Development of neurogenic placodes in Xenopus laevis

The development of neurogenic placodes in Xenopus laevis from the time of neural fold closure to larval stages is described. Placodes were reconstructed from camera lucida drawings of serial sections, and the spatiotemporal pattern of placodal neurogenesis was analyzed using in situ hybridization for 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 trigeminal placodes, a series of dorsolateral placodes (otic placode and five lateral line placodes), a series of epibranchial placodes, and two hypobranchial placodes were identified. Earlier claims that all placodes in anurans develop from a common primordium could not be confirmed. Profundal and trigeminal placodes, however, are partially fused, and all lateral line placodes 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 become intimately associated with the developing heart. All neurogenic placodes strongly express the neuronal differentiation gene XNeuroD. The neuronal determination gene X-NGNR-1, however, is expressed strongly in only some placodes and not in dorsolateral placodes, indicating that neurogenesis in the latter relies on other determination genes. X-Delta-1 is expressed not only 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 patterning of lateral line neuromasts.