The total number, time or origin and kinetics of proliferation of neurons comprising the deep cerebellar nuclei in the rhesus monkey.

The genesis of the neurons that form the cerebellar nuclei was studied by autoradiographic methods in 30 postnatal rhesus monkeys which were exposed to 3H-thymidine at various embryonic (E) and postnatal (P) ages. As a basis for this quantitative analysis, five 2-3 month old monkeys were used for cell counting and estimation of the total number of neurons in each of the cerebellar nuclei. The results show that the cerebellar nuclei on each side contain 131,000 neurons. There are 68,000 neurons in the dentate nucleus, 25,000 neurons in the posterior interposed nucleus, and 19,000 neurons in both the anterior interposed the fastigial nuclei. All of the neurons comprising the deep nuclei are generated during the first half of the 165 days gestation period in this species. Although neurogenesis lasts from E30 through E70, approximately 81% of the neuron population is generated during a one week period between E36 and E40, with the peak of proliferation occurring at E36. Before E45 both large (maximum diameter greater than 35 micrometers) and small (maximum diameter 35 micrometers or less) neurons are produced simultaneously; after this period only small neurons are generated. Although no clearcut spatio-temporal gradients of neurogenesis could be discerned along any of the cardinal axes, each cerebellar nucleus has a somewhat distinctive developmental history in terms of the onset and cessation of neurogenesis and the tempo of cell proliferation. Thus, genesis of neurons destined for the dentate nucleus begins earlier and ends later than proliferation of the neurons that ultimately comprise the fastigial nucleus. Generation of the neurons destined for the anterior and posterior interposed nuclei follows an intermediate time course. The present data on neurogenetic sequences in the deep nuclei could not be correlated with the zonal pattern of reciprocal axonal connections that link the deep nuclei and overlying cerebellar cortex.