Two distinct classes of granulocyte progenitor cells present in normal mouse bone marrow are expressed sequentially in the vivo plasma clot diffusion chamber culture system. By several criteria, progenitor cells giving rise to granulocyte colonies on day 4 of culture (CFU-D4) are different from those giving rise to colonies on day 7 (CFU-D7). These differences include: cell cycle activity as measured by in vitro incubation with cytosine arabinoside, residual concentration in the bone marrow after in vivo treatment of donor mice with cytosine arabinoside or methotrexate, resistance to osmotic lysis, size as determined by velocity sedimentation, and the morphology of the granulocyte colonies to which these cells give rise. The CFU-D7 appears to represent an earlier progenitor cell than the CFU-D4 in the differentiation pathway of the granulocyte and is analagous in many respects to the BFU-E in the erythroid pathway.

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