Cells remain competent to respond to mesoderm-inducing signals present during gastrulation in Xenopus laevis

During gastrulation, the vertebrate embryo is patterned and shaped by compl ex signaling pathways and morphogenetic movements. One of the first regions defined during gastrulation is the prospective notochord, which exhibits s pecific cell behaviors that drive the extension of the embryonic axis. To e xamine the signals involved in notochord formation in Xenopus laevis and th e competence of cells to respond to these signals, we performed cell transp lantation experiments during gastrulation. Labeled cells from the prospecti ve notochord, semitic mesoderm, ventrolateral mesoderm, neural ectoderm, an d epidermis, between stages 9 (pregastrulation) and 12 (late gastrulation), were grafted into the prospective notochord region of the early gastrula. We show that cells from each region are competent to respond to notochord-i nducing signals and differentiate into notochordal tissue. Cells from the p rospective neural ectoderm are the most responsive to notochord-inducing si gnals, whereas cells from the ventrolateral and epidermal regions are the l east responsive. We show that at the end of gastrulation, while transplante d cells lose their competence to form notochord, they remain competent to f orm somites. These results demonstrate that at the end of gastrulation cell fates are not restricted within germ layers. To determine whether notochor d-inducing signals are present throughout gastrulation, grafts were made in to progressively older host embryos. We found that regardless of the age of the host, grafted cells from each region give rise to notochordal tissue. This indicates that notochord-inducing signals are present throughout gastr ulation and that these signals overlap with somite-inducing signals at the end of gastrulation. We conclude that it is the change of competence that r estricts cells to specific tissues rather than the regulation of the induci ng signals. (C) 2000 Academic Press.