Classify and describe the epithelial tissues on the basis of the structural modification of cells. Give examples.

• Epithelial tissue covers the outer and internal organs.
• Epithelium rests on basement membrane.
• It is avascular.
• Based on number of layers, it is two types.
   

A) Simple epithelium:
It is a single layer of cells. It is of 3 types based on shape of cells.
 

i. Simple squamous epithelium:
Cells are flat with ovoid nucleus in the centre.
Example: Alveoli of lungs, Bowman's capsule.

ii. Simple cuboidal epithelium: Cells are cuboidal with spherical nucleus in centre.
Example: Germinal epithelium, PCT of nephron

iii. Simple columnar epithelium: Cells are tall and cylindrical with oval nucleus at the base. It is of two types.
 

a. Ciliated columnar epithelium: Cells are with cilia.
Example: Fallopian tube, cavities of brain and spinal cord

b. Non-ciliated columnar epithelium:
Columnar cells are without cilia.
Example: Lining of stomach and intestine. Cells are with microvilli in intestine.

B) Compound epithelium:
It has more than one layer of cells. Its main function is protection.

i. Stratified keratinised squamous epithelium:
Upper cells are with keratin
Example: Epidermis of skin

ii. Stratified non-keratinised squamous epithelium:
Example: Lining of buccal cavity, pharynx, vagina etc.

iii. Stratified cuboidal epithelium:
Example: Ducts of sweat glands, salivary glands

iv. Transitional epithelium:
Example: Urinary bladder

C) Glandular epithelium: Cuboidal or columar cells specialised for secretion form glandular epithelium.
Example: Exocrine and endocrine glands