Notch signaling is a crucial regulator of SM differentiation of EPDCs, and thus, of formation of a functional coronary system.

After specification of the hepatic endoderm, mammalian liver organogenesis progresses through a series of morphological stages that culminate in the migration of hepatocytes into the underlying mesenchyme to populate the hepatic lobes. Here, we show that in the mouse the transcriptional repressor Tbx3, a member of the T-box protein family, is required for the transition from a hepatic diverticulum with a pseudo-stratified epithelium to a cell-emergent liver bud. In Tbx3-deficient embryos, proliferation in the hepatic epithelium is severely reduced, hepatoblasts fail to delaminate, and cholangiocyte rather than hepatocyte differentiation occurs. Molecular analyses suggest that the primary function of Tbx3 is to maintain expression of hepatocyte transcription factors, including hepatic nuclear factor 4a (Hnf4a) and CCAAT/enhancer binding protein (C/EBP), alpha (Cebpa), and to repress expression of cholangiocyte transcription factors such as Onecut1 (Hnf6) H epatocytes and cholangiocytes constitute the liver parenchyme and the bile-transporting cells of the intrahepatic and extrahepatic bile ducts, respectively. Both cell types derive from a bipotential precursor cell, the hepatoblast, whose specification, expansion, and differentiation is intimately linked with morphogenesis of the liver. 1 Liver development in the mouse begins at embryonic day (E) 8.25 after the formation of the definitive endoderm. Signals from the precardiogenic mesoderm and the underlying septum transversum region act in combination to induce and delineate the hepatic from the neighboring pancreatic and intestinal endoderm. Hepatoblasts activate an early liver gene program and form a thickened columnar epithelium that becomes pseudo-stratified at E9.0. Starting from E9.5, the basal lamina degrades, and finger-like protrusions arise from which individual cells migrate into the underlying mesenchyme and populate the hepatic lobes. Although most hepatoblasts differentiate into hepatocytes, a subset of these cells maintain their precursor character and differentiate into cholangiocytes that form the lining of the bile ducts, starting from E13.5. Thus, differentiation of hepatoblasts into hepatocytes or bile duct cells is temporally and spatially separated, suggesting the existence of localized inducers or repressing mechanisms that direct either fate. 2,3 Phenotypical analysis of mutant mice has provided substantial insight into a molecular network of transcriptional regulators that control distinct subprograms of liver organogenesis. 2,3 Tbx3, a member of the T-box gene family, has recently emerged as an additional player in the genetic circuit underlying the hepatic lineage decision. Heterozygosity of TBX3 causes Ulnar-mammary syndrome in humans, an autosomal-dominant disorder characterized by upper limb skeletal malformations, severe hypoplasia of the breast, and hair and genital defects. 4 Tbx3-homozygous mice present ulnar-mammary syndrome-related features, including severe defects in limb and mammary gland development.

Numerous signals drive the proliferative expansion of the distal endoderm and the underlying mesenchyme during lung branching morphogenesis, but little is known about how these signals are integrated. Here, we show by analysis of conditional double mutants that the two T-box transcription factor genes Tbx2 and Tbx3 act together in the lung mesenchyme to maintain branching morphogenesis. Expression of both genes depends on epithelially derived Shh signaling, with additional modulation by Bmp, Wnt, and Tgfβ signaling. Genetic rescue experiments reveal that Tbx2 and Tbx3 function downstream of Shh to maintain pro-proliferative mesenchymal Wnt signaling, in part by direct repression of the Wnt antagonists Frzb and Shisa3. In combination with our previous finding that Tbx2 and Tbx3 repress the cell-cycle inhibitors Cdkn1a and Cdkn1b, we conclude that Tbx2 and Tbx3 maintain proliferation of the lung mesenchyme by way of at least two molecular mechanisms: regulating cell-cycle regulation and integrating the activity of multiple signaling pathways.