Colonization of trunk neural crest derivatives in avians follows a ventral to dorsal order beginning with sympathetic ganglia, Schwann cells, sensory ganglia and finally melanocytes. Continuous crest emigration underlies this process, which is accounted for by a progressive ventral to dorsal relocation of neural tube progenitors prior to departure. This causes a gradual narrowing of FoxD3, Sox9 and Snail2 expression domains in the dorsal tube that characterize the neural progenitors of the crest and these genes are no longer transcribed by the time melanoblasts begin emigrating. Consistently, the final localization of crest cells can be predicted from their relative ventrodorsal position within the premigratory domain or by their time of delamination. Thus, a dynamic spatiotemporal fate map of crest derivatives exists in the dorsal tube at flank levels of the axis with its midline region acting as a sink for the ordered ingression and departure of progenitors. Furthermore, discrete lineage analysis of the dorsal midline at progressive stages generated progeny in single rather than multiple derivatives, revealing early fate restrictions. Compatible with this notion, when early emigrating ;neural' progenitors were diverted into the lateral ;melanocytic' pathway, they still adopted neural traits, suggesting that initial fate acquisition is independent of the migratory environment and that the potential of crest cells prior to emigration is limited.
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