EFFECT OF PRECOCIOUS AND DELAYED AFFERENT ARRIVAL ON SYNAPSE LOCALIZATION ON THE AMPHIBIAN MAUTHNER CELL (AXONS, NEURONAL SPECIFICITY)
LEBER, STEVEN MARK
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The Mauthner cell (M-cell) of Ambystoma mexicanum receives synapses from the ipsilateral VIIIth nerve (nVIII) only on the ventral surface of the proximal portion of its lateral dendrite. The specific distribution of terminals (club endings) might result from nothing more than where and when during development the growing axons happen to meet the growing dendrites (spatiotemporal model). Alternatively, it might reflect constraints on where the developing axons are capable of growing (axon segregation model) or synapsing (recognition model). As an experimental test of the spatiotemporal model, the normal timing of nVIII ingrowth and synaptogenesis was surgically disrupted and the resulting distribution of club endings was analyzed. To cause precocious ingrowth, ears and nVIII ganglia from older animals (just preceding axon formation) were unilaterally grafted in place of prospective ear/ganglion placodes of younger embryos. The opposite heterochronic transplantation was performed to delay ingrowth. LM and EM examination of embryos after surgery confirmed that synaptogenesis could be made to occur earlier or later than normal. In addition to this experimental work, the normal development of nVIII and the M-cell at these early stages was studied electron microscopically. Mature 21 mm larvae were examined to determine the localization of the club endings. In animals whose nerves had formed early, detailed EM mapping of club endings onto reconstructed M-cells revealed a distribution indistinguishable from that of the control cells. The distribution of club endings in the animals whose nerve had entered late was more variable, but no large or consistent shifts occurred in the location of the synapses, and the largest group of animals had distributions indistinguishable from the control cells. Thus synaptogenesis initiated on M-cells that are either younger or older than normal doses not alter the eventual localization of the synapses. Together with the results of David and Model, these results conflict with the spatiotemporal model and require that the nVIII axons are either restricted to a certain region of neuropil or are capable of recognizing a specific region of the M-cell surface.