Regulation of primary erythropoiesis in the ventral mesoderm of Xenopus gastrula embryo: evidence for the expression of a stimulatory factor(s) in animal pole tissue.

We have previously shown that the animal pole tissue from a st.10+ early gastrula Xenopus embryo stimulates the primary differentiation of erythrocytes in the ventral mesoderm in combination culture. To characterize the nature of this stimulation, various sizes and different portions of animal pole tissue were combined with the ventral mesoderm explants. The erythrocyte differentiation in terms of globin expression depended on the size of the animal pole tissue that was combined with the ventral mesoderm. No difference was observed in the potency of stimulation between the ventral and dorsal halves of animal pole tissue. We also found that animal pole tissue from as late as st.7 failed to stimulate both mRNA and protein levels of globin in the explant. Histological studies of the combination explant with st.7 animal pole tissue showed that epidermis, vesicle structure, and blood-cell-like cells developed in the explant, but very few blood cells expressed globin molecules. However, the stimulation of erythroid differentiation was restored if total (20 ng) or poly(A)+ (0.2 ng) RNA from st.10+ animal pole tissue was previously injected at the 2-cell stage and the resulting animal pole tissue at st.7 was combined with st.10+ ventral mesoderm. Erythroid differentiation was also restored by injection with 1 ng of Xenopus bone morphogenetic protein-4 (XBMP-4) RNA. The effect of an extremely small dose of poly(A)+ RNA on erythroid differentiation suggests that in addition to XBMP-4 there exist substances, expressed later than st.7 in the animal pole region, which can stimulate erythrocyte differentiation in the ventral mesoderm.