Epithelium Tissue.

 Tissue level of organization: Epithelium Tissue.

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Introduction:

• Also called “Epithelium”.

• It forms the covering of organs.

• They are derived from all three embryonic layers.

Epithelium Tissue.

Location of Epithelial Tissue:

• Covering of the organs exposed to the outer environment e.g. skin, GI tract, respiratory tract, urinary and reproductive tract etc.

• Epithelium tissue forms much of the glandular tissue. 

• It forms the internal lining of the cavities and hollow organs.

Basic Functions of Epithelium Tissue:

1. Protects the body from physical, biological and chemical wear and tear.

2. Selective transport: Act as gatekeeper as forms covering the tissues.

3. Secretion: Secretes mucus and other chemicals.

4. Absorption: Intestinal columnar epithelium.

5. Excretion: Sweat Glands.

Structural Features of Epithelium Tissue:

• Epithelium tissue is tightly packed.

• Non vascular: No blood supply (Receives nutrients and oxygen by diffusion)

• Cells shows intracellular junctions for communication.

• Cell has three surfaces;

• Apical: Upper Surface

• Lateral: Side.

• Basal: Lower.

• The basal layer secretes certain substances like collagen and glycoproteins which form basal lamina, basal lamina separates epithelium from underlying connective tissue.

• The underlying connective tissue secretes and forms reticular lamina.

• The reticular lamina and basal lamina together form the basement membrane which acts as a site of attachment for the epithelium tissue.

Classification of Epithelial Tissues:

1. Simple Epithelium:

1. Simple Squamous Epithelium.

2. Simple Cuboidal Epithelium

3. Simple Columnar Epithelium.

2. Stratified Epithelium:

1. Stratified Squamous Epithelium.

2. Stratified Cuboidal Epithelium

3. Stratified Columnar Epithelium.

4. Transitional Epithelium

3. Pseudostratified Epithelium.

Simple Squamous Epithelium:

• The cells are flat polygonal in shape and are tightly packed.

• There is no direct blood supply and the transfer of materials takes place by diffusion from blood vessels of the underlying connective tissue.

• The cells show the presence of intracellular junctions for communication.

• The simple squamous epithelium that lines blood vessels and lymph vessels is specialized for faster chemical transfer and called “Endothelium”.

• The simple squamous epithelium that lines the serous membranes is called “Mesothelium”, it secretes a secretion called “Serous Fluid” which serves as lubricating fluid and acts as a shock absorber.

• The alveoli of the lungs where gases diffuse, segments of kidney tubules, and the lining of capillaries are also made of simple squamous epithelial tissue.

Simple Cuboidal Epithelium:

• The cells are “Cube” like in structure hence called  Cuboidal epithelium.

• Width is more as compared to height.

• This tissue forms internal linings of Thyroid gland, salivary glands, nephrons etc.

Simple Columnar Epithelium:

• The cells have a “Column” like structure hence called  Columnar epithelium.

• Height is more as compared to width.

• This tissue forms internal linings of intestines  etc.

• They are specialized in functions of absorption and excretion.

• Columnar epithelium of the intestine has “microvilli” which increases the surface area of the cells for absorption.

• Goblet cells of the intestinal lining and the lining of parts of the respiratory tract such as the trachea have secretory function and secretes mucus.

Ciliated Columnar Epithelium:

• Ciliated epithelium consists of columnar cells that have cilia on their apical surface.

• Ciliated epithelium lines the nasal cavities, larynx, trachea, and large bronchial tubes.

• The cilia sweep mucus, with trapped dust and bacteria from the inhaled air, toward the pharynx to be swallowed. 

• Another location of ciliated epithelium in women is the lining of the fallopian tubes, the cilia here sweep the ovum, toward the uterus.

Stratified epithelium:

• Stratified epithelia consist of several layers of cells of various shapes. 

• Continual cell division in the lower (basal) layers pushes cells above nearer and nearer to the surface, where they are worn off.

• Basement membranes are usually absent. 

• The main function of stratified epithelium is “Protection”

• There are two main types:

• Keratinized stratified epithelium

• Nonkeratinized stratified epithelium

Keratinised stratified epithelium:

• This is found on dry surfaces subjected to wear and tear, i.e. skin, hair and nails. 

• The surface layer consists of dead epithelial cells that have lost their nuclei and contain the protein keratin. 

• This forms a tough, relatively waterproof protective layer that prevents drying of the live cells underneath. 

Non-keratinised stratified epithelium:

• This protects moist surfaces subjected to wear and tear, and prevents them from drying out. 

• e.g. the conjunctiva of the eyes, the lining of the mouth, the pharynx, the oesophagus and the vagina.

Commonly Asked Questions.

1. Write a note on Epithelial tissue and explain its different types.

2. Explain in detail the structure and functions of epithelium tissue.