A glimpse into the molecular entrails of endoderm formation

The formation of an ancient tissue like the endoderm is expected to utilize common molecular pathways across the phyla, and the studies so far point in that very direction (Table 1). However, this comparative analysis also points to clear gaps in our knowledge, such as the potential role of TGF-β signaling in endoderm formation in more primitive organisms and that of Wnt signaling in vertebrates. In the next several years, we can expect a rapid elucidation of these questions, as well the continuing investigation of more complex issues, such as how the various endodermal organs form. Equally exciting will be attempts to coax stem cells to form endodermal tissue, a likely prerequisite to forming endodermal organs such as the pancreas and liver de novo.