Neurogenesis of the magnocellular basal forebrain nuclei in the rhesus monkey

The time of origin of the neurons that comprise the magnocellular basal forebrain nuclei in rhesus monkeys was determined by using 3H]thymidine autoradiography. Thirteen pregnant animals received an injection of 3H]thymidine between embryonic days 27 (E27) and E50 of their 165 day gestation, and their offspring were sacrificed during the early postnatal period. Neurons within this region were generated in a biphasic pattern. An initial burst of 3H]thymidine-labeled magnocellular neurons was first observed throughout short quiescent period, cells of the remaining anterior basal forebrain (inclusive of magnocellular neurons comprising the vertical limb of the diagonal band and the anteromedial and anterolateral regions of the nucleus basalis) were generated between E36 and E45 with a peak of neurogenesis seen on E40-E43. The intermediate division of the nucleus basalis was generated about the same time, but the peak period of neurogenesis in this region occurred slightly earlier (E36 and E40) and was completed by E43. During the second phase of neurogenesis, neurons within the posterior division of the basal forebrain were generated first, with their genesis virtually completed between E33 and E36. The genesis of all neurons comprising the magnocellular basal forebrain nuclei was completed by E48 of gestation. A general caudal to rostral gradient of neurogenesis was observed within this telencephalic region. In contrast, a neurogenic gradient was not discerned in the radial direction. The present data demonstrate that neurons comprising the basal forebrain magnocellular nuclei in monkeys are generated early in gestation with two peak times of neuronal genesis. These nuclei are among the earliest to be generated in the entire telencephalon, which, like neurons of the thalamus and cortical neurons giving rise to cortical-cortical connections, places them in a strategic position to potentially influence their target neurons within the cortical mantle that are generated later in gestation.