Gradients of neurogenesis in possum neocortex.

Gradients of neurogenesis in neocortex of the brushtailed possum were determined with [3H]thymidine autoradiography, using data taken from 16 possums injected with [3H]thymidine between postnatal (P) days 5-68, and allowed to survive until brain cytoarchitecture was mature. Gradients of neurogenesis shown in neocortex were: (a) an inside-out radial gradient within cortical areas with neurogenesis beginning around P5 in layer 6 and concluding between 1 and 2 months later in layer 2, and (b) a single, simple, regional gradient across all neocortex. On the lateral surface of the hemisphere neurogenesis runs anteroventrolateral to posterodorsomedial, and on the medial surface neurogenesis runs anteroventral to posterodorsal. When plotted for the whole neocortex, the two regional gradients from the lateral and medial surfaces of the hemisphere are seen as parts of the one simple pattern. The time duration of neurogenesis in possum cortex is 1-2 months, and is similar to that seen in monkeys.