Identify & describe epithelial cell surface domains, their specific characteristics/specialisations & functional significance

Epithelial cells are specialized cells that line the surfaces of the body, including the skin, blood vessels, urinary tract, and organs. They serve as a barrier between the inside and outside of the body and have several unique characteristics.

1. Apical Domain: This is the surface of the epithelial cell that faces the lumen (the inside space of a tubular structure) or the external environment. It is specialized for absorption and secretion. The apical domain often has microvilli (small projections that increase surface area for absorption) or cilia (larger projections that move fluid across the cell surface).

2. Lateral Domain: This is the side of the cell where it contacts its neighboring cells. Specializations here include tight junctions, adherens junctions, and desmosomes, which help to seal off the body cavity and prevent leaks, and gap junctions, which allow for communication between cells.

3. Basal Domain: This is the surface of the cell that is opposite the apical domain and is in contact with the basement membrane, a layer of extracellular matrix that provides support and anchorage for the epithelial cell. The basal domain often has infoldings that increase surface area for better attachment to the basement membrane.

4. Basolateral Domain: This is a combination of the lateral and basal domains. It is separated from the apical domain by tight junctions. The basolateral domain is involved in the transport of molecules between the cell and the underlying tissues.

The functional significance of these domains lies in their ability to allow the epithelial cell to carry out its roles effectively. For example, the specializations in the apical domain allow for efficient absorption and secretion, while those in the lateral and basal domains ensure that the cell remains firmly attached to its neighbors and the underlying tissues, and that substances do not leak around the cell.