Figure 7.

Schematic depicting key points during hepatic development and the stage at which specific transcription factors first act. During gastrulation, at ∼7.0 days of gestation in the mouse, the definitive endoderm (yellow) is generated from the primitive ectoderm. Both ventral and dorsal aspects of the definitive embryonic endoderm express HNF-3 and are competent to adopt a hepatic fate. By the four- to six-somite stage a ventral portion of the embryonic endoderm (blue) lies close to the developing heart. During hepatic specification, FGFs (fgf's) from the developing cardiac mesoderm (red) induce the underlying endoderm to commit to a hepatic fate. In response to this signal the endodermal cells proliferate, expand into the surrounding septum transversum, and adopt a stellate morphology. At this point these cells are considered hepatoblasts. The hepatoblasts continue to proliferate and begin to differentiate to become mature hepatocyte in a process that requires HNF-4α.