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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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| Contributed by: AAAS DoSER and the Institute for
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