The Effect of Bleeding on the Number of in Vitro Colony‐Forming Cells in the Bone Marrow

Summary: Bleeding 0.3 ml. daily for 3 days reduced the total number of in vitro colony‐forming cells in the bone marrow of C57Bl and BALB/c mice to less than 50 per cent of that in control mice, but caused a significant increase of erythropoietic cells in the bone marrow. Transfusion of packed red cells combined with bleeding prevented this fall in in vitro colony‐forming cells. Bled mice developed spleen enlargement with some increase in the total number of in vitro colony‐forming cells in the spleen but this did not compensate for the reduced number of such cells in the bone marrow. Splenectomy did not affect this response of in vitro colony‐forming cells in the bone marrow. Cortisone reduced the bone marrow content of in vitro colony‐forming cells. Serum levels of colony‐stimulating factor were not elevated in response to bleeding. The results suggest that in vitro colony‐forming cells are not erythropoietic cells but may share a common ancestor with erythropoietic cells.