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Ted Peters describes the following article as the “most exhaustive and careful parsing of the scientific and moral issues surrounding chimerism in research.”
Stem cells are broadly classified as either adult or embryonic. Technically, even stem cells that come from fetal tissue or umbilical cord blood are classified as adult stem cells, and so most researchers prefer the term tissue stem cells for all stem cells other than those from embryos.
Embryonic stem cells can, in theory, produce any type of tissue in large quantities. Researchers use these cells to study development and disease and, hopefully, to find treatments.
In the near term, stem cells that are genetically identical to someone with, say, diabetes or a neurodegenerative disease can be used to study that disease. Eventually, genetically matched cells could mean that patients receiving cell therapies wouldn't need antirejection drugs.
Scientists want to have a variety of stem cell lines so that they can pick the best ones for their experiments. Some lines are better suited for becoming pancreatic cells, others for neurons (no one is sure why). Moreover, older cell lines are harder to grow, and many contain mutations and chromosomal abnormalities.