Analysis of the neural crest ventral pathway using injected tracer cells

Abstract

Cloned neural crest melanocytes and nonneural crest cells were introduced into chick embryos to study questions of neural crest migration and localization. Previously we showed that cloned quail melanocytes injected directly into the newly formed somites of 2.5-day-old chick embryos were initially contained within the somitic lumen ( Bronner and Cohen, 1979); following release from the somites, the melanocytes migrated along the ventral neural crest pathway. Here we demonstrate that the migratory pattern of melanocytes freshly isolated from the skin of 9- to 11-day-old quail embryos is identical to that of the cloned quail melanocytes grown in vitro for 8 days or more. Like cloned melanocytes, quail skin melanocytes migrated beyond the dorsal root and sympathetic chain ganglia and localized in the vicinity of the adrenal gland, aortic plexus, metanephric mesenchyme, gonads, and gut. This suggests that the choice of the ventral migratory route is not simply a consequence of prolonged stay in tissue culture. Nor is the choice of the ventral pathway a consequence of introducing cells of one species into the environment of another since melanocytes derived from chick neural crest cells migrated analogously to the quail melanocytes. The ventral pathway appears to be selective for neural crest-derived cells since somite cells and single fibroblast cells did not migrate ventrally when placed into the embryo. Quail somite cells remained associated with the somitic mesenchyme and single fibroblast cells migrated under the ectoderm. Injected quail fibroblasts when clumped, however, were found on the ventral pathway indicating that the ventral route is not specific for crest cells and their derivatives. Cloned quail melanocytes injected into older developmental levels of the neural axis did not migrate as far ventrally as those implanted in younger levels where crest cells were only beginning to migrate. This demonstrates that changes in the environment that occur as a function of developmental age limit the extent of ventral migration of injected melanocytes.