Human stem cells are the “starter” cells that act as precursors of mature bodily tissues. Such cells have not yet differentiated (become specialized) into their mature forms. All human beings possess such cells. For example, precursors of mature blood cells are the pluripotent stem cells of the bone marrow. These cells are called “pluripotent” (L. “many” + “powers”) because one of these undifferentiated cells can become any of a variety of different blood cells. These include the various white blood cells that protect against bodily infection, platelets that help the blood to clot, and the red blood cells that carry oxygen throughout the body.
All adult cells once developed from stem cells by the process of cell division, with daughter cells successively becoming more complex than their precursors. However, adult cells that constitute bodily organs have mostly lost the ability to divide. Unlike bone marrow cells, mature cells in the brain, spinal cord, skeletal muscle, heart muscle, and many other organs no longer have any corresponding pluripotent stem cells to repopulate them when they are damaged. Therefore, brain cells (for example) are limited to the number that arose from their original stem cells. Despite some limited exceptions, these are incapable of repair or replacement.
In a stroke, a sudden blockage of the blood supply to a region of the brain destroys brain cells, never to be replaced. Rehabilitation from a stroke involves training other brain centers to take over the function of the damaged region, but there is no natural process that can replace the dead cells. The same problem occurs in the heart, where repeated heart attacks weaken the heart wall. Since heart cells cannot be replaced, there is a limit to how much damage the heart may sustain before permanent disability or death occurs.1-3
What if there were stem cells that could replace damaged brain cells or heart muscle? This could conceivably improve one’s lifespan, or at least the quality of life. The biological possibilities are intriguing. An equally compelling case can be made for the use of stem cells to repair spinal cord injuries, to provide new pancreatic cells in diabetes mellitus, or to cure Parkinson’s disease.
Where would such stem cells come from? One source is from human embryos, composed exclusively of unprogrammed early stem cells, any one of which may become the precursor of adult tissues and organs. Two possible sources for embryonic stem cells are the excess embryos from in-vitro fertilization procedures (often called “frozen embryos” because of the cryogenic process used to preserve them) or embryos derived from human cloning. The ethical dilemma arises from the fact that the harvesting of embryonic stem cells destroys human embryos.
Another source for stem cells is adult tissues such as bone marrow, fat, and even tooth pulp. Numerous studies demonstrate that some adult stem cells are as flexible as embryonic stem cells. Indeed, these ethically non-controversial adult stem cells are currently being used to benefit human patients.4
References:
1. Tortora GJ, Derrickson B. Principles of Anatomy and Physiology. Eleventh ed. New York: John Wiley & Sons; 2006.
2. Martini F. Anatomy & physiology. San Francisco: Benjamin Cummings; 2005. xx, 845 p., [77] p.
3. Medina J. The Outer Limits of Life. Nashville: Thomas Nelson; 1991. 287 p.
4. For more on adult stem cells, see http://www.stemcellresearch.org/
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