Regulation of the growth fraction of CFU-S by an inhibitor produced by bone marrow

Humoral factors were extracted from cell suspensions obtained from rat bone marrow and thymus and fractionated using Amicon filters to obtain crude molecular weight bands of 50–100 K and 30–50 K. These fractions were incubated in vitro with haematopoietic stem cells (CFU-S) obtained from drug-treated mice to evaluate their effects on the proliferation of stem cells. An inhibitor was found in the 50–100 K fraction from normal rat bone marrow that would decrease the proportion of stem cells synthesising DNA in the regenerating bone marrow from ～ 50 to ～ 10%. This inhibitor was not found in the 30–50 K fraction extracted from rat bone marrow or in the 50–100 K fraction extracted from rat thymus and had no effect on the proportion of committed progenitors (CFU-C) synthesis DNA or the proliferation of mitogen-stimulated lymphocytes. The inhibitor has the same properties as that described by Lord et al. 11].