When a sperm joins an egg fertilization takes place. The product of this union is the embryo. Stem cells are cells found in the embryo, which are capable of reproducing any cell type. (pluripotency). In the human, there are about 200 such cell types. Stem cells are also found in adult tissues but in small numbers. E.g. Bone marrow, brain, skin etc. The umbilical cord, which carries nutrients from the mother to the baby also has stem cells. Researchers are trying to use the special ability of stem cells to produce new cells and tissues to repair and reproduce ailing and damaged tissues and organs. At present damaged or non functional tissues and organs are replaced by compatible donor tissues and organs e.g. Renal, Liver and Heart transplants. Sometimes synthetic materials are also used for replacement of heart valves.