Analysis of human blood cell development in the SCID-hu mouse, an in vivo replica of the human fetal hematopoietic system.

Human fetal blood tissues implanted in the SCID immuno-deficient mouse are tolerated and develop considerably. The bone marrow maintains its autonomous hematopoietic function for several months. Conversely, thymic lymphopoiesis can continue over the long term only in the presence of an appropriate source of stem cells. This in vivo replica of the human blood system can be used to test the hematogenic abilities of candidate populations of human hematopoietic stem cells, as suggested by the hematogenic reconstitution of human thymus and marrow in SCID mice obtained from purified CD34+ precursors. In the same model, hematogenic activity of CD34+ cells was limited to the sub-population of these precursors coexpressing the antigen Thy-1. These results, combined with those of in vitro analyses, suggest that the population of CD34+ Thy-1+ components of fetal liver and of the bone marrow is extremely rich in human hematopoietic stem cells.