The mechanism by which left-right (LR) information is interpreted by organ primordia during asymmetric morphogenesis is largely unknown. We show that spleen and pancreatic laterality is dependent on a specialised, columnar mesodermal-derived cell layer referred to here as the splanchnic mesodermal plate (SMP). At early embryonic stages, the SMP is bilateral, surrounding the midline-located stomach and dorsal pancreatic bud. Under control of the LR asymmetry pathway, the left SMP is maintained and grows laterally. Mice carrying the dominant hemimelia (Dh) mutation lack the SMP. Significantly, the mice are asplenic and the pancreas remains positioned along the embryonic midline. In the absence of Fgf10 expression, the spleno-pancreatic mesenchyme and surrounding SMP grow laterally but contain no endodermal component, showing that leftward growth is autonomous and independent of endoderm. In the Bapx1–/–mutants, the SMP is defective. Normally, the SMP is a source for both Fgf9 and Fgf10; however, in the Bapx1 mutant, Fgf10 expression is downregulated and the dorsal pancreas remains at the midline. We conclude that the SMP is an organiser responsible for the leftward growth of the spleno-pancreatic region and that Bapx1 regulates SMP functions required for pancreatic laterality.

The middle ear apparatus is composed of three endochondrial ossicles (the stapes, incus and malleus) and two membranous bones, the tympanic ring and the gonium, which act as structural components to anchor the ossicles to the skull. Except for the stapes, these skeletal elements are unique to mammals and are derived from the first and second branchial arches. We show that, in combination with goosecoid (Gsc), the Bapx1 gene defines the structural components of the murine middle ear.During embryogenesis, Bapx1 is expressed in a discrete domain within the mandibular component of the first branchial arch and later in the primordia of middle ear-associated bones, the gonium and tympanic ring. Consistent with the expression pattern of Bapx1, mouse embryos deficient for Bapx1 lack a gonium and display hypoplasia of the anterior end of the tympanic ring. At E10.5, expression of Bapx1partially overlaps that of Gsc and although Gsc is required for development of the entire tympanic ring, the role of Bapx1 is restricted to the specification of the gonium and the anterior tympanic ring. Thus, simple overlapping expression of these two genes appears to account for the patterning of the elements that compose the structural components of the middle ear and suggests that they act in concert.In addition, Bapx1 is expressed both within and surrounding the incus and the malleus. Examination of the malleus shows that the width, but not the length, of this ossicle is decreased in the mutant mice. In non-mammalian jawed vertebrates, the bones homologous to the mammalian middle ear ossicles compose the proximal jaw bones that form the jaw articulation(primary jaw joint). In fish, Bapx1 is responsible for the formation of the joint between the quadrate and articular (homologues of the malleus and incus, respectively) enabling an evolutionary comparison of the role of a regulatory gene in the transition of the proximal jawbones to middle ear ossicles. Contrary to expectations, murine Bapx1 does not affect the articulation of the malleus and incus. We show that this change in role of Bapx1 following the transition to the mammalian ossicle configuration is not due to a change in expression pattern but results from an inability to regulate Gdf5 and Gdf6, two genes predicted to be essential in joint formation.

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During early stages of pancreatic development, the mesenchyme that contributes to the spleen overlies the dorsal pancreatic endoderm. Here, we show that interactions between splenic mesenchyme and pancreas proceed via a highly orchestrated morphogenetic program. Disruption of morphogenesis, as occurs in the Bapx1(Nkx3.2) −/− embryo, results in transformation of these tissues into well-organized, ectopic gut-like structures. Bapx1 plays a crucial organizing role effecting position and separation of the spleen and pancreas to prevent this metaplastic transformation. Similar transformations occur in organ cultures employing wild-type pancreatic endoderm and spleen mesenchyme, revealing the developmental plasticity of the pancreas and that precise spatial and temporal control of tissue interactions are required for development of both organs. Epithelial-mesenchymal interactions are central to the development of the primitive gut tube endoderm and are necessary to form fully functional parts of the gastrointestinal tract. Such interactions come into play in a stepwise manner during development of the dorsal pancreas and begin with patterning signals from the mesectoderm. Later signals from the notochord repress endodermal SHH and thereby permit expression of pancreatic genes in the dorsal pancreatic anlage. Formation of the dorsal aorta will separate the notochord from the pancreatic endoderm and will in turn provide signals that induce insulin expression (Lammert et al. 2001 and references therein). Finally, splanchnic mesenchyme will accumulate around the dorsal pancreatic bud and will promote growth and differentiation of the developing pancreatic epithelium (Kim and Hebrok 2001). Studies indicate that this accumulated mesenchyme provides permissive, rather than instructive, cues (Edlund 2002). Hence, the primary role of the mesenchymal signals at this stage is to stimulate proliferation of the different pancreatic components and not to determine developmental fate. Apart from being important for interaction with underlying epithelium, the mesenchyme surrounding the dorsal pancreas also contributes to development of the spleen during early stages of development ( Fig. 1A; Kanzler and Dear 2001;Hecksher-Sorensen et al. 2004).The mammalian homeobox gene Bapx1 (Nkx3.2) encodes a putative transcription factor that belongs to the Nkx gene family, most similar to the Drosophila homolog bap (Kim and Nirenberg 1989). Bapx1-null mutants show visceral mesoderm defects manifesting as asplenia and impaired pyloric sphincter formation (Lettice et al. 1999;Tribioli and Lufkin 1999;Akazawa et al. 2000). To better understand the dynamic developmental relationships between the spleen and pancreas, we have analyzed the Bapx1-null mutant, a model in which direct interaction between spleen and pancreas can be addressed. Here we show that heterotopic tissue outgrowths with the differentiated characteristics of gut form in the mutant. These are derived from the transformation of pancreatic endoderm and adjacent mesenchyme. Metaplasia w...

BackgroundThe integration of host genetics, environmental triggers and the microbiota is a recognised factor in the pathogenesis of barrier function diseases such as IBD. In order to determine how these factors interact to regulate the host immune response and ecological succession of the colon tissue-associated microbiota, we investigated the temporal interaction between the microbiota and the host following disruption of the colonic epithelial barrier.Methodology/Principal FindingsOral administration of DSS was applied as a mechanistic model of environmental damage of the colon and the resulting inflammation characterized for various parameters over time in WT and Nod2 KO mice.ResultsIn WT mice, DSS damage exposed the host to the commensal flora and led to a migration of the tissue-associated bacteria from the epithelium to mucosal and submucosal layers correlating with changes in proinflammatory cytokine profiles and a progressive transition from acute to chronic inflammation of the colon. Tissue-associated bacteria levels peaked at day 21 post-DSS and declined thereafter, correlating with recruitment of innate immune cells and development of the adaptive immune response. Histological parameters, immune cell infiltration and cytokine biomarkers of inflammation were indistinguishable between Nod2 and WT littermates following DSS, however, Nod2 KO mice demonstrated significantly higher tissue-associated bacterial levels in the colon. DSS damage and Nod2 genotype independently regulated the community structure of the colon microbiota.Conclusions/SignificanceThe results of these experiments demonstrate the integration of environmental and genetic factors in the ecological succession of the commensal flora in mammalian tissue. The association of Nod2 genotype (and other host polymorphisms) and environmental factors likely combine to influence the ecological succession of the tissue-associated microflora accounting in part for their association with the pathogenesis of inflammatory bowel diseases.