Gap Junctions in Development

The concept of a multicellular organism as an assemblage of cells that work together harmoniously for the good of the whole organism is well established. Over the past 15 years, however, it has become increasingly clear that the individual cells of multicellular animals are not as self-sufficient and independent of each other as once had been believed. Channels that exist between virtually all neighboring cells permit such widespread intercellular exchanges of low-molecular-weight materials that they must be considered syncytial with respect to small molecules. The specialized areas of the membrane that permit these exchanges are clusters of channels whose permeability is limited only by molecular size of the permeant (Simpson et al., 1977; Flagg-Newton et al., 1979), known collectively as gap junctions. Only in the case of molecules close to the exclusion limit can one detect selectivity (Brink and Dewey, 1980).