The gap junction

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Abstract
Gap junctions are intercellular structures that link cells and allow them to exchange ions and small metabolites without recourse to the extracellular space (for reviews and early references, see Goodenough, 1979; Loewen-stein, 1981). They are found in every organ including the nervous system, where they form electrical syn-apses; the only exception being fully differentiated skeletal muscle. Gap-junctional communication is present between all cells in early embryos, regardless of eventual developmental fate, at times when cellular interactions are known to be taking place (Potter et al. 1966). The widespread occurrence of gap junctions has led to the proposal that they play an important part in coordinating cellular signals, including those involved in growth control and embryogenesis.