Developmental regulation of the HSC compartment. Each HSC has a limited life span, and the production of mature cells found in blood follows a ballistic curve from the start to the end of their life. Early in the life of the organism, Ly-bi (blue lines) are more frequent than My-bi (red) HSCs. On average, My-bi HSCs have a longer life span than other subsets of HSCs, leading to an accumulation of My-bi and a loss of Ly-bi HSCs as the organism ages. The bar above the graph shows how the HSC composition is reflected in the ratios of lymphoid to myeloid cells in blood. Because of the differences in self-renewal, My-bi HSCs will preferentially (but not exclusively) outlive the host as has been shown in serial transplants.³