Epitelium

 Covering epitelium

Covering epithelium shows almost :

• no extracellular matrix and 
• epithelial cells are tightly attached to one another by macromolecular adhesion complexes. 
• Some epithelia show a high rate of cell turnover where cell death and cell proliferation are frequent. 
• Some epithelial cells can have apical specializations that allow them to function as sensory receptors, and some animals show complex structures in their epithelial layer, such as hair, feathers or scales.

Epithelia are usually classified according to two features: the number of cell layers and the shape of the cells of the more superficial layer 

•  Simple epithelia are single cell layers where all the cells contact the underlying basal lamina and have an apical free surface. The shape of the cells can be flat (wider than high), cuboidal (as wide as high), or columnar (higher than wide).
•  Pseudostratified epithelia are simple epithelia where all cells contact the basal lamina, but not all cells reach the superficial layer because some are shorter than the others. Thus, this is a simple epithelium that looks stratified. 
• Stratified epithelium contains two or more layers of cells. Only cells of the deeper layer are in contact with the basal lamina and only cells of the upper layer show free surfaces. 

Stratified epithelia can be classified as 

squamous, 

cuboidal and 

columnar, depending on the shape of the cells of the upper layer when observed in transverse view.

Epithelium.

KERATINIZED STRATIFIED SQUAMOUS

Basal lamina: The basal lamina is a layer of extracellular matrix secreted by the epithelial cells, on which the epithelium sits.

SIMPLE SQUAMOUS

Glandular epithelium

Secretion is the release of substances from cells to the extracellular milieu, having these substances a physiological purpose. Secretory cells are usually grouped to from glands, although it is not always so.

Exocrine glands release their products to the external surfaces of the body (for example, skin, respiratory ducts and digestive duct).

 If the products are released into inner tissues and blood stream they are called endocrine glands.

Exocrine glands release their products to internal cavities or to the exterior of the body.

Salivary gland