Role of endothelial cells in early pancreas and liver development

Liver and pancreas initially develop by budding from the embryonic endoderm. The formation of these organs coincides with the appearance of endothelial cells (ECs) adjacent to the endoderm. ECs either develop in situ in organs, or are recruited by organs and are induced to form blood vessels. Recent reports on liver and pancreas have now shown that ECs also induce essential steps in organ formation such as morphogenesis and cell differentiation. This review summarizes reports on EC signaling during organogenesis and cell differentiation.     

Interplay between Wnt2 and Wnt2bb controls multiple steps of early foregut-derived organ development

The vertebrate liver, pancreas and lung arise in close proximity from the multipotent foregut endoderm. Tissue-explant experiments uncovered instructive signals emanating from the neighbouring lateral plate mesoderm, directing the endoderm towards specific organ fates. This suggested that an intricate network of signals is required to control the specification and differentiation of each organ. Here, we show that sequential functions of Wnt2bb and Wnt2 control liver specification and proliferation in zebrafish. Their combined specific activities are essential for liver specification, as their loss of function causes liver agenesis. Conversely, excess wnt2bb or wnt2 induces ectopic liver tissue at the expense of pancreatic and anterior intestinal tissues, revealing the competence of intestinal endoderm to respond to hepatogenic signals. Epistasis experiments revealed that the receptor frizzled homolog 5 (fzd5) mediates part of the broader hepatic competence of the alimentary canal. fzd5 is required for early liver formation and interacts genetically with wnt2 as well as wnt2bb. In addition, lack of both ligands causes agenesis of the swim bladder, the structural homolog of the mammalian lung. Thus, tightly regulated spatiotemporal expression of wnt2bb, wnt2 and fzd5 is central to coordinating early liver, pancreas and swim bladder development from a multipotent foregut endoderm.     

Unique and conserved aspects of gut development in zebrafish

Although the development of the digestive system of humans and vertebrate model organisms has been well characterized, relatively little is known about how the zebrafish digestive system forms. We define developmental milestones during organogenesis of the zebrafish digestive tract, liver, and pancreas and identify important differences in the way the digestive endoderm of zebrafish and amniotes is organized. Such differences account for the finding that the zebrafish digestive system is assembled from individual organ anlagen, whereas the digestive anlagen of amniotes arise from a primitive gut tube. Despite differences of organ morphogenesis, conserved molecular programs regulate pharynx, esophagus, liver, and pancreas development in teleosts and mammals. Specifically, we show that zebrafish faust/gata-5 is a functional ortholog of gata-4, a gene that is essential for the formation of the mammalian and avian foregut. Further, extraembryonic gata activity is required for this function in zebrafish as has been shown in other vertebrates. We also show that a loss-of-function mutation that perturbs sonic hedgehog causes defects in the development of the esophagus that parallel those associated with targeted disruption of this gene in mammals. Perturbation of sonic hedgehog also affects zebrafish liver and pancreas development, and these effects occur in a reciprocal fashion, as has been described during mammalian liver and ventral pancreas development. Together, these data define aspects of digestive system development necessary for the characterization of zebrafish mutants. Given the similarities of teleost and mammalian digestive physiology and anatomy, these findings have implications for developmental and evolutionary studies as well as research of human diseases, such as diabetes, liver cirrhosis, and cancer.     

A bipotential precursor population for pancreas and liver within the embryonic endoderm

The pancreas emerges independently from dorsal and ventral domains of embryonic gut endoderm. Gene inactivation experiments in mice have identified factors required for dorsal pancreas development, but factors that initiate the ventral pancreas have remained elusive. In this study, we investigated the hypothesis that the emergence of the ventral pancreas is related to the emergence of the liver. We find that the liver and ventral pancreas are specified at the same time and in the same general domain of cells. Using embryo tissue explantation experiments, we find that the default fate of the ventral foregut endoderm is to activate the pancreas gene program. FGF signalling from the cardiac mesoderm diverts this endoderm to express genes for liver instead of those for pancreas. No evidence was found to indicate that the cell type choice for pancreas or liver involves a selection for growth or viability. Cardiac mesoderm or FGF induces the local expression of sonic hedgehog, which in turn is inhibitory to pancreas but not to liver. The bipotential precursor cell population for pancreas and liver in embryonic development and its fate selection by FGF has features that appear to be recapitulated in the adult pancreas and are reflected in the evolution of these organs.     

Growth-limiting role of endothelial cells in endoderm development

Endoderm development is dependent on inductive signals from different structures in close vicinity, including the notochord, lateral plate mesoderm and endothelial cells. Recently, we demonstrated that a functional vascular system is necessary for proper pancreas development, and that sphingosine-1-phosphate (S1P) exhibits the traits of a blood vessel-derived molecule involved in early pancreas morphogenesis. To examine whether S1P₁-signaling plays a more general role in endoderm development, S1P₁-deficient mice were analyzed. S1P₁ ablation results in compromised growth of several foregut-derived organs, including the stomach, dorsal and ventral pancreas and liver. Within the developing pancreas the reduction in organ size was due to deficient proliferation of Pdx1⁺ pancreatic progenitors, whereas endocrine cell differentiation was unaffected. Ablation of endothelial cells in vitro did not mimic the S1P₁ phenotype, instead, increased organ size and hyperbranching were observed. Consistent with a negative role for endothelial cells in endoderm organ expansion, excessive vasculature was discovered in S1P₁-deficient embryos. Altogether, our results show that endothelial cell hyperplasia negatively influences organ development in several foregut-derived organs.     

The molecular basis and prospects in pancreatic development

Studies on the signaling mechanism that control the specification of endoderm-derived organs have only recently begun. While many studies revealed genes involved in the differentiation, growth and morphogenesis of the pancreas through studies of mutant mice, still little is known about how endoderm give rise to specific domains. Although many genes are known to have a role in pancreatic differentiation, growth and morphogenesis, few genes are known to take part in the specification of the pancreas so far. Hallmarks as well as gene markers for early development of the pancreas, which are however still very limited, will be useful for dissecting early events in pancreatic specification. Here, I give a summary on the origin of the dorsal and ventral pancreatic progenitors, signals for inductions, and genes so far known to function in pancreatic differentiation. I also give a future prospect in the use of ES cells and other experimental models, towards a comprehensive understanding of gene networks in the progenitor cells or intermediate cell types which arise during various stages of differentiation.     

The endoderm of the mouse embryo arises by dynamic widespread intercalation of embryonic and extraembryonic lineages

The cell movements underlying the morphogenesis of the embryonic endoderm, the tissue that will give rise to the respiratory and digestive tracts, are complex and not well understood. Using live imaging combined with genetic labeling, we investigated the cell behaviors and fate of the visceral endoderm during gut endoderm formation in the mouse gastrula. Contrary to the prevailing view, our data reveal no mass displacement of visceral endoderm to extraembryonic regions concomitant with the emergence of epiblast-derived definitive endoderm. Instead, we observed dispersal of the visceral endoderm epithelium and extensive mixing between cells of visceral endoderm and epiblast origin. Visceral endoderm cells remained associated with the epiblast and were incorporated into the early gut tube. Our findings suggest that the segregation of extraembryonic and embryonic tissues within the mammalian embryo is not as strict as believed and that a lineage previously defined as exclusively extraembryonic contributes cells to the embryo.