Stem cells work to maintain your tissues, but their ability to carry out their job diminishes with age, causing a corresponding decline in tissue function. An understanding of why stem cell decline occurs is important in determining what to do about the problem. For example, if the problem is damage in the stem cells themselves, then replacement is a very viable option. If, however, the issue is caused by broader damage in supporting cell populations that leads important signaling processes and stem cell niches to run awry, a completely different strategy is needed. Researchers do not yet have a full understanding as to the mechanisms by which stem cells decline with age, but they are working on it: "Stem cell aging is a novel concept that developed together with the advances of stem cell biology, especially the sophisticated prospectively isolation and characterization of multipotent somatic tissue stem cells. Although being immortal in principle, stem cells can also undergo aging processes and potentially contribute to organismal aging. The impact of an age-dependent decline of stem cell function weighs differently in organs with high or low rates of cell turnover. Nonetheless, most of the organ systems undergo age-dependent loss of homeostasis and functionality, and emerging evidence showed that this has to do with the aging of resident stem cells in the organ systems. The mechanisms of stem cell aging and its real contribution to human aging remain to be defined. Many antitumor mechanisms protect potential malignant transformation of stem cell by inducing apoptosis or senescence but simultaneously provoke stem cell aging."