ORIGIN OF THE AVIAN NEURAL CREST

Neural crest cells are derived from a population of multipotent stem cells within the neural tube, They emerge shortly after neural tube closure, migrate extensively in the embryo and localize in numerous sites, where they differentiate into neurons and glia of the peripheral nervous system, cartilage and bone of the face, melanocytes and various other cell types, This review summarizes recent experiments from our laboratory delineating the origin and lineage of avian neural crest cells, Neural crest cells arise from the ectoderm, which also gives rise to presumptive epidermal, placodal and neural tube cells, Fate mapping experiments have demon strated that the neural crest arises at the juncture between presumptive epidermis and the neural plate, Inductive interactions between these two early tissues can generate neural crest cells, suggesting that signals travel through the epidermis to generate neural crest cells prior to neural tube closure. Injection of lineage tracer into individual cells reveals that a single neural fold can form all ectodermal derivatives (i,e,, epidermis, neural tube, neural crest). Even after neural tube closure, neuroepithelial cells have the capacity to form multiple neural crest and neural tube derivatives, including both dorsal and ventral phenotypes, suggesting that neural tube and neural crest cells share a common precursor, Further evidence that neural crest and neural tube cells are intimately related comes from experiments in which the cranial neural folds are ablated, The remaining neural tube cells have the capacity to regulate, at least for a limited time, to compensate for missing neural crest cells, These experiments suggest that the early neuroepithelium has no clear segregation with respect to the neural tube or neural crest, With time, dorsalizing and ventralizing signals may cause neural tube cells to acquire specific cell fates.