[Physiological importance of gap junctions]

Some structural features of the different types of intercellular junctions which occur in vertebrate tissues (desmosomes, tight and gap junctions, Table 1) are first mentioned. Then, this review is exclusively concerned with gap junctions. The ubiquitous occurrence of these junctions throughout the phylogenetic scale up to man points to a major functional role. Cells of most organized tissues make cell-to-cell channels, 1-2 nm diameter, that provide a structural hydrophilic pathway for free diffusion of inorganic ions and small molecules. Ionic coupling and metabolic cooperation have been shown to be functional expressions of the direct intercellular communication. The role of gap junctions in nonexcitable tissues is not well established (Chap. III). While these junctions are clearly involved in the regulation of some enzymatic activities and exocrine and endocrine secretions, the cell-to-cell transmission of signal molecules necessary for growth control remains largely hypothetical.