Distribution and identity of the earliest proliferating progeny of colony-forming cells in regenerating murine spleen and bone marrow.

A study was made of the sites of development and the types of cells found in very early hemopoietic colonies in the mouse spleen. Two, 3, and 4 days after transplantation, the proliferating descendants of transplanted bone-marrow cells were identified on radioautographs of spleen sections and on spleen and bone-marrow smears of supralethally irradiated recipient mice which were injected with 3H-TdR at 12, 6, and 0.5 hours before sacrifice. Surprisingly the spleens of nontransplanted, irradiated mice contained proliferating medium and large lymphocytes in the white pulp which increased in numbers during the observation period. The early descendants of transplanted cells that lodged in the spleen could be clearly distinguished from the labeled indigenous cells because they formed discrete nodules or colonies beneath the splenic capsule or in the vicinity of venules and trabeculae of the red pulp. These cells were identifiable on day 2 as transitional cells or unknown hemopoietic blasts and on day 4 included early erythroid cells and small lymphocytes. There was evidence for the traffic of 3H-TdR-labeled cells through the splenic sinusoids.