Lateral line placodes are induced during neurulation in the axolotl

In order to determine the time window for induction of lateral line placodes in the axolotl, we performed two series of heterotopic and isochronic transplantations from pigmented to albino embryos at different stages of embryogenesis 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 (stages 13-27). Whereas grafts from early neurulae typically differentiated only 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 and tailbud stages (stages 13-35). Normal lateral lines containing pigmented neuromasts formed in most embryos when grafts were performed prior to early tailbud stages (stage 24) but not when they were performed later. Our findings 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 decline of inducibility at mid tailbud stages is mainly due to the loss of ectodermal competence.