Much of the interest in various types of stem cells for applications in regenerative medicine is based on the notion that stem cells have ‘self-renewal’ capacity and can be expanded in vitro or in vivo to achieve a particular therapeutic effect. While the self-renewal of embryonic stem cells appears only limited by the inevitable accumulation of genetic abnormalities, the self-renewal properties of adult stem cells are less well defined (Lansdorp, 1997). Upon culture of human hematopoietic ‘candidate’ stem cells, the proliferation rate – the fraction of cells that enter mitosis – and the ability to produce CD34+ progenitor cells markedly decrease with the age of the cell donor (Lansdorp et al., 1993). Such developmental changes in functional properties were found to correlate with the loss of telomere repeats in cells before and after culture (Vaziri et al., 1994). These initial observations were supported by more extensive studies of telomere length in nucleated blood cells enabled by the development of in situ hybridization with fluorescently labeled peptide nucleic acid probe in combination with flow cytometry [flow FISH (Rufer et al., 1998)]. Application of this method to measure the telomere length in nucleated blood cells from humans (Rufer et al., 1999) and baboons (Baerlocher et al., 2003) have yielded remarkably consistent results. Granulocytes and naive T cells showed a biphasic decline in telomere length with age, which was proposed to reflect accumulated cell divisions in the common precursors of both cell types: hematopoietic stem cells. Telomere loss was very rapid in the first year, and continued in humans for more than eight decades at a 30-fold lower rate. These findings suggested that cell divisions in hematopoietic stem cells and T cells result in loss of telomeric DNA and pointed to a dramatic decline in stem cell turnover in early childhood.
A limitation of these previous studies was that conclusions were based on telomere length data acquired in cross-sectional studies which, given the marked heterogeneity in telomere length at any given age (Harley et al., 1990; Hastie et al., 1990; Okuda et al., 2002), complicated the interpretation of the data. Ideally, longitudinal studies of telomere length should be performed. In humans such studies are complicated by practical difficulties of acquiring serial blood samples from healthy newborns. In view of the similarity in leukocyte telomere length dynamics between humans and baboons (Baerlocher et al., 2003), we decided to perform a longitudinal study of telomere length in newborn baboons. The main result of this study is shown in Fig. 1. Blood samples were collected at biweekly intervals for most of the first year of life followed by less frequent sampling thereafter. Note that two animals (17891 and 17966) had considerably longer telomeres at birth (∼25–28 kb) than the other two animals (17927 and 18760; ∼13–15 kb) in support of the previously observed heterogeneity in telomere length in these outbred animals (Baerlocher et al., 2003). At birth, the telomere length in T cells was significantly longer than in granulocytes or B cells, but this difference had almost disappeared at 100 weeks of age. Most likely, more mature T cells divide more frequently than hematopoietic stem cells in this time interval. The loss of telomere length in B cells was similar to that in granulocytes. Whether this reflects significant production of B cells from stem cells with increasingly shorter telomeres and/or turnover of mature B cells remains to be determined. However, the most striking result from this longitudinal study was the marked drop in the telomere attrition rate in all nucleated cells after the first 50–70 weeks of life. These results support our previous conclusions that the turnover of stem cells appears to follow a biphasic pattern (Rufer et al., 1999) compatible with rapid cell divisions in the first year, in which stem cell numbers are most likely increased, followed by a greatly reduced cell division rate. Recent studies in a murine model suggest that the turnover of transplantable hematopoietic stem cells decreases strikingly at 3 weeks after birth (Bowie et al., 2006). Further studies on the factors that regulate the cell cycle status in hematopoietic stem cells during development are warranted.
Strict, but poorly understood, developmental control of cell divisions in the hematopoietic stem cell compartment (Lansdorp, 1995) as well as limitations in their replicative potential imposed by progressive telomere loss (Vaziri et al., 1994) underscore the difficulty of continuing efforts to purify ‘stem cells’ to homogeneity or exploit their ‘self-renewal’ when telomere and developmental status are not taken into account. Studies in the murine model suggest that telomere maintenance alone may not be sufficient to increase the expansion potential of stem cells (Allsopp et al., 2003). However, recent studies of patients with aplastic anemia indicate that even modest reductions in telomerase levels resulting from haploinsufficiency for either the telomerase RNA gene TERC (Vulliamy et al., 2004) or the telomerase reverse transcriptase gene TERT (Yamaguchi et al., 2005) can both result in bone marrow failure. These observations suggest that overexpression of both these genes may be required to increase the telomere length in adult stem cells and, possibly, increase their replicative potential.