Paraxial-fated mesoderm is required for neural crest induction in Xenopus embryos

Neural crest induction is thought to occur by a two-step process. Axially fated mesoderm induces neural plate, which is then recruited to neural crest by nonneural epidermal ectoderm at the neural plate border. This model suggests a rather indirect role for mesoderm in inducing neural crest. We extensively examined the role of mesoderm in neural crest induction by determining which types of mesoderm induce neural crest cells in Xenopus embryos. We found that noggin-dorsalized ventral marginal zone (VMZ) explants differentiate as melanocytes in the absence of axial mesoderm. Dorsalized VMZ is also a potent inducer of melanocytes when juxtaposed to animal cap ectoderm in recombinant explants. Dorsalized VMZ is analogous to the dorsal-lateral marginal zone (DLMZ) region of the embryo. Neural crest-inducing activities of gastrula stage DLMZ and dorsal marginal zone (DMZ) were also compared in recombinant explants. DLMZ was a stronger inducer of neural crest than was DMZ; DLMZ induced high levels of XSlug expression and melanocyte formation in recombinants, whereas DMZ weakly induced neural crest. In whole embryos lacking DLMZ, XSlug expression and melanocyte formation were significantly reduced; in contrast, no significant reduction of XSlug expression or melanocyte formation was seen in embryos lacking a DMZ. These results suggest that paraxial-fated mesoderm plays a central role in neural crest formation by inducing a novel type of lateral neural plate. This lateral neural plate is then recruited to neural crest by adjacent nonneural epidermal ectoderm.