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THE PROLIFERATIVE CAPACITY OF HAEMOPOIETIC COLONY FORMING UNITS IN THE RAT

After transplantation into rats lethally treated with cytotoxic chemicals both bone marrow and spleen CFU in the spleen and spleen derived CFU in the bone marrow expand with doubling times (Td) of approximately 18 hr. However, bone marrow derived CFU in the bone marrow have a Td of 36 hr. Evidence obtained using tritiated thymidine in vitro and methotrexate in vivo show that the proliferation rate of bone marrow derived CFU is similar in both the bone marrow and spleen and calculations suggest that the different Td between these two sites is due to the higher loss of CFU through differentiation in the bone marrow compared to the spleen. These findings further support the hypothesis of an environment in the spleen which favours CFU self‐maintenance over differentiation with the opposite situation occurring in the bone marrow.