Ouroboros on just how many stem cells we have: "It is widely accepted that stem cells are involved in tissue regeneration. It is also widely accepted that (in most organs) stem cells are vanishingly rare. So: if there doesn't happen to be a stem cell adjacent to a site of damage, how can stem cells be involved in the process of tissue repair? There might be more stem cells than we think, because we've been missing them for some reason. This possibility is strongly supported by the recent findings of Zuba-Surma et al., who have discovered a population of tiny pluripotent cells (termed, appropriately, very small embryonic-like, or VSELs) scattered throughout the body. ... Note that both VSEL number and potency diminish with age, consistent with the decrease in proliferative and regenerative capacity that we see in older animals. ... Required skepticism: VSELs are both brand new and (so far as I can tell) idiosyncratic to a single group's work. Before we get too worked up about this, I'd like to see the work reproduced by other labs and in other systems. Hopefully that sort of confirmation is already underway." The easier stem cells become to source, the faster research and development will proceed.