Epithelial cells are divided into two broad categories on the basis of structural modification of the cells:

(A) Simple Epithelium and (B) Compound (Stratified) Epithelium

(A) Simple Epithelium: It is the epithelium tissue which has a single layer of cell. Thus, it is known as simple epithelium tissue. It is further divided into three types on the basis of the cell shape and structures:

i) Squamous ( flattened) cells: These are cells which flattened in shape and they allow materials and gases to pass through them by methods like diffusion and filtration. They can also secrete lubricating substances. They are found in the blood and lymphatic vessels, lining of the heart and air sacs of the lungs.

ii) Cuboidal cells: These cells are shaped like cuboid and they help in the secretion and absorption of gases and other materials in the skin. They also have microvilli present. They are found in ducts like collecting tubule of the kidney.

iii) Columnar epithelium: These cells are present in a tube or column-like shaped and they are used in absorption and secretion of certain enzymes and mucus. They can have microvilli, surface cilia, etc. They are present in the fallopian tube, small intestine, gall bladder, etc.

(B) Compound (stratified) Epithelium: It is the epithelium tissue which has two or more layers of cells lined together. It is also divided like simple epithelium on the basis of structure and it differs from simple epithelium because of the presence of more than one layer of cells.