ment cells of the skin, cartilage and bone in the face and skull, and connective tissue in many parts of the body. Neural crest cells from mice have been cultured in the laboratory.

The fetal liver and blood are rich sources of hematopoietic stem cells, which are responsible for generating multiple cell types in blood, but their properties have not been extensively investigated. Although not part of the fetus, the umbilical cord and placenta are also rich sources of hematopoietic stem cells. Tissue extracted from the fetal pancreas has been shown to stimulate insulin production when transplanted into diabetic mice, but it is not clear whether this is due to a true stem cell, a more mature progenitor cell, or to the presence of fully mature insulin-producing pancreatic islet cells themselves (Beattie et al., 1997). Finally, multipotent cells called primordial germ cells have been isolated from the gonadal ridge, a structure that arises at an early stage of the fetus that will eventually develop into eggs or sperm in the adult. Germ cells can be cultured in vivo and have been shown to give rise to multiple cell types of the three embryonic tissue layers (Shamblott et al., 1998).

Adult stem cells are undifferentiated cells that occur in a differentiated tissue, such as bone marrow or the brain, in the adult body. They can renew themselves in the body, making identical copies of themselves for the lifetime of the organism, or become specialized to yield the cell types of the tissue of origin. Sources of adult stem cells include bone marrow, blood, the eye, brain, skeletal muscle, dental pulp, liver, skin, the lining of the gastrointestinal tract, and pancreas. Studies suggest that at least some adult stem cells are multipotent. For example, it has been reported that stem cells from the bone marrow, a mesodermal tissue, can give rise to the three major types of brain cells, which are ectodermal derivatives (Mezey et al., 2000) and that stem cells from the brain can differentiate into blood cells and muscle tissue (Bjornson et al., 1999), but these findings require verification. It is not clear whether investigators are seeing adult stem cells that truly have plasticity or whether some tissues contain several types of stem cells that each give rise to only a few derivative types. Adult stem cells are rare, difficult to identify and purify,