Fine structure of growth cones in medullary raphe nuclei in the postnatal cat

Morphological aspects of the dynamic processes of growth cone formation and synaptogenesis have been studied in neonatal kitten (2-17 days) medullary raphe nuclei. The formation and elaboration of dendritic growth cones and primary dendritic trunks is actively taking place on the medium size class neurons (stellates) of these nuclei. The dendritic growth cones are morphologically distinctive due to their population of large dense-core vesicles and postsynaptic position. Another growth cone morphology, interpreted as axonal, is also described. This growth cone is typically found in close association or synaptic contact with the dendritic growth cones and contains, in addition to synaptic vesicles, a dense-core vesicle population distinguishable from that of the dendritic growth cone by the presence of a variety of vesicles containing an eccentrically positioned dense particle. No evidence of axo-axonic or dendrodendritic synapses has been found. Synaptogenesis was found to be occurring on somas, dendrites and dendritic growth cones throughout the medullary raphe nuclei, though this phenomenon was more apparent in indistinctly localized subnuclear spaces termed synaptogenic zones. Within these zones large class neurons are found to have greater densities of both axodendritic and axosomatic synapses than medium and small class neurons respectively. Axodendritic synaptic densities on primary and secondary dendrites of large and medium class neurons are greater than their respective axosomatic synapse densities, which may suggest that the latter forms at a later period of development.