© FANATIC STUDIO/GETTY IMAGESWe may perceive ourselves as static beings, but the cells of our bodies are in constant flux. The outer layers of our skin and intestinal tract are replaced every few weeks; red blood cells circulate in our bodies for about 100 days before they are replaced; cells in our liver and fat are longer lived—more than a year for a liver cell, 10 years on average for a fat cell—but still turn over repeatedly during our lifetimes. More slowly, up to half our heart cells may be replaced during a normal lifespan. And, of course, when healthy tissue is lost due to injury, new cells are made to patch up the damage. What are the biological processes responsible for normal cell turnover and organ homeostasis? What controls proper repair after injury? What allows organisms like the salamander to regenerate an amputated limb while humans form scars and struggle to regrow much simpler structures, such as hair?
These and other questions are the target of ongoing research in the field of regenerative medicine. But what we do know, and have known for nearly half a century, is that stem cells are crucial players. Stem cells self-renew to maintain their numbers and differentiate into the specialized cell types that make up our tissues and organs—a function that becomes especially important after stress or injury. The ultimate goal of regenerative medicine is to harness stem cells’ regenerative potential to treat and even cure many of the diseases besetting society today. Despite progress in understanding the potential of these multipotent cells, the unfortunate reality is that we remain far from cures. One possible reason for this is scientists’ failure to sufficiently consider what goes on ...
