Synapse formation and other ultrastructural features of postnatal development in the basilar pontine nuclei of the rat.

The postnatal development of the basilar pontine nuclei (BPN) was studied in the rat from birth through postnatal day 21. Although 3 maturational phases could be discerned within this time span, the events taking place within each phase overlap to some degree and the postnatal maturation of this region is best characterized as a continuum comprised of several simultaneously occurring processes. The early postnatal period (birth to postnatal day 5) is best described as a period of relative immaturity. Although generally similar to the adult BPN cytoarchitectonically, cell density in the early neonate is substantially higher than in the adult and this obscures to some degree the various subdivisions of the pontine gray. Neuronal somata are quite small in cross-sectional area relative to the adult and the neuropil is similarly immature and exhibits wide expanses of extracellular space and few synaptic boutons. These features are in marked contrast to the second developmental period which extends from postnatal days 6 through 13 and is characterized by an intense wave of synaptic proliferation. It is during this period that each of the various types of synaptic boutons that have been described in the adult make their appearance. Curiously, during this same time period, as well as in the early postnatal period, there also appears to be an ongoing process of spontaneous axon terminal elimination. The third developmental period begins on postnatal day 14 and extends through day 21 at which time the cytoarchitectonics, cytology and synaptic organization of the BPN are adult-like. During this period, no additional types of synaptic boutons appear, although the number of boutons continues to increase in a fairly uniform manner across all the various categories of synaptic boutons.