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Epithelial Tissue: Structure, Classification (Part-I)

AIPMT-NEET Biology Aspirants, read out the next AIPMT-NEET Biology Study material/ Notes on Epithelial Tissue. In this we will learn about structure of epithelial tissue, its structure and classication and function in next post. Important for AIPMT-NEET Biology. Free online notes for AIPMT-NEET.

Tissue is a group of cells having a common embryonic origin and specialized in one particular function. So, group of cells which are structurally and functionally similar forms tissue.

On the basis of structure and function, 4 basis types of animal tissue are present: Epithelial tissue, Connective tissue, Muscular tissue and Nervous tissue.

Epithelial tissue is a covering and protective tissue. It covers body surfaces and forms linings of hollow organs, body cavities and ducts throughout the body. It is not covered by any other tissue so it has a free surface always. Epithelial tissue allows the body to interact with both its internal and external environment.

  • Epithelial tissue consists of cells arranged in a continuous sheet, either as single layer or multiple layers.
  • Cells of epithelial tissue are tightly and closely packed and held together by cell junctions. They have only a small amount of cementing material between them and almost no intercellular spaces.
  • Epithelial tissue has 3 major functions: (1) acts as a barrier helping in transfer/ exchange of substances into and out of the body (i.e. exchange between body and external environment )and also between different parts of the body . (2) acts as a secretory surface releasing its products onto its free surface. (3) acts as a protective layer to resist harsh  environmental conditions.
  • It also forms a barrier to keep different body systems separate.
  • Epithelial cells continuously renew and repair itself as they are subjected to damage, so cells have high rate of cell division to replace dead cells.
  • On the basis of function, epithelial tissue divided into two types: 1) covering and lining epithelium or 2) glandular epithelium.
  • Covering epithelium present as outer covering of skin and lining as inner lining of blood vessels, ducts, and interior of digestive, respiratory, urinary and reproductive systems.
  • As glandular epithelium, it is present in secretory portion of thyroid, sweat and adrenal glands.
  • So, important roles of epithelial tissue are protection, filtration, secretion, absorption and excretion. Also with nervous tissue form special organs for smell, hearing, vision and touch.

 

Basic Structure of Epithelial tissue

Basic structure of Epithelial tissue

Basic structure of Epithelial tissue

In a single layer epithelial tissue, epithelial cell has 3 surfaces: Apical (free) surface, Lateral surface and basal surface.

  • Apical surface of epithelial cell faces the outer part of tissue i.e. body surface, body cavity, lumen of internal organ etc. It may contain cilia or microvilli.
  • Lateral surface of epithelial cell are two in no. and faces the adjacent cells on either side. It may contain cell junctions.
  • Basal surface of epithelial cell is opposite the apical surface. It adheres to the extracellular materials such as basement membrane. Hemidesmosomes helps in adherence of basal surface of cell to the basement membrane.

In multi layer epithelial cell, apical surface is the surface of the most superficial layer of cells and basal surface is the surface of the deepest layer of the cells.

  • Basement membrane : attaches to epithelial tissue and provide support to it. It is a thin extracellular layer having 2 layers in it – Basal lamina and Reticular lamina. Basal lamina is secreted by epithelial cells, present below the basal surface and contains proteins like collagen and laminin. Laminin molecules attaches to hemidesmosomes to adhere basal surface to basement membrane. Reticular lamina is present below the basal lamina and it underlies connective tissue. Basement membrane also acts as road for migration of epithelial cells during growth and wound healing, restricts movement of selected molecules between epithelium and connective tissue and helps in filtration of blood in the kidneys.
  • Epithelial tissue has its own nerve supply but it has no blood supply. For blood supply, exchange of nutrients and waste, it is dependent on underlying connective tissue. Exchange occurs through diffusion.

 

Classification of Epithelial tissue

Different types of covering and lining epithelium are classified on the basis of Arrangement of cell layers and Cell shapes.

On the basis of arrangement of cell layers, cells are arranged in one or more layers. Following are types of epithelium:

(1) Simple epithelium – having single layer of cells. Helps in diffusion, osmosis, filtration, secretion and absorption.

Simple epithelium

Simple epithelium

(2) Pseudostratified epithelium-  actually a simple epithelium having single layer but appears as multiple layers because the cell nuclei lie at different levels and not all cells reach to apical surface. It is simple epithelium because all cell rest on basement membrane (pseudo= false). Few have cilia and some secret mucus.

Pseudostratified Epithelium

Pseudostratified Epithelium

(3) Stratified epithelium- having two or more layers of cells. Helps in protection of underlying tissues (stratum=layer).

Stratified epithelium

Stratified epithelium

 

On the basis of cell shapes, following are types of epithelial cells:

  • Squamous cells: these cells are flat and thin. Allows rapid passage of molecules through them.

    Squamous cells

    Squamous cells

  • Cuboidal cells: these cells are as tall as they are wide, i.e. like cube. Have microvilli at apical surface and help in secretion and absorption.

    Cuboidal cells

    Cuboidal cells

  • Columnar cells: these cells are much taller than their width, i.e. like columns. Have cilia or microvilli at apical surface and help in secretion and absorption.

    Columnar cells

    Columnar cells

  • Transitional cells: these cells change their shape from squamous to cuboidal and back as required. These cells help in stretching of organs to larger size and regain its shape. Present in urinary bladder.

 

Combining these two characteristics : arrangement of cell layers with cell shapes; we finally get following types of epithelial tissues (covering and lining) present in human body. Below is the table:

biotrick- neet-Classification of Epithelial tissue

Classification of Epithelial tissue

 

Let’s discuss this classification of epithelial tissue in next post.

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