The six distinct laminae within the mammalian cerebral cortex contain neurons that exhibit a wide variety of specific physiological properties and synaptic connections. This diversity emerges from a restricted progenitor pool within the embryonic cortical ventricular zone. Individual cortical progenitors produce multiple subtypes over a prolonged period during corticogenesis. This article describes classical studies that suggest that neurogenesis in the cerebral cortex is dependent on asymmetric divisions, where one daughter remains in a progenitor state while the other exits to become a mature neuron. The present understanding of the molecular mechanisms regulating both asymmetric cell division and the sequential production of different neuronal subtypes during development is reviewed. However, as yet only a subset of the factors controlling each of these two events has been identified. This suggests that present genetic approaches will soon significantly extend our understanding of these developmental processes.
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