Alteration of the anterior-posterior embryonic axis: the pattern of gastrulation in macrocephalic frog embryos.

The production of an enlarged head or macrocephaly in frog embryos can be achieved by interspecific hybridization or by injection of the contents of the germinal vesicle (GV), the large nucleus of immature oocytes, into the blastocoels of embryos before they gastrulate. The macrocephalic embryos have large suckers and their neural tubes are larger anteriorly but smaller posteriorly as compared to controls. This abnormal syndrome has previously been thought to arise as a result of an axial structure determinant present in the germinal vesicle. When examined during gastrulation, however, Xenopus laevis and Rana pipiens macrocephalic embryos produced by GV injection as well as macrocephalic embryos produced by the hybrid cross, Rana septentrionalis female X Rana catesbeiana male, all exhibit alterations in the pattern of gastrulation. The most striking of these alterations is the persistence throughout gastrulation of a thick blastocoel roof composed of many cell layers, suggesting that there is an inhibition of posterior spreading of the roof normally associated with epiboly. In R. pipiens, the dorsal mesodermal mantle of GV-injected gastrulae is thicker as compared to controls, accounting for a neural plate which is wide at the anterior end. Vital dye mapping experiments on Xenopus laevis embryos show that dye marks placed on regions normally fated to become trunk epidermis become localized anteriorly when the embryos are GV injected, consistent with the idea that ectodermal cells are inhibited from moving posteriorly. These results indicate that the macrocephalic syndrome can be attributed to a localized inhibition of cell rearrangements during gastrulation as opposed to the effects of altered inducers or to axial determinants.