What are Stem Cells?
In explaining stem cells, it is important to clarify the nomenclature. The mere term "stem cell" has itself been widely misunderstood and, in many cases, inaccurately used. There are, in fact, a multitude of different types of stem cells, each one with clear and distinct characteristics.
Frequently, the names of these various types of stem cells are used interchangeably, and incorrectly so. The simple term "stem cell" is often casually employed in a generic sense, when what is usually meant, more correctly, is either "embryonic" stem cell or "adult" stem cell. As representatives of the National Institutes of Health (NIH) have stated,
"The terminology used to describe stem cells in the lay literature is often confusing or misapplied. Second, even among biomedical researchers, there is a lack of consistency in common terms to describe what stem cells are and how they behave in the research laboratory." (From "Stem Cells: Scientific Progress and Future Research Directions," available at http://stemcells.nih.gov).
Clearly, therefore, before one may understand the possible applications and related risks of stem cells, one must first understand the differences between the types of stem cells. Only then may one also be able to apply the correct terms in a correct manner. Herein, we shall offer a clarification of the various types of stem cells, their related terms and their differing characteristics.
In their first comprehensive report on stem cells, published in 2001 at the request of Tommy G. Thompson, then Secretary of Health and Human Services, The National Institutes of Health offered the following definition:
"Put simply, stem cells are self-renewing, unspecialized cells that can give rise to multiple types of all specialized cells of the body. The process by which dividing, unspecialized cells are equipped to perform specific functions - muscle contraction or nerve cell communication, for example - is called differentiation, and is fundamental to the development of the mature organism. It is now known that stem cells, in various forms, can be obtained from the embryo, the fetus, and the adult." (From "Stem Cells: Scientific Progress and Future Research Directions," available at http://stemcells.nih.gov).
The authors of this NIH report also add the observation that, "Like all fields of scientific inquiry, research on stem cells raises as many questions as it answers."
Although this review is not intended to be exhaustive, we shall nevertheless address some fundamental questions about the 3 most basic types of stem cells. As listed above, these are:
- Prenatal Stem Cells (embryonic + fetal stem cells)
- Postnatal Sten Cells (umbilical cord + placental)
- Adult Cells (post natal + full grown)
("Full grown" stem cells are derived from various sites throughout the body, such as from fat, muscle, bone marrow, etc.).
The first pluripotent stem cells to be recognized were fetal stem cells, which were first isolated and grown in culture in 1998. (Ibid.) By this time, adult stem cells had already been in clinical use for nearly 40 years, although they were not yet believed to be pluripotent. Recent evidence, however, now suggests that some adult stem cells are capable of pluripotency. (These findings shall be addressed in more detail in the section on adult stem cells).
From an embryo that is 5 to 7 day old, it is possible to obtain "totipotent" human embryonic stem cells (hESC). From an embryo that is six weeks old or less, it is possible to obtain pluripotent human primordial germ cells (hEG). From fetal tissue (older than 8 weeks of development), one may obtain both pluripotent and multipotent human stem cells (hSC). From the umbilical cord and placenta, one may also obtain both pluripotent and multipotent stem cells. In the past, it was believed that, from adult stem cells, one may obtain only multipotent and monopotent stem cells. Now, however, as mentioned, new research has clearly demonstrated that even adult stem cells exhibit pluripotency.
At the beginning of any research involving embryonic or fetal stem cells, there is first a fertilized egg. Male and female gametes unite during fertilization, such that one cell divides into 2 cells, which in turn divide into 4 cells, which divide into 8 cells, etc. By continuous division and growth, the fertilized egg eventually matures into a recognizable organism. The fertilized egg may be said to be the ultimate stem cell, because it eventually becomes the whole body of a person. The fertilized egg is the only known occurrence of "totipotent" stem cells. (Please see the section on "Key Terms").
Through differentiation into different tissues and organs, the fertilized egg develops into the embryo by the 14th day, which in turn develops into the fetus by the 8th week. Ultimately, a baby is born, and once birth has occurred, the stem cells present in the body are known exclusively as "adult stem cells". As a newborn, each of us enjoyed a very high number of stem cells (namely, "adult" stem cells), the most we'll ever have in our lives. After birth, the number of our stem cells then diminishes rapidly.
When a stem cell divides, each "daughter" cell has the potential either to remain a stem cell or to differentiate into another type of cell with a more specialized function, such as a muscle cell, a heart cell, or a blood cell, etc.. Regardless of the name or the type of stem cell, however, they all share the same characteristic: stem cells are what manufacture the constituents of our bodies. Embryonic and fetal stem cells initially create, and adult stem cells continually regenerate throughout life, the cells of the body. Stem cells are constantly involved not only in the formation, but also in the maintenance, of life. They are needed in every organ and bone and tissue of the body throughout one's entire life. Whether you cut your finger or break a bone, stem cells are responsible for healing wounds and injuries. Even without injury, every few hours, days and weeks, stem cells continuously renew the cells of our bodies, replenishing the constant turnover of blood, intestinal lining, skin and other tissue. As long as we are alive, we have stem cells in our bodies. Even people who live past the age of 100 still have their own stem cells, although to a lesser amount than they did when they were younger. And, whether we are one hour old or 100 years old, the stem cells which we have in our bodies are known as "adult" stem cells. Although they are introduced here for purposes of comparison, adult stem cells shall be addressed in more detail in a separate section.
Up until the end of the 2nd trimester, it is possible to perform surgery in utero, and the baby will be born without any scar from the surgery. This perfect, scarless healing of any surgical incision is the result of "fetal" stem cells, which are much more robust in their pluripotency than are adult stem cells. By contrast, there is no such thing as scarless surgery after birth. Additionally, the hyarluronic acid present in the amniotic fluid is of a special nature such that it allows the embryonic and fetal stem cells to continue replicating with very high efficiency (an advantage also absent in the internal milieu in which adult stem cells exist).
An important point often overlooked is the fact that teratocarcinomas (germ cell tumor cells capable of forming teratomas) may also be derived from embryonic stem cells (pluripotent ECs). The danger that such embryonic stem cells may cause cancer is a reality that has been well known since the very early days of stem cell research. Such a danger does not exist with adult stem cells.
In recent years, the media have given very little coverage of placental and umbilical cord stem cells, emphasizing instead the "advantages" of embryonic stem cells and the "disadvantages" of adult stem cells. Similarly, media reports have focused very little attention on the disadvantages of embryonic stem cells, or on the advantages of adult stem cells.
Herein, this review shall hopefully offer a more balanced, and scientifically complete, comparison of the various types of stem cells, and of the potential advantages, disadvantages and risks which all of them have to offer.