Neural Stem Cells of the Neuroepithelium Direct Newborn Neurons' Axons Electrically: Galvanotropism Precedes Chemotropism

Growing axons are directed not only by chemical signals but also by electric fields in a process known as galvanotropism. The axons of embryonic brain, spinal cord, and retina extend along the extracellular voltage gradient towards the cathode. During embryonic development neuroepithelial cells function as neural stem cells. The neuroepithelial cells have epithelial type sodium channels (ENaC) and the sodium transport by these cells produces positive direct current (DC) potentials within the neuroepithelium. The amplitude of the DC potential is large at the area where neuroepithelial cells proliferate. The central nervous system develops as neuroepithelial cells proliferate, during which period the neurons that issue long-distance travelling axons are born first. Since neuroepithelial cells generate positive DC potentials, the resultant electric field orients the new-born neurons’ axons in the direction against the area where the neuroepithelial cells most actively proliferate.