Changes in the stem cell niche are a good part of the age-related decline in stem cell activity, which explains why old stem cells can perform like young stem cells if put into a young environment, and vice versa. Here researchers compensate for one of those niche changes: "Stem cells reside within a microenvironment of other cells - the niche - that is known to play a role in stem cell function. For example, after a tissue is injured, the niche signals to stem cells to form new tissue. It is believed that stem cells and their niche send signals to each other to help maintain their potency over a lifetime. But while the loss of tissue and organ function during aging has been attributed to decreases in stem cell function, it has been unclear how this decline occurs. [There are] a number of possible scenarios, such as whether the loss of tissue function is due to a decrease in the number of stem cells, to the inability of stem cells to respond to signals from their niche, or to reduced signaling from the niche. ... researchers discovered that as the stem cell niche [in flies] ages, the cells produce a microRNA (a molecule that plays a negative role in the production of proteins from RNA) known as let-7. This microRNA is known to exist in a number of species, including humans, and helps time events that occur during development. This increase in let-7 leads to a domino effect that flips a switch on aging by influencing a protein known as Imp, whose function is to protect another molecule, Upd, which is secreted from a key area of the niche. In short, Upd promotes the signaling that keeps stem cells active and in contact with the niche so that they can self-renew. And as aging advances, increasing expression of let-7 ultimately leads to lower Upd levels, decreasing the number of active stem cells in the niche. What leads to accumulation of let-7 in the niche of aged flies still remains an open question. The researchers also demonstrated they could reverse this age-related loss of stem cells by increasing expression of Imp."