Stages Of Fetal Lung Development Research Articles

Association of Fetal Lung Development Disorders with Adult Diseases: A Comprehensive Review.

Fetal lung development is a crucial and complex process that lays the groundwork for postnatal respiratory health. However, disruptions in this delicate developmental journey can lead to fetal lung development disorders, impacting neonatal outcomes and potentially influencing health outcomes well into adulthood. Recent research has shed light on the intriguing association between fetal lung development disorders and the development of adult diseases. Understanding these links can provide valuable insights into the developmental origins of health and disease, paving the way for targeted preventive measures and clinical interventions. This review article aims to comprehensively explore the association of fetal lung development disorders with adult diseases. We delve into the stages of fetal lung development, examining key factors influencing fetal lung maturation. Subsequently, we investigate specific fetal lung development disorders, such as respiratory distress syndrome (RDS), bronchopulmonary dysplasia (BPD), congenital diaphragmatic hernia (CDH), and other abnormalities. Furthermore, we explore the potential mechanisms underlying these associations, considering the role of epigenetic modifications, transgenerational effects, and intrauterine environmental factors. Additionally, we examine the epidemiological evidence and clinical findings linking fetal lung development disorders to adult respiratory diseases, including asthma, chronic obstructive pulmonary disease (COPD), and other respiratory ailments. This review provides valuable insights for healthcare professionals and researchers, guiding future investigations and shaping strategies for preventive interventions and long-term care.

Autopsy findings in a fetus with monosomy 20 mosaicism

Introduction. Mosaic monosomy 20 is a rare chromosomal aberration, without characteristic clinical features. We present a case of a fetus with monosomy 20 mosaicism revealed after prenatal ultrasound detection of anhydramnios and multiple anomalies. Case outline. The second pregnancy of a 33 years old woman, was terminated at 23rd gestational week, because of the multiple fetal anomalies and anhydramnios, detected by ultrasound. The autopsy of a female fetus revealed multiple congenital anomalies: ventriculomegaly, bilateral choroid plexus cysts, perivascular gliosis in periventricular region of cerebri, hydropericardium, severe cardiomegaly, severe myocardial hypertrophy, hydrothorax, glandular/canalicular stage of fetal lung development, bilateral renal and ureter agenesis (Potter syndrome), bladder aplasia, agenesis of the uterus, fallopian tubes and proximal vagina and valgus deformity of left foot (pes valgus). Fetal growth was adequate for gestational age with no craniofacial dysmorphy or radiographically visible anomalies of the skeleton, without signs of infection. The umbilical cord was too much length for gestational age-48cm. Analysis of fetal karyotype from fetal blood sampling revealed monosomy of chromosome 20 in 10% of analyzed cells in metaphase. Conclusion. Revealing the genetic basis of fetal anomalies is at outmost importance not only for further evaluation of pregnancy, but also for proper genetic informing of patients.

The developmental toxicity of PM2.5 on the early stages of fetal lung with human lung bud tip progenitor organoids

Exposure to air pollution has been proven to be associated with impaired fetal lung development. However, due to the lack of reliable human source models, it is still challenging to deeply understand the human fetal lung development under PM2.5 exposure. Here, we utilized human embryonic stem cell (hESC) line H9 to generate lung bud tip progenitor organoids (LPOs), a process that mimics early stages of fetal lung development including definitive endoderm (DE) formation, anterior foregut endoderm (AFE) differentiation and lung progenitor cell specification, to evaluate potential pulmonary developmental toxicity of PM2.5. We demonstrated that PM2.5 exposure the entire LPOs induction from hESCs significantly affected cellular proliferation of LPOs, and altered the expression of lung progenitor cell markers NKX2.1, SOX2 and SOX9, which are canonically defined subsequently proximal–distal airways specification. To explore the dynamic influences of PM2.5 exposure at different stages of LPOs specification, we also found that PM2.5 exposure significantly affected the expression of several transcriptional factors that are important for the differentiation of DE and AFE. Mechanistically, we suggested PM2.5-induced developmental toxicity to LPOs was partially linked with the Wnt/β-catenin signaling pathway. Therefore, our findings further emphasize the substantial health risks in the development of respiratory system associated with prenatal exposure to PM2.5.

Expression Patterns of Serotonin Receptors 5-HT1A, 5-HT2A, and 5-HT3A during Human Fetal Lung Development.

We analyzed the expression of the serotonin receptors 5-HT1A, 5-HT2A, and 5-HT3A at four different stages of fetal lung development from 12 to 40 weeks of gestation, divided into four groups: the pseudoglandular stage (12-16th week of development; n = 8), the canalicular stage (16th-26th week of development; n = 7), the saccular stage (26th-36th week of development; n = 5), and the alveolar stage (36th-40th week of development; n = 5). The strongest expression of all three receptor types was found in the epithelium of the proximal airways during the pseudoglandular, canalicular, and saccular stages and in a vascular wall. 5-HT1A was also strongly expressed in the smooth muscle cells of the proximal airway. Vascular smooth muscle cells and endothelium occasionally showed a strong expression of 5-HT1A and 5-HT2A. In the alveolar stage, the expression of 5-HT1A, 5-HT2A, and 5-HT3A was detected in both type I (p1) and type II (p2) pneumocytes, with a stronger expression in p2. A significant decrease in percent the 5-HT2A area and in the integrated density was observed at the alveolar stage. On the other hand, a significant decrease in the percentage area but an increase in the integrated density was observed for 5-HT3A toward the alveolar stage, suggesting that a smaller number of cells expressed 5-HT3A but that they (p1 and p2) significantly increased their 5-HT3A expression at the alveolar stage. The results presented provided us with new data on the development and function of the serotonin system in the human fetal lung and gave us insight into their possible involvement in the pathogenesis of lung pathology, particularly that characteristic of the neonatal period.

Comparison of survival of preterm newborn rabbits at 25-28 days of gestation with perinatal therapies at birth transition.

Eligibility of ventilated preterm rabbit model to investigate extreme pulmonary immaturity at birth transition is unknown. By extending this model to early saccular stage of fetal lung development, we evaluated efficacy in survival, lung maturation, and underlying mechanisms of contemporary perinatal therapies. Pregnant New Zealand White rabbit does were given dexamethasone (DEX), or sham injection as control (NDEX), 48 and 24 h before delivery at gestational age (GA) of 25-28 days. At birth, newborn rabbits were anesthetized and randomly allocated to four groups receiving either surfactant or nonsurfactant for both DEX and NDEX, and mechanically ventilated within low tidal volumes. Ranges of time to maintain survival rate ≥ 50% in GA 25-28 days were 59-136, 138-259, 173-288, and 437 to ≥600 min, respectively, each across the four groups. The benefits of DEX and/or surfactant for survival were more obvious in GA 25-26 days, as judged by improved lung mechanics, lower lung injury scores, higher lung surfactant phospholipid pools, and surfactant protein mRNA expression, with DEX-surfactant combination being the most optimal for the outcome. In contrast, those of GA 27-28 days had variable but meaningful responses to the treatment. Cox regression analysis revealed GA, DEX, and surfactant being independently protective factors whereas pneumothorax was a risk factor. The extremely preterm rabbits at GA 25-26 days markedly responded to the perinatal therapies for longer survival, lung maturation and injury alleviation, and were relevant for study of preterm birth transition-associated morbidities and underlying mechanisms.NEW & NOTEWORTHY An extremely preterm rabbit model with gestational age of 25-26 (term 31) days was established by mechanical ventilation with individually adjusted tidal volume at lower ranges. The administration of antenatal glucocorticoids and/or postnatal surfactant achieved significantly longer duration to maintain 50% survival and facilitated lung maturation and protection at early saccular stage. The usefulness of this model should be validated in future investigation of perinatal and neonatal morbidity and mortality at extremely preterm birth transition.

Prenatal administration of all-trans retinoic acid upregulates leptin signaling in hypoplastic rat lungs with experimental congenital diaphragmatic hernia

Pulmonary hypoplasia (PH), characterized by alveolar immaturity, is one of the leading causes of respiratory insufficiency in newborns with congenital diaphragmatic hernia (CDH). Leptin (Lep) and its receptor (Lep-R) play an important role in fetal lung growth by stimulating alveolar differentiation and maturation. Lep and Lep-R are strongly expressed by alveolar cells during the saccular stage of fetal lung development. Lep-deficient mice exhibit decreased alveolarization with reduced pulmonary surfactant phospholipid synthesis, similar to human and nitrofen-induced PH. Prenatal administration of all-trans retinoic acid (ATRA) has been shown to stimulate alveolarization in nitrofen-induced PH. Recent studies have demonstrated that Lep and Lep-R expression in developing lungs is regulated by ATRA. We hypothesized that prenatal treatment with ATRA increases pulmonary Lep and Lep-R expression in the nitrofen model of CDH-associated PH. Time-mated rats received either 100 mg nitrofen or vehicle via oral-gastric lavage on embryonic day 9.5 (E9.5). Control and nitrofen-exposed dams were randomly assigned to either intraperitoneal ATRA (5 mg/kg/d) or placebo administration on E18.5, E19.5 and E20.5. Fetal lungs were harvested on E21.5, and divided into Control+Placebo, Control+ATRA, Nitrofen+Placebo and Nitrofen+ATRA. Alveolarization was assessed using stereo- and morphometric analysis techniques. Surfactant phospholipid synthesis was analyzed by labeling for surfactant protein B (SP-B). Pulmonary gene expression levels of Lep and Lep-R were determined using quantitative real-time polymerase chain reaction. Immunohistochemical staining for Lep and Lep-R was performed to evaluate alveolar protein expression and localization. In vivo administration of ATRA resulted in significantly increased lung-to-body weight ratio with enhanced radial alveolar count and decreased mean linear intercept compared to placebo treatment. Immunofluorescence analysis demonstrated markedly increased pulmonary SP-B expression in Nitrofen+ATRA compared to Nitrofen+Placebo. Relative mRNA expression of Lep and Lep-R was significantly increased in Nitrofen+ATRA compared to Nitrofen+Placebo. Lep and Lep-R immunoreactivity was markedly increased in interstitial and alveolar epithelial cells of Nitrofen+ATRA compared to Nitrofen+Placebo. Increased Lep and Lep-R expression after prenatal administration of ATRA in nitrofen-induced PH suggests that ATRA may have therapeutic potential in attenuating CDH-associated PH by stimulating alveolarization and de novo surfactant production.

Disruption of the Bone Morphogenetic Protein Receptor 2 Pathway in Nitrofen‐Induced Congenital Diaphragmatic Hernia

Congenital diaphragmatic hernia (CDH) remains a major therapeutic challenge despite advances in neonatal resuscitation and intensive care. The high mortality and morbidity in CDH has been attributed to pulmonary hypoplasia and persistent pulmonary hypertension (PH). Bone morphogenetic protein receptor 2 (BMPR2) plays a key role in pulmonary vasculogenesis during the late stages of fetal lung development. BMPR2 is essential for control of endothelial and smooth muscle cell proliferation. Dysfunction of BMPR2 and downstream signaling have been shown to disturb the crucial balance of proliferation of smooth muscle cells contributing to the pathogenesis of human and experimental PH. We designed this study to investigate the hypothesis that BMPR2 signaling is disrupted in nitrofen-induced CDH. Pregnant rats were treated with nitrofen or vehicle on gestational day 9 (D9). Fetuses were sacrificed on D21 and divided into CDH and control. Quantitative real-time polymerase chain reaction, Western blotting, and confocal-immunofluorescence were performed to determine pulmonary gene expression levels and protein expression of BMPR2 and related proteins. Pulmonary Bmpr2 gene expression levels were significantly decreased in nitrofen-induced CDH compared to controls. Western blotting and confocal microscopy revealed decreased pulmonary BMPR2 protein expression and increased activation of p38(MAPK) in CDH compared to controls. The observed disruption of the BMPR2 signaling pathway may lead to extensive vascular remodeling and contribute to PH in the nitrofen-induced CDH model. BMPR2 may therefore represent a potential target for the treatment of PH in CDH.

Decreased pulmonary c-Cbl expression and tyrosine phosphorylation in the nitrofen-induced rat model of congenital diaphragmatic hernia

The high morbidity of newborn infants with congenital diaphragmatic hernia (CDH) is attributed to pulmonary hypoplasia (PH), which is characterized by a failure of alveolar development. The nitrofen-induced CDH model has been widely used to investigate the pathogenesis of PH in CDH. It has previously been shown that the fibroblast growth factor receptor (FGFR) pathway, which is essential for a proper lung development, is disrupted during late gestation of nitrofen-induced CDH. Casitas B-lineage lymphoma (c-Cbl) proteins are known regulators of signal transduction through FGFRs, indicating their important role during alveolarization in developing lungs. Furthermore, it has been demonstrated that tyrosine phosphorylation of c-Cbl proteins has a pivotal role for their physiological function and activity during fetal lung development. We designed this study to test the hypothesis that pulmonary c-Cbl expression and tyrosine phosphorylation status are decreased in the nitrofen-induced CDH model. Timed-pregnant rats received either 100mg nitrofen or vehicle on gestation day 9 (D9). Fetuses were harvested on D18 and D21, and lungs were divided into two groups: control and hypoplastic lungs with CDH (CDH(+)) (n=10 at each time-point, respectively). Pulmonary gene expression levels of c-Cbl were analyzed by quantitative real-time polymerase chain reaction. Western blotting combined with densitometry analysis was used for semi-quantification of protein levels of pulmonary c-Cbl and tyrosine phosphorylation status. Confocal-immunofluorescence staining was performed to evaluate c-Cbl protein expression and distribution. Relative mRNA expression levels of pulmonary c-Cbl were significantly decreased in CDH(+) on D18 and D21 compared to controls. Western blotting showed markedly decreased protein levels of pulmonary c-Cbl and tyrosine phosphorylation status in CDH(+) on D18 and D21. Confocal-immunofluorescence analysis confirmed decreased c-Cbl expression in CDH(+) on D18 and D21 mainly in the distal alveolar epithelium compared to controls. Decreased pulmonary c-Cbl gene and protein expression accompanied by a decreased tyrosine phosphorylation status during the late stages of fetal lung development may result in reduced c-Cbl activity, and thus interfere with the FGFR-mediated alveolarization in the nitrofen-induced CDH model.

Upregulation of fibroblast growth factor receptor 2 and 3 in the late stages of fetal lung development in the nitrofen rat model

Nitrofen model of congenital diaphragmatic hernia (CDH) has been widely used to investigate the pathogenesis of pulmonary hypoplasia (PH). Fibroblast growth factor (FGF) signaling pathway plays a fundamental role in fetal lung development. FGF7 and FGF10, which are critical for lung morphogenesis, have been reported to be downregulated in nitrofen-induced PH. FGF signaling is mediated by a family of four single transmembrane receptors, FGFR1-4. FGFR2 and FGFR3 have been shown to be expressed predominantly in the late stages of developing lungs. In addition, the upregulation of FGFR2 gene expression has been associated with severe defects in lung development and resulted in arrested alveologenesis similar to PH seen in the nitrofen model. Furthermore, FGFR3(-/-)FGFR4(-/-) double mutants showed thinner mesenchyme and larger air spaces. We designed this study to test the hypothesis that FGFR gene expression is upregulated in the late stages of lung development in the nitrofen CDH model. Pregnant rats were exposed to either olive oil or nitrofen on day 9 of gestation (D9). Cesarean section was performed and fetuses were harvested on D18 and D21. Fetal lungs were divided into three groups: control, nitrofen without CDH [CDH(-)], and nitrofen with CDH [CDH(+)] (n = 24 at each time-point). Pulmonary gene expression levels of FGFR1-4 were analyzed by real-time RT-PCR. Immunohistochemistry was also performed to evaluate protein expression/distribution at each time-point. The relative messenger RNA expression levels of pulmonary FGFR2 and FGFR3 on D21 were significantly increased in CDH(-) (6.38 ± 1.93 and 7.84 ± 2.86, respectively) and CDH(+) (7.09 ± 2.50 and 7.25 ± 3.43, respectively) compared to controls (P < 0.05 and P < 0.01, respectively), whereas no significant alteration was observed on D18. There were no differences in FGFR1 and FGFR4 expression at both time-points. Increased immunoreactivity of FGFR2 and FGFR3, mainly in the distal epithelium and mesenchyme, was observed in the nitrofen-induced hypoplastic lungs on D21 compared to controls. Upregulation of FGFR2 and FGFR3 pulmonary gene expression in the late stages of fetal lung development may disrupt FGFR-mediated alveologenesis resulting in PH in the CDH model.

Pleiotrophin Regulates Lung Epithelial Cell Proliferation and Differentiation during Fetal Lung Development via β-Catenin and Dlk1

The role of pleiotrophin in fetal lung development was investigated. We found that pleiotrophin and its receptor, protein-tyrosine phosphatase receptor beta/zeta, were highly expressed in mesenchymal and epithelial cells of the fetal lungs, respectively. Using isolated fetal alveolar epithelial type II cells, we demonstrated that pleiotrophin promoted fetal type II cell proliferation and arrested type II cell trans-differentiation into alveolar epithelial type I cells. Pleiotrophin also increased wound healing of injured type II cell monolayer. Knockdown of pleiotrophin influenced lung branching morphogenesis in a fetal lung organ culture model. Pleiotrophin increased the tyrosine phosphorylation of beta-catenin, promoted beta-catenin translocation into the nucleus, and activated T cell factor/lymphoid enhancer factor transcription factors. Dlk1, a membrane ligand that initiates the Notch signaling pathway, was identified as a downstream target of the pleiotrophin/beta-catenin pathway by endogenous dlk1 expression, promoter assay, and chromatin immunoprecipitation. These results provide evidence that pleiotrophin regulates fetal type II cell proliferation and differentiation via integration of multiple signaling pathways including pleiotrophin, beta-catenin, and Notch pathways.

MicroRNA-127 modulates fetal lung development

MicroRNAs (miRNAs) are small endogenous RNAs and are widely regarded as one of the most important regulators of gene expression in both plants and animals. To define the roles of miRNAs in fetal lung development, we profiled the miRNA expression pattern during lung development with a miRNA microarray. We identified 21 miRNAs that showed significant changes in expression during lung development. These miRNAs were grouped into four distinct clusters based on their expression pattern. Cluster 1 contained miRNAs whose expression increased as development progressed, while clusters 2 and 3 showed the opposite trend of expression. miRNAs in cluster 4 including miRNA-127 (miR-127) had the highest expression at the late stage of fetal lung development. Quantitative real-time PCR validated the microarray results of six selected miRNAs. In situ hybridization demonstrated that miR-127 expression gradually shifted from mesenchymal cells to epithelial cells as development progressed. Overexpression of miR-127 in fetal lung organ culture significantly decreased the terminal bud count, increased terminal and internal bud sizes, and caused unevenness in bud sizes, indicating improper development. These findings suggest that miR-127 may have an important role in fetal lung development.

Effect of insulin-like-growth factor and its receptors regarding lung development in fetal mice

In congenital diaphragmatic hernia (CDH), both mortality and morbidity are mainly caused by pulmonary hypoplasia and persistent pulmonary hypertension. Insulin-like growth factors (IGFs) are one of the growth factors that may play an important role in the fetal lung development. Elucidating the roles of these growth factors regarding fetal lung development would thus provide new insight regarding the optimal therapy for CDH patients. The aim of this study is to investigate the role of IGFs in the fetal lung development. The mRNA expression of IGFs and its receptors was analyzed by real-time RT-PCR from embryonic day (E) 11.5 to E18.5 mice. In addition, the lungs dissected from the E17.5 mice were divided into the following three groups; lungs cultured only in the serum-free medium (group I n = 5), lungs cultured in medium containing either IGF-I (group II n = 5), or IGF-II (group III n = 5). All cultures were investigated by immunohistochemistry, using the antibodies of thyroid transcription factor (TTF)-1, prosurfactant protein (proSp)-C, alpha smooth muscle actin (alpha-SMA), and anti-proliferating cell nuclear antigen (PCNA). The mRNA expression level of both IGF-I and IGF-II was higher during the earlier stage than that of later stage. In contrast, the mRNA expression of both IGF-I receptor (IGF-IR) and IGF-II receptor (IGF-IIR) was higher from the E17.5 to E18.5 than that at any other stage. The number of positive cells for TTF-1, proSp-C, alpha-SMA and PCNA increased more in both groups II and III than in group I. Based on our findings, IGFs are suggested to induce alveolar and vascular maturation in the late stages of fetal lung development. Therefore, the administration of IGFs to the fetal CDH lung may thus be able to effectively improve the symptoms of hypoplastic lung.

Fetal Pulmonary Malformations: Defining Histopathology

Although classification schemes have sought to categorize congenital cystic lung malformations, studies including the pathology of pulmonary malformations occurring specifically during the fetal period are limited. To better characterize such histopathology, we reviewed a total of 23 fetal lung malformations seen at the Children's Hospital of Philadelphia from 1996 to 2004. Twenty-one of the 23 fetal pulmonary malformations could be categorized into 1 of 3 groups based upon the predominant histologic features present within each lesion. Group 1 (9/21) demonstrated tubular airspaces lined by columnar epithelium. Group 2 (6/21) contained airspaces lined by cuboidal epithelium and surrounded by smooth muscle with abundant interstitial mesenchyme. Group 3 (6/21) showed a mixture of relatively mature-appearing airspaces lined by flattened epithelium and scattered dilated bronchiole-like structures. Cysts were of variable size but in all cases showed a respiratory-type lining. Gestational ages ranged from 21 5/7 to 38 2/7 weeks. Patients in groups 1 and 2 were generally younger than those in group 3; however, morphology did not seem to correlate entirely with normal stages of fetal lung development, and group 2 lesions in particular were the least akin to normal fetal lung. In 4 cases a systemic vascular supply to a lobe of lung was identified, providing evidence that such vasculature is embryonic in origin. The histopathology of fetal lung malformations highlights the variability seen in such lesions at all ages, and it is hoped that continued investigations will provide further insight into these enigmatic lesions.

Pleiotrophin Promotes Alveolar Epithelial Cell Proliferation and Migration

Pleiotrophin (PTN) is a heparin-binding cytokine highly expressed during the late stage of embryogenesis and in postnatal central nerve system. It plays important roles in cell proliferation, mitogenesis, angiogenesis, and mesenchymal-epithelial interaction. Our previous DNA microarray and real-time PCR analyses have shown that PTN was highly expressed at the late stage of fetal lung development. Immunostainning revealed that PTN was localized in the mesenchyme adjacent to the developing epithelium, suggesting a role in mesenchymal-epithelial interaction. In the isolated alveolar epithelial cells, PTN significantly increased cell migration and proliferation in a dose-dependent manner. In fetal lung organ culture, PTN influenced lung branching morphogenesis, increased cell proliferation, alveolar epithelial type II cell differentiation, and decreased apoptosis. Our results demonstrate for the first time that PTN may play an important role in fetal lung development. (Supported by NIH R01 HL-52146 and R01 HL-071628 to LL).

Ontogeny and nutritional programming of uncoupling protein‐2 and glucocorticoid receptor mRNA in the ovine lung

This study investigated the developmental and nutritional programming of uncoupling protein-2 (UCP2), glucocorticoid receptor (GR) and 11beta-hydroxysteroid dehydrogenase type 1 (11betaHSD1) mRNA in the sheep lung from the time of uterine attachment to 6 months of age. The effect of maternal nutrient restriction on lung development was determined in early to mid gestation (i.e. 28-80 days gestation, period of maximal placental growth, and embryonic and pseudoglandular stages of fetal lung development) and late gestation (i.e. 110-147 days gestation, period of maximal fetal growth, and canalicular and saccular stages of fetal lung development). Fetal lungs were sampled at 80 and 140 days (term approximately 148 days) gestation, and sheep lungs at 1, 7, 30 days and 6 months. GR and 11betaHSD1 mRNA were maximal at 140 days gestation, whereas UCP2 mRNA peaked at 1 day of age and then declined with postnatal age. Maternal nutrient restriction in both early-to-mid and late gestation had no effect on lung weight, but increased UCP2, GR and 11betaHSD1 mRNA abundance at every sampling age. These findings suggest that the developmental ontogeny of UCP2 mRNA in the ovine lung is under local glucocorticoid hormone action and that maternal nutrient restriction has long-term consequences for UCP2 and GR mRNA abundance in the lung irrespective of its timing.

The role of apoptosis in normal and accelerated lung development in fetal rabbits

Background/Purpose: During fetal development, the mammalian lung undergoes progressive parenchymal involution. Intrauterine tracheal occlusion induces accelerated architectural maturation of the fetal lungs associated with depletion of the surfactant-producing type II cells. This study investigates the spatiotemporal pattern of apoptosis during normal fetal lung development and its modulation in tracheal occlusion-induced accelerated fetal lung growth.Methods: Fetal rabbit lungs were studied at 25 to 31 days' gestational age (DGA; term, 31 DGA), corresponding to late pseudoglandular through terminal air sac stages of fetal lung development. Intrauterine tracheal ligation (TL) was performed at 24 DGA. TL fetuses were monitored until 29 DGA, a time-point previously shown to coincide with significant type II cell depletion. Apoptotic cells were identified by light and electron microscopy, as well as terminal deoxynucleotidyl transferase-mediated dUTP-FITC nick-end labeling (TUNEL). Epithelial (type I and II) cell apoptosis was studied by TUNEL labeling in conjunction with antisurfactant protein and anticytokeratin immunohistochemistry. DNA fragmentation was analyzed by gel electrophoresis. Sham-operated littermates served as controls.Results: The number of apoptotic cells progressively increased with advancing lung growth and architectural maturation (apoptotic index [Al] 1.2 ± 0.7 x 10−3 at 25 DGA v 4.2 ± 1.4 x 10−3 at 31 DGA; P < .05, analysis of variance). In TL fetuses, the apoptotic rate was significantly higher than in non-TL fetuses from the third postligation day on, coinciding with the onset of significantly increased airspace distension (Al 4.9 ± 1.3 x 10−3 in TL v 2.6 ± 0.4 x 10−3 in controls at 29 DGA; P < .05, Student's t test). Apoptosis occurred in parenchymal cells and in isolated cells within the airspaces. The apoptotic activity of type II cells was significantly higher in TL fetuses than C fetuses at 29 DGA (type II Al 25.5 ± 6.3 x 10−3 in TL v 2.3 ± 0.8 x 10−3 in C; P < .001). Electron microscopic studies confirmed the presence of apoptotic nuclei in interstitial macrophages and in degenerating intraluminal type II cells. DNA analysis showed nucleosomal bands.Conclusions: Normal fetal lung development is associated with a progressive increase of epithelial and interstitial apoptotic activity, a process enhanced by TL. Tracheal occlusion induces a significant increase of type II cell apoptosis, which likely contributes to the observed type II cell depletion after TL. We speculate that fetal type II cell apoptosis after TL may be induced by mechanical distension (stretch) of the airspaces.

Ontogeny of Extracellular Matrix Gene Expression by Rat Lung Cells at Late Fetal Gestation

Lung epithelial and mesenchymal cells are separated by a basement membrane. At late fetal gestation, this basement membrane in fenestrated, allowing epithelial cytoplasmic extensions to reach in close proximity of the interstitial fibroblast. The enzymes responsible for this focal basement membrane remodelling, and their cellular origin, remains to be defined. Basement membrane remodelling generally involves a special class of matrix-degrading enzymes, called metalloproteinases. Herein, we report that fetal lung cells originating from both tissue layers, mesoderm and endoderm, express the metalloproteinase genes, MMP-1 or interstitial collagenase, and MMP-3 or stromelysin. The inhibitor of metalloproteinases, TIMP-1, is mainly expressed in fetal lung fibroblasts. During late fetal development, MMP-1 mRNA expression in both cell types increases close to term (day 21, term = 22 days), while that of stromelysin and TIMP-1 remain constant. Both fibroblasts and epithelial cells express fibronectin (FN) mRNA. The expression of the FN gene in epithelial cells decreases slightly at the canalicular stage of lung development (days 19–20), whereas FN expression in fibroblasts is not changed with advancing gestation. Procollagen α₁(I) mRNA is predominantly detected in fibroblasts whereas message for laminin B1 chain is primarily found in epithelial cells. Expression of procollagen α₁(I) mRNA decreases in fibroblasts during the canalicular stage of fetal lung development compared to the pseudoglandular stage (day 18) but increases thereafter at the saccular stage (day 21) of development. Laminin B1 expression in epithelial cells declines with advancing gestation. These data are consistent with a process of basement membrane thinning during the canalicular stage, followed by metalloproteinase-mediated penetration. Further, a progressive reduction in laminin expression is consistent with progressive epithelial differentiation.

Glucocorticoids upregulate tropoelastin expression during late stages of fetal lung development

The production of elastin, an essential extracellular matrix protein of terminal airway interstitium, occurs mostly during early development. Because glucocorticoids influence airway maturation, we studied the effect of dexamethasone (Dex) on tropoelastin expression during fetal lung development. Timed-pregnant rats were treated with Dex (1 mg/kg daily), and fetal lungs were collected 3 days later at 17, 19, and 21 days of gestation. Dex treatment resulted in about a threefold increase in tropoelastin mRNA levels at 19 days concomitant with accelerated airway development. By in situ hybridization, Dex treatment increased the number of tropoelastin-expressing cells and the level of tropoelastin mRNA per cell. In organ culture, Dex increased lung tropoelastin expression and augmented cortisol stimulation of tropoelastin expression. In fetal pulmonary artery smooth muscle cells, 10(-8) M Dex upregulated tropoelastin mRNA expression and increased tropoelastin promoter-chloramphenicol acetyl transferase activity in transient transfections. These data indicate that pharmacologically administered glucocorticoids transcriptionally upregulate fetal lung tropoelastin expression and suggest that steroid hormones may be important regulators of elastin production in vivo.

Late appearance of a type I alveolar epithelial cell marker during fetal rat lung development.

Recent studies in fetal lung using immunological and molecular probes have revealed type I and type II cell phenotypic markers in primordial lung epithelial cells prior to the morphogenesis of these cell types. We have recently developed monoclonal antibodies specific for adult type I cells. To evaluate further the temporal appearance of the type I cell phenotype during alveolar epithelial cell ontogeny, we analyzed fetal lung development using one of our monoclonal antibodies (mAb VIII B2). The epitope recognized by mAb VIII B2 first appears in the canalicular stage of fetal lung development, at approx. embryonic day 19 (E19), in occasional, faintly stained tubules. Staining with this type I cell probe becomes more intense and more widespread with increasing gestational age, during which time the pattern of staining changes. Initially, all cells of the distal epithelial tubules are uniformly labelled along their apical and basolateral surfaces. As morphological differentiation of the alveolar epithelium proceeds, type I cell immunoreactivity appears to become restricted to the apical surface of the primitive type I cells in a pattern approaching that seen in the mature lung. We concurrently analyzed developing fetal lung with an antiserum to surfactant apoprotein-A (alpha-SP-A). Consistent with the findings of others, labeling of SP-A was first detectable in scattered cuboidal cells at E18. Careful examination of the double-labeled specimens suggested that some cells were reactive with both the VIII B2 and SP-A antibodies, particularly at E20. Confocal microscopic analysis of such sections from E20 lung confirmed this impression. Three populations of cells were detected: cells labeled only with alpha-SP-A, cells labeled only with mAb VIII B2, and a smaller subset of cells labeled by both.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of Maternal Dexamethasone on Expression of SP-A, SP-B, and SP-C in the Fetal Rat Lung

Prenatal administration of glucocorticoids has been shown to enhance surfactant production in the fetus. Since the surfactant proteins play an important role in surfactant function and secretion, we wished to determine the effects of maternal glucocorticoid administration on their fetal expression and appearance. Daily dexamethasone (DEX) (1 mg/kg/day) or 0.9% saline was administered to timed-pregnant rats on gestational days 14 through 16 or on day 16 with sacrifice on day 17 (term day 22), and on gestational days 14 through 18, or days 16 through 18, or day 18 with sacrifice on day 19. SP-A content was determined in lung homogenates from treated and control male and female fetal rats by an enzyme-linked in lung homogenates from treated and control male and female fetal rats by an enzyme-linked immunosorbent assay. The abundance of mRNAs for SP-A, SP-B, and SP-C per fixed amount of total cellular RNA was also determined in lungs from treated and control male and female fetal rats by Northern blot analysis. In litters sacrificed on day 17, DEX administered on days 14 through 16 and on day 16 resulted in significant increases in SP-A content. Expression of SP-A mRNA, which was not detectable in control fetuses on day 17, became clearly apparent after either 1 or 3 d of DEX treatment. The abundance of mRNAs for SP-B and SP-C also increased in day-17 fetuses after either 1 or 3 d of DEX treatment.(ABSTRACT TRUNCATED AT 250 WORDS)