Abstract

The time of origin and spatio-temporal pattern of neuron distribution in the nucleus accumbens septi and the lateral and medial septal nuclei were determined in the rhesus monkey. Autoradiographic analysis of 2 to 5 month old monkeys that had been exposed to a pulse of [ 3H]thymidine during embryonic or early postnatal days showed that neurons destined for the nucleus accumbens septi are generated over a period of approximately 50 days (between embryonic day 36 and 85 of the 165 day gestational period) whereas the neurons destined for the medial and lateral septal nuclei are produced for only 25 days (from embryonic day 36 to 62). The peak of proliferation of neurons destined for all three nuclei occurs synchronously around embryonic day 45. A number of small cells with round, densely stained nuclei were labeled in specimens injected after embryonic day 85; however, following combined light- and electron-microscopic analysis, these late generated cells were classified as glia. Neurons destined for the nucleus accumbens septi generated on different embryonic days did not exhibit spatio-temporal gradients in their distribution. In contrast, neurons destined to populate the lateral and medial septal nuclei displayed a prominent ‘outside-to-inside’ spatio-temporal gradient in which earlier generated neurons occupied positions closer to the pial surface and later forming neurons were positioned closer to the ventricular surface. Based on this evidence regarding the time span of neuronal production and the absence of spatio-temporal gradients, the mode of development of the nucleus accumbens septi more closely resembles the pattern of neurogenesis reported for the primate neostriatum than that in the adjacent septal nuclei. These findings indicate that the nucleus accumbens septi and neostriatum in primates may have a common embryological origin.

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