These inductive effects were restricted to c-kit+ endoderm-enriched EB-derived populations, suggesting that Hex functions at the level of hepatic specification of endoderm in this model. Microarray analysis revealed that Hex regulated the expression of a broad spectrum of hepatocyte-related
genes, including fibrinogens, apolipoproteins, and cytochromes. When added to the endoderm-induced EBs, bone morphogenetic protein 4 acted synergistically with Hex in the induction of expression of Alb, Afp, carbamoyl phosphate synthetase, transcription factor 1, and CCAAT/enhancer binding protein α. These findings indicate that Hex plays a pivotal role during induction of liver development from endoderm in this in vitro model and suggest that this strategy may provide important insight into the generation of functional hepatocytes from ESCs. (HEPATOLOGY 2010.) In the Wnt inhibitor mouse embryo, the liver is first detected as an outgrowth bud of proliferating endodermal cells in the ventral foregut on day
8 of gestation.1–3 The liver develops in close proximity to the cardiac mesoderm, which produces fibroblast growth factor 1 and 2, which in turn are required for the outgrowth of the ventral foregut endoderm2, GSK126 clinical trial 4 and the induction of several liver-specific genes, including albumin (Alb) and α-fetoprotein (Afp).5 In addition to fibroblast growth factors, bone morphogenetic protein 4 (BMP-4) expressed in the septum transversum mesenchyme6 has been shown to be essential for early liver development.
In the absence of BMP-4, the foregut endoderm does not thicken, and consequently a distinct Rebamipide liver bud does not form. In spite of the lack of liver bud formation in BMP-4–null embryos, Alb expression is induced, suggesting that this factor may play a role in the proper movement of hepatoblasts into the developing liver. Beyond the induction stage, numerous other transcription factors are required for endoderm patterning and organ development. Among these, the hematopoietically expressed homeobox gene Hex (also known as Prh)7–9 is of particular interest, because it has been shown to play a pivotal role in hepatic development. Hex is expressed at multiple sites in the developing embryo, including the yolk sac and the region of gut endoderm that gives rise to the liver and thyroid bud.10–12 Analysis of Hex-null embryos demonstrated that formation of the liver bud initiates in the absence of a functional protein and that expression of liver-specific genes including Alb, Afp, and Ttr is up-regulated in this endodermal population.13, 14 Whereas the early stages of morphogenesis to a columnar structure can be detected in these mutant embryos, development beyond day 9.