EFFECT OF CYCLOPHOSPHAMIDE ON THE HEMATOPOIETIC MICROENVIRONMENTAL FACTORS WHICH INFLUENCE HEMATOPOIETIC STEM CELL PROLIFERATION

Abstract

ABSTRACT Transplanted hematopoietic stem cells (HSC) regenerate more rapidly in the femoral marrow of lethally irradiated hosts pretreated with cyclophosphamide (CY) 4 days prior to X‐irradiation than they do in that of uninjected irradiated hosts (control). On the other hand, regeneration of HSC transplanted into irradiated hosts given CY 7 days before X‐irradiation is slower than in controls. The microenvironment in the femoral marrow was studied at various times after giving CY. Four days after injecting CY, the number of colony forming units (CFU), total nucleated hematopoietic cells, and mature myeloid and erythroid cells in the femoral marrow is markedly reduced. Seven days after injecting CY, the number of CFU in the femoral marrow is still reduced, the total nucleated cell count is back to normal, but the number of mature myeloid elements in the marrow are significantly increased. These observations suggest the conclusion that the rate of proliferation of HSC is modulated by the number of mature myeloid cells in the microenvironment.