The Fate of Transfused H³ Thymidine‐labeled Bone‐marrow Cells in Irradiated Recipients*

It has been established by chromosome marker studies, histochemical, immunological, and cytological methods that the hemopoietic tissues of animals exposed to lethal doses of whole-body irradiation can be repopulated by transfused autologous, homologous or heterologous bone marrow cells. However, the morphology of the cell responsible for the regeneration of hematopoietic activity in the various hemopoietic organs has not been identified. It has been shown that the bone marrow contains the cell or cells capable of regenerating all types of hemopoietic tissues. In order to identify transfused cells, one must have a label which persists through successive divisions. Odell and Smith labeled the donors with S35 methionin and were thus able to follow the accumulation of the donor marrow cells in the recipients lungs and subsequently their releases to the bone marrow and spleen. However, this compound has a relatively rapid turnover in the labeled cells and thus a relatively limited capability of serial studies to observe mitosis and differentiation. Tritiated thymidine is ideal for this purpose since it is incorporated solely into DNA and is diluted only by mitosis. In addition the high resolution with tritium makes it certain that one is observing nuclear labeling. Bond et al. have studied the migration of labeled cells from a non-irradiated parabiotic rat to the irradiated member. Nagal and Knisely have studied the fate of in vitro labeled autologous and homologous bone marrow cells after transfusion into two patients. Balner et al have labeled bone marrow cells in vivo with H3TDR and transfused these labeled cells into irradiated homologous newborn mice and studied their distribution as a function of time following transfusion by [unintelligible] of autoradiography.