Epithelial Tissue

Epithelial tissues are characterized by the absence of blood vessels. Instead, it receives the required nutrition through diffusion. They form the outer covering as well as the inner lining of many organs. In epithelial tissues, all cells are densely packed with little intercellular matrix between them. Epithelial tissue is classified into two types – simple epithelium and compound epithelium. Moreover, epithelial tissue can be classified based on its shape as well – cuboidal, squamous, columnar.

Where are the epithelial tissues found in the human body?

• The lining of the blood vessels

• The lining of the mouth

• Kidney tubules

• Skin

• Lung alveoli

Structure and functions of the epithelial tissues -

• The main function of the epithelial tissues is to act as a barrier and separate different organs and systems from each other.

• There is no space between the cells of epithelial tissues

• The cells are permeable. This makes it possible for them to exchange materials between different parts of the body and also between the body and the external environment.

• The epithelial tissues remain separated from the tissues beneath them because of a thin membrane over them.

  

Squamous Epithelium:

In cells lining blood vessels or lung alveoli, where transportation of substances occurs through a selectively permeable surface, there is a simple flat kind of epithelium. This is called the simple squamous epithelium (squama means scale of skin). Simple squamous epithelial cells are extremely thin and flat and form a delicate lining. The oesophagus and the lining of the mouth are also covered with squamous epithelium.

Stratified Squamous:

The skin, which protects the body, is also made of squamous epithelium. Skin epithelial cells are arranged in many layers to prevent wear and tear. Since they are arranged in a pattern of layers, the epithelium is called stratified squamous epithelium.

Ciliated Columnar Epithelium:

Where absorption and secretion occur, as in the inner lining of the intestine, tall epithelial cells are present. This columnar (meaning ‘pillar-like’) epithelium facilitates movement across the epithelial barrier. In the respiratory tract, the columnar epithelial tissue also has cilia, which are hair-like projections on the outer surfaces of epithelial cells. These cilia can move, and their movement pushes the mucus forward to clear it. This type of epithelium is thus ciliated columnar epithelium.

Cuboidal Epithelium:

Cuboidal epithelium (with cube-shaped cells) forms the lining of kidney tubules and ducts of salivary glands, where it provides mechanical support. Epithelial cells often acquire additional specialisation as gland cells, which can secrete substances at the epithelial surface. Sometimes a portion of the epithelial tissue folds inward, and a multicellular gland is formed. This is glandular epithelium.

1. Simple epithelial tissue
   1. Squamous epithelial tissue
   2. Cuboidal epithelial tissue
   3. Columnar epithelium
   4. Ciliated epithelium
   5. Glandular epithelium
   6. Sensory epithelial tissue
   7. Germinal epithelial tissue
2. Compound epithelial tissue
   a. Stratified epithelium
   b. Transitional epithelium

• Cell junctions 
  Types of Cell junction
  - Tight junctions (TJs)
  - Hemidesmosomes (HDs)
  - Gap Junctions (GJs)
  - Adherens Junctions (AJs)
  - Desmosomes (Ds)

Epithelial Tissue:

• The covering or protective tissues in the animal body are epithelial tissues

• Epithelial tissue cells are tightly packed and form a continuous sheet with almost no intercellular spaces.

• Permeability of the cells of various epithelia plays an important role in regulating the exchange of materials between the body and the external environment and also between different parts of the body.

• All epithelium is usually separated from the underlying tissue by an extracellular fibrous basement membrane.

1) Squamous epithelium:

• Present in the oesophagus and the lining of the mouth (simple), in the skin (stratified i.e. arranged in many layers)
• Prevents wear and tear
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2) Cuboidal epithelium:

• Forms the lining of kidney tubules and ducts of salivary glands

• Provides mechanical support

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3) Columnar epithelium:

• Present in the inner lining of the intestine (tall columnar), respiratory tract (ciliated columnar)

• Helps in absorption and secretion

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4) Glandular epithelium

• Secrete substances at the epithelial surface.

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