"Gene activity is what distinguishes a heart cell from a brain cell or a kidney cell. The stem cells in a 5-day-old embryo can develop into any of these specialized cell types, but this "pluripotent" state doesn't last long. As a stem cell picks up chemical signals from its surroundings, different genes get switched on or silenced, and the cell becomes more specialized. Specialized cells can no longer switch roles as needed, and few can reproduce. That's why heart attacks and brain injuries are so devastating. They leave the injured organ powerless to repair itself."

Read "Insights From Harvard Medical School"