Adult hemoglobins are synthesized in yolk sac microenvironment obtained from murine cultured blastocysts.

The capability of yolk sac (YS) hematopoietic stem cells to produce in vitro either primitive or definite erythrocytes under the influence of stimulating agents was analyzed. Two systems were used: either 9-day-old yolk sacs were explanted and cultured or 3-day-old blastocysts were cultured under conditions in which they give rise only to YS tissue. In both systems, YS tissue produced only primitive erythrocytes. When the stimulating agents (spleen cell-conditioned medium or erythropoietin) were added to the medium, YS cultures gave rise to definitive erythrocytes. These data confirm the intrinsic ability of the YS stem cells to undergo primitive erythropoiesis; furthermore, they provide evidence that YS hemopoietic progenitor cells do not require any direct interaction with cells of the embryo to undergo definitive erythropoiesis but only stimulation by agents, such as pokeweed mitogen spleen conditioned medium or erythropoietin; the YS microenvironment does not counteract this capacity.