Epithelial cells

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 epitheilal cells:

1. Paracellular (between cells)
2. Transcellular (through cells)

Paracelluar tranmission

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 transcelluar 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)

Transmembrane transmittion

In water movement the main protein needed to cross the membrane is Aquaporin.

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 reabsorbtion in the presence of vasopressin.

Other proteins to aid transcellular transport include from lumen to blood or vicea-versa:

• CFTR
• Na⁺/K⁺ Pump
• Various ion channels