Regional identity is established before gastrulation in the Xenopus embryo

Regional differences in cell recognition properties along the animal-vegetal axis of Xenopus laevis embryos were investigated by using an in vitro cell sorting assay. Dissociated cells were obtained from defined regions of blastula- and early gastrula-stage embryos. Binary combinations of cells from different regions, or from the same region at different ages, were aggregated in stationary culture. Labelling of one population in each pair with a cell-autonomous dye allowed the degree of sorting out in the resulting aggregates to be scored. In combinations of cells from animal caps (prospective ectoderm) and vegetal masses (prospective endoderm), sorting was detectable at the equivalent of late blastula stage and increased with developmental age to the gastrula stage. In addition, cells from the same region but different stages sorted from each other, indicating temporal changes in regional identity. Marginal cells (prospective mesoderm) sorted strongly from vegetal cells but only weakly from animal cells. These results indicate the presence of regional identity in the form of specific recognition properties in cells of the Xenopus blastula. We suggest that these properties act to establish and maintain coherent cell populations, corresponding to the primary germ layers, prior to gastrulation. These results account for the gradual restriction in developmental capacity of blastomeres seen previously in single-cell transplantation experiments from this laboratory.