In vitro analysis of epiblast tissue potency for hematopoietic cell differentiation.

In murine embryogenesis, all cells that will constitute the embryonic structures originate from the epiblast (primitive ectoderm) tissue, the epithelial cell sheet of the gastrulating embryo. The cells of this tissue are totipotent at the beginning of gastrulation, but at the end of this period are specified to particular cell lineages. Thus, it is likely that during murine gastrulation, the potency of epiblast cells that were originally totipotent becomes restricted as development progresses. However, the mechanisms of this process are unknown. We have investigated this process in vitro, focusing on the hematopoietic cell lineage. To detect the hematogenic potency of the epiblast tissue, we established an in vitro culture system in which the hematopoietic cell differentiation of the epiblast tissue was supported by a stromal cell layer. With this culture system, we investigated the process by which this potency becomes spatially and temporally restricted during gastrulation. The results showed that hematogenic potency resides in the entire epiblast of the early- to mid-gastrulating embryo, but becomes restricted to the posterior half of the epiblast at the headfold stage. Furthermore, we showed that this process is altered by exogenous bone morphogenetic protein-4 (BMP-4) or activin A, which may be mesoderm inducers in Xenopus embryogenesis.