The first pluripotent embryonic cells were isolated from teratocarcinomas, a spontaneous tumor of the testes in mice and humans. The tumors resembled a disorganized fetus consisting of a wide variety of tissues including hair, muscle, bone, and teeth. Developmental biologists discovered teratocarcinomas could be artificially induced by transferring mouse embryos to extrauterine sites and that they contained undifferentiated stem cells. These embryonic carcinoma (EC) stem cells, once isolated, could grow in culture without losing the capacity to differentiate. When these cells were introduced into a blastocyst, they formed chimeras and could contribute to all somatic tissues. EC cells exhibit an unstable karyotype, which reduces their experimental value despite their capacity to differentiate into all three germ layers.
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