Hematopoietic progenitors are retained in the yolk sac despite circulation.

Numbers of BFU-E, CFU-E, CFU-mac, and HPP-CFC hematopoietic progenitors and erythroblasts in the yolk sac versus the embryo proper were determined. Fold enrichment was calculated as the ratio of progenitors in yolk sac to embryo proper divided by the ratio of erythroblasts in the yolk sac to embryo proper. If progenitors circulated as freely as erythroblasts, the expected enrichment would have been 1. However, significantly more of all 3 types of progenitors were found in the yolk sac than would be expected, even as the fetal liver was becoming hematopoietic (E10.5). Three to 4 embryos were pooled for each experiment, and 3 to 6 independent experiments were performed for each time point. Error bars indicate SEM.