ADULT STEM CELLS

The embryo usually becomes a fetus about eight weeks after fertilizationon. At this time another type of cell—the adult %tem cell—emerges. The discovery that these cells were present in many formed adult tissues and organs changed the paradigm of past assumptions of how the body repairs itself. In the past. i was assumed that skin cells near the area of a cut or scraped knee would rush to repair the offended area. Brain cells, muscles, and some other cells were thought to never repair themselves. But recent research on adult stem cells has changed the picture. Adult tissues and organs have stem cells that are believed to play a role in the regeneration of damaged tissues and organs (see Table 1.1, Stem Cells in Adult Organs and Tissues). Chapers 5. 6. and 7 explore stern cell development and stem cell harvesting in each of the areas noted in Table 1.1. The adult stem cell is undifferentiated and unspecialized. However. Table 1.1 shows that these adult cells do exist in specialized and differentiated tissues and organs. They can renew themselves and differentiate to

yield all the specialized cell types of the tissue from which they originate. For example, adult stem cells from the skin can form cells related to theskin, although they cannot form all the cells of the body like the embryonic stem cells can. Embryonic stem cells are identified in the 1CM. but finding adult stem
cells is much more difficult. For example. in certain organs, they may be present in no more than one out of 15.000 cells. Although the exact source of these adult stem cells is unclear, three ideas have been proposed to explain their origin:

I. They are embryonic cells that were set aside when the tissue first developed.
2. They may be pan of a migrating group of embryonic cells that became part of the organs or tissues durin early divisions.
3. They may have developed after embryonic formation in some process of dedifferentiation.

  The adult stem cells appear to be less plastic than the embryonic stem cell (ESC). For example. when both adult and embryonic stern cells were injected into mice, adult stern cells went to the area where they belong. Stem cells from the bone marrow gravitated there: those that were brain cells went to the brain. The ESCs did not go to any differentiated tissue hut clumped together to form tumor-like masses. In addition, in the test tube, adult stem cells differentiate into only a small number of cells, while ESCs become many cell types representing all germ layers. However, several scientists are now challenging the dogma of the limited plasticity of adult stern cells.
  While adult cells may lead to the first important therapeutic uses. embryonic cells present the possibility of devising therapies for most diseases and disorders. Testifying before the U.S. Senate Subcommittee on
Stem Cell Research in 1998, Dr. James Thomson outlined a future made possible by embryo stern cell technology. He described how the cells could enable standardized production of large purified populations of norm al human cells to provide limitless sources of tissue for pharmacologic research and transplantation therapies. Future clinical targets for the repair of bLood, bone, and other tissues include blood cell repopulation, osteoarthritis, Parkinson’s disease, diabetes, spinal cord injury, stroke, burns, and myocardial infarction. Study of embryo lines could provide insight into abnormal development that cannot be accessed in the intact embryo. These studies are having an important impact in clinical research on birth defects, infertility, and pregnancy loss.
    Research using embryonic stem cells is controversial because it requires the embryo to be destroyed. invoking ethical questions for those

wishing to uphold the sanctity of life. Adult stem cells are very important because they do not give rise to the ethical questions related to destruction of the embryo. However, the difficulty of harvesting them and their limited plasticity are major hurdles for researchers. The potential for U.S. patient populations to benefit from stem cell—based therapies is overw helming, as shown in Table 1.2, Patients and Possibilities. Researchers
around the world are working on the problems: when these are overcome, we may see the anticipated miracle cures.
  Although research on stem cells seems to have developed only in recent decades, the idea of the blank slate of cells with the potential to take shape as many different types of cells has roots in historical thought. Chapter 2 traces the exciting ideas of how the slates of life were perceived in earlier times.