Progress made in the last 20 years clearly indicates that the cell surface is an extremely dynamic structure involved in fundamental processes such as cell motility, innervation, and cell adhesion. Of particular interest is the finding that, in several tissues, the cell surface is differen tiated at the intercellular region, thereby providing communicating chan nels between apposing cells. Although our actual knowledge of the precise structures and mecha in the complex process of intercellular communication is still nisms involved scanty, evidence has been presented that ions and molecules...