The Stem Cell Niche

"A stem cells' immediate neighborhood, a specialized environment also known as the stem cell niche, provides crucial support needed for stem cell maintenance. But nothing lasts forever, found scientists at the Salk Institute for Biological Studies. During the aging process, the level of support drops off, diminishing the stem cells' ability to replenish themselves (self-renew) indefinitely."

"...results suggest that over time the niche is changing to the point where you start losing self-renewing stem cells"