Cell interactions within nascent neural crest cell populations transiently promote death of neurogenic precursors

We have previously shown that cultured trunk neural crest cell populations irreversibly lose neurogenic ability when dispersal is prevented or delayed, while the ability to produce other crest derivatives is retained (Vogel, K. S. and Weston, J. A. (1988) Neuron 1, 569-577). Here, we show that when crest cells are prevented from dispersing, cell death is increased and neurogenesis is decreased in the population, as a result of high cell density. Control experiments to characterize the effects of high cell density on environmental conditions in culture suggest that reduced neurogenesis is the result of cell-cell interactions and not changes (conditioning or depletion) of the culture medium. Additionally, we show that the caspase inhibitor zVAD-fmk, which blocks developmentally regulated cell death, rescues the neurogenic ability of high density cultures, without any apparent effect on normal, low-density cultures. We conclude, therefore, that increased cell interaction at high cell densities results in the selective death of neurogenic precursors in the nascent crest population. Furthermore, we show that neurogenic cells in cultured crest cell populations that have dispersed immediately are not susceptible to contact-mediated death, even if they are subsequently cultured at high cell density. Since most early migrating avian crest cells express Notch1, and a subset expresses Delta1 (Wakamatsu, Y., Maynard, T. M. and Weston, J. A. (2000) Development 127, 2811-2821), we tested the possibility that the effects of cell contact were mediated by components of a Notch signaling pathway. We found that neurogenic precursors are eliminated when crest cells are co-cultured with exogenous Delta1-expressing cells immediately after they segregate from the neural tube, although not after they have previously dispersed. We conclude that early and prolonged cell interactions, mediated at least in part by Notch signaling, can regulate the survival of neurogenic cells within the nascent crest population. We suggest that a transient episode of cell contact-mediated death of neurogenic cells may serve to eliminate fate-restricted neurogenic cells that fail to disperse promptly in vivo.