Colony-forming units in the spleen and bone marrow of young (C57BL/6xDBA/2)F1 hybrid mice.

Spleen or bone marrow cells from (C57BL/6 X DBA/2)F1 hybrid mice ranging in age from 1 to 32 days were transplanted into syngeneic irradiated adult recipients. By enumeration of macroscopic hemopoietic spleen colonies 8 days after transplantation, the colony-forming unit(CFU) content of the spleen and bone marrow of the young donor mice was determined. The CFU concentration in the bone marrow, initially at a level of 15.1--17.0 CFU/105 nucleated cells, increased during the third week of life and reached a value comparable to that of the adult during week 4. In contrast, the CFU concentration of the spleen decreased during the first 2 weeks and reached a plateau at a level of 4.45--6.66 CFU/105 nucleated cells during the third and fourth weeks of age. This was still significantly higher than that of the adult hybrid. The rise in prominence of the bone marrow as a hemopoietic organ during this period appeared to be due to an increase in total cellularity coupled with an increase in CFU concentration in this hybrid. While the cellularity of the spleen also increased during this time, the CFU concentration decreased, resulting in little change in the total CFU content of this organ.