Effects of cyclophosphamide on murine bone marrow and splenic megakaryocyte‐CFC, granulocyte‐macrophage‐CFC, and peripheral blood cell levels

The effect of cyclophosphamide (CY) on megakaryocytopoiesis in mice was examined with assays of megakaryocyte colony‐forming cells (Meg‐CFC) in bone marrow and spleen and simultaneous determinations of peripheral blood counts, after a single intraperitoneal dose (200 mg/kg) of CY. Significant rebound thrombocytosis (170% of normal) occurred at day 11 after injection with CY, although only modest preceding thrombocytopenia (70% of normal) was observed. After an initial 3–5‐day period of suppression, total megakaryocyte colony‐forming cells (Meg‐CFC) in both bone marrow and spleen of CY‐treated mice demonstrated rebound increases at 5 and 7 days, respectively, after administration of the drug. Granulocyte‐macrophage colony‐forming cells (GM‐CFC) exhibited alterations which were similar to those of Meg‐CFC, suggesting similar sensitivities of Meg‐CFC and GM‐CFC to CY. The increase in Meg‐CFC in both bone marrow and spleen preceded development of thrombocytosis by 4–6 days. This suggests that increased platelet counts in CY‐treated mice are attributable, at least in part, to alterations in feedback mechanisms which control megakaryocytopoiesis, with resultant stimulation of the megakaryocyte progenitor compartment.