Rho signalling restriction by the RhoGAP Stard13 integrates growth and morphogenesis in the pancreas.

The development of functional organ architecture relies on coordinated morphogenesis and growth. In the developing pancreas, the branching epithelium is organised in discrete domains, delineating one specific domain of progenitor cells at the tip of the branches. The molecular mechanisms underlying the coordinated action of branching and proliferation in organ formation are largely unknown. Here, we identify the RhoGAP protein Stard13 as an essential regulator of pancreas tissue architecture in the mammalian embryo. Conditional ablation of Stard13 expression in the pancreas disrupts epithelial morphogenesis and tip-domain organisation, resulting in hampered proliferation of tip progenitors and subsequent organ hypoplasia. Stard13 acts by regulating Rho signalling spatially and temporally during pancreas development. Our findings provide new insights into the mechanisms that shape pancreatic epithelium to create a mature organ and establish a functional link between Rho-mediated control of epithelial remodelling and organ size determination, involving reciprocal interaction of actin-MAL/SRF and MAPK signalling pathways.

A system for ex vivo culturing of embryonic pancreas.

The pancreas controls vital functions of our body, including the production of digestive enzymes and regulation of blood sugar levels. Although in the past decade many studies have contributed to a solid foundation for understanding pancreatic organogenesis, important gaps persist in our knowledge of early pancreas formation. A complete understanding of these early events will provide insight into the development of this organ, but also into incurable diseases that target the pancreas, such as diabetes or pancreatic cancer. Finally, this information will generate a blueprint for developing cell-replacement therapies in the context of diabetes. During embryogenesis, the pancreas originates from distinct embryonic outgrowths of the dorsal and ventral foregut endoderm at embryonic day (E) 9.5 in the mouse embryo. Both outgrowths evaginate into the surrounding mesenchyme as solid epithelial buds, which undergo proliferation, branching and differentiation to generate a fully mature organ. Recent evidences have suggested that growth and differentiation of pancreatic cell lineages, including the insulin-producing ß-cells, depends on proper tissue-architecture, epithelial remodeling and cell positioning within the branching pancreatic epithelium. However, how branching morphogenesis occurs and is coordinated with proliferation and differentiation in the pancreas is largely unknown. This is in part due to the fact that current knowledge about these developmental processes has relied almost exclusively on analysis of fixed specimens, while morphogenetic events are highly dynamic. Here, we report a method for dissecting and culturing mouse embryonic pancreatic buds ex vivo on glass bottom dishes, which allow direct visualization of the developing pancreas (Figure 1). This culture system is ideally devised for confocal laser scanning microscopy and, in particular, live-cell imaging. Pancreatic explants can be prepared not only from wild-type mouse embryos, but also from genetically engineered mouse strains (e.g. transgenic or knockout), allowing real-time studies of mutant phenotypes. Moreover, this ex vivo culture system is valuable to study the effects of chemical compounds on pancreatic development, enabling to obtain quantitative data about proliferation and growth, elongation, branching, tubulogenesis and differentiation. In conclusion, the development of an ex vivo pancreatic explant culture method combined with high-resolution imaging provides a strong platform for observing morphogenetic and differentiation events as they occur within the developing mouse embryo.