ScienceDaily notes progress in our understanding of why stem cell function declines with age: "A stem cells' immediate neighborhood, a specialized environment also known as the stem cell niche, provides crucial support needed for stem cell maintenance. ... During the aging process, the level of support drops off, diminishing the stem cells' ability to replenish themselves (self-renew) indefinitely. ... in older flies a steep decline in the growth factor unpaired (upd), which is necessary to maintain stem cells, results in fewer stem cells in the testis of the fruit fly Drosophila. Identifying the reasons for reduced upd expression could reveal how aging leads to changes in stem cell behavior, and counteracting these changes may slow the loss of adult stem cells during aging. ... Taken together, our results suggest that over time the niche is changing to the point where you start losing self-renewing stem cells." Understanding these mechanisms is a step on the path to doing something about the loss of regenerative capacity with age.