Epithelial cells are polarised cells which are tightly bound together to form a sheet which is found on top of a basement membrane. These sheets protect the body from the external environment by forming a barrier. They are found in the skin, intestine, liver, pancreas, gallbladder, lungs and kidney. The cells can be various shapes and project cilia for surface transport and microvilli for absorption. Although the cell's primary function is protection, they are also involved in absorption, maintaining the body's internal environment and secretion.
There are 2 ways of crossing epithelial cells:
- Paracellular (between cells)
- Transcellular (through cells)
The epithelial cells are joint by junctions which are known as 'tight' or 'leaky', this is determined by the ability to allow small ions through. Known as cation selectivity.Tight junctions main functions are to stop transcellular transported molecules move back across the membrane and limits lateral diffusion which keeps polarity.
Proteins found in the tight junction:
- Occludin (tetraspanning protein)
- Claudin family ( ~20 types)
- Junction Adhesion Molecule (JAM, one transmembrane span)
These have a 6 transmembrane spanning domains and have 9 known types in the family. the best known is AQ3 and AQ4 in the basal membrane of the collecting duct which allows water reabsorption in the presence of vasopressin.
Epithelial cells are positions in many organs and are arranged into cohesive sheets called 'epithelia'. They are reliant on each other and cover surfaces and salivary glands. The cohesive sheets keep the cytoskeleton attached to neighbouring cells and also keep the epithelium to the extracellular matrix material. Epithelial cells are categorised into three groups based on their shape; cuboidal, squamous, and columnar. Epithelial cells perform a wide range of functions such as absorption (shown by the intestine), protection (shown by the skin), secretion (shown by the glands), excretion (shown by the kidneys) and gas exchange (demonstrated by the lungs and blood vessels). To perform these differing functions, the epithelial cells must arrange themselves into different structures; the squamous, for example, is found in the lungs and blood vessels and is a thin layer of flattened cells that facilitate gas exchange. The cuboidal and columnar are involved in absorption and therefore are much taller structures. Both of these structures need the aid of other organelles such as the rough endoplasmic reticulum or the Golgi apparatus or mitochondria to provide energy in the form of ATP to enlarge the cell. Epithelial cells have a large supply of stem cells in order to replace cells that are damaged by harsh environmental conditions.