The effect of chronic biphrenectomy on lung growth and maturation in fetal lambs. Morphologic and morphometric studies

Pathogenesis and Physiologic Mechanisms of Neonatal Pulmonary Hypertension: Preclinical Studies

Neonatal pulmonary hypertension (PH) is a devastating disorder of the pulmonary vasculature characterized by elevated pulmonary vascular resistance and mean pulmonary arterial pressure. Occurring predominantly because of maldevelopment or maladaptation of the pulmonary vasculature, PH in neonates is associated with suboptimal short-term and long-term outcomes because its pathobiology is unclear in most circumstances, and it responds poorly to conventional pulmonary vasodilators. Understanding the pathogenesis and pathophysiology of neonatal PH can lead to novel strategies and precise therapies. The review is designed to achieve this goal by summarizing pulmonary vascular development and the pathogenesis and pathophysiology of PH associated with maladaptation, bronchopulmonary dysplasia, and congenital diaphragmatic hernia based on evidence predominantly from preclinical studies. We also discuss the pros and cons of and provide future directions for preclinical studies in neonatal PH.

The effect of tracheal occlusion in congenital diaphragmatic hernia in the nitrofen rat lung explant model

PURPOSE

Here, we establish a tracheal occlusion (TO) model with rat lung explants in nitrofen-induced pulmonary hypoplasia in the congenital diaphragmatic hernia (CDH).

METHODS

We extracted lungs from rats on an embryonic day 18. We mimicked TO in the lung explants by tying the trachea. We assessed lung weight, morphometry, and abundance of Ki-67, Active caspase-3, and Prosurfactant Protein C (proSP-C) with immunofluorescence.

RESULTS

Lung weight was higher in TO + than TO - on day 1. Abundance of Ki-67 was higher in TO + than TO - (0.15 vs. 0.32, p = 0.009 for day 1, 0.07 vs. 0.17, p = 0.004 for day 2, 0.07 vs. 0.12, p = 0.044 for day 3), and Active caspase-3 was higher in TO + than TO - on day 2 and day 3 (0.04 vs. 0.03 p = 0.669 for day 1, 0.03 vs. 0.13 p < 0.001 for day 2, 0.04 vs. 0.17 p = 0.008 for day3). However, proSP-C protein abundance was lower in TO + than TO - (67.9 vs. 59.1 p = 0.033 for day 1, 73.5 vs. 51.6 p = 0.038 for day 2, 83.1 vs. 56.4 p = 0.009 for day 3).

CONCLUSIONS

The TO model in lung explants mimics the outcomes of current surgical models of TO and further studies can reveal the cellular and molecular effects of TO in CDH lungs.

The Cellular and Molecular Effects of Fetoscopic Endoluminal Tracheal Occlusion in Congenital Diaphragmatic Hernia

Congenital diaphragmatic hernia (CDH) is a complex disease associated with pulmonary hypoplasia and pulmonary hypertension. Great strides have been made in our ability to care for CDH patients, specifically in the prenatal improvement of lung volume and morphology with fetoscopic endoluminal tracheal occlusion (FETO). While the anatomic effects of FETO have been described in-depth, the changes it induces at the cellular and molecular level remain a budding area of CDH research. This review will delve into the cellular and molecular effects of FETO in the developing lung, emphasize areas in which further research may improve our understanding of CDH, and highlight opportunities to optimize the FETO procedure for improved postnatal outcomes.

Contractile activity of skeletal musculature involved in breathing is essential for normal lung cell differentiation, as revealed inMyf5−/−:MyoD−/− embryos

In the current study, the role of contractile activity of respiratory muscles in fetal lung growth and cell differentiation was examined using Myf5-/-:MyoD-/- mouse embryos. As previously found, Myf5-/-:MyoD-/- mouse embryos had no respiratory musculature. Consequently, they suffered from pulmonary hypoplasia and died shortly after birth. The hypoplastic lung had decreased proliferation and increased apoptotic index as early as embryonic day 14.5. By contrast, only at the last gestational day, the number of lung cells expressing platelet derived growth factor B and insulin growth factor I was decreased, while the gradient of the thyroid transcription factor 1 was not maintained. Type II pneumocytes had a failure in glycogen utilization and surfactant storage and secretion but were able to synthesize the surfactant-associated proteins. Type I pneumocytes were readily detectable using an early differentiation marker (i.e., Gp38). However, the late differentiation of type I pneumocytes never occurred, as revealed by transmission electron microscopy. Together, our findings suggest that pulmonary distension due to fetal breathing-like movements plays an important role not only in lung growth but also in lung cell differentiation.

Diaphragmatic function in infants with surgically corrected anomalies

Infants with surgically correctable anomalies, abdominal wall defects (AWD) or congenital diaphragmatic hernia (CDH) may have poor postnatal diaphragmatic function, because the low intra-abdominal pressure experienced by such patients in utero could result in impaired diaphragmatic development. Our objective was to compare postoperative diaphragmatic function of infants with CDH or AWD to that of gestational age-matched controls. Diaphragmatic function was assessed by measurement of the transdiaphragmatic pressure and maximum inspiratory pressure at the mouth generated during crying against an occlusion. In addition, the transdiaphragmatic pressure produced by unilateral and/or bilateral magnetic stimulation of the phrenic nerves (TwPdi) was examined. Lung volume was assessed by measurement of functional residual capacity (FRC) using a helium gas dilution technique. Ten infants with CDH, 26 with AWD infants (19 gastroschisis, seven exomphalos), and 36 gestational age-matched controls were studied. Compared with their matched controls, the eight CDH infants with left-sided defects had significantly lower left (p < 0.01) and right (p < 0.05) TwPdi and FRC (p < 0.01), and the gastroschisis infants, but not those with exomphalos, had significantly lower left and right TwPdi (p < 0.05). There were no significant differences in transdiaphragmatic pressure and maximum inspiratory pressure at the mouth between the CDH or AWD infants and the controls. Diaphragmatic function postoperatively is impaired in infants with CDH or gastroschisis.

Pulmonary hypertension syndrome in young chickens challenged with frozen and autoclaved cultures of Enterococcus faecalis

Enterococcus faecalis, when administered in a growth medium or sterile saline, will cause pulmonary hypertension syndrome (PHS) in chickens. The objective of this study was to determine if frozen and/or autoclaved cultures of E. faecalis retain ability to evoke PHS. In Trial 1, chicks were inoculated with 3.6 x 10(7) E. faecalis (IA) in tryptic soy broth (TSB) from either a live culture or one that had been autoclaved (120 degrees C for 20 min). Controls received TSB. Autoclaved and live cultures produced the same degree of PHS in a majority of the birds. Trial 2 used the same protocol, except a frozen (-70 degrees C for 60 min) culture of E. faecalis was compared with the control. The results agreed with those of Trial 1, i.e., the frozen culture also produced PHS. Trial 3 was conducted to determine if E. faecalis caused PHS by producing and releasing some unknown substance into the supernatant. Incidence of PHS was based on percentage of birds exhibiting ascites fluid at 24 hr after challenge. Controls received sterile, frozen, or autoclaved TSB. As compared with controls, those birds that received challenge with E. faecalis alone, supernatant alone, and E. faecalis plus supernatant from live cultures exhibited similar incidence of ascites, whereas birds that received E. faecalis plus supernatant and supernatant alone from cultures that had been either frozen or autoclaved exhibited elevated incidence of ascites as compared with controls. Also, with frozen and autoclaved cultures, those birds that received only pelleted E. faecalis exhibited incidence of ascites that did not differ from controls. Apparently, E. faecalis produces PHS in chicks by producing and releasing an unknown toxin.

Fetal pulmonary development: the role of respiratory movements

The lung develops before birth as a collapsible, liquid-filled, organ. Throughout the later stages of gestation the fetal lungs are maintained at a level of expansion that is considerably greater than the level achieved as a result of passive equilibration between lung recoil and the chest wall. Fetal breathing movements (FBM) are a feature of normal fetal life and, as such, are used clinically in the assessment of fetal wellbeing. By opposing lung recoil, FBM help to maintain the high level of lung expansion that is now known to be essential for normal growth and structural maturation of the fetal lungs. During 'apnoeic' periods between successive episodes of FBM, active laryngeal constriction has the effect of opposing lung recoil by resisting the escape of lung liquid via the trachea. The prolonged absence or impairment of FBM is likely to result in a reduced mean level of lung expansion which can lead to hypoplasia of the lungs. There is clinical evidence, disputed by some, that the absence of FBM exacerbates the effects of other factors that are associated with lung hypoplasia, such as premature rupture of fetal membranes and oligohydramnios. Even in the absence of such factors, prolonged or repeated reductions or abolition of FBM may contribute to impairments of fetal lung development; FBM can be inhibited by fetal hypoxaemia, hypoglycaemia, maternal alcohol consumption, maternal smoking, intra-amniotic infection and maternal consumption of sedatives or narcotic drugs. Abnormal growth of the fetal lungs has relevance for postnatal respiratory health as it is now recognised that there may be only a limited capacity after birth for the restoration of normal pulmonary architecture following impaired intra-uterine lung development.

Tracheal ligation does not correct the surfactant deficiency associated with congenital diaphragmatic hernia

INTRODUCTION

Experimental tracheal ligation (CDH + TL) has been shown to reverse the profound lung hypoplasia associated with congenital diaphragmatic hernia (CDH) and to normalize gas exchange. The aim of this study was to determine whether this experimental therapy would correct the surfactant deficiency present in the fetal lamb model of CDH.

METHODS

The CDH lamb model was created at 80 days' gestation, and tracheal ligation was performed at 110 days. At term, the lambs were delivered and were ventilated for 30 minutes. The lambs were killed, a pressure-volume curve performed, and the lungs lavaged to measure total phospholipid content. Finally, type II pneumocytes were isolated, and surfactant synthesis was assessed by the incorporation of tritiated choline into phosphatidylcholine.

RESULTS

CDH + TL resulted in a lung significantly larger than that of CDH alone. The lungs of the former also had better oxygenation and ventilation. However, lung compliance was reduced compared with controls. Total alveolar phospholipid was dramatically lower, with a decrease in the proportion of phosphatidylcholine present. Surfactant synthesis by the isolated type II pneumocyte was significantly impaired.

CONCLUSION

Occlusion of the fetal trachea produces a lung comparable in size to a normal control lung. However, broncheoalveolar lavage analysis shows a marked reduction in total phospholipid, with a decrease in surfactant synthesis by the type II pneumocyte. The normalization of gas exchange reported for this animal model may be only a transient phenomenon. Further studies are required to assess the impact of this surfactant deficiency on long-term lung function.

Alveolar epithelial composition and architecture of the late fetal pulmonary acinus: an immunocytochemical and morphometric study in a rat model of pulmonary hypoplasia and congenital diaphragmatic hernia

The aim of the present study was to compare the architecture and alveolar epithelial cell composition of the pulmonary acinus in hypoplastic and normal fetal rat lungs. For this purpose, a rat model of pulmonary hypoplasia in association with congenital diaphragmatic hernia (CDH) induced by Nitrofen (100 mg on day 10 of pregnancy) was studied. Sections (5 microns) from lungs of control and Nitrofen-exposed fetal Sprague Dawley rats with or without CDH aged 18-22 days (vaginal plug on day 1, birth on day 23) were stained with hematoxylin and eosin. To identify developing alveolar epithelial cells, sections were incubated with anti-surfactant protein A (SP-A; rabbit anti-mouse) or preimmunization serum (indirect immunofluorescence). On days 18 and 19, control lungs and exposed lungs from fetuses with and without CDH looked similar (pseudoglandular stage of lung development). The prospective pulmonary acinus consisted of acinar tubules with small round lumens, lined by cuboid, fluorescent type II cells. Morphometric analysis on day 19 showed significantly smaller lung volumes and lung tissue volumes after Nitrofen exposure. On day 20 (canalicular stage), some tubules were slightly dilated and lined by cuboid and thinner fluorescent cells; these dilated tubules were less numerous in lungs from exposed fetuses with CDH. On days 21 and 22 (saccular stage), the saccular lining consisted of cuboid to thin fluorescent cells in exposed lungs from fetuses with and without CDH, and fluorescent (low) cuboid cells interspersed with dark zones (type I cell areas) in control lungs. In the exposed lungs from fetuses with CDH, the lumens of all airspaces were frequently slit-like, and the septa were thicker. These phenomena gave the lungs a primitive, compact aspect. Morphometric analysis on day 22 showed smaller lung volumes and lung tissue volumes, smaller airspace/tissue ratios, smaller epithelial surface areas, and more type II cells per surface area in Nitrofen-exposed lungs than in normal control lungs. The results suggest that Nitrofen-exposed, and thus hypoplastic, fetal rat lungs are retarded with respect to the differentiation of cuboid type II cells into squamous type I cells whether or not CDH is present, and with respect to the development of the future airspaces between days 20 and 22 if CDH is present.

Ultrastructural features of alveolar epithelial cells in the late fetal pulmonary acinus: a comparison between normal and hypoplastic lungs using a rat model of pulmonary hypoplasia and congenital diaphragmatic hernia

The aim of this study was to describe and compare the ultrastructural features and functional maturity of alveolar epithelial cells in hypoplastic and normal fetal rat lungs. Pulmonary hypoplasia in association with congenital diaphragmatic hernia was induced in fetuses by administration of 2,4-dichlorophenyl-p-nitrophenylether (Nitrofen) to pregnant Sprague Dawley rats (100 mg on day 10 of gestation). Lung tissue of Nitrofen-exposed and control fetal rats aged 19-22 days (vaginal plug day 1, birth day 23) was embedded in Epon. Semithin (1 micron) toluidine blue-stained sections were examined by light microscopy; ultrathin sections (approximately 80 nm) were studied via transmission electron microscopy. In bronchoalveolar lavage fluid from control and Nitrofen-exposed fetuses (day 22), phospholipid fractions and surfactant protein A content were measured semiquantitatively. On day 19 both control and Nitrofen-exposed lungs contained only cuboid alveolar epithelial cells; from day 20 there were cuboid, low cuboid, and thinner epithelial cells. The (low) cuboid cells contained large glycogen fields, some precursory stages of multilamellar bodies (MLBs), and just a few mature MLBs on day 19 and 20; smaller glycogen fields, more precursory stages, and more mature MLBs on day 21; and little or no glycogen but many precursory stages and mature MLBs on day 22. The thinner cells contained little or no glycogen and a few precursory stages of MLBs on days 20-22; very thin cells on day 22 contained neither glycogen nor any precursory stages of MLBs. MLBs and tubular myelin were seen in the lumens of future air spaces from day 20 onward. Nitrofen-exposed lungs differed from control lungs in that inclusion bodies (IBs) were less numerous in (low) cuboid alveolar cells on days 19 and 20, and more glycogen was seen on day 22. In addition intra- and extracellular "MLBs" in exposed lungs more often had an unusual appearance, i.e., a confluent structure and higher electron density. However, despite morphologic differences, there was no clear difference in phospholipid composition and SP-A content per mol phospholipid in bronchoalveolar lavage fluid. We conclude that morphologically hypoplastic lungs are less mature near term, without an apparent effect on surfactant composition.

The effect of maternal exercise on somatic growth and lung development of fetal rats: morphologic and morphometric studies

Our study was aimed at elucidating the effect of strenuous maternal exercise (running at a speed of 20 m/min) on fetal somatic growth and lung development. Dams were separated into three groups: (1) exercising during the entire course of pregnancy; (2) exercising from the 16th to the 20th gestational day; and (3) controls treated in the same manner as the exercising animals, but not exercised. There were no differences in the mother's body weight (mother's body weight--total body weight of the fetuses) among the three groups. Overall growth and lung development in fetuses from mothers that exercised during the entire course of pregnancy showed markedly decreased size and weight of body and lung, smaller air spaces and thicker future alveolar walls, and fewer osmiophilic lamellated bodies in alveolar type II cells. The morphometric data of fetuses from mothers that had exercised only during late gestation were similar to those of control animals. The ratio of fetal lung volume to body weight was the same in all three groups. These findings indicate that long-term strenuous maternal exercise induces retardation of fetal lung development, possibly accompanied by a disturbance of overall growth. Maternal exercise represents another means of manipulating lung development.

Effects of anti-mouse EGF antiserum on prenatal lung development in fetal mice

To clarify the precise role of epidermal growth factor (EGF) on fetal lung development, rabbit anti-mouse EGF (anti-mEGF) antiserum was administered to pregnant mice from days 10 to 17 during late gestation. Control mice were administered either normal rabbit serum (NRS) or physiological saline (PS). Serum EGF was not detected in fetuses from anti-mEGF antiserum treated mothers, but the level in NRS treated control animals was 4.73 +/- 0.66 ng/mL. One day prior to birth, the fetuses were removed and their body and lung weights were measured. There was no difference between body weights and lung weights of anti-mEGF antiserum treated animals and NRS-treated control animals. On light microscopic morphometry, there was no obvious difference between pulmonary architecture of anti-mEGF antiserum treated animals and NRS treated control animals. On transmission electron microscopy, osmiophillic lamellar inclusion bodies were less prominent in the type II epithelial cells in anti-mEGF antiserum treated animals. Electron microscopic morphometric study revealed that the osmiophillic lamellar inclusion bodies in type II epithelial cells of anti-mEGF antiserum treated animals were fewer in number and had decreased area fraction. These findings support the previous finding that EGF promotes epithelial cell differentiation of the fetal lung without affecting body weight and lung weight.

Altered diaphragm function modifies neonatal lung growth: biologic morphometric assessment

Thoracic volume relationships supported by the diaphragm are important mechanical components of neonatal lung growth. Alterations modify regional lung growth. We used stereologic morphometry to study effects of altered diaphragm function on alveolar growth in neonatal pigs. Nine piglets (1 mo) were divided into three groups: unilateral phrenectomy, noncompliant patch replacement of diaphragm, and sham. Seven days later lungs were fixed in situ with 2.5% glutaraldehyde by airway installation at 20 cm H2O. Five 1-mm cubes were cut from lung corresponding to apex/RUQ(A), base/RLQ(B), apex/LUQ(C), and base/LLQ(D) and prepared for light microscopy (n = 25/quad/animal). Stereologic morphometry involved point counting for air volume density and point intersect to determine the surface area density of the alveolar spaces. Results were analyzed for variance and by Tukey range testing. Variance of air volume % between groups-quadrants B, C, D had decreased air volume % in phrenectomy group compared with patch and sham group (P < .05). Variance of air volume % within groups, for phrenectomy group A (81.5%), is different from all others (B = 70.6%, C = 75.5%, D = 66.7%); C is different from D (P < .05). For patch and sham group, D (P = 74.8%, S = 80.7%) is different from A (P = 84.1%, S = 86.6%) and C (P = 86.2%, S = 84.4%). Variance of surface area density between groups--quadrant D had increased surface area density % in phrenectomy group compared to sham group (P < .05). Variance of surface area density % within groups, for phrenectomy group, A (37.9 mm-1) is different from D (55.7 mm-1).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of in utero phrenic nerve section on the development of collagen and elastin in lamb lungs

Interference with fetal breathing movements is known to retard morphologic development of the lung and to reduce compliance. We hypothesized that the lower compliance might be in part due to effects on lung structural proteins. We studied the effects of phrenic nerve section in utero on lung compliance and on the lung contents of collagen, elastin, and DNA. At 110 to 112 days of gestation, one fetal lamb in each of 12 twin pregnancies had either both phrenic nerves cut (PX) or a sham operation (S). The other twin was left unoperated (Upx, Us) as a control. They were killed 14 to 22 days later, and the concentrations in lung parenchyma of collagen (as hydroxyproline HPro), elastin, and DNA were measured, together with lung compliance and dry and wet weight. Paired comparisons were made (PX versus Upx and S versus Us). Both operated groups (PX, S) had smaller lungs with lower water content than did their unoperated twins. Absolute static compliance in PX was reduced, but compliance relative to lung weight was unchanged, and there was no significant difference between S and Us. There were no significant effects of PX on the concentrations of HPro, elastin, and DNA, or on the elastin/collagen ratio. Compliance was not correlated with either HPro or elastin content. HPro content increased significantly with gestational age in all groups. It is concluded that phrenic nerve section retards the increase of lung compliance and possibly air space, but it does not affect the overall rate of lung cell proliferation or of deposition of elastin or collagen.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanical forces contribute to neonatal lung growth: the influence of altered diaphragm function in piglets

Neonatal lung growth is controlled in part by mechanical forces. Altered mechanical forces precipitated by phrenectomy or prosthetic replacement of the diaphragm result in altered thoracic volume relationships, which, in turn, change lung distending pressures and or thoracic volume. These effects might contribute to regional lung growth. We postulated a relationship between altered thoracic mechanical forces and changes in lung growth and asked if altered diaphragm function influenced regional lung growth. Piglets (28d, 7-8kg), were assigned to left transthoracic phrenectomy (P), prosthetic diaphragm replacement (PDR), or sham (S), (n = 6). After a mean 10 days, piglets were studied with tracheostomy and regional pleural pressure transducers. Integrated lung volumes (LV) were recorded with intrapleural pressure (Pip). Dynamic compliance (Cdyn) was calculated (dV/dP). After sacrifice continuous pressure volume (P/V) curves were generated. Lungs were then cut into 4 quadrants based on relationship to R/L bronchus and processed for DNA content and total protein indexes. Analysis of data were made within and between groups. Body weight and gain were similar in all. LV, Pip, Cdyn, and P/V were not significantly different in PDR and P compared with S. Pip differences between thoracic regions within each group were significant for PDR and showed LU less than RU, LL less than RL (P less than 0.05). RU and RL Pip in the PDR group were the same as S. Pip in the P group were decreased in the RU, LU, and LL but only the LL approached significance. Whole lung wet weights were decreased (P less than .05) in P compared to PDR and S.(ABSTRACT TRUNCATED AT 250 WORDS)

Human fetal upper respiratory tract function as revealed by ultrasonography

The respiratory functions of the oropharynx, larynx, and trachea of normal human fetuses in utero were explored by means of real-time, two-dimensional ultrasonography combined with color-flow and spectral Doppler analysis. Coronal and transverse images revealed the maturation of coordinated respiratory activity of these structures in the late second and early third trimesters. Observation of the effects of these movements upon the flow of amniotic fluid in the fetal respiratory tract provides an appreciation of the early role of the upper airway in the modulation of fetal breathing.

Role of lung fluid volume in growth and maturation of the fetal sheep lung

We studied the effects of alterations in lung fluid volume on growth and maturation of the fetal lung. In a chronic fetal sheep preparation, right fetal lung volume was decreased by drainage of lung fluid while the volume of the left lung was expanded by mainstem bronchus ligation leading to lung fluid retention. After an experimental period of 25 d (from 105 to 129 d of gestation, term = 145 d), the right (deflated) lung was significantly hypoplastic and contained less DNA than the controls; 175.15 +/- 55.18 vs. 346.77 +/- 61.97 mg, respectively; P less than 0.001. In contrast, the left (expanded) lung was significantly hyperplastic and contained more DNA than the controls; 390.74 +/- 103.53 vs. 238.85 +/- 33.32 mg, respectively; P = 0.001. Biochemical indices of lung maturation, including total phospholipids, phosphatidylcholine, and disaturated phosphatidylcholine content expressed per unit of tissue DNA, were no different when comparing the hypoplastic, hyperplastic, and control lungs. These findings demonstrate that fetal lung cell multiplication is influenced by local distension with lung fluid, while the biochemical maturation of fetal lung surfactant is under systemic control.