The development of spatial distributions of CFU‐S and in‐vitro CFC in femora of mice of different ages

The radial distributions of spleen colony forming units (CFU-S) and in-vitro colony forming cells (in-vitro-CFC) were measured in the diaphyseal marrow cavity of femora removed from 3-, 5- and 11-week-old mice. The distributions observed in 11-week-old mice confirm earlier findings that the highest concentrations of CFU-S exist near bone surfaces whereas the concentration of in-vitro-CFC increases to a peak value approximately 300 microns from the femoral axis with a low value at the bone surface. The gradients of the distributions in all three age groups are very similar suggesting that spatial organization in marrow is established by 3 weeks at the latest and, as the marrow cavities grow, so the distributions extend into the new space following their respective gradients. The peak of CFU-S concentration at the bone surfaces in all age groups coincides with increased rates of DNA synthesis and a low self-renewal capacity. Conversely, CFU-S nearer the centre of the cavity maintain a low turnover but have a high self-renewal capacity. Measurements made on 1-week-old mice show that the marrow contains a lower average concentration of CFU-S in the femoral cavity compared to older mice. However, these CFU-S have both a high rate of turnover and a high self-renewal capacity. It appears that these better quality CFU-S remain in a central location while the rest of the population ages and expands in association with growing bone regions.