Tissues

7 Epithelial Tissues

  • 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.

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