Human Embryonic Stem Cells

The study of human stem cells has barely begun and what is known is summarized in this section. The vast majority of experimental data discussed here are the results of experiments in mice. ES cells from the mouse have been intensely investigated since their discovery 18 years ago. Therefore, what is said about human ES cells assumes in part that their fundamental properties will resemble those of mouse ES cells. While on the surface this assumption appears to be reasonable it will have to be proven through intensive further investigation.

There is an abundance of stem cell lines from mammals including some from human beings. ES cells are valuable scientifically because they combine three properties not found together in other cell lines. First, they appear to replicate indefinitely without undergoing senescence (aging and death) or mutation of the genetic material. They are thus a large-scale and valuable source of cells. Second, ES cells appear genetically normal, both by a series of genetic tests and functionally as shown by the creation of mice with genomes derived entirely from ES cells. In mice these cells are developmentally totipotent; when inserted into an early embryo, they join the host cells to create a normal mouse, differentiating into every cell type of the body (it is this property that earns them the name “stem cell of the body”). ES cells can also differentiate into many cell types in tissue culture, including neurons, blood cells and cardiac and skeletal muscle. The normal embryo has about 100 cells with the properties of ES cells that exist for about one day and then develop into more advanced cell types. The isolation and subsequent growth of ES cells in culture allow scientists to obtain millions of these cells in a single tissue culture flask, making something once rare and precious now readily available to researchers. It is worth noting here the striking parallel to recombinant DNA and monoclonal antibody technologies, both of which have amplified rare and precious biological entities. Like those technologies, ES cell technology may well be transformative in opening scientific arenas that to date have been closed.

The isolation, culture, and partial characterization of stem cells isolated from human embryos was reported in November of 1998. The ability of the cells to maintain their pluripotent character even after 4 to 5 months of culturing was demonstrated. There is concern that this feature of these cells could also lead to cancerous growth. Thus far there are no data indicating the induction of malignant tumors, although there is some evidence for benign hyperproliferation (overgrowth of cells).

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