A critical period for conversion of ectodermal cells to a neural crest fate

Previously, we found that interactions between neural and nonneural ectoder m can generate neural crest cells, with both the ectodermal and the neuroep ithelial cells contributing to induced population (M. A. J. Selleck and M. Bronner-Fraser 1995, Development 121, 525-538). To further characterize the ability of ectodermal cells to form neural crest, we have challenged their normal fate by transplanting them into the neural tube. To ensure that the ectoderm was from nonneural regions, we utilized extraembryonic ectoderm ( the proamnion) and transplanted it into the presumptive midbrain of 1.5-day -old chick embryos. We observed that the grafted ectoderm has the capacity to adopt a neural crest fate, responding within a few hours of surgery by t urning on neural crest markers HNK-1 and Slug. However, the competence of t he ectoderm to respond to neural crest-inducing signals is time limited, de clining rapidly in donors older than the 10-somite stage. Similarly, the in ductive capacity of the host midbrain declines in a time-dependent fashion. Our results show that extraembryonic ectoderm has the capacity to form neu ral crest cells given proper inducing signals, expressing both morphologica l and molecular markers characteristic of neural crest cells. (C) 2000 Acad emic Press.