Where do hematopoietic stem cells come from?

The experimental model constituted by a quail embryo grafted on a chick yolk sac has produced undisputable evidence according to which hematopoietic stem cells (HSC) colonizing the blood-forming rudiments are of intraembryonic origin in birds. Appropriate cell culture systems now make it feasible to demonstrate that ontogeny of the mouse hematopoietic system also involves at least two generations of HSC, one formed in the yolk sac and the other in the embryo, only the latter having lymphoid potential. Furthermore the developmental relationships between endothelial and hematopoietic cells are being analyzed in the avian model. Two distinct endothelial lineages are detected by means of interspecific transplantations, a dorsal one, somitic in origin, which is purely endothelial and a ventral one, splanchnopleural in origin, which is associated with the production of hematopoietic cells.