Migration and differentiation of neural crest cells and their derivatives: in vivo and in vitro studies on the early development of the avian peripheral nervous system.

After a period of extensive migration through the vertebrate embryo, neural crest cells differentiate into a great variety of cell types, including all the elements of the peripheral nervous system. We have studied crest cell migration in quail-chick chimeras in which quail cells can be identified by means of a stable natural nuclear marker. The results of interspecific grafts of neural primordium, performed systematically at different levels of the neuraxis, have established the sites of origin of the principal peripheral ganglia. In addition, they suggest that the cholinergic and adrenergic phenotypes are not predetermined in the neural crest before migration, but are the result of multiple cellular interactions. Furthermore, the phenotype expression of young differentiating autonomic ganglia is labile and can be modified if the latter are subjected to an appropriate cellular environment by grafting into a younger host embryo. The results of experiments in which fragments of neural crest, sensory and autonomic ganglia were transplanted are presented in terms of a model of crest cell-line segregation. As a step towards the analysis of the cellular interactions occurring during autonomic neuron differentiation, we have also studied neuronal development in tissue cultures of neural crest, taken from the cranial and trunk levels of quail embryos and grown in the presence or absence of other embryonic tissues. The results confirm that both levels of the crest are potentially able to give rise to cells that can make acetylcholine and catecholamines. However, whereas acetylcholine-synthesizing ability is apparently a very early feature of autonomic neuron precursors, the ability to produce catecholamines is acquired later as a result of interactions with other cell types, in particular with mesenchymal derivatives. Although production of both neurotransmitters can be considerably stimulated by associating crest with any of several young embryonic rudiments, only when trunk crest is cultured with the sclerotomal moiety of the somite is biochemical differentiation accompanied by extensive morphological and cytochemical neuronal maturation.