Animal Tissue - Types, Characteristics and Function

We know that the body of animals is made up of different types of tissues which perform specific functions. For example; contraction and relaxation of muscle cells result in the movement of body parts. Blood and lymph carry substances from one part to another part of a body. Nerve cells respond to external and internal stimuli. Thus, muscle, blood, nerve, etc are the examples of tissue found in our body. A group of animal cells having the same
origin, structure and function is called animal tissue. Animal tissue can be broadly classified into following four types:

(a) Epithelial Tissue (Epithelium)

Epithelial Tissue (Greek; epi = upon: thele = nipple) is a simplest kind of animal tissue that occurs as a protective covering. The animal tissue which covers the surface of the body and lies in the various body cavities and internal organs including the blood vessels is called epithelial tissue.

Characteristics: The characteristics of epithelial tissues are:

• Epithelial tissue covers most organs and cavities of the body
• The cells of epithelium are lightly packed and form continuous sheets.
• There is almost no intercellular space between the epithelial cells.
• All the types of Epithelial Tissues rest on an extracellular fibrous basement membrane.
• The cells are single or multilayered.
• Blood vessels are absent.
• The Epithelial cells possess the power of division and regeneration.

Function: Epithelial tissue performs the following functions:

• It protects the underlying tissue from mechanical injury, entry of germs, dryng up and harmful chemicals.
• Epithelial lining of the intestine absorbs water and digested food.
• Epithelial tissue helps in the elimination of nitrogenous and other waste products.
• Epithelial lining of digestive glands and endocrine glands secrete useful secretion such as mucus, gastric juice, etc.
• Epithelial lining of alveoli (lungs) brings about exchange of oxygen and carbon dioxide.

Types of Epithelial Tissue:

Based on the cell layers and shape of cells, the epithelial tissues are classified as follows:

1. Pavement or Squamous epithelium

Characteristics: The characteristics of squamous epithelial tissues are:

• It is composed of thin cells which are like the tiles of a floor.
• The cells have centrally placed nucleus which is rounded or oval.
• All the cells rest on delicate and noncellular basement membrane.
• It may be single layered or multilayered of cells.

Function: Squamous epithelial tissue performs the following functions:

• Protection
• Secretion
• Excretion

2. Cuboidal epithelium

Characteristics: The characteristics of cuboidal epithelial tissues are:

• It is composed of cubical cells.
• The cells have centrally placed rounded nucleus.
• All the cells rest on delicate and non-cellular basement membrane.
• Some cells of this epithelium may become specialized as glands may secrete substances

Function:

• Protection
• Secretion
• Absorption
• Excretion
• Gamete formation

3. Columnar epithelium 

Characteristics:

• It is composed of elongated cells which are placed side by side like columns.
• The nucleus is oval shaped and generally placed near the base.
• All cells rest on delicate and noncellular basement membrane.

Function:

• Protection
• Secretion
• Absorption

Glandular epithelium

Characteristics:

• The cells of the epithelium are columnar in shape.
• The nucleus is oval shaped.
• All cells rest on delicate and noncellular basement membrane.

Function:

• Secretion

b) Muscular Tissue:

The tissue which forms the muscles of the body is called muscular tissue. It is formed of elongated cells called muscle fibres that are joined by connective tissue. Muscles are responsinle for the movement of the body parts.

(c) Connective Tissue: 

The tissue that connects or binds one tissue to another or supports them in order to keep them in proper place is called connective tissue. This tissue is also called joining tissue. It also fills up the space between the organs.

The major function of connective tissue are to bind, support and pack together different organs of the body.

(d) Nervous Tissue:

Nervous tissue which contains densely packed nerve cells is called neurons. It is present in the brain, spinal cord and nerves. The neurons are specialized for conduction of nerve impulses. They receive stimuli from different places of the body and conduct impulses to the brain and spinal cord.