From stem cells to tissue-specific differentiation.
Stem cells have been isolated from embryonic, foetal and adult sources. Embryonic stem cells, derived from the pre-implantation embryo, can be expanded indefinitely in vitro. When reintroduced into the blastocyst they contribute to all lineages in vivo. In vitro, differentiated derivatives of embryonic stem cells are obtained by manipulating culture conditions. Embryonic germ cells, derived from the primordial germ cells, display the same degree of pluripotential differentiation. Adult stem cells that repopulate the tissue of origin throughout life possess the ability to differentiate into phenotypes not restricted to the tissue and, in some cases, to the germ layer from which they derive. Although the pluripotential differentiation capability of these stem-cell populations is supported by an increasing amount of evidence, a better understanding of the mechanisms that control differentiation is required for their exploitation in the treatment of human diseases.