Cellular composition of erythropoietic cell populations and aggregate cell cultures derived from early chick blastodiscs.

Light and electron microscopy of suspensions of cells prepared by dispersing chick blastodiscs at primitive-streak and head-fold stages showed the presence of numerous yolk granules, yolk-rich endodermal cells and occasional presumed ecto- and mesodermal cells. Several cell fractions prepared from this suspension by sedimentation through discontinuous Ficoll gradients were of similar composition. No enrichment of any particular cell type which might account for either differential sedimentation or erythropoietic potential of the fractions could be recognized. Two fractions, EP 1, and EP 2, were cultured as cell aggregates on vitelline membranes. EP 1 produced highly organized blood islands containing developing erythrocyte cells, organized endothelium, fibroblasts, thrombocytes and occasional granulocytes. Blood islands derived from EP 2, on the other hand, contained essentially only aggregates of erythroblasts embedded in endoderm. It is tentatively suggested that EP1 contains young multipotential hematopoietic precursors while EP 2 has only older blood-cell precursors committed to erythrocyte development. No cellular basis for the resolution of EP 1 into two complementary subfractions could be recognized.