An autoradiographic study of the postnatal development of sensorimotor and visual components of the corticopontine system.

Standard autoradiographic tracing methods which employed intracortical injections of 3H-leucine were used to describe the time course of development in those components of the corticopontine system which originate in sensorimotor (SM) or visual (VIS) cortices. These studies were performed using male or female pigmented rats ranging in age from 1 to 24 days (birth = day 0). Labeled axons could easily be recognized within the cerebral peduncle at the level of the basilar pons on postnatal day 1 following either an SM or a VIS cortical injection. Labeling over the basilar pontine neuropil suggestive of axonal ingrowth could also be seen on day 1 following SM injections but not until day 3 in cases with VIS injections. During the latter part of the first postnatal week, both SM and VIS axon terminal fields were established in the pontine neuropil in locations generally similar to those described in the adult. However, at this time they occupied a larger area and appeared more diffusely organized than in the adult. Subsequently during the second postnatal week, both SM and VIS projection fields gradually became more focused and circumscribed until by day 16 in the case of the SM system, and day 18 for the VIS projection, an adultlike configuration was attained. At no time during development was there any evidence of a more substantial contralateral projection than that described for the adult. Correlation of these events with previous Golgi observations and ongoing electron microscopic studies on pontine synaptogenesis suggest that the focusing or constriction of corticopontine axon terminal fields might be the result of a selective degeneration process in which the presynaptic (corticopontine axon) and postsynaptic (basilar pontine neurons) elements both play an active role. Furthermore, the absence of a stage in development characterized by an increase in the density of the contralateral component of the projection confirms the interpretation of earlier studies which suggested that the aberrant contralateral corticopontine projections observed following neonatal cortical lesions were the result of sprouting from the intact corticopontine system and not due to failure of retraction in one component of a system which in fact distributes bilaterally for a time during development.