Adult Stem Cells
A typical adult stem cell is one which, on division, results in one daughter cell that can further differentiate and replenish a whole compartment or tissue, while the other remains fully “stem” and self-renewing. This is called asymmetric division, and the stem cell is known as multipotent. (See Figure 1, p. 21.)
The best-studied example of this type of adult stem cell is the hematopoietic stem cell, which produces a trillion differentiated blood cells per day for the life of the animal. This remarkable property is possible because of a complex interaction between the stem cell and other cells and structures within the bone marrow microenvironment called the niche. However, other stem cells exist in the embryo from the moment of conception; the first eight to 16 divisions produce totipotent stem cells by symmetrical division. The word totipotent is used here because each cell may produce a full individual animal because the placental elements are present. Identical twins are the best example. Once the blastocyst has formed with an inner cell mass (destined to become the fetus) and an outer membrane (the early placenta), then the embryonal stem cells (ESCs) from the inner cell mass are called pluripotent. This means that all organs and tissues may be produced but no longer an intact individual. (See Figure 2, p. 22.)
Recently, another cell, the mesenchymal stem cell (MSC), has been applied to the treatment of AD. The concept is completely different from HSC in that pretreatment with chemotherapy is not required, and the MSCs are thought to home in on damaged tissue and exert a local paracrine healing effect.