INTRODUCTION

the cells of animals are grouped into tissues with characteristic functions such as movement, secretion, protection etc.

CLASSIFICATION OF ANIMAL TISSUE

In the bodies of animals, there are four major categories of tissues, which are as follows; 

1. epithelial tissue
2. connective tissue
3. muscle tissue
4. nervous tissue.

EPITHELIAL TISSUE

• Epithelial tissue covers the outside of body and lines the organs and cavities such as the surface of the skin, the inner surface of airways, digestive tract, Genito-urinary tract and surface of soft organs etc.
• The cells in this tissue are very closely packed together, thereby establishing a barrier between the external environment and the organ it covers. 
• This tissue has many types on the basis of the shape of cells as well as the number of cell layers. Some of the salient types include: 

Squamous epithelium 

• Squamous epithelium comprises of a single layer of flat cells. 
• It is found in the lungs, heart and blood vessels. Here, it allows the movement of materials across it. 

Cuboidal epithelium 

• Cuboidal epithelium comprises of a single layer of cube-shaped cells.
•  It is found in kidney tubes, small glands etc, where it is responsible for the production of secretions.

Columnar epithelium 

• Columnar epithelium is composed of cells that are elongated in shape. 
• It is found in alimentary canal, gall bladder etc where it produces secretions.

Ciliated columnar epithelium 

• Ciliated columnar epithelium has elongated cells with cilia. 
• It is present in trachea and bronchi and propels mucous.

Stratified squamous epithelium 

• Stratified squamous epithelium consists of multiple layers of flat cells. 
• It is present in the lining of oesophagus and mouth and also over the skin and provides protection to the enclosed contents.

FUNCTIONS OF EPITHELIAL TISSUE

Generally, epithelial tissues are responsible for two main functions i.e. protective and secretory.

• Cells of epithelial tissue are tightly packed together thereby creating a boundary between the internal and external environment.
• Epithelial tissue also regulates the inflow and outflow of material between the internal and external environment.
• Epithelial tissues, in various parts of the body, are also responsible for the production and release of various secretions for example; digestive enzymes in the digestive tract.

CONNECTIVE TISSUE

• As the name implies, connective tissue serves as a “connecting” material, supporting and binding other tissues together. 
• Unlike epithelial tissue, connective tissue has cells scattered throughout an extracellular matrix.

FOR EXAMPLE 

Common examples of connective tissue are;

1. cartilage (found around the ends of bones, in external ear, nose, trachea etc). 
2. bone  
3. blood. 

The adipose tissue (found around kidneys, under skin, in abdomen etc.) is also a type of connective tissue. It provides energy and support to the visceral organs.

MUSCLE TISSUE

• Muscle tissue comprises of bundles of long cells called muscle fibers. 
• It is the most abundant tissue in an animal.
•  The cells of this tissue possess the ability to contract. 

TYPES OF MUSCLE TISSUE

There are three kinds of muscle tissue.

1. Skeletal muscles
2. Smooth muscles
3. Cardiac muscles

SKELETAL MUSCLES

• Skeletal muscles are also termed as straited muscles because they exhibit a striated (stripped) appearance when viewed under a microscope.
• Skeletal muscles or striated muscles consist of multi-nucleate cells and are present attached to the bones.
• Skeletal muscles are voluntary in nature and their movement can be controlled by an individual i.e. picking a glass using hand muscles.
•  They are responsible for the movements of bones and are the basis for the mobility of animals. 

SMOOTH MUSCLES

• Smooth muscles are also termed as non-striated muscles because they do not exhibit a striated (stripped) appearance under a microscope.
• Smooth muscles consist of mono-nucleate cells and are found in the walls of alimentary canal, urinary bladder, blood vessels etc. 
• Smooth muscles are involuntary in nature and their movement cannot be controlled by an individual I.e. movement of smooth muscles within the gastro-intestinal tract.
• Smooth muscles are responsible for the movement of substances within the passageways of the body for example; peristalsis which moves food throughout the alimentary canal.

CARDIAC MUSCLES

• Cardiac muscles are present in the wall of heart. 
• The cells of cardiac muscles are also striated but there is a single nucleus in each cell. 
• Cardiac muscles are involuntary in nature and are constantly producing movement in a rhythmic fashion leading to the production of heartbeat. 
• Cardiac muscles are the most active muscles of the body and keep on moving from the beginning of one’s life till death.

NERVOUS TISSUE

• We know that an animal’s survival depends on its ability to respond appropriately to the stimuli from its environment. 
• This ability requires the transmission of information among the parts of body. 
• Nervous tissue performs this task via forming a communication system.
• This tissue is mainly composed of nerve cells or neurons, which are specialized to conduct messages in the form of nerve impulses. 
• Nervous tissues are present in the brain, spinal cord and nerves.