Induction and development of neural crest in Xenopus laevis

Neural crest cells are a migratory embryonic cell population that form at t he border between the neural plate and the future epidermis. This border, t he neural plate border, corresponds to the neural fold. The neural fold sur rounds the entire neural plate, but only the lateral and posterior portions of the fold give rise to neural crest cells, while the anterior neural fol d differentiates as forebrain. This review focuses on neural crest developm ent in Xenopus laevis embryos, and analyzes aspects of the induction of the neural crest in Xenopus, summarizing available information relating to the expression of several genes in the neural crest. Two models for neural cre st induction are discussed. In the first model, the neural crest is induced by the interaction between the neural plate and the epidermis. In the seco nd, the specification of the neural plate border arises as a consequence of a gradient of BMP activity. The role of posteriorizing signals on neural c rest specification is also discussed. Finally, we propose that the specific ation and differentiation of the neural crest is controlled by a cascade of transcription factors, encoded and expressed from a hierarchy of 1 C, gene s. A set of extracellular signals establishes the positional information in the ectoderm, which activates Pre-pattern genes (Gli, Xiro, Zic, Dlx, etc. ) across extended and overlapping domains. A local combination of these gen es at the neural plate border activates the cascade of neural crest specifi cation, while different sets of genes are activated at both sides of the ne ural folds (in the epidermis and the neural plate). The genes activated in regions adjacent to the neural plate border have an inhibitory effect on th e neural crest transcription program.