Stem cells are undifferentiated (unspecialised) cells that can transform into specialised cells and can divide to produce more stem cells. They have the potential to develop into many different cell types in the body during early life and growth. In addition, in many tissues they serve as an internal repair system, dividing essentially without limit to replenish other cells as long as the person or animal is still alive. There are two types of stem cells: Embryonic stem cells, found in the early stage of embryonic development, can differentiate into all the specialised cells of the body, such as muscle cells, red blood cells, and nerve cells. Adult stem cells, which are found in some adult tissues, can act as a repair system for the body.