Mesenchymal Wnt signaling promotes formation of sternum and thoracic body wall

Pentalogy of Cantrell in Two Neonate Littermate Puppies: A Spontaneous Animal Model Suggesting Familial Inheritance

Developmental anomalies are an important cause of stillbirth and early perinatal death in companion animals. Many of these disorders remain poorly understood and provide an opportunity as a spontaneous animal model for human disease. Pentalogy of Cantrell is a rare congenital syndrome described in human neonates. It is a ventral midline closure defect with a proposed familial inheritance in humans. This syndrome involves five defects, including the thoracoabdominal wall, sternal, diaphragmatic, pericardial and cardiac malformations. Diverse expressions of these defects have been described in humans and sporadically in domestic animals. This severe syndrome commonly harbors a poor prognosis, posing an ethical and surgical dilemma. To better understand this syndrome and its presentation in dogs, we describe two rare cases of Pentalogy of Cantrell in a litter of papillon dogs. The affected puppies had anomalies compatible with the Pentalogy of Cantrell, including thoracoabdominal schisis, ectopia cordis, sternal cleft, pericardial agenesis, and diaphragmatic defects. The diagnosis was confirmed by advanced imaging (computed tomography) and postmortem examinations. The family history of this litter was explored and other cases in domestic animals were reviewed. This is the first report of the complete Pentalogy of Cantrell with ectopia cordis in the dog and the only report on papillons. Similar to human cases, possible familial inheritance and suspected male gender bias were observed. Further research on this novel animal model, its pathogenesis and its hereditary basis, may be helpful in better understanding this rare developmental disorder.

Disruption of BMP4 signaling is associated with laryngeal birth defects in a mouse model

Laryngeal birth defects are considered rare, but they can be life-threatening conditions. The BMP4 gene plays an important role in organ development and tissue remodeling throughout life. Here we examined its role in laryngeal development complementing similar efforts for the lung, pharynx, and cranial base. Our goal was to determine how different imaging techniques contribute to a better understanding of the embryonic anatomy of the normal and diseased larynx in small specimens. Contrast-enhanced micro CT images of embryonic larynx tissue from a mouse model with Bmp4 deletion informed by histology and whole-mount immunofluorescence were used to reconstruct the laryngeal cartilaginous framework in three dimensions. Laryngeal defects included laryngeal cleft, laryngeal asymmetry, ankylosis and atresia. Results implicate BMP4 in laryngeal development and show that the 3D reconstruction of laryngeal elements provides a powerful approach to visualize laryngeal defects and thereby overcoming shortcomings of 2D histological sectioning and whole mount immunofluorescence.

The role of Evi/Wntless in exporting Wnt proteins

Intercellular communication by Wnt proteins governs many essential processes during development, tissue homeostasis and disease in all metazoans. Many context-dependent effects are initiated in the Wnt-producing cells and depend on the export of lipidated Wnt proteins. Although much focus has been on understanding intracellular Wnt signal transduction, the cellular machinery responsible for Wnt secretion became better understood only recently. After lipid modification by the acyl-transferase Porcupine, Wnt proteins bind their dedicated cargo protein Evi/Wntless for transport and secretion. Evi/Wntless and Porcupine are conserved transmembrane proteins, and their 3D structures were recently determined. In this Review, we summarise studies and structural data highlighting how Wnts are transported from the ER to the plasma membrane, and the role of SNX3-retromer during the recycling of its cargo receptor Evi/Wntless. We also describe the regulation of Wnt export through a post-translational mechanism and review the importance of Wnt secretion for organ development and cancer, and as a future biomarker.

Epithelial Wntless regulates postnatal alveologenesis

Alveologenesis requires the coordinated modulation of the epithelial and mesenchymal compartments to generate mature alveolar saccules for efficient gas exchange. However, the molecular mechanisms underlying the epithelial-mesenchymal interaction during alveologenesis are poorly understood. Here, we report that Wnts produced by epithelial cells are crucial for neonatal alveologenesis. Deletion of the Wnt chaperone protein Wntless homolog (Wls) disrupts alveolar formation, resulting in enlarged saccules in Sftpc-Cre/Nkx2.1-Cre; Wlsloxp/loxp mutants. Although commitment of the alveolar epithelium is unaffected, α-SMA+ mesenchymal cells persist in the alveoli, accompanied by increased collagen deposition, and mutants exhibit exacerbated fibrosis following bleomycin challenge. Notably, α-SMA+ cells include a significant number of endothelial cells resembling endothelial to mesenchymal transition (EndMT), which is also present in Ager-CreER; Wlsloxp/loxp mutants following early postnatal Wls deletion. These findings provide initial evidence that epithelial-derived Wnts are crucial for the differentiation of the surrounding mesenchyme during early postnatal alveologenesis.

Making and shaping endochondral and intramembranous bones

Skeletal elements have a diverse range of shapes and sizes specialized to their various roles including protecting internal organs, locomotion, feeding, hearing, and vocalization. The precise positioning, size, and shape of skeletal elements is therefore critical for their function. During embryonic development, bone forms by endochondral or intramembranous ossification and can arise from the paraxial and lateral plate mesoderm or neural crest. This review describes inductive mechanisms to position and pattern bones within the developing embryo, compares and contrasts the intrinsic vs extrinsic mechanisms of endochondral and intramembranous skeletal development, and details known cellular processes that precisely determine skeletal shape and size. Key cellular mechanisms are employed at distinct stages of ossification, many of which occur in response to mechanical cues (eg, joint formation) or preempting future load‐bearing requirements. Rapid shape changes occur during cellular condensation and template establishment. Specialized cellular behaviors, such as chondrocyte hypertrophy in endochondral bone and secondary cartilage on intramembranous bones, also dramatically change template shape. Once ossification is complete, bone shape undergoes functional adaptation through (re)modeling. We also highlight how alterations in these cellular processes contribute to evolutionary change and how differences in the embryonic origin of bones can influence postnatal bone repair.

Compares and contrasts Endochondral and intramembranous bone development

Reviews embryonic origins of different bones

Describes the cellular and molecular mechanisms of positioning skeletal elements.

Describes mechanisms of skeletal growth with a focus on the generation of skeletal shape

Cantrell Syndrome (Thoracoabdominal Ectopia Cordis; Anomalous Umbilical Cord; Diaphragmatic, Pericardial and Intracardiac Defects) in the Pig (Sus scrofa domesticus)

Pentalogy of Cantrell (PC) is a complex body wall defect, involving the abdominal wall, sternum, diaphragm, pericardium and heart. We report on six stillborn piglets with anomalous umbilical cord (UC), cranio-umbilical abdominal defect, cleft sternum, incomplete diaphragm and pericardium, ectopia cordis and intracardiac anomalies. Anomalous UC was defined as a single umbilical artery (SUA) and/or short cord, or as an UC with atypical coiling pattern. The embryonic period, in which all the anomalies found in these piglets develops, coincides with that of UC formation in the pig. We propose that anomalous UC should be considered a sixth defect in Cantrell syndrome (CS), considering that the insult leading to the classical malformations of PC and UC abnormalities is the same or that the sequence of malformations itself may alter the early fetoplacental blood flow and therefore the normal development of the UC angioarchitecture. CS has not been reported previously in animals.

Complete Tracheal Ring Deformity. A Translational Genomics Approach to Pathogenesis

Rationale: Complete tracheal ring deformity (CTRD) is a rare congenital abnormality of unknown etiology characterized by circumferentially continuous or nearly continuous cartilaginous tracheal rings, variable degrees of tracheal stenosis and/or shortening, and/or pulmonary arterial sling anomaly.Objectives: To test the hypothesis that CTRD is caused by inherited or de novo mutations in genes required for normal tracheal development.Methods: CTRD and normal tracheal tissues were examined microscopically to define the tracheal abnormalities present in CTRD. Whole-exome sequencing was performed in children with CTRD and their biological parents ("trio analysis") to identify gene variants in patients with CTRD. Mutations were confirmed by Sanger sequencing, and their potential impact on structure and/or function of encoded proteins was examined using human gene mutation databases. Relevance was further examined by comparison with the effects of targeted deletion of murine homologs important to tracheal development in mice.Measurements and Main Results: The trachealis muscle was absent in all of five patients with CTRD. Exome analysis identified six de novo, three recessive, and multiple compound-heterozygous or rare hemizygous variants in children with CTRD. De novo variants were identified in SHH (Sonic Hedgehog), and inherited variants were identified in HSPG2 (perlecan), ROR2 (receptor tyrosine kinase-like orphan receptor 2), and WLS (Wntless), genes involved in morphogenetic pathways known to mediate tracheoesophageal development in mice.Conclusions: The results of the present study demonstrate that absence of the trachealis muscle is associated with CTRD. Variants predicted to cause disease were identified in genes encoding Hedgehog and Wnt signaling pathway molecules, which are critical to cartilage formation and normal upper airway development in mice.

Elimination of Wnt Secretion From Stellate Cells Is Dispensable for Zonation and Development of Liver Fibrosis Following Hepatobiliary Injury

Alterations in the Wnt signaling pathway including those impacting hepatic stellate cells (HSCs) have been implicated in liver fibrosis. In the current study, we first examined the expression of Wnt genes in human HSC (HHSCs) after treatment with a profibrogenic factor TGF-β1. Next, we generated HSC-specific Wntless (Wls) knockout (KO) using the Lrat-cre and Wls-floxed mice. KO and littermate controls (CON) were characterized for any basal phenotype and subjected to two liver fibrosis protocols. In vitro, TGF-β1 induced expression of Wnt2, 5a and 9a while decreasing Wnt2b, 3a, 4, and 11 in HHSC. In vivo, KO and CON mice were born at normal Mendelian ratio and lacked any overt phenotype. Loss of Wnt secretion from HSCs had no effect on liver weight and did not impact β-catenin activation in the pericentral hepatocytes. After 7 days of bile duct ligation (BDL), KO and CON showed comparable levels of serum alkaline phosphatase, alanine aminotransferase, aspartate aminotransferase, total and direct bilirubin. Comparable histology, Sirius red staining, and immunohistochemistry for α-SMA, desmin, Ki-67, F4/80, and CD45 indicated similar proliferation, inflammation, and portal fibrosis in both groups. Biweekly administration of carbon tetrachloride for 4 or 8 weeks also led to comparable serum biochemistry, inflammation, and fibrosis in KO and CON. Specific Wnt genes were altered in HHSCs in response to TGF-β1; however, eliminating Wnt secretion from HSC did not impact basal β-catenin activation in normal liver nor did it alter the injury-repair response during development of liver fibrosis.

Pentalogy of Cantrell

Pentalogy of Cantrell is a constellation of five congenital defects that pose a unique challenge for surgeons. Defects of the heart, pericardium, diaphragm, sternum, and anterior abdominal wall are pathognomonic. Although the incidence is low, it is critical to identify it in a timely fashion in order to adequately address all aspects. Early diagnosis, supportive care, and strategic surgical planning with a multidisciplinary team are all key components in managing patients with Pentalogy of Cantrell. In this text we sought to explore the evolution of both the understanding and treatment for this complex entity and provide current recommendations to today's pediatric caregivers.

Bronchopulmonary Dysplasia: Crosstalk Between PPARγ, WNT/β-Catenin and TGF-β Pathways; The Potential Therapeutic Role of PPARγ Agonists

Bronchopulmonary dysplasia (BPD) is a serious pulmonary disease which occurs in preterm infants. Mortality remains high due to a lack of effective treatment, despite significant progress in neonatal resuscitation. In BPD, a persistently high level of canonical WNT/β-catenin pathway activity at the canalicular stage disturbs the pulmonary maturation at the saccular and alveolar stages. The excessive thickness of the alveolar wall impairs the normal diffusion of oxygen and carbon dioxide, leading to hypoxia. Transforming growth factor (TGF-β) up-regulates canonical WNT signaling and inhibits the peroxysome proliferator activated receptor gamma (PPARγ). This profile is observed in BPD, especially in animal models. Following a premature birth, hypoxia activates the canonical WNT/TGF-β axis at the expense of PPARγ. This gives rise to the differentiation of fibroblasts into myofibroblasts, which can lead to pulmonary fibrosis that impairs the respiratory function after birth, during childhood and even adulthood. Potential therapeutic treatment could target the inhibition of the canonical WNT/TGF-β pathway and the stimulation of PPARγ activity, in particular by the administration of nebulized PPARγ agonists.

Synchronized mesenchymal cell polarization and differentiation shape the formation of the murine trachea and esophagus

Tube morphogenesis is essential for internal-organ development, yet the mechanisms regulating tube shape remain unknown. Here, we show that different mechanisms regulate the length and diameter of the murine trachea. First, we found that trachea development progresses via sequential elongation and expansion processes. This starts with a synchronized radial polarization of smooth muscle (SM) progenitor cells with inward Golgi-apparatus displacement regulates tube elongation, controlled by mesenchymal Wnt5a-Ror2 signaling. This radial polarization directs SM progenitor cell migration toward the epithelium, and the resulting subepithelial morphogenesis supports tube elongation to the anteroposterior axis. This radial polarization also regulates esophageal elongation. Subsequently, cartilage development helps expand the tube diameter, which drives epithelial-cell reshaping to determine the optimal lumen shape for efficient respiration. These findings suggest a strategy in which straight-organ tubulogenesis is driven by subepithelial cell polarization and ring cartilage development.

Bone development in laboratory mammals used in developmental toxicity studies

Evaluation of the skeleton in laboratory animals is a standard component of developmental toxicology testing. Standard methods of performing the evaluation have been established, and modification of the evaluation using imaging technologies is under development. The embryology of the rodent, rabbit, and primate skeleton has been characterized in detail and summarized herein. The rich literature on variations and malformations in skeletal development that can occur in the offspring of normal animals and animals exposed to test articles in toxicology studies is reviewed. These perturbations of skeletal development include ossification delays, alterations in number, shape, and size of ossification centers, and alterations in numbers of ribs and vertebrae. Because the skeleton is undergoing developmental changes at the time fetuses are evaluated in most study designs, transient delays in development can produce apparent findings of abnormal skeletal structure. The determination of whether a finding represents a permanent change in embryo development with adverse consequences for the organism is important in study interpretation. Knowledge of embryological processes and schedules can assist in interpretation of skeletal findings.

Abrogation of TGF-beta signalling in TAGLN expressing cells recapitulates Pentalogy of Cantrell in the mouse

Pentalogy of Cantrell (PC) is a rare multi-organ congenital anomaly that impedes ventral body wall closure and results in diaphragmatic hernia, intra- and pericardial defects. The underlying cellular and molecular changes that lead to these severe developmental defects have remained unknown largely due to the lack of representative animal models. Here we provide in depth characterization of a mouse model with conditional ablation of TGFβRII in Transgelin (Tagln) expressing cells. We show that Tagln is transiently expressed in a variety of cells that participate in the embryonic development and patterning of ventral structures. Genetic ablation of TGFβRII in these cells leads to ventral midline closure defect, diaphragmatic hernia, dilated cardiac outflow tract and aberrant cardiac septation, providing a reliable model to study the morphological changes leading to PC. We show that myogenisis in the diaphragm is independent of TGFβ and the diaphragmatic hernia arises from fibroblast-specific migration defect. In the dorsal body wall Tagln expression is initiated after the closure process, revealing a remarkable difference between ventral and dorsal body walls development. Our study demonstrates the use of micro-CT scanning to obtain a 3-dimensional high-resolution overview of embryonic anomalies and provides the first mechanistic insight into the development of PC.

Exposure to sub-lethal dose of a combination insecticide during early embryogenesis influences the normal patterning of mesoderm resulting in incomplete closure of ventral body wall of chicks of domestic hen

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Chlorpyrifos and cypermethrin treatment induced developmental anomalies in chicks.

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Protrusion of visceral organs and microphthalmia were the major anomalies observed.

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Treated embryos were conspicuous with incomplete ventral body wall and sternum.

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Altered expression pattern of E-cadherin, Shh, bmp4, Wnt11 and Pitx2 were recorded.

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Impairment of major regulators of development is suspected to induce VBWD.

Pesticide exposure to the non target groups especially during embryonic development has quite often resulted in congenital malformations. A commercially available combination insecticide (Ci, 50% chlorpyrifos and 5% cypermethrin) is known to induce ventral body wall defects (VBWDs) wherein abdominal viscera protrude out of the ventral body wall. Herein, an attempt was made to understand the mechanistic insight into Ci induced VBWDs. For this, before incubation, the chick embryos were dosed with the test chemical and then at different developmental stages of incubation, they were monitored for the changes in the expression of certain genes, which are indispensable for the ventral body wall closure since they regulate the cell fate, proliferation and survival. Concurrently, histopathological changes during the embryonic development were examined to corroborate the above observations. The results of mRNA profiling revealed a significant downregulation of Shh on day 4 and upregulation on day 10, while bmp4, Pitx2, E-cadherin, Wnt11, Wnt6, Pxn, MyoD1, Caspase-3, AHR, Cyp3A4, showed a significant upregulation on day 4 and/or on day 10. N-cadherin, fgf8, bmp1 showed no significant changes. The possible means by which these skewed expression patterns of regulatory molecules culminated into the VBWD are discussed.

Notum attenuates Wnt/β-catenin signaling to promote tracheal cartilage patterning

Tracheobronchomalacia (TBM) is a common congenital disorder in which the cartilaginous rings of the trachea are weakened or missing. Despite the high prevalence and clinical issues associated with TBM, the etiology is largely unknown. Our previous studies demonstrated that Wntless (Wls) and its associated Wnt pathways are critical for patterning of the upper airways. Deletion of Wls in respiratory endoderm caused TBM and ectopic trachealis muscle. To understand mechanisms by which Wls mediates tracheal patterning, we performed RNA sequencing in prechondrogenic tracheal tissue of Wls^f/f;Shh^Cre/wt embryos. Chondrogenic Bmp4, and Sox9 were decreased, while expression of myogenic genes was increased. We identified Notum, a deacylase that inactivates Wnt ligands, as a target of Wls induced Wnt signaling. Notum's mesenchymal ventral expression in prechondrogenic trachea overlaps with expression of Axin2, a Wnt/β-catenin target and inhibitor. Notum is induced by Wnt/β-catenin in developing trachea. Deletion of Notum activated mesenchymal Wnt/β-catenin and caused tracheal mispatterning of trachealis muscle and cartilage as well as tracheal stenosis. Notum is required for tracheal morphogenesis, influencing mesenchymal condensations critical for patterning of tracheal cartilage and muscle. We propose that Notum influences mesenchymal cell differentiation by generating a barrier for Wnt ligands produced and secreted by airway epithelial cells to attenuate Wnt signaling.

Cantrell Syndrome-A Rare Complex Congenital Anomaly: A Case Report and Literature Review

Cantrell syndrome (CS) or pentalogy of Cantrell is defined as a rare condition involving a midline anterior abdominal wall defect, a distal sternal cleft, a defect of the anterior diaphragm, and a defect of the apical pericardium with pericardio-peritoneal communication, as well as intracardiac anomalies. We report the case of a male newborn with type 2 CS diagnosed during intrauterine life based on ultrasonographic evaluation. Clinical examination at birth revealed an abdominal wall defect with extrathoracic displacement of the heart and a diastasis of the sagittal suture. Postnatal echocardiography revealed tricuspid atresia, partial extrathoracic and extra-abdominal displacement of the heart and liver, a large ventricular septal defect, severe subpulmonary stenosis, hypoplasia of the pulmonary artery, and a large hourglass-shaped left ventricle secondary to narrowing of the heart at the level of its extrathoracic displacement. Computed tomography showed additional abnormalities including increased left ventricular volume with extrathoracic apical aneurysmal dilatation below the xiphoid process at the level of anterior abdominal wall, a hypoplastic right ventricle, partial transparietal herniation of the left hepatic lobe adjacent to a left ventricular diverticulum, and an adrenal hematoma. The newborn received intensive medical management during his first week of life; however, surgical management had to be postponed owing to his unstable condition. Eventually, it was performed on the 14th day of life, but unfortunately, the newborn died shortly after the procedure.

Transgelin-expressing myofibroblasts orchestrate ventral midline closure through TGFβ signalling

Ventral body wall (VBW) defects are among the most common congenital malformations, yet their embryonic origin and underlying molecular mechanisms remain poorly characterised. Transforming growth factor beta (TGFβ) signalling is essential for VBW closure, but the responding cells are not known. Here, we identify in mouse a population of migratory myofibroblasts at the leading edge of the closing VBW that express the actin-binding protein transgelin (TAGLN) and TGFβ receptor (TGFβR). These cells respond to a temporally regulated TGFβ2 gradient originating from the epithelium of the primary body wall. Targeted elimination of TGFβR2 in TAGLN⁺ cells impairs midline closure and prevents the correct subsequent patterning of the musculature and skeletal components. Remarkably, deletion of Tgfbr2 in myogenic or chondrogenic progenitor cells does not manifest in midline defects. Our results indicate a pivotal significance of VBW myofibroblasts in orchestrating ventral midline closure by mediating the response to the TGFβ gradient. Altogether, our data enable us to distinguish highly regulated epithelial-mesenchymal signalling and successive cellular migration events in VBW closure that explain early morphological changes underlying the development of congenital VBW defects.

Summary: A population of migratory myofibroblasts at the leading edge of the closing ventral body wall expresses cytoskeletal components and TGFβR2 and responds to an epithelial TGFβ2 morphogen gradient to drive midline closure.

Posttranslational modification of β-catenin is associated with pathogenic fibroblastic changes in bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD) is a common complication of premature birth. The histopathology of BPD is characterized by an arrest of alveolarization with fibroblast activation. The Wnt/β-catenin signaling pathway is important in early lung development. When Wnt signaling is active, phosphorylation of β-catenin by tyrosine kinases at activating sites, specifically at tyrosine 489 (Y489), correlates with nuclear localization of β-catenin. We examined fetal lung tissue, lung tissue from term newborns, and lung tissue from infants who died with BPD; we found nuclear β-catenin phosphorylation at Y489 in epithelial and mesenchymal cells in fetal tissue and BPD tissue, but not in the lungs of term infants. Using a 3D human organoid model, we found increased nuclear localization of β-catenin phosphorylated at Y489 (p-β-catenin^Y489) after exposure to alternating hypoxia and hyperoxia compared with organoids cultured in normoxia. Exogenous stimulation of the canonical Wnt pathway in organoids was sufficient to cause nuclear localization of p-β-catenin^Y489 in normoxia and mimicked the pattern of α-smooth muscle actin (α-SMA) expression seen with fibroblastic activation from oxidative stress. Treatment of organoids with a tyrosine kinase inhibitor prior to cyclic hypoxia-hyperoxia inhibited nuclear localization of p-β-catenin^Y489 and prevented α-SMA expression by fibroblasts. Posttranslational phosphorylation of β-catenin is a transient feature of normal lung development. Moreover, the persistence of p-β-catenin^Y489 is a durable marker of fibroblast activation in BPD and may play an important role in BPD disease pathobiology.

Prenatal diagnosis of focal dermal hypoplasia: Report of three fetuses and review of the literature

Focal dermal hypoplasia (FDH) is a rare syndrome characterized by pleiotropic features knowing to involve mostly skin and limbs. Although FDH has been described in children and adults, the cardinal signs of the fetal phenotype are not straightforward impacting the quality of the prenatal diagnosis. We describe in depth the ultrasound, radiological, macroscopical, and histological phenotype of three female fetuses with a severe form of FDH, propose a review of the literature and an attempt to delineate minimal and cardinal signs for FDH diagnosis. This report confirms the variability of FDH phenotype, highlights unreported FDH features, and allows delineating evocative clinical associations for prenatal diagnosis, namely intrauterine growth retardation, limbs malformations, anterior wall/diaphragm defects, and eye anomalies. © 2016 Wiley Periodicals, Inc.

Endodermal Wnt signaling is required for tracheal cartilage formation

Tracheobronchomalacia is a common congenital defect in which the walls of the trachea and bronchi lack of adequate cartilage required for support of the airways. Deletion of Wls, a cargo receptor mediating Wnt ligand secretion, in the embryonic endoderm using ShhCre mice inhibited formation of tracheal-bronchial cartilaginous rings. The normal dorsal-ventral patterning of tracheal mesenchyme was lost. Smooth muscle cells, identified by Acta2 staining, were aberrantly located in ventral mesenchyme of the trachea, normally the region of Sox9 expression in cartilage progenitors. Wnt/β-catenin activity, indicated by Axin2 LacZ reporter, was decreased in tracheal mesenchyme of Wls(f/f);Shh(Cre/+) embryos. Proliferation of chondroblasts was decreased and reciprocally, proliferation of smooth muscle cells was increased in Wls(f/f);Shh(Cre/+) tracheal tissue. Expression of Tbx4, Tbx5, Msx1 and Msx2, known to mediate cartilage and muscle patterning, were decreased in tracheal mesenchyme of Wls(f/f);Shh(Cre/+) embryos. Ex vivo studies demonstrated that Wnt7b and Wnt5a, expressed by the epithelium of developing trachea, and active Wnt/β-catenin signaling are required for tracheal chondrogenesis before formation of mesenchymal condensations. In conclusion, Wnt ligands produced by the tracheal epithelium pattern the tracheal mesenchyme via modulation of gene expression and cell proliferation required for proper tracheal cartilage and smooth muscle differentiation.