The nervous system is composed of a complex network of interacting neurons. During development, distinct neuronal populations are generated by proliferating precursors according to highly reproducible spatial and temporal patterns. Traditional studies suggested that neurogenesis was primarily regulated by intrinsic cellular programs, unresponsive to extracellular growth factors or target organ influences. Using culture systems of dividing peripheral and central precursors, we have found that neurogenesis is indeed subject to regulation by extracellular signals, and that these molecules are population-specific. Extracellular factors control two distinct processes that contribute to proliferation. First, factors may stimulate cells to undergo another round of cell division (mitogenesis). Second, molecular signals may promote the survival of dividing precursors that would otherwise die (trophism). Thus, unexpectedly, trophic requirements occur far earlier than previously considered. Finally, recent studies indicate that dividing precursors possess long neuronal processes that extend into target regions. Consequently, dividing precursors may simultaneously be engaged in axonal pathfinding, providing an opportunity for distant target tissues to regulate the growth of the innervating population through mitogenic and trophic mechanisms.