Muscle specification in the Xenopus laevis gastrula-stage embryo.

Recent fate maps of the Xenopus laevis gastrula show that mesodermal tissue surrounding the blastopore gives rise to muscle (Keller [1991] Methods Cell Biol 36:61-113; Lane and Smith [1999] Development 126:423-434). In a significant deviation from earlier data, the new maps demonstrate that cells in the ventral half of the gastrula are precursors to a significant portion of trunk somites. However, these posterior somites are not formed until tadpole stages (stages 38-44). We therefore set out to determine the timing of muscle specification within the ventral half of the gastrula. Our approach was to generate a series of tissue explants from gastrula-stage embryos and then culture them to either stage 28 (tailbud) or stage 44 (tadpole). At each endpoint, the presence of muscle in explants was assessed with a muscle-specific antibody. Interestingly, we found that muscle tissue is detected in ventral explants. However, these explants must be cultured to the tadpole stage. This is perhaps not unexpected, as this is the point at which this tissue normally gives rise to muscle. We further show that muscle specification of the involuting marginal zone does not change over the course of gastrulation. Together, these results suggest that dorsalizing signals emanating from the midline during gastrulation are not necessary for muscle specification of the ventral half of the involuting marginal zone.