DEGENERATION AND REGENERATION OF THE VIIITH CRANIAL NERVE IN SALAMANDER (AMBYSTOMA MEXICANUM) LARVAE (MAUTHNER CELL, REINNERVATION, HORSERADISH PEROXIDASE, VERTIBULAR NERVE, AXONAL REGENERATION)

Abstract

To investigate nerve regeneration in the central nervous system of a lower vertebrate, the vestibular or VIIIth cranial nerve (nVIII) root was transected unilaterally in salamander, Ambystoma mexicanum, larvae. Regenerating nVIII fibers were identified by applying the intracellular marker horseradish peroxidase (HRP) to the vestibular appartus. To gain insight into the fate of denervated postsynaptic sites as well as the process of reinnervation at the cellular level, degeneration and regeneration of nVIII axons which connect with an identified neuron, the Mauthner cell (M-cell), were followed at the electron microscopic level.;Axons of the ipsilateral nVIII provide major sensory input to the M-cell; their terminals are restricted to a well defined region of the cell's dendritic arbor. Ultrastructural observations show that transected nVIII axons initially demonstrate a filamentous pattern of degeneration which progresses within hours to an electron dense form. By 48 hours most of the terminals have become separated from the M-cell surface. Following detachment, no vacated postsynaptic sites were observed. The presence of coated pits along the M-cell surface adjacent to degenerating nVIII terminals and a striking reduction in the surface area of denervated dendritic regions suggest that the M-cell itself may play a role in the removal of postsynaptic sites. At two weeks postlesion, numerous HRP labelled fibers were observed within the nVIII tract and by four weeks the axons formed a projection that closely resembled the normal nVIII projection in that there was a large number of fibers confined to the nVIII tract and they occupied its full rostrocaudal extent. In addition, by two weeks, the regenerating nVIII axons contact the M-cell and by four weeks, the regenerated terminals are numerous and many demonstrate a morphology which is indistinguishable from normal nVIII terminals. Furthermore, at all times the regenerated contacts were found only on the correct region of the M-cell. Since there were no vacated postsynaptic sites following nVIII degeneration, new postsynaptic sites are presumably induced by the regenerating fibers.