Interesting research from the Harvard University Gazette: "The rules governing mammalian organ repair and regeneration are so widely varied as to suggest at first glance that there are no rules: Blood has such an enormous regenerative capacity that you can literally give it away by the pint and be none the worse for wear; rip a hole in your skin and new skin will cover it; donate a portion of your liver and it will regenerate; but lose a kidney or suffer damage to your pancreas, and what's lost is lost. ... a new study [helps] to explain the variation both in organ regenerative capacity and in organ size determination as well. The findings also underscore the value of embryonic stem cells as tools to study normal development. Comparing development of the liver, which can regenerate to compensate for damage, and the pancreas, which cannot, [researchers] found that the ultimate size and regenerative capacity of certain organs, e.g., the pancreas, is determined by the specific number of progenitor cells that are set aside during a very early time in development - about day 10 in the mouse." The more we learn, the closer we come to greatly improving regenerative capabilities.