In the 1960s, scientists first found that the bone marrow contains adult stem cells by discovering hematopoietic stem cells (HSCs). HSCs are stem cells from which all the erythrocytes, leukocytes, and platelets develop. Soon afterwards, mesenchymal stem cells (MSCs) were discovered that develop into osteocytes, chondrocytes, and other connective tissues (Kirschstein et al. 2001). Also in the same decade, neural stem cells were found in the brains of rats. However, this discovery was not readily accepted by the scientific community as the common belief then was that neurogenesis did not occur in the adult mammalian brain. This belief was changed in the 1990s when scientists learned that such progenitors can differentiate into astrocytes, oligodendrocytes, and neurons (Carpenter et al. 1999; Reynolds et al. 1992).