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Brief Introduction to Adult Stem Cells by Sasha Bakhru

Adult stem cells are undifferentiated cells found in different parts of the body, such as the brain, bone marrow, skeletal muscle, and liver, among others. Also known as somatic stem cells, they are primarily responsible for injury repairs and maintenance of the body tissues. The exact origins of the somatic stem cells in mature tissue are not yet known. There are number of key differences between adult stem cells and embryonic stem cells. Adult stem cells have some immunoprivilege because they lack certain proteins on the outer leaflet of their plasma membrane specifically, MHC class II receptors, that may trigger an immune response. Also, somatic stem cells have less potential for expansion than embryonic stem cells, though there are more wellknown methods to control differentiation. Recently, adult stem cells were found to have some plasticity (Fig. 2.2), meaning that stem cells found in one part of the body may be able to differentiate into cell in different location (Kirschstein et al. 2001)

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).

Adult Neural Stem Cells

Until the early 1990s, it was believed that neurogenesis did not occur in the mature adult brain. However, in 1992, multipotent cells with the ability to indefinitely selfrenew were isolated from the striatal tissue of the adult mouse brain (Reynolds et al. 1992). Soon after, similar multipotent neural progenitors, termed neural stem cells, were isolated in the brains of other mammals, most notably in humans, from regions including the subventricular zone (Palmer et al. 2001).

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