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 known as the epitheila, often used to seperate body tissue to external environment found in:

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

Classification

An epithelial tissue can be made of one layer of cells or more than one layer of cells, which are classified as simple or stratified. The epithelial cells can be categorised further with regards to their shape. The cells can be flat, roughly cube shaped or tall, which are squamous, cuboidal and columnar respectively.

Properties & Structure

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

Cilia are often found on the Apical membrane (the membrane that is exposed to the lumen) these significantly increase surface area to aid absorbtion, 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.