Proliferation of erythroid and granulocyte progenitors in the spleen as a function of stem cell dose

A study of the kinetics of cellular proliferation, in the morphologically unrecongizable haemopoietic progenitor cell compartment, as a function of injected CFU-S dose has been carried out in the spleens of lethally X-irradiated mice using 3H-TdR labelling. Amplification in this proliferating cell compartment was observed to decline as CFU-S dose increased. The number of divisions in the differentiated line arising from CFU-S up to the first appearance of recognizable erythroid precursors were calculated to be 9-2, 12-5, 15 and 17 for the 2, 0-35, 0-05 and 0-007 femur equivalent doses respectively. The growth of cell populations arising from CFU-S was biphasic, with a rapid initial phase having a doubling time of about 6-3 hr, and a slow phase of doubling time around 1 day. Analysis of the rapid phase by the FLM method gave a cycle time of 5-6 hr, Recognizable labelled erythroid precursors were detected at the same time as, or just after, the change in slope of the growth curve. Significant numbers of proliferating (labelled) granulocytes only appeared in the spleens of animals receiving the higher marrow doses (2 and 0-35 femur). The erythroid to granulocyte ratio was also a decreasing function of marrow dose.