The germ line (the cell lineage from which reproductive cells are derived) is replicatively immortal — distinct from the soma, where most cells capable of division have a strictly limited replicative capacity, if they’re not entirely postmitotic. Why?
Shawn Ahmed reviews evidence that in the nematode C. elegans, the mechanisms that promote longevity in the soma (sirtuins, as well as players in the IGF-1 signaling axis) are distinct from those that confer immortality on the germ line.
A dichotomy exists between germ and somatic cells in most organisms, such that somatic cell lineages proliferate for a single generation, whereas the germ cell lineage has the capacity to proliferate from one generation to the next, indefinitely. Several theories have been proposed to explain the unlimited replicative life span of germ cells, including the elimination of damaged germ cells by apoptosis or expression of high levels of gene products that prevent aging in somatic cells. These theories were tested in the nematode Caenorhabditis elegans by examining the consequences of eliminating either apoptosis or the daf-16, daf-18 or sir-2.1 genes that promote longevity of postmitotic somatic cells. However, germ cells of strains deficient for these activities displayed an unlimited proliferative capacity. Thus, C. elegans germ cells retain their youthful character via alternative pathways that prevent or eliminate damage that accumulates as a consequence of cell proliferation.
Understanding the proliferative perseverance of the germ line is particularly important when one considers the way in which germ cells violate one of the time-honored justifications for a replicatively limited soma, namely, that an unlimited replicative capacity would place a cell at risk for initiating cancer (see our earlier article on that subject, p16 vs p16: Preventing cancer, limiting self-renewal). Certainly, germ-line derived tumors are not unheard of, but they are rare, even though everyone’s got germ cells — so these lineages serve as an example of replicative immortality that does not confer an unusual risk of carcinogenesis.