Current Status of Human Stem Cell Research

“Stem cells” is a term to describe precursor cells that can give rise to multiple tissue types. There are important distinctions, however, regarding how developmentally plastic these cells are; that is, how many different paths they can follow and to what portion of a functioning organism they can contribute. Totipotent stem cells are cells that can give rise to a fully functional organism as well as to every cell type of the body. Pluripotent stem cells are capable of giving rise to virtually any tissue type, but not to a functioning organism. Multipotent stem cells are more differentiated cells (that is, their possible lineages are less plastic/more determined) and thus can give rise only to a limited number of tissues. For example, a specific type of multipotent stem cell called a mesenchymal stem cell has been shown to produce bone, muscle, cartilage, fat, and other connective tissues.

There are many potential sources for stem cells. Embryonic stem cells are derived from the inner cell mass of a blastocyst (a very early embryo). Embryonic germ cells are collected from fetal tissue at a somewhat later stage of development (from a region called the gonadal ridge), and the cell types that they can develop into may be slightly limited. Adult stem cells are derived from mature tissue. Even after complete maturation of an organism, cells need to be replaced (a good example is blood, but this is true for muscle and other connective tissue as well, and may be true for at least some nervous system cells). Because these give rise to a limited number of cell types, they are perhaps more accurately referred to as multipotent stem cells, as discussed above.

Knowledge about stem cell science and potential applications has been accumulating for more than 30 years. In the 1960s, it was recognized that certain mouse cells had the capacity to form multiple tissue types, and the discovery of bona fide stem cells from mice occurred in 1971. Limited types of stem cell therapies are already in use. The most well-known therapy is the stem cell transplant (a form of a bone marrow transplant) for cancer patients. In this therapy, stem cells that can give rise to blood cells (red and white cells, and platelets) are given to patients to restore tissue destroyed by high dose chemotherapy or radiation therapy. But it has been only recently that scientists have understood stem cells well enough to consider the possibilities of growing them outside the body for long periods of time. With that advance, rigorous experiments can be conducted, and the possibility of manipulating these cells in such a way that specific tissues can be grown is real. It is impossible to project when actual treatments or cures might emerge from such research, but the paths this research might take and potential applications have been much discussed. To understand the potential clinical applications, it is critical to understand the research that is taking place now.

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