervous Tissue.
1. Epithelial Tissue
The covering or protective tissues in the animal body are epithelial tissues. It is the simplest protective tissue of the animal body. It covers most organs and cavities of the body.
(i) lt forms a barrier to keep different body systems separated from each other. In these tissues cells are tightly packed and form a continuous sheer. There is almost no intercellular space between them. They have a very small amount of cementing material between them.
(ii) The epithelium is separated from underlying tissue by an extracellular fibrous basement membrane containing collagen.
On the basis of the shape of the cells and their arrangement, epithelial tissues are further classified as follow:
(i) Squamous Epithelium
Squamous epithelial tissue constitutes the skin which protects the body. It is further categorized as:
(a) Simple Squamous Epithelium
(i) It is single-layered and closely fitted. The cells are very thin and flat and appear as tiles over a floor.
(ii) It forms a delicate lining of blood vessels and lung alveoli, where substance transport occurs through a selectively permeable membrane.
(iii) It also covers oesophagus and lining of the mouth.
(b) Stratified Squamous Epithelium
(i) It is found in the outer side of skin as it is highly resistant to mechanical injury and is water-proof.
(ii) Cells are arranged in many layers to prevent their wear and tear.
(ii) Cuboidal Epithelium
(a) It is made up of cube-shaped cells, which have round nuclei.
(b) It forms the lining of kidney tubules and ducts of salivary glands, where it provides mechanical support. It also forms germinal epithelium of gonads.
(c) It also helps in absorption, excretion and secretion.
(iii) Columnar Epithelium
(a) It is made up of tall, pillar-like cells, with elongated nuclei.
(b) It is usually found in the inner lining of the intestine, where absorption and secretion occur.
(c) It facilitates movement across the epithelial barrier.
(iv) Ciliated Columnar Epithelium
(a) When columnar epithelial cells possess cilia (hair-like projections), it is called ciliated columnar epithelium.
(b) The cilia have the ability to move. Their movement pushes substances like mucus forward.
(c) It is found in the respiratory tract and also lines oviducts, sperm ducts, kidney tubules, etc.
(v) Glandular Epithelium
(a) Gland cells secrete substances at the epithelial surface.
(b) Sometimes, a portion of epithelial tissue folds inward. This results in the formation of a multicellular gland. Its tissue is called glandular epithelium.
Functions of Epithelial Tissue:
(i) It protects the underlying cells from drying, injury, infections and also from the harmful effects of chemicals.
(ii) It plays a vital role in regulating the exchange of materials between the body and the external environment and between different body parts.
(iii) It helps in the absorption of water and nutrients and in the diffusion of gases.
(iv) It helps in the elimination of waste products from the body.
2. Connective Tissue
This tissue is specialised to connect various body organs with each other. For e.g., it connects two or more bones to each other muscles to bones, binds different tissues together and also gives support to various parts of the body. The cells of connective tissue arc loosely packed, living and embedded in an intercellular matrix that may either be jelly-like, fluid, dense or rigid in nature. The nature of the matrix differs in concordance with the function of the particular connective tissue.
It is a fluid connective tissue that links different parts of the body. It helps to maintain the continuity of body. It contains fluid matrix called plasma and blood cells such as RBCs (Red Blood Corpuscles or Cells), WBC (White Blood Corpuscles) and platelets suspended in it. Plasma also contains proteins, salts and hormones.
Blood transports nutrients, gases, hormones and vitamins to various tissues of the body. It carries excretory products from tissues to excretory organs. It also conducts heat and regulates body temperature.
Properties shown by different blood cells in the body are as follows:
(a) RBCs Help in the transport of respiratory gases, oxygen and carbon dioxide with the help of haemoglobin to and from the various parts of our body. The average lifespan of RBCs is 120 days.
(b) WBCs Also called leucocytes, fight with diseases by producing antibodies.
(c) Blood platelets Also called thrombocytes, help in the clotting of blood.
It is very strong and non-flexible tissue. It is porous, highly vascular, mineralized, hard and rigid. Its matrix is made up of proteins and is rich in salts of calcium and phosphorus. It forms the framework that supports the body. It also anchors the muscles and supports the main organs.
They connect one bone to other bone. A ligament is very elastic and has considerable strength. It contains very little matrix. Ligaments strengthen joints and permit normal movement. Their overstretching leads to sprain.
They are strong and inelastic structures, which join skeletal muscles to bones. These are composed of white fibrous tissues with limited flexibility, but great strength.
It is a specialized connective tissue matrix having widely spaced cells. It has a solid matrix called chondrin which is composed of proteins and sugars. Cartilage provides smoothness to the bone surfaces at the joints. It is present in nose, ear, trachea and larynx. We can fold the cartilage of the ears, but we cannot bend the bones in our arms.
(vi) Areolar Tissue
It is a supporting and packing tissue found between the organs lying in a body cavity. It is located between skin and muscles, around blood vessels and nerves and in the bone marrow. It is a loose and cellular tissue. It fills the space inside the organs, supports internal organs. It helps in the repair of tissue.
(vii) Adipose Tissue
It serves as a fat reservoir, keeps visceral organs in position. It acts as an insulator due to the storage of fats. It is located below the skin in between the internal organs.
3. Muscular Tissue
Muscle cells form the active contractile tissue of the body known as muscle tissue. Muscle tissue functions to produce force and cause motion, either locomotion or movement within internal organs. Different types of muscular tissues are given below:
(i) Striated Muscles
The muscles present in our limbs which move or stop as per our will are called striated muscles. These are also called as voluntary muscles as we can move them by conscious will. Mostly these are attached to bones and help in body movement, e.g. muscles of limbs. Hence, they are also called skeletal muscles. The cells constituting their muscles are long, cylindrical, unbranched and multinucleate (having many nuclei). Under the microscope, striated muscles show alternate light and dark bands or striations. Thus, they are also known as striated muscles.
(ii) Unstriated Muscles or Smooth Muscles
The muscles which we cannot move as per our will are called unstriated or smooth muscles. There are also called involuntary muscles. For example, movement of food in the alimentary canal, contraction and relaxation of blood vessels, iris of the eye, muscles present in ureters and in bronchi of lungs. The cells constituting these muscles are long, with a pointed end (spindle-shaped) and uninucleate (single nucleus). These muscles do not show any dark or light band. Hence, they are also called unstriated muscles.
(iii) Cardiac Muscles
These are involuntary muscles present only in our heart. They perform rhythmic contraction and relaxation throughout life. The cells constitution cardiac muscles are cylindrical uninucleate and branched. Cardiac muscles have stripes of light and dark bands.
4. Nervous Tissue
The tissue that receives a stimulus and transmits it from one part of the tissue to other, are nervous tissues. The cells that constitute nervous tissue are called nerve cells or neuron. These are highly specialized for receiving a stimulus and then transmitting it very rapidly from one place to another within the body itself. Brain, spinal cord and nerves are composed of nervous tissues.
An individual nerve cell or a neuron may be up to a metre long and is composed of three major parts:
(i) Cell body: It consists of cytoplasm, nucleus and cell membrane.
(ii) Axon: It is a single long conducting fibre extending from the neuron. It transmits impulse away from the cell body.
(iii) Dendrites: These are short branched fibres of a neuron, which receive nerve impulses.
Note: Synapse is a region of the union of the axon of one neuron with the dendrite of next. This allows the transfer of nerve impulse generated to and fro in the body.
Many nerve fibres bound together by connective tissue make up a nerve. Nerve impulse allows us to move our muscles according to our will. Combination of nerve and muscle tissue In animals is of fundamental importance as causes rapid movement in response to stimuli.