Fates of the blastomeres of the 32-cell-stage Xenopus embryo.

A detailed fate map of all of the progeny derived from each of the blastomeres of the 32-cell-stage South African clawed frog embryo (Xenopus laevis), which were selected for stereotypic cleavages, is presented. Individual blastomeres were injected with horseradish peroxidase and all of their descendants in the late tailbud embryo (stages 32 to 34) were identified after histochemical processing of serial tissue sections and whole-mount preparations. The progeny of each blastomere were distributed characteristically, both in phenotype and location. Most organs were populated largely by the descendants of particular sets of blastomeres, the progeny of each often being restricted to defined spatial addresses. Thus, the descendants of any one blastomere were distinct and predictable when embryos were preselected for stereotypic cleavages. However, variations among embryos were common and the frequencies with which one may expect organs to contain progeny from any particular blastomere are reported. The differences in the fates of the 16-cell-stage blastomeres and their 32-cell-stage daughter blastomeres are outlined and can be grouped into three general categories. The two daughter cells may give rise to equal numbers of cells in a particular organ, one daughter cell may give rise to many more of the cells in an organ derived from the mother blastomere, or one daughter cell may give rise to all of the progeny in an organ derived from the mother blastomere. Thus, cell fates are segregated during cleavage stages in both symmetric and asymmetric manners, and the lineages exhibit a diversification mode (G. S. Stent, 1985, Philos. Trans R. Soc. London Ser. B 312, 3-19) of cell division.