Authors: Masayuki Yamashita.
Growing axons are directed not only by chemical signals but also by electric fields in a process known as galvanotropism. Axons of embryonic neurons extend along the extracellular voltage gradient towards the cathode. During embryonic development neuroepithelial cells function as neural stem cells. The neuroepithelial cell has epithelial type sodium channels (ENaC), and the sodium transport via ENaC of retinal neuroepithelial cells produces extracellular positive direct current (DC) potentials within the retinal neuroepithelium. The amplitude of the positive DC potential is large at the periphery of the embryonic retina, and almost null at the ventral part of the optic cup, where the future optic disc is formed. Retinal ganglion cells are first born at the central part of the optic cup and they extend their axons along the endogenous voltage gradient; the disruption of the DC potential by blocking ENaC results in erroneous path finding of newborn retinal ganglion cell axons. Retinal ganglion cell axons can also be oriented by exogenous electric fields in vitro. Galvanotropism may be used to reform an optic nerve from the retinal ganglion cells that are generated from ES cells and iPS cells.
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