Genesis of the primate neostriatum: [ 3H]thymidine autoradiographic analysis of the time of neuron origin in the rhesus monkey

Abstract

The time of neuron origin and the spatio-temporal pattern of neuronal distribution in the caudate nucleus and putamen were examined in autoradiograms from 2- to 5- month-old monkeys that were exposed to a single pulse of [ 3H]thymidine on various embryonic or early postnatal days. Heavily labeled neurons, representing cells that entered their last cell division at the time of [ 3H]thymidine injection, were found in both the caudate and putamen in the animals injected between embryonic days 36 and 80, but not in monkeys injected at earlier or later stages. A peak in neuronal proliferation occurs between embryonic days 43 and 50. Thus, all of the neurons that ultimately compose the neostriatum are generated during a 45-day period in the first half of gestation, which lasts 165 days in the Rhesus monkey. Two classes of neurons, distinguished by their size and morphology, were discernible in autoradiograms stained with toluidine blue: small neurons with maximum cell body diameters ranging from 13 to 22 μm and larger cells with maximum diameters of 25–40 μm. The larger cells are generated over the first 7–8 days of striatal neurogenesis, while small cells are produced throughout the entire period. In agreement with previous [ 3H]thymidine autoradiographic studies in rodents, no spatio-temporal gradient related to the time of neuron origin was found in the primate neostriatum. However, small neurons generated on a given day were usually arranged in scattered clusters which, in a single coronal section, consisted of 5 to over 40 labeled neurons. These isochronously generated cell aggregates may represent a basic structural unit of the neostriatum.