Fibroblast growth factor 18 gives growth and directional cues to airway cartilage

FGF18

FGF18 was discovered in 1998. It is a pleiotropic growth factor that stimulates major signalling pathways involved in cell proliferation and growth, and is involved in the development and homeostasis of many tissues such as bone, lung, and central nervous system. The gene consists of five exons that code for a 207 amino acid glycosylated protein. FGF18 is widely expressed in developing and adult chickens, mice, and humans, being seen in the mesenchyme, brain, skeleton, heart, and lungs. Knockout studies of FGF18 in mice lead to perinatal death, characterised by distinct phenotypes such as cleft palate, smaller body size, curved long bones, deformed ribs, and reduced crania. As can be expected from a protein involved in so many functions FGF18 is associated with various diseases such as idiopathic pulmonary fibrosis, congenital diaphragmatic hernia, and most notably various types of cancer such as breast, lung, and ovarian cancer.

Development and Disorders of the Airway in Bronchopulmonary Dysplasia

Bronchopulmonary dysplasia (BPD), a disorder characterized by arrested lung development, is a frequent cause of morbidity and mortality in premature infants. Parenchymal lung changes in BPD are relatively well-characterized and highly studied; however, there has been less emphasis placed on the role that airways disease plays in the pathophysiology of BPD. In preterm infants born between 22 and 32 weeks gestation, the conducting airways are fully formed but still immature and therefore susceptible to injury and further disruption of development. The arrest of maturation results in more compliant airways that are more susceptible to deformation and damage. Consequently, neonates with BPD are prone to developing airway pathology, particularly for patients who require intubation and positive-pressure ventilation. Airway pathology, which can be divided into large and small airways disease, results in increased respiratory morbidity in neonates with chronic lung disease of prematurity.

FGF18 promotes human lung branching morphogenesis through regulating mesenchymal progenitor cells

Fibroblast growth factor (FGF) signaling is known to play an important role in lung organogenesis. However, we recently demonstrated that FGF10 fails to induce branching in human fetal lungs as is observed in mouse. Our previous human fetal lung RNA sequencing data exhibited increased FGF18 during the pseudoglandular stage of development, suggestive of its importance in human lung branching morphogenesis. Whereas it has been previously reported that FGF18 is critical during alveologenesis, few studies have described its implication in lung branching, specifically in human. Therefore, we aimed to determine the role of FGF18 in human lung branching morphogenesis. Human fetal lung explants within the pseudoglandular stage of development were treated with recombinant human FGF18 in air-liquid interface culture. Explants were analyzed grossly to assess differences in branching pattern, as well as at the cellular and molecular levels. FGF18 treatment promoted branching in explant cultures and demonstrated increased epithelial proliferation as well as maintenance of the double positive SOX2/SOX9 distal bud progenitor cells, confirming its role in human lung branching morphogenesis. In addition, FGF18 treated explants displayed increased expression of SOX9, FN1, and COL2A1 within the mesenchyme, all factors that are important to chondrocyte differentiation. In humans, cartilaginous airways extend deep into the lung up to the 12th generation of branching whereas in mouse these are restricted to the trachea and main bronchi. Therefore, our data suggest that FGF18 promotes human lung branching morphogenesis through regulating mesenchymal progenitor cells.

Tracheal Ring Formation

The trachea is a long tube that enables air passage between the larynx and the bronchi. C-shaped cartilage rings on the ventral side stabilise the structure. On its esophagus-facing dorsal side, deformable smooth muscle facilitates the passage of food in the esophagus. While the symmetry break along the dorsal-ventral axis is well understood, the molecular mechanism that results in the periodic Sox9 expression pattern that translates into the cartilage rings has remained elusive. Here, we review the molecular regulatory interactions that have been elucidated, and discuss possible patterning mechanisms. Understanding the principles of self-organisation is important, both to define biomedical interventions and to enable tissue engineering.

Mammalian tracheal development and reconstruction: insights from in vivo and in vitro studies

The trachea delivers inhaled air into the lungs for gas exchange. Anomalies in tracheal development can result in life-threatening malformations, such as tracheoesophageal fistula and tracheomalacia. Given the limitations of current therapeutic approaches, development of technologies for the reconstitution of a three-dimensional trachea from stem cells is urgently required. Recently, single-cell sequencing technologies and quantitative analyses from cell to tissue scale have been employed to decipher the cellular basis of tracheal morphogenesis. In this Review, recent advances in mammalian tracheal development and the generation of tracheal tissues from pluripotent stem cells are summarized.

Congenital tracheal defects: embryonic development and animal models

Tracheal anomalies are potentially catastrophic congenital defects. As a newborn begins to breathe, the trachea needs to maintain an appropriate balance of elasticity and rigidity. If the tracheal cartilages are disorganized or structurally weak, the airways can collapse, obstructing breathing. Cartilage rings that are too small or too rigid can also obstruct breathing. These anomalies are frequently associated with craniofacial syndromes, and, despite the importance, are poorly understood. In this review, we summarize the spectrum of pathological phenotypes of the trachea and correlate them with the molecular events uncovered in mouse models.

The control of lung branching morphogenesis

Branching morphogenesis generates epithelial trees which facilitate gas exchange, filtering, as well as secretion processes with their large surface to volume ratio. In this review, we focus on the developmental mechanisms that control the early stages of lung branching morphogenesis. Lung branching morphogenesis involves the stereotypic, recurrent definition of new branch points, subsequent epithelial budding, and lung tube elongation. We discuss current models and experimental evidence for each of these steps. Finally, we discuss the role of the mesenchyme in determining the organ-specific shape.

Incoherent Feedforward Regulation via Sox9 and ERK Underpins Mouse Tracheal Cartilage Development

Tracheal cartilage provides architectural integrity to the respiratory airway, and defects in this structure during embryonic development cause severe congenital anomalies. Previous genetic studies have revealed genes that are critical for the development of tracheal cartilage. However, it is still unclear how crosstalk between these proteins regulates tracheal cartilage formation. Here we show a core regulatory network underlying murine tracheal chondrogenesis from embryonic day (E) 12.5 to E15.5, by combining volumetric imaging of fluorescence reporters, inhibitor assays, and mathematical modeling. We focused on SRY-box transcription factor 9 (Sox9) and extracellular signal-regulated kinase (ERK) in the tracheal mesenchyme, and observed a synchronous, inverted U-shaped temporal change in both Sox9 expression and ERK activity with a peak at E14.5, whereas the expression level of downstream cartilage matrix genes, such as collagen II alpha 1 (Col2a1) and aggrecan (Agc1), monotonically increased. Inhibitor assays revealed that the ERK signaling pathway functions as an inhibitory regulator of tracheal cartilage differentiation during this period. These results suggest that expression of the cartilage matrix genes is controlled by an incoherent feedforward loop via Sox9 and ERK, which is supported by a mathematical model. Furthermore, the modeling analysis suggests that a Sox9-ERK incoherent feedforward regulation augments the robustness against the variation of upstream factors. The present study provides a better understanding of the regulatory network underlying the tracheal development and will be helpful for efficient induction of tracheal organoids.

The Fgf8 subfamily (Fgf8, Fgf17 and Fgf18) is required for closure of the embryonic ventral body wall

The closure of the embryonic ventral body wall in amniotes is an important morphogenetic event and is essential for life. Defects in human ventral wall closure are a major class of birth defect and a significant health burden. Despite this, very little is understood about how the ventral body wall is formed. Here, we show that fibroblast growth factor (FGF) ligands FGF8, FGF17 and FGF18 are essential for this process. Conditional mouse mutants for these genes display subtle migratory defects in the abdominal muscles of the ventral body wall and an enlarged umbilical ring, through which the internal organs are extruded. By refining where and when these genes are required using different Cre lines, we show that Fgf8 and Fgf17 are required in the presomitic mesoderm, whereas Fgf18 is required in the somites. This study identifies complex and multifactorial origins of ventral wall defects and has important implications for understanding their origins during embryonic development.

Mechanisms of larynx and vocal fold development and pathogenesis

The larynx and vocal folds sit at the crossroad between digestive and respiratory tracts and fulfill multiple functions related to breathing, protection and phonation. They develop at the head and trunk interface through a sequence of morphogenetic events that require precise temporo-spatial coordination. We are beginning to understand some of the molecular and cellular mechanisms that underlie critical processes such as specification of the laryngeal field, epithelial lamina formation and recanalization as well as the development and differentiation of mesenchymal cell populations. Nevertheless, many gaps remain in our knowledge, the filling of which is essential for understanding congenital laryngeal disorders and the evaluation and treatment approaches in human patients. This review highlights recent advances in our understanding of the laryngeal embryogenesis. Proposed genes and signaling pathways that are critical for the laryngeal development have a potential to be harnessed in the field of regenerative medicine.

Maternal Exposure to PM2.5 Affects Fetal Lung Development at Sensitive Windows

Lung development continues from the embryonic period to adulthood. Previous epidemiological studies have noted that maternal exposure of atmospheric pollutants during the sensitive windows disturbed the lung development and increased the risk of lung diseases after birth, but the experimental evidence was insufficient. In the present study, we exposed plug-positive mice to PM_2.5 (3 mg/kg b.w.) by oropharyngeal aspiration every other day, and intended to test whether maternal PM_2.5 exposure affected prenatal lung development in the offspring. First, maternal PM_2.5 exposure decreased embryo weight and crown-rump length at E18.5 but not in earlier developmental stages (E0-E16.5). Second, maternal PM_2.5 exposure did not prevent lung-bud and tracheal specification, and did not cause abnormalities in branching morphogenesis, distal lung epithelium, and mesenchyme differentiation in earlier stages of lung development (E0-E16.5). However, the exposure significantly disturbed the distal lung epithelium and mesenchyme differentiation of lung, led to reduced intact rings of trachea, and suppressed the expression of lung development-related genes (Nkx2.1, Tbx4, Tbx5, and Sox9) at E18.5. Finally, we found that the exposure not only increased PM_2.5-bound metal content (Pb and Cu) but also caused inflammatory response in the placenta, which transmitted from the mother to the fetus and contributed to the developmental abnormalities.

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.

Decrease of miR-195 Promotes Chondrocytes Proliferation and Maintenance of Chondrogenic Phenotype via Targeting FGF-18 Pathway

Slow growth and rapid loss of chondrogenic phenotypes are the major problems affecting chronic cartilage lesions. The role of microRNA-195 (miR-195) and its detailed working mechanism in the fore-mentioned process remains unknown. Fibroblastic growth factor 18 (FGF-18) plays a key role in cartilage homeostasis; whether miR-195 could regulate FGF-18 and its downstream signal pathway in chondrocyte proliferation and maintenance of chondrogenic phenotypes still remains unclear. The present research shows elevated miR-195 but depressed FGF-18 expressed in joint fluid specimens of 20 patients with chronic cartilage lesions and in CH1M and CH3M chondrocytes when compared with that in joint fluid specimens without cartilage lesions and in CH1W and CH2W chondrocytes, respectively. The following loss of function test revealed that downregulation of miR-195 by transfection of miR-195 inhibitors promoted chondrocyte proliferation and expression of a type II collagen α I chain (Col2a1)/aggrecan. Through the online informatics analysis we theoretically predicted that miR-195 could bind to a FGF-18 3' untranslated region (3'UTR), also, we verified that a miR-195 could regulate the FGF-18 and its downstream pathway. The constructed dual luciferase assay further confirmed that FGF-18 was a direct target of miR-195. The executed anti-sense experiment displayed that miR-195 could regulate chondrocyte proliferation and Col2a1/aggrecan expression via the FGF-18 pathway. Finally, through an in vivo anterior cruciate ligament transection (ACLT) model, downregulation of miR-195 presented a significantly protective effect on chronic cartilage lesions. Evaluating all of the outcomes of the current research revealed that a decrease of miR-195 protected chronic cartilage lesions by promoting chondrocyte proliferation and maintenance of chondrogenic phenotypes via the targeting of the FGF-18 pathway and that the miR-195/FGF-18 axis could be a potential target in the treatment of cartilage lesions.

Studying Wnt Signaling During Patterning of Conducting Airways

Wnt signaling pathways play critical roles during development of the respiratory tract. Defining precise mechanisms of differentiation and morphogenesis controlled by Wnt signaling is required to understand how tissues are patterned during normal development. This knowledge is also critical to determine the etiology of birth defects such as lung hypoplasia and tracheobronchomalacia. Analysis of earliest stages of development of respiratory tract imposes challenges, as the limited amount of tissue prevents the performance of standard protocols better suited for postnatal studies. In this paper, we discuss methodologies to study cell differentiation and proliferation in the respiratory tract. We describe techniques such as whole mount staining, processing of the tissue for confocal microscopy and immunofluorescence in paraffin sections applied to developing tracheal lung. We also discuss methodologies for the study of tracheal mesenchyme differentiation, in particular cartilage formation. Approaches and techniques discussed in the current paper circumvent the limitation of material while working with embryonic tissue, allowing for a better understanding of the patterning process of developing conducting airways.

Mesodermal lineages in the developing respiratory system

The life-sustaining air-blood interface of the respiratory system requires the exquisite integration of the epithelial lining with the mesenchymal capillary network, all supported by elastic smooth muscle and rigid cartilage keeping the expandable airways open. These intimate tissue interactions originate in the early embryo, where bidirectional paracrine signaling between the endoderm epithelium and adjacent mesoderm orchestrates lung and trachea development and controls the stereotypical branching morphogenesis. Although much attention has focused on how these interactions impact the differentiation of the respiratory epithelium, relatively less is known about the patterning and differentiation of the mesenchyme. Endothelial cells, smooth muscle cells, and chondrocytes together with other types of mesenchymal cells are essential components of a functional respiratory system, and malformation of these cells can lead to various congenital defects. In this review, we summarize the current understanding of mesenchymal development in the fetal trachea and lung, focusing on recent findings from animal models that have begun to shed light on the poorly understood respiratory mesenchyme lineages.

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.

Role of SOX2 in foregut development in relation to congenital abnormalities

The uptake of the two essential ingredients for life, oxygen and nutrients, occurs primarily through the oral cavity, but these two lifelines need to be separated with high accuracy once inside the body. The two systems, the gas exchange pulmonary system and the gastro-intestinal feeding system, are derived from the same primitive embryonic structure during development, the foregut, which need to be separated before birth. In certain newborns, this separation occurs not or insufficiently, leading to life threatening conditions, sometimes incompatible with life. The development of the foregut, trachea and lungs is influenced and coordinated by a multitude of signaling cascades and transcription factors. In this review, we will highlight the development of the foregut and pulmonary system and focus on associated congenital abnormalities in light of known genetic alterations with specific attention to the transcription factor SOX2.

Interactions between FGF18 and retinoic acid regulate differentiation of chick embryo limb myoblasts

During limb development Pax3 positive myoblasts delaminate from the hypaxial dermomyotome of limb level somites and migrate into the limb bud where they form the dorsal and ventral muscle masses. Only then do they begin to differentiate and express markers of myogenic commitment and determination such as Myf5 and MyoD. However the signals regulating this process remain poorly characterised. We show that FGF18, which is expressed in the distal mesenchyme of the limb bud, induces premature expression of both Myf5 and MyoD and that blocking FGF signalling also inhibits endogenous MyoD expression. This expression is mediated by ERK MAP kinase but not PI3K signalling. We also show that retinoic acid (RA) can inhibit the myogenic activity of FGF18 and that blocking RA signalling allows premature induction of MyoD by FGF18 at HH19. We propose a model where interactions between FGF18 in the distal limb and retinoic acid in the proximal limb regulate the timing of myogenic gene expression during limb bud development.

Lung mesenchymal expression of Sox9 plays a critical role in tracheal development

Background

Embryonic lung development is instructed by crosstalk between mesenchyme and epithelia, which results in activation of transcriptional factors, such as Sox9, in a temporospatial manner. Sox9 is expressed in both distal lung epithelium and proximal lung mesenchyme. Here, we investigated the effect of lung mesenchyme-specific inducible deletion of Sox9 during murine lung development.

Results

Transgenic mice lacking Sox9 expression were unable to breathe and died at birth, with noticeable tracheal defects. Cartilage rings were missing, and the tracheal lumen was collapsed in the mutant trachea. In situ hybridization showed an altered expression pattern of Tbx4, Tbx5 and Fgf10 genes and marked reduction of Collagen2 expression in the tracheal mesenchyme. The tracheal phenotype was increasingly severe, with longer duration of deletion. Lymphatic vasculature was underdeveloped in the mutant trachea: Prox1, Lyve1, and Vegfr3 were decreased after Sox9 knockout. We also found that compared with normal tracheal epithelium, the mutant tracheal epithelium had an altered morphology with fewer P63-positive cells and more CC10-positive cells, fewer goblet cells, and downregulation of surfactant proteins A and C.

Conclusion

The appropriate temporospatial expression of Sox9 in lung mesenchyme is necessary for appropriate tracheal cartilage formation, lymphatic vasculature system development, and epithelial differentiation. We uncovered a novel mechanism of lung epithelium differentiation: tracheal cartilage rings instruct the tracheal epithelium to differentiate properly during embryonic development. Thus, besides having a mechanical function, tracheal cartilage also appears to be a local signaling structure in the embryonic lung.

ERK activation is required for hydrostatic pressure-induced tensile changes in engineered articular cartilage

The objective of this study was to identify ERK 1/2 involvement in the changes in compressive and tensile mechanical properties associated with hydrostatic pressure treatment of self-assembled cartilage constructs. In study 1, ERK 1/2 phosphorylation was detected by immunoblot, following application of hydrostatic pressure (1 h of static 10 MPa) applied at days 10-14 of self-assembly culture. In study 2, ERK 1/2 activation was blocked during hydrostatic pressure application on days 10-14. With pharmacological inhibition of the ERK pathway by the MEK1/ERK inhibitor U0126 during hydrostatic pressure application on days 10-14, the increase in Young's modulus induced by hydrostatic pressure was blocked. Furthermore, this reduction in Young's modulus with U0126 treatment during hydrostatic pressure application corresponded to a decrease in total collagen expression. However, U0126 did not inhibit the increase in aggregate modulus or GAG induced by hydrostatic pressure. These findings demonstrate a link between hydrostatic pressure application, ERK signalling and changes in the biomechanical properties of a tissue-engineered construct.

The role of Sox genes in lung morphogenesis and cancer

The human lung consists of multiple cell types derived from early embryonic compartments. The morphogenesis of the lung, as well as the injury repair of the adult lung, is tightly controlled by a network of signaling pathways with key transcriptional factors. Lung cancer is the third most cancer-related death in the world, which may be developed due to the failure of regulating the signaling pathways. Sox (sex-determining region Y (Sry) box-containing) family transcriptional factors have emerged as potent modulators in embryonic development, stem cells maintenance, tissue homeostasis, and cancerogenesis in multiple processes. Recent studies demonstrated that the members of the Sox gene family played important roles in the development and maintenance of lung and development of lung cancer. In this context, we summarize our current understanding of the role of Sox family transcriptional factors in the morphogenesis of lung, their oncogenic potential in lung cancer, and their potential impact in the diagnosis, prognosis, and targeted therapy of lung cancer.

Emerging drugs for osteoarthritis

INTRODUCTION

Osteoarthritis (OA), the most prevalent form of joint disease, affects as much as 13% of the world's population. In the USA, it is the leading cause of disability in people over age 65 and is characterized by progressive cartilage loss, bone remodeling, osteophyte formation and synovial inflammation with resultant joint pain and disability. There are no treatments marketed for structural disease modification; current treatments mainly target symptoms, with > 75% of patients reporting need for additional symptomatic treatment.

AREAS COVERED

Drugs in later development (Phase II - III) for OA pain and joint structural degeneration are reviewed. Topics that are not covered in this article are procedural-based (e.g., arthrocentesis, physical therapy), behavioral-based (e.g., weight loss, pain coping techniques) or device-based (e.g., knee braces, surgical implants) treatments.

EXPERT OPINION

More in-depth understanding of the pathophysiology of the disease, as well as elucidation of the link between clinical symptomatology and structural changes in the joint will likely lead to the development of novel target classes with promising efficacy in the future. Efficacy notwithstanding, there remain significant hurdles to overcome in clinical development of these therapeutics, inherent in the progression pattern of the disease as well as challenges with readouts for both pain and structure modification trials.

Profiling target genes of FGF18 in the postnatal mouse lung: possible relevance for alveolar development

Better understanding alveolarization mechanisms could help improve prevention and treatment of diseases characterized by reduced alveolar number. Although signaling through fibroblast growth factor (FGF) receptors is essential for alveolarization, involved ligands are unidentified. FGF18, the expression of which peaks coincidentally with alveolar septation, is likely to be involved. Herein, a mouse model with inducible, lung-targeted FGF18 transgene was used to advance the onset of FGF18 expression peak, and genome-wide expression changes were determined by comparison with littermate controls. Quantitative RT-PCR was used to confirm expression changes of selected up- and downregulated genes and to determine their expression profiles in the course of lung postnatal development. This allowed identifying so-far unknown target genes of the factor, among which a number are known to be involved in alveolarization. The major target was adrenomedullin, a promoter of lung angiogenesis and alveolar development, whose transcript was increased 6.9-fold. Other genes involved in angiogenesis presented marked expression increases, including Wnt2 and cullin2. Although it appeared to favor cell migration notably through enhanced expression of Snai1/2, FGF18 also induced various changes consistent with prevention of epithelial-mesenchymal transition. Together with antifibrotic effects driven by induction of E prostanoid receptor 2 and repression of numerous myofibroblast markers, this could prevent alveolar septation-driving mechanisms from becoming excessive and deleterious. Last, FGF18 up- or downregulated genes of extracellular matrix components and epithelial cell markers previously shown to be up- or downregulated during alveolarization. These findings therefore argue for an involvement of FGF18 in the control of various developmental events during the alveolar stage.

A large animal model of the fetal tracheal stenosis/atresia spectrum

BACKGROUND

Treatment of congenital tracheal stenosis/atresia remains essentially unresolved. Previous models of this disease entity have been restricted to rodents and the chick. We sought to establish the principles of a large, surgical animal model of this spectrum of fetal anomalies.

METHODS

Fetal lambs (n = 8) underwent open surgery at 90-112 days gestation. Their cervical tracheas were encircled by a biocompatible polytetrafluoroethylene wrap, so as to extrinsically restrict their external diameter by 25%. Survivors (n = 7) were killed at different time points post-operatively before term. The manipulated tracheal segments were compared with their respective proximal portions (controls). Analyses included morphometry, histology and quantitative extracellular matrix measurements.

RESULTS

At necropsy, the typical gross appearance of tracheal stenosis/atresia was present in all manipulated tracheal segments. Histological findings included the virtual disappearance of the membranous portion of the trachea, along with infolding, fragmentation, and/or posterior fusion of cartilaginous rings, often with disappearance of the airway mucosa. There were significant decreases in diameter (P < 0.001) and total collagen levels (P = 0.005) on the manipulated trachea compared with the control portions. No significant differences were observed in overall elastin or glycosaminoglycan contents. A significant time-dependent increase in elastin was noted on the control, but not the experimental side.

CONCLUSIONS

In a surgical ovine model, controlled extrinsic compression of the fetal trachea leads to morphological and biochemical findings compatible with the congenital tracheal stenosis/atresia spectrum. This simple and easily reproducible prenatal model can be instrumental in the development of emerging therapies for these congenital anomalies.

Lung organogenesis

Developmental lung biology is a field that has the potential for significant human impact: lung disease at the extremes of age continues to cause major morbidity and mortality worldwide. Understanding how the lung develops holds the promise that investigators can use this knowledge to aid lung repair and regeneration. In the decade since the "molecular embryology" of the lung was first comprehensively reviewed, new challenges have emerged-and it is on these that we focus the current review. Firstly, there is a critical need to understand the progenitor cell biology of the lung in order to exploit the potential of stem cells for the treatment of lung disease. Secondly, the current familiar descriptions of lung morphogenesis governed by growth and transcription factors need to be elaborated upon with the reinclusion and reconsideration of other factors, such as mechanics, in lung growth. Thirdly, efforts to parse the finer detail of lung bud signaling may need to be combined with broader consideration of overarching mechanisms that may be therapeutically easier to target: in this arena, we advance the proposal that looking at the lung in general (and branching in particular) in terms of clocks may yield unexpected benefits.