It is now widely accepted that the plasma membranes of epithelial cells display a marked degree of polarity. This polarity is reflected in a number of parameters, e.g. the localization of certain proteins to basolateral membranes. Polarity is also illustrated by the presence of discrete membrane specializations, including desmosomes, tight junctions and gap junctions. These specializations provide strong support for the idea of membrane "domains" within the polarized membrane of the epithelial cell. This presentation relates to the proteins found in gap junction membranes. The issues include a clear identification of these proteins in different cells and an analysis of how a cell-to-cell channel is constructed by these proteins.
The plasma membranes of most animal cells contain cell-to-cell channels, which provide for the direct, passive exchange of small molecules between cells. Collections of these channels are identified as "gap junctions." This form of intercellular communication is thought to be important in a wide variety of biological processes, including cellular differentiation, proliferation, and tissue homeostasis.