Cortisol enhances structural maturation of the hypoplastic fetal lung in sheep

Fetal Hippocampal Connectivity Shows Dissociable Associations with Maternal Cortisol and Self-Reported Distress during Pregnancy

Maternal stress can shape long-term child neurodevelopment beginning in utero. One mechanism by which stress is transmitted from mothers to their offspring is via alterations in maternal cortisol, which can cross the placenta and bind to glucocorticoid receptor-rich regions in the fetal brain, such as the hippocampus. Although prior studies have demonstrated associations between maternal prenatal stress and cortisol levels with child brain development, we lack information about the extent to which these associations originate prior to birth and prior to confounding postnatal influences. Pregnant mothers (n = 77) completed questionnaires about current perceived stress, depressive symptoms, and anxiety symptoms, provided three to four salivary cortisol samples, and completed a fetal resting-state functional MRI scan during their second or third trimester of pregnancy (mean gestational age = 32.8 weeks). Voxelwise seed-based connectivity analyses revealed that higher prenatal self-reported distress and higher maternal cortisol levels corresponded to dissociable differences in fetal hippocampal functional connectivity. Specifically, self-reported distress was correlated with increased positive functional coupling between the hippocampus and right posterior parietal association cortex, while higher maternal cortisol was associated with stronger positive hippocampal coupling with the dorsal anterior cingulate cortex and left medial prefrontal cortex. Moreover, the association between maternal distress, but not maternal cortisol, on fetal hippocampal connectivity was moderated by fetal sex. These results suggest that prenatal stress and peripheral cortisol levels may shape fetal hippocampal development through unique mechanisms.

Impact of maternal late gestation undernutrition on surfactant maturation, pulmonary blood flow and oxygen delivery measured by magnetic resonance imaging in the sheep fetus

Restriction of fetal substrate supply has an adverse effect on surfactant maturation in the lung and thus affects the transition from in utero placental oxygenation to pulmonary ventilation ex utero. The effects on surfactant maturation are mediated by alteration in mechanisms regulating surfactant protein and phospholipid synthesis. This study aimed to determine the effects of late gestation maternal undernutrition (LGUN) and LGUN plus fetal glucose infusion (LGUN+G) compared to Control on surfactant maturation and lung development, and the relationship with pulmonary blood flow and oxygen delivery ( D O 2 ) measured by magnetic resonance imaging (MRI) with molecules that regulate lung development. LGUN from 115 to 140 days' gestation significantly decreased fetal body weight, which was normalized by glucose infusion. LGUN and LGUN+G resulted in decreased fetal plasma glucose concentration, with no change in fetal arterial P O 2 compared to control. There was no effect of LGUN and LGUN+G on the mRNA expression of surfactant proteins (SFTP) and genes regulating surfactant maturation in the fetal lung. However, blood flow in the main pulmonary artery was significantly increased in LGUN, despite no change in blood flow in the left or right pulmonary artery and D O 2 to the fetal lung. There was a negative relationship between left pulmonary artery flow and D O 2 to the left lung with SFTP-B and GLUT1 mRNA expression, while their relationship with VEGFR2 was positive. These results suggest that increased pulmonary blood flow measured by MRI may have an adverse effect on surfactant maturation during fetal lung development. KEY POINTS: Maternal undernutrition during gestation alters fetal lung development by impacting surfactant maturation. However, the direction of change remains controversial. We examined the effects of maternal late gestation maternal undernutrition (LGUN) on maternal and fetal outcomes, signalling pathways involved in fetal lung development, pulmonary haemodynamics and oxygen delivery in sheep using a combination of molecular and magnetic resonance imaging (MRI) techniques. LGUN decreased fetal plasma glucose concentration without affecting arterial P O 2 . Surfactant maturation was not affected; however, main pulmonary artery blood flow was significantly increased in the LGUN fetuses. This is the first study to explore the relationship between in utero MRI measures of pulmonary haemodynamics and lung development. Across all treatment groups, left pulmonary artery blood flow and oxygen delivery were negatively correlated with surfactant protein B mRNA and protein expression in late gestation.

MMP-2 and MMP-9 levels in plasma are altered and associated with mortality in COVID-19 patients

Respiratory symptoms are one of COVID-19 manifestations, and the metalloproteinases (MMPs) have essential roles in the lung physiology. We sought to characterize the plasmatic levels of matrix metalloproteinase-2 and 9 (MMP-2 and MMP-9) in patients with severe COVID-19 and to investigate an association between plasma MMP-2 and MMP-9 levels and clinical outcomes and mortality. MMP-2 and MMP-9 levels in plasma from patients with COVID-19 treated in the ICU (COVID-19 group) and Control patients were measured with the zymography. The study groups were matched for age, sex, hypertension, diabetes, BMI, and obesity profile. MMP-2 levels were lower and MMP-9 levels were higher in a COVID-19 group (p < 0.0001) compared to Controls. MMP-9 levels in COVID-19 patients were not affected by comorbidity such as hypertension or obesity. MMP-2 levels were affected by hypertension (p < 0.05), but unaffected by obesity status. Notably, hypertensive COVID-19 patients had higher MMP-2 levels compared to the non-hypertensive COVID-19 group, albeit still lower than Controls (p < 0.05). No association between MMP-2 and MMP-9 plasmatic levels and corticosteroid treatment or acute kidney injury was found in COVID-19 patients. The survival analysis showed that COVID-19 mortality was associated with increased MMP-2 and MMP-9 levels. Age, hypertension, BMI, and MMP-2 and MMP-9 were better predictors of mortality during hospitalization than SAPS3 and SOFA scores at hospital admission. In conclusion, a significant association between MMP-2 and MMP-9 levels and COVID-19 was found. Notably, MMP-2 and MMP-9 levels predicted the risk of in-hospital death suggesting possible pathophysiologic and prognostic roles.

Identification of Betamethasone-Regulated Target Genes and Cell Pathways in Fetal Rat Lung Mesenchymal Fibroblasts

Preterm birth is characterized by severe lung immaturity that is frequently treated antenatally or postnatally with the synthetic steroid betamethasone. The underlying cellular targets and pathways stimulated by betamethasone in the fetal lung are poorly defined. In this study, betamethasone was compared with corticosterone in steroid-treated primary cultures of fetal rat lung fibroblasts stimulated for 6 hours and analyzed by whole-cell transcriptome sequencing and glucocorticoid (GC) receptor (GR) chromatin immunoprecipitation sequencing (ChIP-Seq) analysis. Strikingly, betamethasone stimulated a much stronger transcriptional response compared with corticosterone for both induced and repressed genes. A total of 483 genes were significantly stimulated by betamethasone or corticosterone, with 476 stimulated by both steroids, indicating a strong overlap in regulation. Changes in mRNA levels were confirmed by quantitative PCR for eight induced and repressed target genes. Pathway analysis identified cell proliferation and cytoskeletal/cell matrix remodeling pathways as key processes regulated by both steroids. One target, transglutaminase 2 (Tgm2), was localized to fetal lung mesenchymal cells. Tgm2 mRNA and protein levels were strongly increased in fibroblasts by both steroids. Whole-genome GR ChIP-Seq analysis with betamethasone identified GC response element-binding sites close to the previously characterized GR target genes Per1, Dusp1, Fkbp5, and Sgk1 and near the genes identified by transcriptome sequencing encoding Crispld2, Tgm2, Hif3α, and Kdr, defining direct genomic induction of expression in fetal lung fibroblasts via the GR. These results demonstrate that betamethasone stimulates specific genes and cellular pathways controlling cell proliferation and extracellular matrix remodeling in lung mesenchymal fibroblasts, providing a basis for betamethasone's treatment efficacy in preterm birth.

Effect of lung hypoplasia on the cardiorespiratory transition in newborn lambs

Newborns with lung hypoplasia (LH) commonly have limited respiratory function and often require ventilatory assistance after birth. We aimed to characterize the cardiorespiratory transition and respiratory function in newborn lambs with LH. LH was induced by draining fetal lung liquid in utero [110-133 days (d), term = 147d, n = 6]. At ~133d gestation, LH and Control lambs (n = 6) were instrumented and ventilated for 3 h to monitor blood-gas status, oxygenation, ventilator requirements, and hemodynamics during the transition from fetal to newborn life. Lambs with LH had significantly reduced relative wet and dry lung weights indicating hypoplastic lungs compared with Control lambs. LH lambs experienced persistent hypercapnia and acidosis during the ventilation period, had lower lung compliance, and had higher alveolar-arterial differences in oxygen and oxygenation index compared with Control lambs. As a result, LH lambs required greater respiratory support and more supplemental oxygen. Following delivery, LH lambs experienced periods of significantly lower pulmonary artery blood flow and higher carotid artery blood flow in association with the lower oxygenation levels. The detrimental effects of LH can be attributed to a reduction in lung size and poorer gas exchange capabilities. This study has provided greater understanding of the effect of LH itself on the physiology underpinning the transition from fetal to newborn life. Advances in this area is the key to identifying improved or novel management strategies for babies with LH starting in the delivery room, to favorably alter the fetal-to-newborn transition toward improved outcomes and reduced lifelong morbidity.NEW & NOTEWORTHY Current clinical management of newborns with lung hypoplasia (LH) is largely based on expert opinion rather than scientific evidence. We have generated physiological evidence for detrimental effects of LH on hemodynamics and respiratory function in newborn lambs, which mimics the morbidity observed in LH newborns clinically. The unfavorable consequences of LH can be attributed to a reduction in lung size and poorer gas exchange capabilities.

Does environmental stress affect cortisol biodistribution in freshwater mussels?

As of today, regulation and physiological purpose of steroid hormones in invertebrates such as mussels are not completely understood. Many studies were able to show their presence, but their origin and genesis are not clear. Nevertheless, knowledge about changes in steroid hormone biodistribution in reaction to treatments could improve our understanding of their physiological functions in these species. Cortisol is a corticosteroid, which is frequently used as a stress biomarker in vertebrates, like fish or higher organisms. The aim of the study was to optimize cortisol extraction from various tissues of mussels, to develop a quantitative ELISA test system, and to study changes in biodistribution of cortisol in reaction to negative and positive stimulation treatments. As model organism, we used Anodonta anatina, a widespread freshwater mussel species native to Europe. We quantified cortisol concentrations in hepatopancreas, mantle, gills, gonads and the foot muscle. Tissue-specific reactions to environmental influences, simulated with the chemical stressors copper (II) chloride and sodium chloride, were assessed. During the 24-hours treatment, we additionally observed changes in cortisol regulation in response to feeding activity of the mussels. Besides, we found highly significant variations in the biodistribution of cortisol in different tissues, with a peak in the hepatopancreas. Whole body cortisol did not increase in the treated groups. However, balancing of all measured tissues showed redistribution of more than 10% of total body cortisol from the hepatopancreas to all other tissues during copper (II) chloride stressor treatment, but also when mussels ingested feed, compared to the non-fed control group. No redistribution was observed during sodium chloride treatment. We conclude that there can be a redistribution of cortisol in mussels, depending on external influences.

Glucocorticoids influence versican and chondroitin sulphate proteoglycan levels in the fetal sheep lung

BACKGROUND

Prenatal glucocorticoid treatment decreases alveolar tissue volumes and facilitates fetal lung maturation, however the mechanisms responsible are largely unknown. This study examines whether changes in versican levels or sulphation patterns of chondroitin sulphate (CS) side chains, are associated with glucocorticoid-induced reductions in peri-alveolar tissue volumes.

METHODS

Lung tissue was collected from 1) fetal sheep at 131 ± 0.1 days gestational age (GA) infused with cortisol (122-131d GA) to prematurely induce a pre-parturient-like rise in circulating cortisol, 2) fetal sheep at 143d GA bilaterally adrenalectomised (ADX) at 112d GA to remove endogenous cortisol and 3) fetal sheep at 124d GA in which bolus doses (2 × 11.4 mg) of betamethasone were administered to the pregnant ewe. The level and distribution of versican and CS glycosaminoglycans (GAG) were determined using immunohistochemistry (IHC). Fluorophore assisted carbohydrate electrophoresis (FACE) was used to determine changes in CS sulphation patterns.

RESULTS

Cortisol infusion significantly decreased chondrotin-6-sulphate levels (C-6-S) to 16.4 ± 0.7 AU, compared with saline-infused fetuses (18.9 ± 0.7 AU: p = 0.04) but did not significantly alter the level of versican or chondroitin-4-sulphate (C-4-S). ADX significantly increased the level of C-4-S (28.2 ± 2.2 AU), compared with sham-operated fetuses (17.8 ± 2.0 AU; p = 0.006) without altering versican or C-6-S levels. Betamethasone significantly decreased versican, C-4-S and C-6-S in the fetal sheep lung (19.2 ± 0.9 AU, 24.9 ± 1.4 AU and 23.2 ± 1.0 AU, respectively), compared with saline-exposed fetuses (24.3 ± 0.4 AU, p = 0.0004; 33.3±0.6 AU, p = 0.0003; 29.8±1.3 AU, 0.03, respectively).

CONCLUSIONS

These results indicate that glucocorticoids alter versican levels and CS side chain microstructure in alveolar lung tissue. Betamethasone appears to have a greater impact on versican and CS side chains than cortisol.

Effect of betamethasone, surfactant, and positive end-expiratory pressures on lung aeration at birth in preterm rabbits

Antenatal glucocorticoids, exogenous surfactant, and positive end-expiratory pressure (PEEP) ventilation are commonly provided to preterm infants to enhance respiratory function after birth. It is unclear how these treatments interact to improve the transition to air-breathing at birth. We investigated the relative contribution of antenatal betamethasone, prophylactic surfactant, and PEEP (3 cmH₂O) on functional residual capacity (FRC) and dynamic lung compliance (C_DL) in preterm (28 day GA) rabbit kittens at birth. Kittens were delivered by cesarean section and mechanically ventilated. FRC was calculated from X-ray images, and C_DL was measured using plethysmography. Without betamethasone, PEEP increased FRC recruitment and C_DL Surfactant did not further increase FRC, but significantly increased C_DL Betamethasone abolished the benefit of PEEP on FRC, but surfactant counteracted this effect of betamethasone. These findings indicate that low PEEP levels are insufficient to establish FRC at birth following betamethasone treatment. However, surfactant reversed the effect of betamethasone and when combined, these two treatments enhanced FRC recruitment irrespective of PEEP level.

Ventilation-induced lung injury is not exacerbated by growth restriction in preterm lambs

Intrauterine growth restriction (IUGR) and preterm birth are frequent comorbidities and, combined, increase the risk of adverse respiratory outcomes compared with that in appropriately grown (AG) infants. Potential underlying reasons for this increased respiratory morbidity in IUGR infants compared with AG infants include altered fetal lung development, fetal lung inflammation, increased respiratory requirements, and/or increased ventilation-induced lung injury. IUGR was surgically induced in preterm fetal sheep (0.7 gestation) by ligation of a single umbilical artery. Four weeks later, preterm lambs were euthanized at delivery or delivered and ventilated for 2 h before euthanasia. Ventilator requirements, lung inflammation, early markers of lung injury, and morphological changes in lung parenchymal and vascular structure and surfactant composition were analyzed. IUGR preterm lambs weighed 30% less than AG preterm lambs, with increased brain-to-body weight ratio, indicating brain sparing. IUGR did not induce lung inflammation or injury or alter lung parenchymal and vascular structure compared with AG fetuses. IUGR and AG lambs had similar oxygenation and respiratory requirements after birth and had significant, but similar, increases in proinflammatory cytokine expression, lung injury markers, gene expression, and surfactant phosphatidylcholine species compared with unventilated controls. IUGR does not induce pulmonary structural changes in our model. Furthermore, IUGR and AG preterm lambs have similar ventilator requirements in the immediate postnatal period. This study suggests that increased morbidity and mortality in IUGR infants is not due to altered lung tissue or vascular structure, or to an altered response to early ventilation.

Glucocorticoid regulation of lung development: lessons learned from conditional GR knockout mice

Glucocorticoid (GC) steroid hormones have well-characterized roles in the regulation of systemic homeostasis, yet less understood is their known role in utero to mature the developing respiratory system in preparation for birth. During late gestation, endogenously produced GCs thin the interstitial tissue of the lung, causing the vasculature and future airspaces to come into close alignment, allowing for efficient gas exchange at birth. More potent synthetic GCs are also used worldwide to reduce the severity of respiratory distress suffered by preterm infants; however, their clinical benefits are somewhat offset by potential detrimental long-term effects on health and development. Here, we review the recent literature studying both global and conditional gene-targeted respiratory mouse models of either GC deficiency or glucocorticoid receptor ablation. Although some discrepancies exist between these transgenic mouse strains, these models have revealed specific roles for GCs in particular tissue compartments of the developing lung and identify the mesenchyme as the critical site for glucocorticoid receptor-mediated lung maturation, particularly for the inhibition of cell proliferation and epithelial cell differentiation. Specific mesenchymal and epithelial cell-expressed gene targets that may potentially mediate the effect of GCs have also been identified in these studies and imply a GC-regulated system of cross talk between compartments during lung development. A better understanding of the specific roles of GCs in specific cell types and compartments of the fetal lung will allow the development of a new generation of selective GC ligands, enabling better therapeutic treatments with fewer side effects for lung immaturity at birth in preterm infants.

Regulation of fetal lung development in response to maternal overnutrition

With the worldwide obesity epidemic, the proportion of women entering pregnancy overweight or obese has increased significantly in recent years. Babies born to obese women are at an increased risk of respiratory complications at birth and in childhood. In addition to maternal diabetes, there are a number of metabolic changes that the fetus of an overnourished mother experiences in utero that may modulate lung development and represent the mechanisms underlying the increased risk of respiratory complications. Herein we highlight a series of factors associated with the intrauterine environment of an overnourished mother that may impact on fetal lung development and lead to an increased risk of complications at birth or in postnatal life.

The oncogene Trop2 regulates fetal lung cell proliferation

The factors regulating growth of the developing lung are poorly understood, although the degree of fetal lung expansion is critical. The oncogene Trop2 (trophoblast antigen 2) is upregulated during accelerated fetal lung growth, and we hypothesized that it may regulate normal fetal lung growth. We investigated Trop2 expression in the fetal and neonatal sheep lung during accelerated and delayed lung growth induced by alterations in fetal lung expansion, as well as in response to glucocorticoids. Trop2 expression was measured using real-time PCR and localized spatially using in situ hybridization and immunofluorescence. During normal lung development, Trop2 expression was higher at 90 days gestational age (GA; 4.0 ± 0.8) than at 128 days GA (1.0 ± 0.1), decreased to 0.5 ± 0.1 at 142 days GA (full term ∼147 days GA), and was positively correlated to lung cell proliferation rates (r = 0.953, P < 0.005). Trop2 expression was regulated by fetal lung expansion, but not by glucocorticoids. It was increased nearly threefold by 36 h of increased fetal lung expansion (P < 0.05) and was reduced to ∼55% of control levels by reduced fetal lung expansion (P < 0.05). Trop2 expression was associated with lung cell proliferation during normal and altered lung growth, and the TROP2 protein colocalized with Ki-67-positive cells in the fetal lung. TROP2 was predominantly localized to fibroblasts and type II alveolar epithelial cells. Trop2 small interfering RNA decreased Trop2 expression by ∼75% in cultured fetal rat lung fibroblasts and decreased their proliferation by ∼50%. Cell viability was not affected. This study demonstrates that TROP2 regulates lung cell proliferation during development.

Steroid metabolome in plasma from the umbilical artery, umbilical vein, maternal cubital vein and in amniotic fluid in normal and preterm labor

The boost in placental production of CRH in late pregnancy is specific for human. CRH receptors are expressed in the fetal zone of the fetal adrenal (FZFA). Hence, we evaluated the associations between the steroid metabolome and gestational age (GA). The levels of 69 steroids and steroid polar conjugates such as 3beta-hydroxy-5-ene steroids (3betaOH5S), 3-oxo-4-ene steroids (3O4S), progesterone 5alpha/beta-reduced metabolites, 20alpha-hydroxy-metabolites of C21 steroids, C19 5alpha/beta-reduced metabolites, 7alpha/beta-hydroxy-metabolites of 3betaOH5S, estrogens and 16alpha-hydroxy-metabolites of 3betaOH5S and 3O4S, were measured by GC-MS in plasma from the umbilical artery (UA), umbilical vein (UV), and maternal cubital vein (MV) and in amniotic fluid (AF) in 12 women at normal labor and 38 women at preterm labor due to pathologies unrelated to steroid status. Using multivariate regression, prediction models for GA were completed for the individual body fluids. The conjugated 3betaOH5S (the key products of the FZFA), estrogens, some polar conjugates of progesterone 5alpha/beta-reduced metabolites and some steroid 7alpha/beta- and 16alpha-hydroxy-metabolites showed strong positive correlations with the GA. The predictivity decreased in the following sequence UV (R=0.950), UA (R=0.945), MV (R=0.895), and AF (R=0.891). Although the predictivity of steroids in maternal blood was slightly less effective when compared with the UV and UA, it was the best solution for further practice.

Thrombospondin-1 expression and localization in the developing ovine lung

Fetal lung growth is critically dependent on the degree to which the lungs are expanded by liquid, although the mechanisms involved are unknown. As thrombospondin-1 (TSP-1) can regulate cell proliferation, attachment, spreading and angiogenesis, we investigated the effects of alterations in fetal lung expansion on TSP-1 expression in sheep. TSP-1 mRNA levels were investigated using Northern blot analysis and in situ hybridization, whereas the protein levels were determined by immunohistochemistry. Early growth response 1 (EGR1) mRNA levels were measured by quantitative real-time PCR. TSP-1 was expressed in type-II alveolar epithelial cells and fibroblasts and its mRNA levels increased from 100.0 +/- 14.0% in control fetuses to 347.5 +/- 73.6% at 36 h of increased lung expansion (P < 0.05), and were reduced to 39.4 +/- 6.1% of control levels (100.0 +/- 20.4%) at 20 days of decreased lung expansion (P < 0.05). The percentage of cells positive for TSP-1 mRNA increased from 1.9 +/- 0.4% to 5.2 +/- 0.8% at 36 h of increased fetal lung expansion (P < 0.01). The proportion of tissue stained positive for TSP-1 protein doubled at 36 h of increased lung expansion (23.3 +/- 2.2%) compared to controls (11.7 +/- 3.2%; P < 0.05). Conversely, at 20 days of decreased lung expansion, the percentage of tissue that stained positive for TSP-1 was halved (25.7 +/- 3.2%) compared to controls (39.8 +/- 3.3%; P < 0.05). The increase in TSP-1 expression may be due to increased mRNA levels of the transcription factor EGR1 at 36 h of increased lung expansion (2.7 +/- 0.7-fold of control levels (1.0 +/- 0.2); P < 0.05). Given the known functions of TSP-1 and its localization within the lung, we speculate that TSP-1 may have a significant role in regulating fetal lung growth.

Decreased lung fibroblast growth factor 18 and elastin in human congenital diaphragmatic hernia and animal models

RATIONALE

Lung hypoplasia in congenital diaphragmatic hernia (CDH) seems to involve impaired alveolar septation. We hypothesized that disturbed deposition of elastin and expression of fibroblast growth factor 18 (FGF18), an elastogenesis stimulus, occurs in CDH.

OBJECTIVES

To document FGF18 and elastin in human CDH and ovine surgical and rat nitrofen models and to use models to evaluate the benefit of treatments.

METHODS

Human CDH and control lungs were collected post mortem. Diaphragmatic hernia was created in sheep at 85 days; fetal lungs were collected at 139 days (term = 145 days). Pregnant rats received nitrofen at 12 days; fetal lungs were collected at 21 days (term = 22 days). Some of the sheep fetuses with hernia underwent tracheal occlusion (TO); some of the nitrofen-treated pregnant rats received vitamin A. Both treatments are known to promote lung growth.

MEASUREMENTS AND MAIN RESULTS

Coincidental with the onset of secondary septation, FGF18 protein increased threefold in control human lungs, which failed to occur in CDH. FGF18 labeling was found in interstitial cells of septa. Elastin staining demonstrated poor septation and markedly decreased elastin density in CDH lungs. Consistently, lung FGF18 transcripts were diminished 60 and 83% by CDH in sheep and rats, respectively, and elastin density and expression were diminished. TO and vitamin A restored FGF18 and elastin expression in sheep and rats, respectively. TO restored elastin density.

CONCLUSIONS

Impaired septation in CDH is associated with decreased FGF18 expression and elastic fiber deposition. Simultaneous correction of FGF18 and elastin defects by TO and vitamin A suggests that defective elastogenesis may result, at least partly, from FGF18 deficiency.

Role of the physicochemical environment in lung development

Mechanical forces, exerted on lung tissue via alterations in lung expansion are a major determinant of fetal lung development, having a potent effect on the rate of cellular proliferation, the differentiated state of alveolar epithelial cells and the three-dimensional tissue structure. As a result, much research is currently focused on understanding the molecular mechanisms involved. 2. Although it is likely that mechanical forces exert similar influences on lung development after birth, the types of forces applied to the air-filled lung are very different and more complex. For example, lung aeration causes surface tension to form, which greatly increases lung recoil, leading to a reduction in interstitial tissue and pleural pressures, as well as lung expansion. 3. Because of the loss of the distending influence of lung liquid, the chest wall assumes the role of maintaining resting lung volumes after birth by acting as an external brace that opposes lung recoil. As a result, the distribution of force throughout lung tissue changes markedly. 4. Little is known of how changing the mechanical environment of the lung influences its development after birth, but this has important implications for understanding the impact of assisted ventilation on patients, particularly very preterm infants, who are often ventilated using high positive pressures. 5. Although the application of positive internal distending pressures may, in part, duplicate the fetal environment, the effect of gas versus liquid is unknown and high positive airway pressures are known to adversely affect cardiopulmonary physiology. Understanding the role of mechanical forces in regulating lung development as well as pulmonary physiology in the fetus and newborn is central to improving the care and management of infants suffering respiratory failure.

The Molecular Basis for Abnormal Human Lung Development

Our understanding of lung development in the past two decades has moved from an anatomical to a histological basis and, most recently, to a molecular basis. Tissue interactions specify tracheal and lung primordia formation, program branching morphogenesis of the airway epithelium and regulate epithelial differentiation. In addition, lung development is influenced by mechanical and humoral factors. The regulatory molecules involved in morphogenetic signaling include growth and transcription factors and extracellular matrix molecules. These morphogenetic signals are responsible for lung patterning and differentiation. We will provide a brief overview of molecular signaling during early respiratory formation, airway branching, pulmonary vascularization and epithelial differentiation. We will then review aberrant morphogenetic signaling in human lung abnormalities, such as tracheoesophageal fistula, congenital diaphragmatic hernia, pulmonary hyperplasia, alveolar capillary dysplasia, congenital cystic adenomatoid malformation and bronchopulmonary dysplasia.

Fluoxetine during pregnancy: impact on fetal development

Women are at greatest risk of suffering from depression during the childbearing years and thus may either become pregnant while taking an antidepressant or may require a prescription for one during pregnancy. The antidepressant fluoxetine (FX) is a selective serotonin reuptake inhibitor (SSRI), which increases serotonin neurotransmission. Serotonin is involved in the regulation of a variety of physiological systems, including the sleep–wake cycle, circadian rhythms and the hypothalamic–pituitary–adrenal axis. Each of these systems also plays an important role in fetal development. Compared with other antidepressant drugs, the SSRIs, such as FX, have fewer side effects. Because of this, they are now frequently prescribed, especially during pregnancy. Clinical studies suggest poor neonatal outcome after exposure to FX in utero. Recent studies in the sheep fetus describe the physiological effects of in utero exposure to FX with an 8 day infusion during late gestation in the sheep. This is a useful model for determining the effects of FX on fetal physiology. The fetus can be studied for weeks in its normal intrauterine environment with serial sampling of blood, thus permitting detailed studies of drug disposition in both mother and fetus combined with monitoring of fetal behavioural state and cardiovascular function. Fluoxetine causes an acute increase in plasma serotonin levels, leading to a transient reduction in uterine blood flow. This, in turn, reduces the delivery of oxygen and nutrients to the fetus, thereby presenting a mechanism for reducing growth and/or eliciting preterm delivery. Moreover, because FX crosses the placenta, the fetus is exposed directly to FX, as well as to the effects of the drug on the mother. Fluoxetine increases high-voltage/non-rapid eye movement behavioural state in the fetus after both acute and chronic exposure and, thus, may interfere with normal fetal neurodevelopment. Fluoxetine also alters hypothalamic function in the adult and increases the magnitude of the prepartum rise in fetal cortisol concentrations in sheep. Fetal FX exposure does not alter fetal circadian rhythms in melatonin or prolactin. Studies of the effects of FX exposure on fetal development in the sheep are important in defining possible physiological mechanisms that explain human clinical studies of birth outcomes after FX exposure. To date, there have been insufficient longer-term follow-up studies in any precocial species of offspring exposed to SSRIs in utero. Thus, further investigation of the long-term consequences of in utero exposure to FX and other SSRIs, as well as the mechanisms involved, are required for a complete understanding of the impact of these agents on development. This should involve studies in both humans and appropriate animal models.

Regulation of alveolar epithelial cell phenotypes in fetal sheep: roles of cortisol and lung expansion

Our aim was to determine whether cortisol's effect on alveolar epithelial cell (AEC) phenotypes in the fetus is mediated via a sustained alteration in lung expansion. Chronically catheterized fetal sheep were exposed to 1) saline infusion, 2) cortisol infusion (122–131 days' gestation, 1.5–4.0 mg/day), 3) saline infusion plus reduced lung expansion, or 4) cortisol infusion plus reduced lung expansion. The proportions of type I and II AECs were determined by electron microscopy, and surfactant protein (SP)-A, -B, and -C mRNA levels were determined by Northern blot analysis. Cortisol infusions significantly increased type II AEC proportions (to 38.2 ± 2.2%), compared with saline-infused fetuses (23.8 ± 2.4%), and reduced type I AEC proportions (to 59.0 ± 2.2%), compared with saline-infused fetuses (70.4 ± 2.4%). Reduced lung expansion also increased type II AEC proportions (to 52.9 ± 3.5%) and decreased type I AEC proportions (to 34.2 ± 3.7%), compared with control, saline-infused fetuses. The infusion of cortisol into fetuses exposed to reduced lung expansion tended to further increase type II (to 60.3 ± 2.1%, P = 0.066) and reduce type I AEC (to 26.6 ± 2.3%, P = 0.07) proportions. SP-A, -B, and -C mRNA levels changed in parallel with the changes in type II AEC proportions. These results indicate that cortisol alters the proportion of type I and type II AECs via a mechanism unrelated to the degree of fetal lung expansion. However, reductions in fetal lung expansion appear to have a greater impact on the proportion of AECs than cortisol.