Studies on the Defective Haematopoietic Microenvironment of Sl/Sld Mice

Summary. Femurs and spleens of both Sl/Sld and +/+ origin were implanted into both Sl/Sld and +/+ hosts. Eight weeks later the +/+ grafts incorporated more radioactive iron than did the Sl/Sld ones. The number of colony forming units (CFU) in the marrow of +/+ femur grafts and in +/+ spleen grafts was several fold greater than that in Sl/Sld grafts. X‐irradiation of femur and spleen donors with 950 R reduced the number of CFU in the grafts to 40–60%, whereas grafts from donors exposed to 2500 R supported no CFU growth at all. The haematopoietic cells in the grafts after 6 weeks were almost entirely of host origin. These studies suggest that Sl/Sld mice have a congenital defect in some stromal elements of their haematopoietic tissues which are essential for the normal growth of CFU. These stromal factors are likely to be cellular, are less radiosensitive than are the CFU, and support the growth of CFU rather than actually differentiate into these cells. These stromal factors are present in both spleens and femurs of mice. The model described here appears to be a useful one for further characterizing stromal factors essential for supporting haematopoiesis.