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Key Terms
Totipotency: This is the ability of a stem cell to differentiate into all possible types of cells in the body. Its potential is total (from the Latin "totus", meaning "total"). Only the fertilized egg (zygote) is considered to be totipotent. The term "totipotency" in regard to plants (which is the ability to grow an entire plant from a shoot or cutting) has been used for many years, but only with the advent of stem cell therapy has the term entered the biomedical lexicon. It is believed that, at least theoretically, all living cells have the potential for totipotency, since all living cells carry the necessary genetic information; but the exact mechanisms by which totipotency is "triggered" remain unknown.
Pluripotency: After the fertilized egg has developed into an embryo, its totipotency has yielded to pluripotency. Pluripotent stem cells are able to develop into all possible types of cells in the body, except those needed to develop a fetus. Embryonic, fetal, and post-natal (umbilical plus placental) stem cells are pluripotent. Usually isolated from embryos a few days old, embryonic stem cells are used to create pluripotent stem cell "lines", grown in the laboratory. Pluripotent stem cell lines have also been developed from fetal tissue (older than 8 weeks of development in gestation). As authors of the previously cited NIH report have written, "A single pluripotent stem cell has the ability to give rise to types of cells that develop from the three germ layers (mesoderm, endoderm, and ectoderm), from which all the cells of the body arise." The 2001 NIH report also states that, "The only known sources of human pluripotent stem cells are those isolated and cultured from early human embryos and from fetal tissue that was destined to be part of the gonads." As we shall see in a later section, the known sources of human pluripotent stem cells are today, in 2005, more than they were in 2001.
Multipotency: After birth, most of the adult stem cells that are present no longer exhibit pluripotency, but instead exhibit multipotency. Multipotent stem cells are capable of differentiating into multiple types of cells, but not all possible types. Most adult stem cells have been previously believed to be either multipotent or monopotent, and this has constituted the crux of the argument against the therapeutic use of adult stem cells. As we shall later see in more detail, this argument is not always valid.
Monopotency: Many types of adult stem cells exhibit "monopotency", meaning that they are capable of differentiating into only one particular type of cell. This "specialized" feature has typically been considered one of the major drawbacks of adult stem cells.
Of all the various types of stem cells, the NIH has stated that,
"Pluripotent stem cells offer the greatest possibility of a renewable source of replacement cells and tissues to treat a myriad of diseases, conditions and disabilities, including Parkinson's and Alzheimer's diseases, spinal cord injury, stroke, burns, heart disease, diabetes, osteoarthritis and rheumatoid arthritis." (From "The Promise of Stem Cells", available at http://stemcells.nih.gov).
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