Upper panel: during growth (grey shade), the progenitor cell pool in the hematopoietic system undergoes massive expansion through asymmetric replication of hematopoietic stem cells. In this way, the system can accommodate the tremendous turnover in peripheral blood cells. In contrast, during growth, the progenitor cell pool in skeletal muscle undergoes a modest expansion through replication of muscle stem cells, as the turnover of skeletal muscle cells is small. Lower panel: reflecting the expansions of the progenitor cell pools in the respective systems, TL undergoes rapid attrition in the hematopoietic system but only modest attrition in skeletal muscle. The slow and parallel attritions of TLs in stem cells, as expressed in leukocyte TL and in skeletal muscle TL during adult life, are the outcome of ’maintenance’ replicative activities of stem cells/progenitor cells in these systems.