Germ and Stem Cells
Stem cells are found in different places throughout the body and participate in tissue homeostasis by replacing differentiated cells that died; due to their high place in each tissue's hierarchy, they are interesting subjects for study in the context of aging. Human stem cells can express telomerase (Chiu et al., 1996; Sugihara et al., 1999) indicating that the most actively dividing cell lines in the body overcome telomere shortening--though somatic stem cells can show senescence in vitro (Smith and Schofield, 1994) and in vivo (Martin et al., 2000). Interestingly, a correlation between mean telomeres and age is found in the first two decades for muscle satellite cells but not afterwards (Decary et al., 1997). So, one hypothesis is that somatic cells can only divide a limited amount of times but are constantly being replenished by stem cells. Interestingly, one study found a correlation between stem cell turnover and mice lifespan (de Haan and Van Zant, 1999), meaning that perhaps stem cell senescence influences organismic senescence (Snyder and Loring, 2005). At present it is speculative to consider this hypothesis but certainly much work will be done in the future attempting to relate stem cells to aging.
As previously mentioned, the doctrine of the immortal germplasm claims that germ cells are immortal and can divide forever (Weismann, 1891; Kirkwood, 1977). A prediction of such hypothesis is that the germ cells should have increased stress resistance and repair mechanisms (Kirkwood, 1977). Experimental evidence, however, is contradictory: the soma of Drosophila has been reported to be more sensitive to mutagens (Vogel and Zijlstra, 1987); increased DNA repair has been documented in male mice germ cell (Walter et al., 1994), but using ionizing radiation no difference in sensibility was found between mice male germ cells and bone marrow (van Loon et al., 1993). It has also been proposed that meiosis and gametogenesis can have recombinational and other genetic events that contribute to a rejuvenation not possible in differentiated somatic cells (Medvedev, 1981; Holliday, 1984). Yet little or no evidence exists to support such claims. Furthermore, the common notion that germ cells have improved DNA repair mechanisms and thus avoid aging is itself debatable (reviewed in Walter et al., 2003).
http://www.senescence.info/cells.html
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