The very early mouse embryo is initially covered by a layer of cells called visceral endoderm that despite its name (visceral means associated with internal organs) was only thought to give rise to extraembryonic structures. Now, in this issue of Developmental Cell, Kwon et al. (2008) show unexpectedly that a significant population of visceral endoderm cells does contribute, along with epiblast-derived endoderm, to the embryo proper.
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