Autonomous mesoderm formation in blastocoelic roof explants from inverted Xenopus embryos.

Xenopus eggs, artificially fertilized, were prevented from undergoing equilibrium rotation by incubation in medium containing ficoll. Three orientations were selected: normal, with animal pole uppermost; inverted, with vegetal pole away from gravity; and an off-axis orientation, with embryos tilted approximately 90 degrees from the animal-vegetal axis. At blastula stage 8, cells forming the blastocoelic roof were cultured in isolation as explants. These cells are normally fated to from epidermis ventrally and neural derivatives dorsally. Unexpectedly, in the fragments originating from inverted or 90 degrees-off-axis embryos, axial structures were found: notochord, somites, neural cells, cement glands, and sometimes sensory organs. Inverted eggs could be exploited in studies of mesodermal specification.