Induction of hemopoiesis in rat embryonic bone transplanted into adult subcutaneous connective tissue.

Five pairs of immature, non-hemopoietic femur and tibia from 17-day-old gestating female rat fetuses, whose sex was determined by chromosomal analysis of liver cells, were transplanted into subcutaneous tissues of adult male rats. The original bones were about 3 mm in length and they grew to about 17 mm length at 4 wereks after transplantation. Bone deformation was not evident after transplantation and bone marrow hemopoiesis developed. Bone marrow cytohistologic observations were made on smears, and chromosome analyses were performed on bone marrow cells. Active erythro-, myelo- and megakaryopoiesis were conducted by cells of recipient adult rats. Sex chromosome analysis of cartilage cells from the epiphyses of transplanted bones demonstrated that the growing bones were composed of cells from the grafted embryo. The results thus strongly suggest that the transition of hemopoiesis from liver to bone marrow in late embryonic development is conducted by stem cells migrating through circulating blood and settling in the bone marrow and not by in situ cells differentiating in the bone marrow stroma.