It's only one kind of stem cell -- embryonic stem cells -- that's controversial. Procuring embryonic stem cells requires the destruction of an embryo. The embryos that are used were marked for the trash can, as they have been rejected by an in vitro fertilization expert and donated to laboratories by the parents. That back story doesn't satisfy those who believe that life starts at the moment of conception. Since harvesting stem cells comes at the expense of an embryo that could have become a person, then some people believe that the ends of stem cell research don't satisfy the means. In recent years, scientists have been experimenting with ways to make adult stem cells behave more like embryonic stem cells, as well as determining if there's a way to extract embryonic stem cells without harming the embryo.
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