Fig. 8.

A model for neurogenesis by multipotent stem cellsin vitro. Our working hypothesis is that multipotent stem cells produce neurons in four steps during which neuron number and differentiation are regulated. Both EGF and bFGF induce multipotent stem cells to proliferate. Basic FGF can stimulate committed neuronal precursors (e.g., neuron/astrocyte or neuron only), which are derived from multipotent stem cells, to produce neurons. In either case, insulin may act to recruit multipotential or bipotential precursors to a postmitotic neuronal fate. Subsequently, two possible models (lineage model in italic type and birthdate model in bold type) may serve to explain how IGF-I and BDNF act to induce the differentiation of postmitotic neuronal precursors. See Discussion for details of the two models.