Velocity sedimentation and cell cycle characteristics of granulopoietic progenitor cells (CFUc) in canine blood and bone marrow: influence of mobilization and CFUc depletion.

Populations of granulopoietic progenitor cells, as measured by the colony formation technic (CFUc), of canine bone marrow and blood were compared by velocity sedimentation and tritiated thymidine (3H-TdR) induced cell death rates. Blood CFUc were smaller than those of the bone marrow and had a lower proportion in S-phase, as reflected by these methods. CFUc which were mobilized into the peripheral blood by dextran sulfate had unaltered velocity sedimentation and cell death fraction rates. Removal of CFUc from the blood by 24-hour leukapheresis resulted in doubling of 3H-TdR suicide of bone marrow CFUc, whereas that of blood CFUc was not affected. Depending on the degree of CFUc depletion, granulopoietic progenitor cells which were larger than normal appeared in the blood and bone marrow. It is concluded that there is a subpopulation of CFUc within the bone marrow which is ready for discharge into the circulation. These cells are characterized by small size and a lower proportion in S-phase. Some bone marrow/blood barrier retains large CFUc as well as those in active proliferation within the bone marrow. One might speculate that enlarged CFUc in the bone marrow are a subpopulation which normally provides only a small influx to the total CFUc pool.