Neurogenesis in septum, amygdala and hippocampus in the marsupial brushtailed possum (Trichosurus vulpecula).

The times of origin of neurons in the septum, amygdala and hippocampus of the marsupial brushtailed possum, Trichosurus vulpecula, were determined with 3H thymidine autoradiography. The long time period for neurogenesis in the brushtailed possum facilitated analysis of neurogenetic gradients in the brain. A series of 20 possums were injected with 3H thymidine from postnatal (P) days 5-95 and were allowed to survive until brain cytoarchitecture was mature. Our results indicate that septal neurogenesis was complete by P21 (38 days after conception) with a medial to lateral gradient of neurogenesis evident in the lateral septal division. Neurogenesis in the amygdala was complete in the basal, central and medial amygdaloid nuclei by P21, and in the lateral amygdaloid nucleus by P46, with a medial to lateral gradient of neurogenesis evident in basal, central and lateral amygdaloid nuclei. In the hippocampus, neurogenesis of pyramidal cells was complete in field CA3 by P50, and in field CA1 by P55. Early forming pyramidal cells (P5-12) were distributed adjacent to the stratum oriens in fields CA1 and CA3, with later forming pyramidal cells distributing in the middle of the stratum pyramidale (P21-32) and on the edge adjacent to the stratum radiatum (P46). Neurogenesis of dentate granule cells extended over a long period of time, from P5, at least until P82, with the earliest forming granule cells (P5-12) distributed adjacent to the stratum moleculare, and the latest forming granule cells (P82) adjacent to the hilus.