Early Neural Specification by the Planar Signal in Xenopus laevis Development

Abstract

Neural differentiation in amphibian embryos had been thought to start after gastrulation because of vertical neural induction from the underlying dorsal involuting marginal zone (DIMZ). However, recent studies show that another mode of induction, namely the planar induction, also directs neural differentiation in Xenopus laevis. Here, we attempted to specify when the planar induction occurs. From middle blastula to early gastrula stages, explants that included animal cap (AC) and dorsal noninvoluting marginal zone (DNIMZ), but not DIMZ were prepared. Neural differentiation was detected in such explants (named AC-DNIMZ explants) from the early gastrula (St. 10 ) but not from the late blastula (St. 9) or earlier embryos, indicating that the planar induction occurs at about St. 10 . To assess the further effect of planar induction on neural differentiation, Keller sandwiches were prepared from St. 10 embryos, and cultured for 2∼6 hr in vitro, after which the explants were separated into AC-DNIMZ and DIMZ parts. AC-DNIMZ part differentiated a little larger neural tissues, suggesting that the planar neural induction during the gastrula stages promote neural differentiation. Neural tissues in these explants could be detected by histology and immunohistochemistry using a monoclonal antibody specific for neural tissues, as opposed to earlier studies that indicated the planar induction to be insufficent for inducing morphological neural structures.