There are two types of stem cells, embryonic and adult. Embryonic stem cells are exogenous in that they are harvested from outside sources, namely, fertilized human eggs. Unlike embryonic stem cells, adult or multipotent stem cells are endogenous. They are present within our bodies and serve to maintain and repair the tissues in which they are found. Adult stem cells are found in many organs and tissues, including the skin. In fact, human skin is the largest repository of adult stem cells in the body. Skin stem cells reside in the basal layer of the epidermis where they remain dormant until they are activated by tissue injury or disease.