SUMMARYAn important unresolved question in skeletal muscle plasticity is whether satellite cells are necessary for muscle fiber hypertrophy. To address this issue, a novel mouse strain (Pax7-DTA) was created which enabled the conditional ablation of >90% of satellite cells in mature skeletal muscle following tamoxifen administration. To test the hypothesis that satellite cells are necessary for skeletal muscle hypertrophy, the plantaris muscle of adult Pax7-DTA mice was subjected to mechanical overload by surgical removal of the synergist muscle. Following two weeks of overload, satellite cell-depleted muscle showed the same increases in muscle mass (approximately twofold) and fiber cross-sectional area with hypertrophy as observed in the vehicle-treated group. The typical increase in myonuclei with hypertrophy was absent in satellite cell-depleted fibers, resulting in expansion of the myonuclear domain. Consistent with lack of nuclear addition to enlarged fibers, long-term BrdU labeling showed a significant reduction in the number of BrdU-positive myonuclei in satellite cell-depleted muscle compared with vehicle-treated muscle. Single fiber functional analyses showed no difference in specific force, Ca 2+ sensitivity, rate of cross-bridge cycling and cooperativity between hypertrophied fibers from vehicle and tamoxifen-treated groups. Although a small component of the hypertrophic response, both fiber hyperplasia and regeneration were significantly blunted following satellite cell depletion, indicating a distinct requirement for satellite cells during these processes. These results provide convincing evidence that skeletal muscle fibers are capable of mounting a robust hypertrophic response to mechanical overload that is not dependent on satellite cells.

To test the hypothesis that aging has negative effects on stem-cell homing and engraftment, young or old C57BL/6 bone marrow (BM) cells were injected, using a limiting-dilution, competitive transplantation method, into old or young Ly5 congenic mice. Numbers of hematopoietic stem cells (HSCs) and progenitor cells (HPCs) recovered from BM or spleen were measured and compared with the numbers initially transplanted. Although the frequency of marrow competitive repopulation units (CRUs) increased approximately 2-fold from 2 months to 2 years of age, the BM homing efficiency of old CRUs was approximately 3-fold lower than that of young CRUs. Surprisingly, the overall size of individual stem-cell clones generated in recipients receiving a single CRU was not affected by donor age.

A temporal decline in tissue stem cell functionality may be a key component of mammalian aging. The tumor suppressor p53 has recently been implicated as a potential regulator of aging. We examined age-associated hematopoietic stem cell (HSC) dynamics in mice with varying p53 activities. Reduced p53 activity in p53 ؉/؊ mice was associated with higher numbers of proliferating hematopoietic stem and progenitor cells in old age compared with aged wild-type (p53 ؉/؉ ) mice. We also assessed HSC dynamics in a p53 mutant mouse model (p53 ؉/m ) with higher apparent p53 activity than wild-type mice. The p53 hypermorphic (p53 ؉/m ) mice display phenotypes of premature aging. Many aged p53 ؉/m organs exhibit reduced cellularity and atrophy, suggesting defects in stem-cell regenerative capacity. HSC numbers from old p53 ؉/m mice fail to increase with age, unlike those of their p53 ؉/؉ and p53 ؉/؊ counterparts. Moreover, transplantation of 500 HSCs from old p53 ؉/m mice into lethally irradiated recipients resulted in reduced engraftment compared with old wild-type p53 ؉/؉ and p53 ؉/؊ HSCs. Thus, alteration of p53 activity affects stem-cell numbers, proliferation potential, and hematopoiesis in older organisms, supporting a model in which aging is caused in part by a decline in tissue stem cell regenerative function. IntroductionNormal organ size and cellular composition represent a regulated balance between cell death and cell replacement through the proliferation and differentiation of immature cells. In many tissues, adult tissue stem cells are recruited to replace tissue lost to "natural turnover" or during damage and regeneration. Stem cells are defined as immature cells with the ability to self-renew and to produce more differentiated daughter cells. 1 Despite the selfrenewal capacity of stem cells, a number of studies suggest that over time adult tissue stem cells exhibit functional aging and gradually lose the ability to successfully regenerate tissue, thus driving tissue attrition and reduced regeneration, commonly accepted hallmarks of aging. [2][3][4][5] A number of stem-cell aging studies have focused on the hematopoietic stem cell (HSC) compartment. The first studies to suggest stem-cell aging involved serial transplantation of whole bone marrow that supported only 4 to 5 rounds of transplantation. 2,6 Given that the HSC compartment facilitates this regeneration, these findings suggested an exhaustion of the stem-cell pool. Later mouse studies revealed some interesting trends associated with HSC aging: the number of HSCs increased while their proliferative capacity decreased with age. 7,8 Results from studies comparing HSCs in different mouse strains indicate that HSC functional decline can be correlated with lifespan; a negative correlation has also been shown between lifespan and proliferative capacity. 9,10 Because aging in somatic cells is associated with an accumulation of DNA damage, gene products that regulate the DNA damage response are candidate regulators of aging. One such candidate is p53, a potent t...

The extensive self-renewal capacity of hematopoietic stem cells (HSCs) implies that this cell population may not age and thus may provide undiminished replenishment of blood cells throughout the lifespan of an organism. In contrast, accumulating experimental evidence supports the premise that HSCs show signs of aging and may have a limited functional lifespan. We summarize here the evidence for HSC aging, discuss the possible molecular mechanisms that may be involved and show evidence of a genetic connection between the effects of age on blood-forming cells and the longevity of mice. We speculate that age-related functional decline in adult tissue HSCs limits longevity in mammals.