Development of neurogenic placodes in Xenopus laevis

The development of neurogenic placodes in Xenopus laevis from the time of n eural fold closure to larval stages is described. Placodes were reconstruct ed from camera lucida drawings of serial sections, and the spatiotemporal p attern of placodal neurogenesis was analyzed using in situ hybridization fo r the genes X-NGNR-1,XNeuroD, X-MyT1, and X-Delta-1, all of which have been implicated in the regulation of neurogenesis. Olfactory, profundal, and tr igeminal placodes, a series of dorsolateral placodes (otic placode and five lateral line placodes), a series of epibranchial placodes, and two hypobra nchial placodes were identified. Earlier claims that all placodes in anuran s develop from a common primordium could not be confirmed. Profundal and tr igeminal placodes, however, are partially fused, and all lateral line placo des arise from a common precursor. Epibranchial and hypobranchial placodes develop ventral to other placodes and dorsal and ventral to the pharyngeal pouches, respectively. Hypobranchial placodes give rise to neurons that bec ome intimately associated with the developing heart. All neurogenic placode s strongly express the neuronal differentiation gene XNeuroD. The neuronal determination gene X-NGNR-1, however, is expressed strongly in only some pl acodes and not in dorsolateral placodes, indicating that neurogenesis in th e latter relies on other determination genes. X-Delta-1 is expressed not on ly in the neurogenic parts of the placodes but also in the primordia of the lateral lines. This suggests that Delta-Notch-mediated lateral inhibition may be involved not only in placodal neurogenesis, but also in the patterni ng of lateral line neuromasts. (C) 2000 Wiley-Liss, Inc.