The covering or protective tissues in the animal body are epithelial tissues.
Epithelial tissue is the simplest tissue that forms the lining of all layers and organs in the body.
The cells of this tissue are tightly packed and they form a continuous sheet.
Cells of epithelium contain very little or no intercellular matrix.
Epithelium covers most organs and cavities within the body.
It also forms a barrier to keep different body systems separate.
The skin and lining of buccal cavity, blood vessels, alveoli and kidney tubules are made of epithelial tissue.
They have no intercellular spaces.
It is highly permeable and plays a significant role in the exchange of substances across the cells thus maintaining osmoregulation.
Functions:
It helps in regulating the exchange of materials between the body and the external environment and also between different parts of the body
The cells of the body surface i.e., epidermis form the outer layer of skin. These cells protect the underlying cells from drying, injury, and chemical effects.
They also protect the body from viral or bacterial infection.
Some epithelial tissues perform secretory function. They secrete a variety of substances such as sweat, saliva (mucus), enzymes, etc.
Epithelial tissues help in absorption of water and nutrients.
Inside the body, epithelial cells form lining of mouth and alimentary canal and protect these organs.
Epithelial tissues help in elimination of waste products.
Types of epithelial tissue:
Depending upon the shape and function of the cells, the epithelial tissues are classified as follows:
Squamous Epithelium:
Structure:
Squamous epithelium is made up of thin flat, irregular-shaped cells which fit together like floor tiles.
Occurrence:
In cells lining blood vessels or lung alveoli, where transportation of substances occurs through a selectively permeable surface.
It forms the delicate lining of cavities of mouth, oesophagus, nose, pericardium, alveoli, etc. covering of the tongue and skin.
Functions:
It protects the underlying parts of body from mechanical injury, entry of germs, chemicals and drying.
Stratified squamous Epithelium:
Cell of this tissue are arranged in many layers.
Occurrence:
This is found in skin and covers the external dry surface of the skin.
Functions:
They are arranged in many layers to prevent wear and tear.
Cuboidal Epithelium
It consists of cube-like cells.
Occurrence:
The cuboidal epithelium is found in kidney tubules, thyroid vesicles and in glands like salivary glands, sweat glands and exocrine pancreas.
Functions:
It provides mechanical support.
It helps in absorption, excretion and secretion.
Columnar Epithelium
The columnar epithelium consists of cells which are taller than broader.
Occurrence:
It forms the lining of stomach, small intestine and colon, forming mucous membrane.
In the respiratory tract, the columnar epithelial tissue also has cilia, which are hair-like projections on the outer surfaces of epithelial cells.
Functions:
Its main functions include absorption and secretion of mucus by goblet cells or mucous membrane.
It facilitates movement across the epithelial barrier
Glandular Epithelium
The columnar epithelium is often modified to form glands which secrete chemicals.
Ciliated Epithelium
Structure:
Certain cuboidal or columnar cells have free border which bear thread-like cytoplasmic outgrowths, called cilia. Such cells form the ciliated epithelium.
Ciliated cuboidal epithelium is found in the perm ducts. The ciliated columnar epithelium lines the trachea (wind-pipe), bronchi (lungs), kidney tubules and oviducts.
Functions:
The rhythmic, concerted beating of the cilia moves solid particles in one direction through the ducts.