Numbers of neurons in the developing principal sensory nucleus of the trigeminal nerve: enhanced survival of early-generated neurons over late-generated neurons.

The overproduction and subsequent death of neurons is a common phenomenon in the developing vertebrate central nervous system (CNS). We tested the hypothesis that the survival of a neuronal subpopulation is related to its time of origin. Neuronal survival was examined in a well-defined CNS structure, the principal sensory nucleus of the trigeminal nerve (PSN) of the rat. The changes in the total number of PSN neurons and in the numbers of early- and late-generated neurons (i.e., neurons heavily labeled by a single injection of [3H] thymidine on G12 or G14, respectively), between gestational day (G) 16 and postnatal day (P) 10 were determined. The total number of neurons in the PSN rose prenatally to a maximum of 40,600 on G18.5. The increase in neuronal number correlates to the period of migration. More than half of the neurons that successfully migrated to the PSN were lost by P10. The patterns for the changes in the numbers of early- and late-generated neurons were similar; however, there were significant differences between the two subpopulations. The maximum number of early-generated neurons (4,250) was attained on G18.2 and subsequently 58.9% of these neurons were lost. In contrast, the maximum number of late-generated neurons (5,050) was attained on G20.0 and 66.6% of these neurons were lost by P10. Therefore, it appears that the survivability of early generated neurons is greater than for late-generated neurons. This enhanced survivability presumably results from a competitive advantage that early-generated neurons have for forming synapses or gaining access to trophic factor(s) that are in limited supply.