In our body, cells can function for different amounts of time. For instance, red blood cells live for 4 months, whereas intestinal epithelial cells only live for 1 week. Tissue specific stem cells serve for the continuous replacement of these missing cells. Growth of hairs and nails also relies on stem cells. Other organs, such as the heart or the pancreas, are composed of long-living cells that do not need replacement. These organs typically also lack competent stem cells and thus cannot regenerate well after damage.