Specificity of VIII^th nerve regeneration in lower vertebrates

From the initial studies of Sperry (Am. J. Physiol, 144:735–741, 1945) to more recent investigations, the regenerative capacity of the VIII^th cranial nerve in nonmammalian vertebrates has been noted for its robust and accurate recovery of functional connections after transection. The VIII^th cranial nerve contains nerve fibers that link functionally distinct sensory epithelial to various areas within the central nervous system (CNS), yet after transection these multiple components of the nerve navigate back to their original central target areas, without innervating inappropriate nuclei. A number of factors may be required to establish and direct VIII^th nerve regeneration. Cellular interactions appear to be necessary for the initiation of outgrowth and the maintenance of neural connections. The release of chemotropic substances from target cells has been postulated as the most likely mechanism guiding the reinnervation of central targets. Furthermore, the growth characteristics of these neurons in tissue culture, without target cells present, indicates that intrinsically regulated growth features may also contribute to the process of VIII^th nerve regeneration.