Decrease in the ability of bone marrow CFUs to produce macroscopic colonies of haematopoietic tissue in the spleens of partially irradiated young rats.

Experiments were carried out on Wistar rats aged from 3--5 weeks, in order to determine the repopulation capacity of bone marrow CFUs after total body and partial body irradiation. CFUs were derived either from the own marrow, or from bone marrow of isologous donors. In totally irradiated hosts, macroscopic spleen colonies were formed after transplantation of both autologous and isologous marrow cells, whereas no visible colonies were detectable in partially irradiated rats, in spite of an increased haematopoietic activity of the spleen and higher concentration of circulating CFUs. Intravenous application of trypsin resulted in an increase in the number of CFUs in peripheral blood in both intact and partially irradiated rats. CFUs derived from bone marrow of PBR rats form MSC only when transplanted into isologous TBR recipients, but not at autorepopulation. The possibility is discussed that a repressor is released from unirradiated bone marrow of young rats which inhibits the colony-forming capacity of migrating CFUs.