Lateral line placodes are induced during neurulation in the axolotl

In order to determine the time window for induction of lateral line placode s in the axolotl, we performed two series of heterotopic and isochronic tra nsplantations from pigmented to albino embryos at different stages of embry ogenesis and assessed the distribution of pigmented neuromasts in the hosts at later stages. First, ectoderm from the prospective placodal region was transplanted to the belly between early neurula and mid tailbud stages (sta ges 13-27). Whereas grafts from early neurulae typically differentiated onl y into epidermis, grafts from late neural fold stages on reliably resulted in differentiation of ectopic pigmented neuromasts. Second, belly ectoderm was transplanted to the prospective placodal region between early neurula a nd tailbud stages (stages 13-35). Normal lateral lines containing pigmented neuromasts formed in most embryos when grafts were performed prior to earl y tailbud stages (stage 24) but not when they were performed later. Our fin dings indicate that lateral line placodes, from which neuromasts originate, are already determined at late neural fold stages (first series of grafts) but are inducible until early tailbud stages (second series of grafts). A further series of heterochronic transplantations demonstrated that the decl ine of inducibility at mid tailbud stages is mainly due to the loss of ecto dermal competence. (C) 2001 Academic Press.