Ontogeny of medullary raphe nuclei in the rabbit brain stem: a Golgi study.

Nuclei raphe obscurus, pallidus, and magnus, found in the midline and paramedian regions of the medulla, were examined in rabbits between day 22 of gestation and adulthood. At day 22 of gestation (total gestation period of 32 days), the neurons were small with sparsely branched processes. Between day 26 of gestation and 6 days of age, the dendrites expanded, increased the number and length of their branches, and developed abundant spines. During this period, the cell bodies grew in size. From postnatal day 6 to adulthood, a mature pattern of dendritic branching was achieved, and the number of spines on the dendrites was reduced. Nuclei raphe obscurus, pallidus and magnus each possessed cells with unique morphological appearances early in ontogeny, but showed a similar pattern of enhanced dendritic branching with numerous spines through early neonatal development, followed by a reduction in the number and size of the spines. Despite the morphological heterogeneity of the medullary raphe nuclei, their general pattern of dendritic development is similar, suggesting that afferent connections to these nuclei may be formed during cellular maturation by a process similar to that described for the reticular formation and other central neurons.