Disrupted TGF-ß signaling: a link between bronchopulmonary dysplasia and alveolar type 1 cells.

Bronchopulmonary dysplasia (BPD) is a chronic lung disease common in extreme preterm infants and is characterized by alveolar simplification. Current BPD research mainly focuses on alveolar type 2 (AT2) cells, myofibroblasts, and the endothelium. However, a notable gap exists in the involvement of AT1 cells, which constitute a majority of the alveolar surface area. In this issue of the JCI, Callaway and colleagues explored the role of TGF-ß signaling in AT1 cells for managing the AT1-to-AT2 transition and its involvement in the integration of mechanical forces with the pulmonary matrisome during development. The findings implicate AT1 cells in the pathogenesis of BPD.

Long-Service-Life Rigid Polyurethane Foam Fillings for Spent Fuel Transportation Casks.

Soft materials bearing rigid, lightweight, and vibration-dampening properties offer distinct advantages over traditional wooden and metal-based fillings for spent fuel transport casks, due to their low density, tunable structure, excellent mechanical properties, and ease of processing. In this study, a novel type of rigid polyurethane foam is prepared using a conventional polycondensation reaction between isocyanate and hydroxy groups. Moreover, the density and size of the pores in these foams are precisely controlled through simultaneous gas generation. The as-prepared polyurethane exhibits high thermal stability exceeding 185 °C. Lifetime predictions based on thermal testing indicate that these polyurethane foams could last up to over 60 years, which is double the lifetime of conventional materials of about 30 years. Due to their occlusive structure, the mechanical properties of these polymeric materials meet the design standards for spent fuel transport casks, with maximum compression and tensile stresses of 6.89 and 1.37 MPa, respectively, at a testing temperature of -40 °C. In addition, these polymers exhibit effective flame retardancy; combustion ceased within 2 s after removal of the ignition source. All in all, this study provides a simple strategy for preparing rigid polymeric foams, presenting them as promising prospects for application in spent fuel transport casks.

Contemporary and emerging technologies for research in children's rare and interstitial lung disease.

Although recent decades have seen the identification, classification and discovery of the genetic basis of many children's interstitial and rare lung disease (chILD) disorders, detailed understanding of pathogenesis and specific therapies are still lacking for most of them. Fortunately, a revolution of technological advancements has created new opportunities to address these critical knowledge gaps. High-throughput sequencing has facilitated analysis of transcription of thousands of genes in thousands of single cells, creating tremendous breakthroughs in understanding normal and diseased cellular biology. Spatial techniques allow analysis of transcriptomes and proteomes at the subcellular level in the context of tissue architecture, in many cases even in formalin-fixed, paraffin-embedded specimens. Gene editing techniques allow creation of "humanized" animal models in a shorter time frame, for improved knowledge and preclinical therapeutic testing. Regenerative medicine approaches and bioengineering advancements facilitate the creation of patient-derived induced pluripotent stem cells and their differentiation into tissue-specific cell types which can be studied in multicellular "organoids" or "organ-on-a-chip" approaches. These technologies, singly and in combination, are already being applied to gain new biological insights into chILD disorders. The time is ripe to systematically apply these technologies to chILD, together with sophisticated data science approaches, to improve both biological understanding and disease-specific therapy.

Exposure to bisphenol A induces neurotoxicity associated with synaptic and cytoskeletal dysfunction in neuro-2a cells.

Bisphenol A (BPA) has been reported to injure the developing and adult brain. However, the underlying mechanism still remains elusive. This study used neuro-2a cells as a cellular model to investigate the neurotoxic effects of BPA. Microtubule-associated protein 2 (MAP2) and tau protein maintain microtubule normal function and promote the normal development of the nervous system. Synaptophysin (SYP) and drebrin (Dbn) proteins are involved in regulating synaptic plasticity. Cells were exposed to the minimum essential medium (MEM), 0.01% (v/v) DMSO, and 150 µM BPA for 12, 24, or 36 h. Morphological analysis revealed that the cells in the BPA-treated groups shrank and collapsed compared with those in the control groups. CCK-8 and lactate dehydrogenase assay (LDH) assays showed that the mortality of neuro-2a cells increased as the BPA treatment time was prolonged. Ultrastructural analysis further revealed that cells demonstrated nucleolar swelling, dissolution of nuclear and mitochondrial membranes, and partial mitochondrial condensation following exposure to BPA. BPA also decreased the relative protein expression levels of MAP2, tau, and Dbn. Interestingly, the relative protein expression levels of SYP increased. These results indicated that BPA inhibited the proliferation and disrupted cytoskeleton and synaptic integrity of neuro-2a cells.

Exposure to fluoride induces apoptosis in the liver, kidney, and heart of Xenopus laevis by regulating the Caspase-8/3 signaling pathway.

Fluoride compounds are abundant and widely distributed in the environment at various concentrations, which can seriously injure the human body. In this study, we aim to evaluate the effects of excessive fluoride exposure on the liver, kidney, and heart tissues of healthy female Xenopus laevis by administering NaF (0, 100, and 200 mg/L) in drinking water for 90 days. The expression level of procaspase-8, cleaved-caspase-8, and procaspase-3 proteins were determined by Western blot. Compared with the control group, the group exposed to NaF exhibited expression levels of procaspase-8, cleaved-caspase-8, and procaspase-3 proteins that were considerably upregulated at a concentration of 200 mg/L in the liver and kidney. The cleaved-caspase-8 protein expression in the group exposed to a high concentration of NaF was lower than that in the control group in heart. Histopathological results by hematoxylin and eosin staining showed that excessive NaF exposure caused necrosis of hepatocytes and vacuolization degeneration. Granular degeneration and necrosis in renal tubular epithelial cells were also observed. Moreover, hypertrophy of myocardial cells, atrophy of myocardial fibers and disorder of myocardial fibers were detected. These results demonstrated that NaF-induced apoptosis and the mediated death receptor pathway activation ultimately damaged the liver and kidney tissues. This finding offers a fresh perspective on the effects of F-induced apoptosis in X. laevis.

Arbuscular mycorrhizal fungi enhanced the growth, phosphorus uptake and Pht expression of olive (Olea europaea L.) plantlets.

Olive (Olea europaea L.) is a highly mycotrophic species that has been introduced and cultivated in China for half a century. The arbuscular mycorrhizal fungi (AMF) is extremely valuable as a kind of biofertilizer to promote the health and vigor of olive plants. However, it is still unclear how native AMF impact growth and mineral nutrients, especially phosphorus absorption in the area where olive trees were introduced in China. In the present study, through a pot experiment, the effects of native AMF on the growth, phosphorus uptake and expression levels of four phosphate transporter genes (Pht) of olive plantlets were characterized. We found that (1) typical AMF colonization was observed within the roots of inoculated olive plantlets, and the growth of plantlets was significantly promoted; (2) some indigenous consortia (AMF1 and AMF2) notably promoted the absorption of phosphorus, fertilizers significantly increased the foliar content of nitrogen, and both AMF inoculation and fertilization had no significant effect on the uptake of potassium; and (3) AMF inoculation enhanced the expression of phosphate transporter genes in inoculated olive roots. This work demonstrates the effectiveness of native AMF on the cultivation of robust olive plantlets and highlights the role of AMF in increasing phosphorus uptake. There is great potential in using native AMF consortia as inoculants for the production of healthy and robust olive plantlets.

Bisphenol A induces apoptosis in response to DNA damage through c-Abl/YAPY357/ p73 pathway in P19 embryonal carcinoma stem cells.

Bisphenol A (2,2-bis(4'-hydroxyphenyl) propane, BPA) is a well-known endocrine-disrupting compound that is widely used in various daily products and exhibits embryonic development toxicity and genotoxicity. However, the affected signaling pathways involved in embryonic development especially the interactions of involved proteins remain unclear. In our previous study (Ge et al., 2021), BPA induces DNA damage and apoptosis in Xenopus embryos, resulting in multiple malformations of larvae. However, the signaling pathways induced for apoptosis response to DNA damage are still not well elucidated. Here, we systematically elucidated the enriched pathways affected by BPA and illustrated the interactions of involved proteins. Results indicated that BPA affected multiple embryonic development pathways including Hippo, TGF-ß, Wnt, and Notch pathways. Furthermore, the protein-protein interaction network suggested that the c-Abl/YAPY357/p73 pathway may play a key role in apoptosis induction in response to DNA damage. P19 embryonal carcinoma stem cells, as a developmental toxicity model, were treated with different BPA concentrations to establish an in vitro model to verify the role of the c-Abl/YAPY357/p73 pathway in apoptosis. BPA triggered DNA damage and significantly upregulated the expression levels of c-Abl, phosphorylated YAPY357, phosphorylated p73Y99, and cleaved caspase-3 protein (p < 0.05), thus decreasing cell viability and transcriptionally activating the p73 target genes Bax and Puma. These data suggested that BPA activated the c-Abl/YAPY357/p73 pathway in response to DNA damage. Imatinib, an inhibitor of tyrosine kinase c-Abl, significantly downregulated the elevated expression levels of p-YAPY357, p-p73Y99 and cleaved caspase-3 (p < 0.05) caused by BPA and then ameliorated the cell index of P19 cells in the BPA-treated group. Therefore, this substance restrained the phosphokinase activity of c-Abl and suppressed the c-Abl/YAPY357/p73 pathway. Results showed that the c-Abl/YAPY357/p73 pathway served as a mechanism for caspase-3 activation that induced the apoptosis response to DNA damage stress.

E3 ubiquitin ligase FBXW7 balances airway cell fates.

The airway epithelium is composed of multiple cell types each with designated roles. A stereotyped ratio of these cells is essential for proper airway function. Imbalance of airway cell types underlies many lung diseases, including chronic obstructive pulmonary disease (COPD) and asthma. While a number of signals and transcription factors have been implicated in airway cell specification, how cell numbers are coordinated, especially at the protein level is poorly understood. Here we show that in the mouse trachea which contain epithelial cell types similar to human airway, epithelium-specific inactivation of Fbxw7, which encodes an E3 ubiquitin ligase, led to reduced club and ciliated cells, increased goblet cells, and ectopic P63-negative, Keratin5-positive transitory basal cells in the luminal layer. The protein levels of FBXW7 targets including NOTCH1, KLF5 and TGIF were increased. Inactivation of either Notch1, Klf5 but not Tgif genes in the mutant background led to attenuation of selected aspects of the phenotypes, suggesting that FBXW7 acts through different targets to control different cell fates. These findings demonstrate that protein-level regulation by the ubiquitin proteasome system is critical for balancing airway cell fates.

A census of the lung: CellCards from LungMAP.

The human lung plays vital roles in respiration, host defense, and basic physiology. Recent technological advancements such as single-cell RNA sequencing and genetic lineage tracing have revealed novel cell types and enriched functional properties of existing cell types in lung. The time has come to take a new census. Initiated by members of the NHLBI-funded LungMAP Consortium and aided by experts in the lung biology community, we synthesized current data into a comprehensive and practical cellular census of the lung. Identities of cell types in the normal lung are captured in individual cell cards with delineation of function, markers, developmental lineages, heterogeneity, regenerative potential, disease links, and key experimental tools. This publication will serve as the starting point of a live, up-to-date guide for lung research at https://www.lungmap.net/cell-cards/. We hope that Lung CellCards will promote the community-wide effort to establish, maintain, and restore respiratory health.

17ß-Estradiol and estrogen receptor α protect right ventricular function in pulmonary hypertension via BMPR2 and apelin.

Women with pulmonary arterial hypertension (PAH) exhibit better right ventricular (RV) function and survival than men; however, the underlying mechanisms are unknown. We hypothesized that 17ß-estradiol (E2), through estrogen receptor α (ER-α), attenuates PAH-induced RV failure (RVF) by upregulating the procontractile and prosurvival peptide apelin via a BMPR2-dependent mechanism. We found that ER-α and apelin expression were decreased in RV homogenates from patients with RVF and from rats with maladaptive (but not adaptive) RV remodeling. RV cardiomyocyte apelin abundance increased in vivo or in vitro after treatment with E2 or ER-α agonist. Studies employing ER-α-null or ER-ß-null mice, ER-α loss-of-function mutant rats, or siRNA demonstrated that ER-α is necessary for E2 to upregulate RV apelin. E2 and ER-α increased BMPR2 in pulmonary hypertension RVs and in isolated RV cardiomyocytes, associated with ER-α binding to the Bmpr2 promoter. BMPR2 is required for E2-mediated increases in apelin abundance, and both BMPR2 and apelin are necessary for E2 to exert RV-protective effects. E2 or ER-α agonist rescued monocrotaline pulmonary hypertension and restored RV apelin and BMPR2. We identified what we believe to be a novel cardioprotective E2/ER-α/BMPR2/apelin axis in the RV. Harnessing this axis may lead to novel RV-targeted therapies for PAH patients of either sex.

Estrogen receptor-α prevents right ventricular diastolic dysfunction and fibrosis in female rats.

Although women are more susceptible to pulmonary arterial hypertension (PAH) than men, their right ventricular (RV) function is better preserved. Estrogen receptor-α (ERα) has been identified as a likely mediator for estrogen protection in the RV. However, the role of ERα in preserving RV function and remodeling during pressure overload remains poorly understood. We hypothesized that loss of functional ERα removes female protection from adverse remodeling and is permissive for the development of a maladapted RV phenotype. Male and female rats with a loss-of-function mutation in ERα (ERαMut) and wild-type (WT) littermates underwent RV pressure overload by pulmonary artery banding (PAB). At 10 wk post-PAB, WT and ERαMut demonstrated RV hypertrophy. Analysis of RV pressure waveforms demonstrated RV-pulmonary vascular uncoupling and diastolic dysfunction in female, but not male, ERαMut PAB rats. Similarly, female, but not male, ERαMut exhibited increased RV fibrosis, comprised primarily of thick collagen fibers. There was an increased protein expression ratio of TIMP metallopeptidase inhibitor 1 (Timp1) to matrix metalloproteinase 9 (Mmp9) in female ERαMut compared with WT PAB rats, suggesting less collagen degradation. RNA-sequencing in female WT and ERαMut RV revealed kallikrein-related peptidase 10 (Klk10) and Jun Proto-Oncogene (Jun) as possible mediators of female RV protection during PAB. In summary, ERα in females is protective against RV-pulmonary vascular uncoupling, diastolic dysfunction, and fibrosis in response to pressure overload. ERα appears to be dispensable for RV adaptation in males. ERα may be a mediator of superior RV adaptation in female patients with PAH.NEW & NOTEWORTHY Using a novel loss-of-function mutation in estrogen receptor-α (ERα), we demonstrate that female, but not male, ERα mutant rats display right ventricular (RV)-vascular uncoupling, diastolic dysfunction, and fibrosis following pressure overload, indicating a sex-dependent role of ERα in protecting against adverse RV remodeling. TIMP metallopeptidase inhibitor 1 (Timp1), matrix metalloproteinase 9 (Mmp9), kallikrein-related peptidase 10 (Klk10), and Jun Proto-Oncogene (Jun) were identified as potential mediators in ERα-regulated pathways in RV pressure overload.

Myofibroblast contraction is essential for generating and regenerating the gas-exchange surface.

A majority (~95%) of the gas-exchange surface area is generated through septa formation during alveologenesis. Disruption of this process leads to alveolar simplification and bronchopulmonary dysplasia (BPD), a prevalent disorder in premature infants. Although several models have been proposed, the mechanism of septa formation remains under debate. Here we show that inactivation of myosin light chain kinase (MLCK), a key factor required for myofibroblast contraction, disrupted septa formation, supporting the myofibroblast contraction model of alveologenesis. The alveoli simplification phenotype was accompanied by decreased yes-associated protein (YAP), a key effector in the Hippo mechanotransduction pathway. Expression of activated YAP in Mlck-mutant lungs led to partial reversal of alveolar simplification. In the adult, although Mlck inactivation did not lead to simplification, it prevented reseptation during compensatory regrowth in the pneumonectomy model. These findings revealed that myofibroblast reactivation and contraction are requisite steps toward regenerating the gas-exchange surface in diseases such as BPD and chronic obstructive pulmonary disease (COPD).

Smooth Muscle Differentiation Is Essential for Airway Size, Tracheal Cartilage Segmentation, but Dispensable for Epithelial Branching.

Airway smooth muscle is best known for its role as an airway constrictor in diseases such as asthma. However, its function in lung development is debated. A prevalent model, supported by in vitro data, posits that airway smooth muscle promotes lung branching through peristalsis and pushing intraluminal fluid to branching tips. Here, we test this model in vivo by inactivating Myocardin, which prevented airway smooth muscle differentiation. We found that Myocardin mutants show normal branching, despite the absence of peristalsis. In contrast, tracheal cartilage, vasculature, and neural innervation patterns were all disrupted. Furthermore, airway diameter is reduced in the mutant, counter to the expectation that the absence of smooth muscle constriction would lead to a more relaxed and thereby wider airway. These findings together demonstrate that during development, while airway smooth muscle is dispensable for epithelial branching, it is integral for building the tracheal architecture and promoting airway growth.

E3 ubiquitin ligase MDM2 acts through p53 to control respiratory progenitor cell number and lung size.

The respiratory lineage initiates from the specification of NKX2-1+ progenitor cells that ultimately give rise to a vast gas-exchange surface area. How the size of the progenitor pool is determined and whether this directly impacts final lung size remains poorly understood. Here, we show that epithelium-specific inactivation of Mdm2, which encodes an E3 ubiquitin ligase, led to lethality at birth with a striking reduction of lung size to a single vestigial lobe. Intriguingly, this lobe was patterned and contained all the appropriate epithelial cell types. The reduction of size can be traced to the progenitor stage, when p53, a principal MDM2 protein degradation target, was transiently upregulated. This was followed by a brief increase of apoptosis. Inactivation of the p53 gene in the Mdm2 mutant background effectively reversed the lung size phenotype, allowing survival at birth. Together, these findings demonstrate that p53 protein turnover by MDM2 is essential for the survival of respiratory progenitors. Unlike in the liver, in which genetic reduction of progenitors triggered compensation, in the lung, respiratory progenitor number is a key determinant factor for final lung size.

Pdgfra marks a cellular lineage with distinct contributions to myofibroblasts in lung maturation and injury response.

Pdgfra-expressing (Pdgfra+) cells have been implicated as progenitors in many mesenchymal tissues. To determine lineage potential, we generated PdgfrartTA knockin mice using CRISPR/Cas9. During lung maturation, counter to a prior study reporting that Pdgfra+ cells give rise equally to myofibroblasts and lipofibroblasts, lineage tracing using PdgfrartTA;tetO-cre mice indicated that ~95% of the lineaged cells are myofibroblasts. Genetic ablation of Pdgfra+ cells using PdgfrartTA-driven diphtheria toxin (DTA) led to alveolar simplification, demonstrating that these cells are essential for building the gas exchange surface area. In the adult bleomycin model of lung fibrosis, lineaged cells increased to contribute to pathological myofibroblasts. In contrast, in a neonatal hyperoxia model of bronchopulmonary dysplasia (BPD), lineaged cells decreased and do not substantially contribute to pathological myofibroblasts. Our findings revealed complexity in the behavior of the Pdgfra-lineaged cells as exemplified by their distinct contributions to myofibroblasts in normal maturation, BPD and adult fibrosis.

FGF receptors control alveolar elastogenesis.

Alveologenesis, the final step of lung development, is characterized by the formation of millions of alveolar septa that constitute the vast gas-exchange surface area. The genetic network driving alveologenesis is poorly understood compared with earlier steps in lung development. FGF signaling through receptors Fgfr3 and Fgfr4 is crucial for alveologenesis, but the mechanisms through which they mediate this process remain unclear. Here we show that in Fgfr3;Fgfr4 (Fgfr3;4) global mutant mice, alveolar simplification is first observed at the onset of alveologenesis at postnatal day 3. This is preceded by disorganization of elastin, indicating defects in the extracellular matrix (ECM). Although Fgfr3 and Fgfr4 are expressed in the mesenchyme and epithelium, inactivation in the mesenchyme, but not the epithelium, recapitulated the defects. Expression analysis of components of the elastogenesis machinery revealed that Mfap5 (also known as Magp2), which encodes an elastin-microfibril bridging factor, is upregulated in Fgfr3;4 mutants. Mfap5 mutation in the Fgfr3;4 mutant background partially attenuated the alveologenesis defects. These data demonstrate that, during normal lung maturation, FGF signaling restricts expression of the elastogenic machinery in the lung mesenchyme to control orderly formation of the elastin ECM, thereby driving alveolar septa formation to increase the gas-exchange surface.

Generation of a Mouse Full-length Balancer with Versatile Cassette-shuttling Selection Strategy.

Balancer chromosomes are important tools for a variety of genetic manipulations in lower model organisms, owing to their ability to suppress recombination. In mouse, however, such effort has not been accomplished, mostly due to the size of the chromosomes and the complexity of multiple step chromosomal engineering. We developed an effective and versatile cassette-shuttling selection (CASS) strategy involving only two selection markers to achieve the sequential production of multiple large inversions along the chromosome. Using this strategy, we successfully generated the first full-length balancer in mice and showed that Balancer 17M-GFP can efficiently suppress recombination. Our study has not only generated a useful genetic resource, but also provided a strategy for constructing mammalian balancer chromosomes.

Disruption of the Golgi protein Otg1 gene causes defective hormone secretion and aberrant glucose homeostasis in mice.

BACKGROUND: Concerted hormone secretion is essential for glucose homeostasis and growth. The oocyte testis gene 1 (Otg1) has limited information in mammals before. Human OTG1 has been identified as an antigen associated with cutaneous T cell lymphoma, while worm Otg1 is recently reported to be a vesicle trafficking regulator in neurons. To understand the physiological role of Otg1 and its potential relation to hormone secretion, we characterized a mutation caused by the piggyBac transposon (PB) insertion in mice. RESULTS: Oocyte testis gene 1 encodes a Golgi localized protein that is expressed with a broad tissue distribution in mice. The PB insertion effectively blocks Otg1 expression, which results in postnatal lethality, growth retardation, hypoglycemia and improved insulin sensitivity in mice. Otg1 mutants exhibit decreased levels of insulin, leptin and growth hormone in the circulation and reduced hepatic IGF-1 expression. Decreased expression of Otg1 in pituitary GH3 cells causes reduced grow hormone expression and secretion, as well as the traffic of the VSVG protein marker. CONCLUSIONS: Our data support the hypothesis that Otg1 impacts hormone secretion by regulating vesicle trafficking. These results revealed a previously unknown and important role of Otg1 in hormone secretion and glucose homeostasis in mammals.

A three-dimensional study of alveologenesis in mouse lung.

Alveologenesis is the final step of lung maturation, which subdivides the alveolar region of the lung into smaller units called alveoli. Each of the nascent dividers serves as a new gas-exchange surface, and collectively they drastically increase the surface area for breathing. Disruption of alveologenesis results in simplification of alveoli, as is seen in premature infants diagnosed with bronchopulmonary dysplasia (BPD), a prevalent lung disease that is often associated with lifelong breathing deficiencies. To date, a majority of studies of alveologenesis rely on two-dimensional (2D) analysis of tissue sections. Given that an overarching theme of alveologenesis is thinning and extension of the epithelium and mesenchyme to facilitate gas exchange, often only a small portion of a cell or a cellular structure is represented in a single 2D plane. Here, we use a three-dimensional (3D) approach to examine the structural architecture and cellular composition of myofibroblasts, alveolar type 2 cells, elastin and lipid droplets in normal as well as BPD-like mouse lung. We found that 2D finger-like septal crests, commonly used to depict growing alveolar septae, are often artifacts of sectioning through fully established alveolar walls. Instead, a more accurate representation of growing septae are 3D ridges that are lined by platelet-derived growth factor receptor alpha (PDGFRA) and alpha smooth muscle actin (α-SMA)-expressing myofibroblasts, as well as the elastin fibers that they produce. Accordingly in 3D, both α-SMA and elastin were each found in connected networks underlying the 3D septal ridges rather than as isolated dots at the tip of 2D septal crests. Analysis through representative stages of alveologenesis revealed unappreciated dynamic changes in these patterns. PDGFRA-expressing cells are only α-SMA-positive during the first phase of alveologenesis, but not in the second phase, suggesting that the two phases of septae formation may be driven by distinct mechanisms. Thin elastin fibers are already present in the alveolar region prior to alveologenesis, suggesting that during alveologenesis, there is not only new elastin deposition, but also extensive remodeling to transform thin and uniformly distributed fibers into thick cables that rim the nascent septae. Analysis of several genetic as well as hyperoxia-induced models of BPD revealed that the myofibroblast organization is perturbed in all, regardless of whether the origin of defect is epithelial, mesenchymal, endothelial or environmental. Finally, analysis of relative position of PDGFRA-positive cells and alveolar type 2 cells reveal that during alveologenesis, these two cell types are not always adjacent to one another. This result suggests that the niche and progenitor relationship afforded by their close juxtaposition in the adult lung may be a later acquired property. These insights revealed by 3D reconstruction of the septae set the foundation for future investigations of the mechanisms driving normal alveologenesis, as well as causes of alveolar simplification in BPD.