Epithelial Cells

An epithelium can be defined as a ‘sheet’ of cells coating the external surface of a structure or covering a cavity. Epithelial cells are what make up this cellular layer, often used to seperate body tissue from the external environment it's found in:

1. Skin
2. Intestine
3. Lungs
4. Kidney
   

Classification

An epithelial tissue can be made up of one or more layers of cells, which are classified as simple or stratified. The epithelial cells can be categorised further with regards to their shape: flat, which are known as squamous; roughly cubed, or cuboidal; and finally the taller ones are known as columnar.

Properties & Structure

There are two sides to an epithelial cell, the apical membrane which is usually exposed to the extracellular fluid in the lumen and the basal membrane which is secured to another cell.

Cilia are often found on the apical membrane and these significantly increase surface area to aid absorption, and even in some organs are used as a brush boarder to prevent infection.

In a layer of epithelia there are junctions between each cell known as 'tight' or 'leaky' junctions. The tight junctions limit small molecules or ions moving laterally, creating and upholding membrane domains.