Bipotent mesendoderm, a source of both mesoderm and endoderm, is an established entity in some embryos. However, little is known about this potential precursor of the definitive endoderm in mammalian embryos, although fate mapping hints at its presence in the organizer region. On , Tada et al. report their characterization of bipotent mouse mesendoderm isolated from ES cells in vitro. For their study, the authors generated ES cell lines that carry a GFP gene in the goosecoid (Gsc) gene locus- Gsc is expressed specifically in the organizer region from which definitive endoderm arises. By treating these ES lines with activin or nodal under defined conditions, the researchers induced a largely Gsc+population that contained Gsc+, E-cadherin+,PDGFRα+ cells that could differentiate into both endoderm and mesoderm. These studies bring the possibility of producing mature definitive endoderm cells - such as pancreatic islets - for therapeutic use a step closer.
