WHERE DO HEMATOPOIETIC STEM-CELLS COME FROM

The experimental model constituted by a quail embryo grafted on a chic k yolk sac has produced undisputable evidence according to which hemat opoietic stem cells (HSC) colonizing the blood-forming rudiments are o f intraembryonic origin in birds. Appropriate cell culture systems now make it feasible to demonstrate that ontogeny of the mouse hematopoie tic system also involves at least two generations of HSC, one formed i n the yolk sac and the other in the embryo, only the latter having lym phoid potential. Furthermore the developmental relationships between e ndothelial and hematopoietic cells are being analyzed in the avian mod el. Two distinct endothelial lineages are detected by means of intersp ecific transplantations, a dorsal one, semitic in origin, which is pur ely endothelial and a ventral one, splanchnopleural in origin, which i s associated with the production of hematopoietic cells.