Some Properties of the Circulating Hemopoietic Stem Cells

Some properties of hemopoietic stem cells (CFU) circulating in the peripheral blood of the normal mouse (PCFU) were studied. Those parameters were chosen that are well-known characteristics of bone marrow stem cell populations. Leukocytes separated from the peripheral blood were the source of PCFU. The radiosensitivity (D₀ value), growth curve, seeding efficiency (f number), and the turnover state (number of PCFU in S phase measured by ³H-thymidine killing technique in vitro) of PCFU were compared to the same parameters for bone marrow CFU. Several characteristic changes in these parameters were found for PCFU: (1) Lower radiosensitivity (D₀ = 140 R), (2) Higher seeding efficiency (f number 20.9%), (3) Higher sensitivity to ³H-thymidine in vitro (killing effect 32%). The doubling time was the same both for CFU and PCFU (21 hr), but the logarithmic growth of the PCFU population started by approximately 36 hr later than that of the marrow-derived CFU population. All these findings support the idea that PCFU represent a subpopulation of the heterogeneous bone marrow CFU population.