In mammals, stem cells are defined as a unique cell population characterized by nearly unlimited self-renewal and capacity to differentiate via progenitor cells into terminally differentiated somatic cells. Stem cells may be of embryonic or adult origin. Adult stem cells are located in many specialized tissues, including the liver, skin, brain, fat, bone marrow, and muscle. As a result of stem cell activity, adult tissues are continuously renewed, even in the absence of injury, to ensure maintenance of cell type throughout the life of the animal. Pluripotency in mammals is restricted to the zygote, early embryonic cells, primordial germ cells, and the stem cells derived from embryonic carcinomas. Embryonic stem (ES) cells are derived from the inner cell mass of the blastocyst. In contrast to adult stem cells, ES cells are pluripotent, contribute to all three primary germ layers (endoderm, mesoderm, and ectoderm), indefinitely proliferate, and maintain an undifferentiated phenotype. Embryonic germ (EG) cells are derived from primordial germ cells, which are progenitor cells of the sperm and egg in the adult animal. EG cells reintroduced to the early embryo are capable, like ES cells, of colonizing fetal cell lineages and also possess the ability to differentiate in vitro to a variety of cell types. ES and EG cell lines from mammalian species other than the mouse have not been reported in published literature to successfully colonize the germ line of chimeric animals. The potential of stem-cell technology makes it a valuable and exciting science. Adult and embryonic stem cells may possess the ability to restore or replace tissue that has been damaged by disease or injury. Pluripotent, in vitro cell lines offer an opportunity to study the early stages of embryonic development not accessible in utero and are a powerful tool to facilitate genetic modification of animal genomes.

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