Multi-lineage 'stem' cells in the mammalian embryo.

The term 'stem' cell has acquired a rather more restricted meaning in cell biology than in embryology as a result of studies on the growth kinetics of renewing tissues in mature organisms. It is normally used in an embryological context as a synonym for 'progenitor' cell. Methods of establishing the existence of multi-lineage progenitor cells in mammals are examined briefly before the occurrence and properties of such cells in both embryonic and extra-embryonic tissues of the mouse conceptus are reviewed. Various attributes of 'stem' cells that can be obtained from outgrowths of blastocysts in vitro are also discussed.