Control of development by steady ionic currents.

A wide variety of developing systems drive substantial steady ionic currents through themselves. These currents are driven by a certain separation of ion leaks from ion pumps along the membranes of developing cells. Moreover, they seem to be more than epiphenomena. In part they may act back, via self-electrophoresis and/or specific ion gradients, to further segregate the membrane components that drive them; in part they may act to segregate and locally modify other components and thus act as prime agents of differentiation. Among the systems in which evidence for such action has been reported are: 1)Fucoid eggs. There is evidence that early calcium ion currents through these eggs are self-amplifying and induce local growth at the locus of calcium ion entry. 2)Cecropia (insect) follicles. There is evidence that large transfollicular currents maintain enough of a voltage drop between nurse and oocyte cells to help drive macromolecules into the oocytes. 3)Amputated amphibian limbs. There is evidence that large natural stump currents somehow initiate regeneration. 4)Frog embryos. The growth of isolated, embryonic Xenopus neuroblasts and myoblasts can be directed by voltage gradients so small (down to 10 to 30 mV/mm) that they may well exist within the embryo. 5)Wounded mammalian skin. Natural currents through incisions in guinea pig skin establish voltage gradients within the adjoining epidermis that are so large (100 to 200 mV/mm) they may direct wound closure.