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A relatively undifferentiated cell in any tissue responsible for replenishing cells lost from a tissue through normal wear and tear, in the case of adult stem cells, or responsible for virtually all of a fetus's cells, in the case of embryonic stem cells (ES cells); these cells replicate (by mitosis) seemingly for ever, either in the body or in tissue culture, without wearing out the ends of the chromosomes. Embryonic stem cells are controversial because they are derived from human embryos, sometimes by the process of somatic cell nuclear transfer, which involves creating embryos or embryo-like forms of a kind that could be used for cloning. The word stem comes from analogy with the stem of a plant giving rise to branches and further branches, representing the ever-increasing number of different paths a stem cell can differentiate into
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