Tracheal ligation increases cell proliferation but decreases surfactant protein in fetal murine lungs in vitro

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.

Fetal tracheal occlusion for congenital diaphragmatic hernia

Congenital diaphragmatic hernia (CDH) remains one of the most elusive birth defects to treat. Despite greater knowledge of disease and advances in technology, approximately one-third of CDH children born today still die. Consequently, clinicians and researchers have struggled to find the optimal treatment strategies for CDH. Without further innovations in postnatal treatment, many have focused an antenatal approach to improve pulmonary function. Fetoscopic Endoluminal Tracheal Occlusion (FETO) for CDH has evolved to the bedside after decades of research. While still under clinical investigation, FETO remains a promising adjunct to the treatment of CDH.

Quantifying stretch and secretion in the embryonic lung: Implications for morphogenesis

Branching in the embryonic lung is controlled by a variety of morphogens. Mechanics is also believed to play a significant role in lung branching. The relative roles and interactions of these two broad factors are challenging to determine. We considered three hypotheses for explaining why tracheal occlusion triples branching with no overall increase in size. Both hypotheses are based on tracheal occlusion blocking the exit of secretions. (H1) Increased lumen pressure stretches tissues; stretch receptors at shoulders of growing tips increase local rate of branching. (H2) Blocking exit of secretions blocks advective transport of morphogens, leading to (H2a) increased overall concentration of morphogens or (H2b) increased flux of morphogens at specific locations. We constructed and analyzed computational models of tissue stretch and solute transport in a 3D lung geometry. Observed tissue stresses and stretches were predominantly in locations unrelated to subsequent branch locations, suggesting that tissue stretch (H1) is not the mechanism of enhancement of branching. Morphogen concentration in the mesenchyme (H2a) increased with tracheal occlusion, consistent with previously reported results. Morphogen flux at the epithelial surface (H2b) completely changed its distribution pattern when the trachea was occluded, tripling the number of locations at which it was elevated. Our results are consistent with the hypothesis that tracheal occlusion blocks outflow of secretions, leading to a higher number of high-flux locations at branching tips, in turn leading to a large increase in number of branching locations.

Antenatal fetal VEGF therapy to promote pulmonary maturation in a preterm rabbit model

AIM

To assess the effects of fetal tracheal administration of VEGF on pulmonary maturation in a preterm rabbit model.

METHODS

On day 26 (term=31days), fetal rabbits received recombinant rat VEGF (30microg in 70microL normal saline) or placebo (normal saline 70microL) intratracheally, with or without subsequent tracheal occlusion. Non-operated littermates served as internal controls. Fetuses were harvested on day 28 for morphometric study of the lungs or for mechanical ventilation and measurement of lung mechanics. In total, 96 fetuses from 42 does were used, 47 for ventilation and 49 for morphometry.

RESULTS

In fetuses receiving intratracheal VEGF, an increase in immunoreactivity for Flk-1 was observed throughout the lung parenchyma. Tracheal occlusion (TO) adversely affected pulmonary mechanics as compared to un-occluded controls. That effect is partly reversed by intratracheal VEGF. Intratracheal injection of VEGF without tracheal occlusion improves lung mechanics but no more than what was observed in placebo injected controls.

CONCLUSION

Antenatal intratracheal VEGF administration was associated with an increase in Flk-1 immunoreactivity. It also improves lung mechanics, however more so when the trachea is occluded. Without TO, the effects were comparable to placebo controls.

Gene expression analysis after prenatal tracheal ligation in fetal rat as a model of stimulated lung growth

PURPOSE

Prenatal tracheal occlusion or ligation (TL) has been proven to accelerate lung growth, but the mechanism of this is poorly understood. To increase understanding of the biological mechanisms involved in growth stimulation after TL in the fetal lung, we performed Global gene expression analysis using microarray technology.

MATERIAL AND METHODS

Sprague-Dawley rats underwent surgery on gestational day 19. After a small hysterotomy, the trachea was mobilized and tied. As controls, we used littermates to manipulated fetuses. On day 21, fetuses were removed and lungs harvested. Global gene expression analysis was performed using Affymetrix Platform and the RAE 230 set arrays (Affymetrix Inc, Santa Clara, Calif). For validation of microarray data, we performed real time polymerase chain reaction (PCR) of the most significant upregulated or downregulated genes, combined with immunohistochemical (IHC) analysis of lung sections.

RESULTS

In the group that underwent TL, several growth factors had an increased expression including connective tissue growth factor (CTGF), insulin-like growth factor 1 (IGF-1), and fibroblast growth factor 18 (FGF-18). Some of the genes that were downregulated in the group that underwent TL compared with controls were surfactant protein A (SP-A), apolipoprotein E (Apo-E), and phospholipase group II A2 (plg2a2). These results could be confirmed with real time PCR and IHC studies.

DISCUSSION

Tracheal occlusion or ligation is a well-documented stimulator of fetal lung growth, and the present study provides novel insights into the underlying molecular mechanisms, with increased expression of genes and proteins with growth factor activity. One of these growth factors, CTGF, has never been previously described in this model. Also, decreased levels of genes involved in surfactant metabolism were observed, providing molecular insights into the decreased surfactant production that is known to occur in TL. Increased understanding of the molecular mechanisms that control lung growth may be the key to develop novel therapeutic techniques to stimulate prenatal and/or postnatal lung growth.

Increased distension stimulates distal capillary growth as well as expression of specific angiogenesis genes in fetal mouse lungs

Tracheal occlusion (TO) performed surgically in utero near the end of gestation causes a rapid increase in the distension of future airspaces, resulting in accelerated lung development. The authors hypothesize that TO stimulates microvascular growth concomitant with a rapid increase in the expression of genes implicated in angiogenesis. Mouse fetuses underwent in utero surgery (TO or sham-TO surgery) at 16.5 days of gestation, whereupon development was allowed to continue for a further 1 or 24 hours. Microvascular changes were assessed by immunohistochemical staining of fetal lung sections for platelet endothelial cell adhesion molecule-1. Levels of vascular endothelial growth factor-A (VEGF-A; isoforms 120, 164 and 188), VEGF receptors 1 and 2 (VEGFR-1 and -2), angiopoietins 1 and 2, and Tie2 mRNAs were determined by quantitative real-time polymerase chain reaction (PCR). The authors observed more intercapillary interconnection, less isolated capillaries, and a more extended capillary network inside septa of lungs that underwent 24 h of TO versus sham-TO. Moreover, the authors observed a significant increase in mRNA levels of VEGF 188 and VEGFR-1 as early as 1 hour following TO and of VEGFR-1 and angiopoietin 1 after 24 hours. Together, these results suggest that surgically applied stretching quickly enhances the expression of specific angiogenesis and vessel maintenance genes, which seems to result in the maturation and organization of a more extensive and complex capillary network.

Tracheal occlusion increases the rate of epithelial branching of embryonic mouse lung via the FGF10-FGFR2b-Sprouty2 pathway

Tracheal occlusion during lung development accelerates growth in response to increased intraluminal pressure. In order to investigate the role of internal pressure on murine early lung development, we cauterized the tip of the trachea, to occlude it, and thus to increase internal pressure. This method allowed us to evaluate the effect of tracheal occlusion on the first few branch generations and on gene expression. We observed that the elevation of internal pressure induced more than a doubling in branching, associated with increased proliferation, while branch elongation speed increased 3-fold. Analysis by RT-PCR showed that Fgf10, Vegf, Sprouty2 and Shh mRNA expressions were affected by the change of intraluminal pressure after 48h of culture, suggesting mechanotransduction via internal pressure of these key developmental genes. Tracheal occlusion did not increase the number of branches of Fgfr2b-/- mice lungs nor of wild type lungs cultured with Fgfr2b antisense RNA. Tracheal occlusion of Fgf10(LacZ/-) hypomorphic lungs led to the formation of fewer branches than in wild type. We conclude that internal pressure regulates the FGF10-FGFR2b-Sprouty2 pathway and thus the speed of the branching process. Therefore pressure levels, fixed both by epithelial secretion and boundary conditions, can control or modulate the branching process via FGF10-FGFR2b-Sprouty2.

Tracheal occlusion: A review of obstructing fetal lungs to make them grow and mature

Fetal lung growth and functional differentiation are affected strongly by the extent that pulmonary tissue is distended (expanded) by liquid that naturally fills developing future airspaces. Methods that prevent normal egress of this lung fluid through the trachea magnify mechanical stretching of lung parenchymal cells, thereby promoting lung development. Indeed, experimental observations demonstrate that in utero tracheal occlusion (TO) performed on fetuses during the late canalicular-early saccular stage potently stimulates pulmonary growth and maturation. In this review, we present the four principle non-human animal models of TO/obstruction and discuss them in relation to their utility in elucidating lung development, in remedying congenital diaphragmatic hernia (CDH) as well as in investigating the stretching effects on growth and remodeling of the fine vasculature.

Tracheal occlusion in fetal rats alters expression of mesenchymal nuclear transcription factors without affecting surfactant protein expression

BACKGROUND/PURPOSE

Mesenchymal nuclear transcription factors (MNTF) are involved in lung development and maturation and regulate surfactant protein (SP) expression. Prolonged (>2 weeks) fetal tracheal occlusion (TO) has been shown to accelerate lung growth and inhibit pulmonary surfactant synthesis. The effects of TO on SP expression and MNTF, however, have not been formally assessed. The objectives of this study were to evaluate the effects of short-term (3 days) TO on normal lung growth and protein expression of pulmonary MNTF involved in SP synthesis.

METHODS

At E19 (term, 22 days), 2 fetuses per time-dated Sprague-Dawley rats underwent either TO (n = 23) or a sham (n = 22) operation. Lungs were harvested 72 hours post surgery. Pulmonary SP-A; SP-B; SP-C messenger RNA (mRNA) expression; and SP-A and SP-B, Hoxb5, thyroid transcription factor 1, and retinoic X receptor-alpha protein expression were analyzed.

RESULTS

Lung weight was significantly increased by TO (TO 0.32 +/- 0.02g vs SHAM 0.14 +/- 0.01 g; P < .001), resulting in 123% increase of the lung-to-body-weight ratio. No difference of SP-A-mRNA (177 +/- 4.3 TO vs 169 +/- 4.4 SHAM; P = .25), SP-B-mRNA (87.7 +/- 0.2 TO vs 87.4 +/- 0.02 SHAM; P = .33), and SP-C-mRNA (186.5 +/- 3.2 TO vs 183.2 +/- 2.7 SHAM; P = .45) expression was found. Surfactant protein A (175.6 +/- 25.3 TO vs 192.5 +/- 19.8 SHAM; P = .59) and SP-B (163.4 +/- 5.2 TO vs 166.8 +/- 9.3 SHAM; P = .75) protein expression were similar in both groups; however, Hoxb5 (70.3 +/- 18.9 TO vs 130.6 +/- 5.1 SHAM; P = .02) and thyroid transcription factor 1 (102.6 +/- 19 TO vs 181.1 +/- 6.3 SHAM; P = .007) expression were significantly decreased. Retinoic X receptor-alpha expression tended to be increased by TO (171.9 +/- 6.0 TO vs 155.4 +/- 6.7 SHAM; P = .06).

CONCLUSIONS

Short-term TO late in gestation induces rapid lung growth. Surfactant protein-mRNA and protein expression are not significantly altered. Thyroid transcription factor 1 and Hoxb5 are down-regulated by TO, suggesting that duration and timing of occlusion are important in balancing the effects of TO on lung growth vs lung maturation.

Control of basement membrane remodeling and epithelial branching morphogenesis in embryonic lung by Rho and cytoskeletal tension

Local alterations in the mechanical compliance of the basement membrane that alter the level of isometric tension in the cell have been postulated to influence tissue morphogenesis. To explore whether cell tension contributes to tissue pattern formation in vivo, we modulated cytoskeletal force generation in embryonic mouse lung (embryonic days 12-14) rudiments using inhibitors of Rho-associated kinase (ROCK), myosin light chain kinase, myosin ATPase, and microfilament integrity, or a Rho stimulator (cytotoxic necrotizing factor-1). Tension inhibition resulted in loss of normal differentials in basement membrane thickness, inhibition of new terminal bud formation, and disorganization of epithelial growth patterns as well as disruption of capillary blood vessels. In contrast, increasing cell tension through Rho activation, as confirmed by quantitation of myosin light chain phosphorylation and immunohistocytochemical analysis of actin organization, accelerated lung branching and increase capillary elongation. These data suggest that changes in cytoskeletal tension mediated by Rho signaling through ROCK may play an important role in the establishment of the spatial differentials in cell growth and extracellular matrix remodeling that drive embryonic lung development.

Rescue of the Hypoplastic Lung by Prenatal Cyclical Strain

We determined the effects of sustained and cyclical prenatal mechanical strain on the hypoplastic lung of the ovine model of congenital diaphragmatic hernia. Over a period of 4 weeks in late gestation, repeated cyclical tracheal occlusion for 23 hours with 1-hour release stimulated minimal growth, but promoted maturation with the development of a saccular lung. In contrast, a cycle consisting of 47 hours with 1-hour release induced optimal lung growth and morphologic maturation of the hypoplastic lung parenchyma. Sustained occlusion resulted in exaggerated lung growth, exceeding that of unaffected controls, and abnormal alveolar development. The extent of induction of lung growth by mechanical strain was inversely proportional to the number of alveolar type II cells remaining in the lung epithelium. These studies show that, although mechanical strain is capable of inducing lung growth and differentiation, cyclical strain is a prerequisite for normal development and that mechanically induced growth occurs at the expense of the alveolar type II cell. We conclude that cyclical strain may allow optimal alveolar development while maintaining a population of alveolar type II cells and may thus facilitate an improvement in postnatal lung function in infants with congenital diaphragmatic hernia.

The effect of gastroschisis on experimental diaphragmatic hernia in fetal rabbits

PURPOSE

In this study, the authors analyzed the effect of experimentally induced gastroschisis on pulmonary hypoplasia in fetal rabbits with congenital diaphragmatic hernia (CDH).

METHODS

Twenty-three pregnant rabbits underwent fetal surgery on gestational day 24 through 27. A left diaphragmatic hernia was created in 1 fetus (DH group) from each rabbit, and a left diaphragmatic hernia with gastroschisis was created in another fetus (GS group). The fetuses were delivered on gestational day 27 through 33. Histologic and morphometric examination of the lungs in each group was done.

RESULTS

In the DH group, the lungs were hypoplastic with a decrease in lung weight to body weight ratio and an increase pulmonary arterial medial wall thickness. The alveolar septae were markedly thickened with increased interstitial tissue and diminished alveolar air spaces. In the GS group, the alveolar septae were thickened but narrower than those of DH group, and air spaces were increased. The pulmonary arterial wall was markedly thickened in the DH group but only slightly thickened in the GS group.

CONCLUSIONS

Pulmonary hypoplasia seen in newborn rabbits after experimentally induced diaphragmatic hernia is less severe in those rabbits with both gastroschisis and DH.

In vivo tracheal occlusion in fetal mice induces rapid lung development without affecting surfactant protein C expression

Fetal tracheal occlusion (TO) reverses lung hypoplasia by inducing rapid lung growth. Although increases in lung size accompanied by increased numbers of alveoli and capillaries have been reported, effects of TO on lung development have not been formally assessed. In the present study, the objective was to verify our prediction that the main effect of TO would be to accelerate fetal lung development. We have developed and characterized a new fetal mouse model of TO to best realize this goal. At embryonic day 16.5, pregnant CD1 mice were operated under general anesthesia. One fetus per dam was selected to undergo surgical TO with a surgical clip or a sham operation. The fetuses were delivered 24 or 36 h postsurgery. The maturation of lung parenchyma, evaluated by counting the generations of alveolar saccules from the terminal bronchiole to the pleura, was significantly accelerated in the TO group with a complexity of the gas exchange region comparable with postnatal days 1 and 3 after 24 or 36 h of TO. Cellular proliferation and apoptosis peaks, assessed by immunohistochemistry directed against PCNA and the active form of caspase-3, were significantly increased 24 h after surgery in the TO group compared with the sham group. However, in situ hybridization showed no significant difference in the density of type II pneumocytes expressing surfactant protein C mRNA. Our results show that brief TO during late gestation in fetal mice induces accelerated lung development with minimal effects on surfactant protein C mRNA expression.

Control of embryonic lung branching morphogenesis by the Rho activator, cytotoxic necrotizing factor 1

BACKGROUND

Lung development is sensitive to physiological stresses, and its development may be impaired by physical distortion, as in patients with congenital diaphragmatic hernia. Yet, little is known about how mechanical forces can influence lung morphogenesis. Studies with cultured cells suggest that cytoskeletal tension may play a key role in growth control. Since the small GTPase Rho plays an important role in the control of cell tension generation, we carried out studies to test the hypothesis that changes in Rho-mediated cell tension may influence branching morphogenesis.

METHODS

Embryonic lung buds from timed pregnant Swiss Webster mice were microdissected on Embryonic Day 12 (E12), and whole organs were cultured in serum-free medium in the presence of the Rho activator cytotoxic necrotizing factor 1 (CNF-1) for 48 h. Serial measurements of the degree of epithelial branch formation and tissue maturation were performed using light microscopy and computerized image analysis.

RESULTS

At 48 h, embryonic lungs treated with 2 ng/ml CNF-1 increased their terminal bud count by 236 +/- 18% (P = 0.01) compared with 132 +/- 2% for untreated controls. However, dose-response experiments revealed biphasic behavior: at a higher dose of CNF-1 (200 ng/ml), bud number was actually decreased relative to controls (43 +/- 1%, P < 0.001). Histological analysis revealed that individual glands appeared to be more highly developed at low-dose CNF-1, whereas the high dose produced gland contraction.

CONCLUSIONS

These data support a potential role for Rho and cytoskeletal tension in control of epithelial pattern formation during lung development.

Surfactant in respiratory distress syndrome and lung injury

A deficiency in alveolar surfactant due to immaturity of alveolar type II epithelial cells causes respiratory distress syndrome (RDS). In contrast to animals, the fetal maturation of surfactant in human lungs takes place before term, exceptionally large quantities of surfactant accumulating in the amniotic fluid. The antenatal development of surfactant secretion is very variable but corresponds closely to the risk of RDS. The variation in SP-A and SP-B genes, race, sex and perinatal complications influence susceptibility to RDS. Surfactant therapy has improved the prognosis of RDS remarkably. Abnormalities in alveolar or airway surfactant characterize many lung and airway diseases. In the acute respiratory distress syndrome, deficiencies in surfactant components (phospholipids, SP-B, SP-A) are evident, and may be caused by pro-inflammatory cytokines (IL-1, TNF) that decrease surfactant components. The resultant atelectasis localizes the disease, possibly allowing healing (regeneration, increase in surfactant). In the immature fetus, cytokines accelerate the differentiation of surfactant, preventing RDS. After birth, however, persistent inflammation is associated with low SP-A and chronic lung disease. A future challenge is to understand how to inhibit or redirect the inflammatory response from tissue destruction and poor growth towards normal lung development and regeneration.

Effect of lung fluid composition on type II cellular activity after tracheal occlusion in the fetal lamb

BACKGROUND/PURPOSE

Fetal tracheal occlusion (TO) causes accelerated lung growth. However, prolonged TO is associated with a decline in the type II cell number. Type II cell function after TO is unclear. Herein, the authors examine type II cell function after TO and the role of tracheal fluid.

METHODS

Fetal lambs (term, 145 days) underwent TO at 122 days. Tracheal pressure was recorded daily. In one group of animals (TF; n = 6), lung fluid was aspirated, measured, and reinfused daily. In their respective twins, NS group, lung fluid was replaced milliliter per milliliter with normal saline (NS; n = 6). At death near term, lung weight was obtained, and tissues were processed for stereologic volumetry. Type II cells were quantitated using antisurfactant protein B immunohistochemistry. Surfactant protein B-mRNA expression was studied by Northern analysis. Wilcoxon signed rank test and single factor analysis of variance (ANOVA) were used for statistical analysis (P<.05 was significant).

RESULTS

In both experimental groups, intratracheal pressure rose from 1.9+/-1.0 torr to 3.7 to 4.8 torr by day 1, and remained constant thereafter. Lung fluid volume increased from 11.9+/-4.2 on day 0 to 36.8+/-8.0 mL/kg in TF, and to 28.4+/-9.3 mL/kg in NS by day 1 (P<.05). At death, lung weight/body weight ratio was higher in TF (5.45% +/- 0.91%) than in NS (4.40% +/- 0. 67%) or control (3.83%+/-0.58%; P<.05). Type II numerical density was substantially reduced after TO: 57.7+/-12.8 x 10(6)/mL (TF) and 45.0 +/-25.9 x 10(6)/mL (NS), versus 82.3+/-13.6 x 10(6)/mL in controls. Ultrastructurally, remaining type II cells in TF were enlarged and engorged with lamellar bodies; in NS, they were smaller and contained fewer lamellar bodies. Surfactant protein B mRNA expression was significantly decreased in NS, but not in TF, compared with controls.

CONCLUSIONS

Type II cell function as well as overall lung growth are stimulated by TO. Lung growth after TO is therefore not unavoidably detrimental to type II cells. After isobaric saline exchange of lung fluid, type II cell function is severely inhibited, confirming the role of tracheal fluid composition in type II stimulating type II cell function.

Requirements for fetal surgery: the diaphragmatic hernia model

Fetal surgery for congenital diaphragmatic hernia and other fetal conditions can only be considered if (1) the morbidity of antenatal intervention is acceptable, (2) the diagnosis of the condition can be made accurately, (3) the condition can be differentiated from other, non-surgical anomalies. In addition, (4) the natural evolution of the disease, if left untreated, should be predictable, and the condition should be lethal or severely debilitating, (5) there should not exist adequate postnatal treatment, and (6) the proposed in utero operation should be technically feasible. Open fetal surgery has proven too invasive to be justified for the treatment of diaphragmatic hernia, and progress in postnatal therapy (including ECMO) has dramatically improved the neonatal outcome in all but a severe subgroup of patients. Recently, advances in endoscopic fetal surgery (which appears to be less stressful for the fetus and the gravid uterus) and a new approach to accelerate fetal lung growth and maturation have renewed the feasibility of in utero intervention for diaphragmatic hernia.

Pulmonary hypoplasia in the myogenin null mouse embryo

Although fetal breathing movements are required for normal lung development, there is uncertainty concerning the specific effect of absent fetal breathing movements on pulmonary cell maturation. We set out to evaluate pulmonary development in a genetically defined mouse model, the myogenin null mouse, in which there is a lack of normal skeletal muscle fibers and thus skeletal muscle movements are absent in utero. Significant decreases were observed in lung:body weight ratio and lung total DNA at embryonic days (E)14, E17, and E20. Reverse transcriptase/polymerase chain reaction, in situ immunofluorescence, and electron microscopy revealed early lung cell differentiation in both null and wild-type lungs as early as E14. However at E14, myogenin null lungs had decreased 5'-bromo-2-deoxyuridine incorporation compared with that of wild-type littermates, whereas at E17 and E20, increased Bax immunolabeling and terminal deoxyribonucleotidyl transferase-mediated dUTP-biotin nick-end labeling staining were detected in the myogenin null mice but not in the wild-type littermates. These observations highlight the importance of skeletal muscle contractile activity in utero for normal lung organogenesis. Null mice lacking the muscle-specific transcription factor myogenin exhibit a secondary effect on lung development such that decreased lung cell proliferation and increased programmed cell death are associated with lung hypoplasia.

Lung growth response after tracheal occlusion in fetal rabbits is gestational age-dependent

In utero tracheal occlusion (TO) is a potent stimulus of fetal lung growth, and is currently being applied in clinical trials to treat severe forms of pulmonary hypoplasia. The aim of this study was to examine the effect of timing of TO on pulmonary growth and maturation rates. Fetal rabbits (term = 31 d) were subjected to in utero tracheal clipping at 24 (late pseudoglandular stage) or 27 d of gestation (late canalicular/early terminal sac stage). Sham-operated littermates served as controls (C). Animals were killed at time intervals ranging from 1 to 6 d (early group) or 1 to 3 d (late group) after occlusion. Lung growth was measured by computerized stereologic volumetry and 5'-bromo-2'-deoxyuridine (BrdU) pulse labeling. Pneumocyte II population kinetics were analyzed using a combination of anti-surfactant protein-A and BrdU immunohistochemistry and computer-assisted morphometry. Statistical analysis was performed using unpaired Student's t test. Early TO was followed by an initial 3-d stagnation of growth and subsequently a dramatic acceleration of growth (BrdU-labeling index [LI] 10.1 +/- 0. 6% in TO versus 2.7 +/- 0.5% in C at 29 d, P < 0.001). In contrast, late TO induced an immediate and sustained moderate increase of lung growth (BrdU-LI 2.8 +/- 0.9% in TO versus 1.1 +/- 0.2% in C at 30 d, P < 0.05), associated with relatively more pronounced air-space distension. Whereas late TO caused no significant alterations in type II cell density or proliferation, early TO was followed by a marked increase in type II cell proliferation, paradoxically associated with dramatic reduction of type II cell density after 29 d. The effects of intrauterine TO on fetal lung growth and type II cell kinetics critically depend on the gestational age, and thus on the maturity of the lungs at the time of surgery. These findings have important clinical implications with respect to the timing of fetal interventions aimed at promoting lung growth. The fetal rabbit provides an invaluable model to study the mechanics and age dependency of TO-induced lung growth.

The effects of prenatal intraamniotic surfactant or dexamethasone administration on lung development are comparable to changes induced by tracheal ligation in an animal model of congenital diaphragmatic hernia: studies of lung glycogen content, elastic fiber density, and collagen content

BACKGROUND/PURPOSE

A new noninvasive therapeutic strategy, which consisted of prenatal intraamniotic administration of porcine surfactant or dexamethasone, was previously used to prevent the functional and structural immaturity of lungs associated with congenital diaphragmatic hernia (CDH), and its effects on lung development were comparable with the changes induced by tracheal ligation (TL). The purpose of this study is to verify if this novel therapeutic modality has any effect in the elevated concentration of lung glycogen and altered contents of lung elastic fiber and collagen promoted by CDH.

METHODS

A pilot study was performed to investigate in the rabbit model if the infused drugs in the amniotic cavity were aspirated by the CDH and non-CDH fetuses, and if there was correspondence between lung immaturity and high glycogen concentration in lung tissue. Experimental groups consisted of 50 pregnant rabbits that underwent surgery on gestational day 24 or 25 to create left-sided diaphragmatic hernias in 56 fetuses, which were divided in groups according to the procedures: CDH (n = 12), CDH plus TL (n = 16), CDH plus intraamniotic administration of Curosurf (40 mg, n = 12), and CDH plus intraamniotic administration of dexamethasone (n = 16). On gestational day 30, the fetuses were delivered by cesarean section, and 28 normal unoperated fetuses served as controls. The lungs were weighed and submitted to biochemical determination of glycogen, morphometric evaluation of elastic fibers, and colorimetric analysis of collagen.

RESULTS

In all CDH and non-CDH fetuses of the pilot study, the amniotic content was massively aspirated into the lungs and trachea. There was an increase in lung glycogen content of fetuses at 24 days' gestation in comparison with 20-day gestational age fetuses, followed by a decrease in the near full-term fetuses. In the fetuses of the experimental groups, CDH decreased the lung weight to body weight ratios of lungs ipsilateral to the hernia. These changes were reversed by TL but not by intraamniotic administration of surfactant or dexamethasone. Lung glycogen concentrations in the lungs of CDH fetuses were significantly higher than those in the control group. These changes were reversed by intraamniotic administration of surfactant but not by dexamethasone administration or TL. In the lungs ipsilateral to the hernia, surfactant administration promoted a significant decrease in glycogen content to levels lower than control lungs. CDH promoted a decrease in the linear density of elastic fibers in both lungs, ipsilateral and contralateral to the hernia. This alteration was partially corrected by TL and surfactant administration, although dexamethasone administration had no effect. The concentrations of collagen in both lungs were increased significantly by CDH, and these alterations could not be reversed by TL. In the lungs ipsilateral to the hernia, intraamniotic administration of surfactant or dexamethasone promoted a significant decrease in the lung concentration of collagen but not to control levels.

CONCLUSIONS

The positive effects of intraamniotic surfactant or dexamethasone administration on lung maturity of fetuses with CDH were observed. This therapy may be a substitute for TL.

Tracheal occlusion in the fetal rat: a new experimental model for the study of accelerated lung growth

BACKGROUND

Prenatal tracheal occlusion accelerates fetal lung growth, but the mechanism of this phenomenon is unknown. Previous animal models have been limited by expense, lack of species-specific molecular probes, or the stage of lung development when studies could be performed. To provide a model that is more amenable to systematic analysis, we have developed an in vivo rat model of prenatal tracheal occlusion.

METHODS

Time-dated pregnant rats underwent laparotomy at 19 days' gestational age (term, 22 days). The fetal head and neck were exteriorized through a hysterotomy, and the trachea was ligated under a dissecting microscope. The fetus was returned to the amniotic cavity, and the uterine and maternal abdominal incisions were closed. The dam and the fetuses were killed at 21.5 days' gestational age, and the fetal lungs were assessed for lung growth and compared with nonoperated littermate controls.

RESULTS

Thirty-two of 50 manipulated fetuses survived. Of the 32 survivors, successful tracheal ligation was confirmed in 20, and these 20 fetuses were compared with 33 littermate controls. Fetal body weight (4.81+/-0.26 g v 4.87+/-0.41 g) and heart weight (0.05+/-0.01 g v 0.05+/-0.01 g) were not significantly different between ligated fetuses and littermate controls, whereas the wet lung weight (0.30+/-0.06 g v 0.13+/-0.02 g, P<.01), lung-to-body-weight ratio (6.34+/-1.16% v 2.64+/-0.41%, P<.01), dry lung weight (17.4+/-2.45 mg v 12.1+/-1.87 mg, P<.01), total lung DNA (1210+/-371 microg v 828+/-208 microg, P<.01), and total lung protein (14.3+/-5.3 mg v 8.7+/-1.7 mg, P<.01) were increased significantly in the ligated fetuses. The enlarged lung demonstrated normal histology findings after inflation fixation.

CONCLUSIONS

Prenatal tracheal occlusion during the canalicular stage of lung development accelerates lung growth in the rat. In comparison with other large animal models, this relatively inexpensive small animal model has the distinct advantages of a short gestation, a large number of fetuses per litter, the availability of a developmental model of congenital diaphragmatic hernia, and the availability of well-defined molecular probes to investigate the mechanism of tracheal occlusion-induced lung growth.

Glucose uptake in dilated small intestine

BACKGROUND/PURPOSE

The development of dilated small intestine in patients with short bowel syndrome results in increased mucosal surface area. This study examines whether the incremental increase in surface area leads to a proportional increase in absorptive function of the small intestine.

METHODS

Partial obstruction of the small intestine was created in rats by placing an intussusception valve in the proximal jejunum. Rats that underwent sham operations served as controls. One week postoperatively, the small intestine proximal and distal to the valve was removed. The intestinal diameter proximal and distal to the obstruction was measured. The rate of glucose uptake was measured by the everted sleeve technique. The results were analyzed by analysis of variance (ANOVA).

RESULTS

The intestine proximal to the valve was significantly dilated and thickened when compared with the intestine distal to the valve. The wet mass per centimeter of the dilated segment was 2.5 times that of the control group (P<.001). The glucose uptake capacity of the dilated segment was slightly higher than that of the control group (540 v 420 nmol/min/cm, P<.05). However, the specific glucose uptake rate was reduced significantly in the intestine proximal to the valve (247 v 335 nmol/min/cm2, P<.01).

CONCLUSIONS

Although the partial obstruction of small intestine resulted in a substantial increase in the intestinal surface area, the absorptive capacity of the dilated intestine per unit surface area was decreased significantly. This translated ultimately into a slight increase in the overall functional absorptive capacity of glucose in the small intestine. These results suggest that dilated small intestine may not enhance mucosal absorption.

Effects of mechanical forces on lung-specific gene expression

Fetal breathing movements (FBM) are necessary for fetal lung growth and maturation. The authors analyzed fetal rat lungs cultured with or without lung distension and tracheal ligation, and examined the effects of mechanical stretch on a human pulmonary epithelial cell line (NCI-H441) that shows regulated expression of surfactant proteins (SP-A, SP-B). Cells were grown on silastic membranes and mounted in a Flexercell Strain Unit. Cyclic deformation simulating FBM was achieved by applying a vacuum of 22 kPa (5%-15% radial deformation) at 50 cycles per minute for 2 to 24 hours. Results indicate that static distension for as little as 4 hours decreased steady-state SP-A and SP-B mRNA levels in whole lung (n = 5-6, P < .01). In contrast, cyclic stretching of H441 cells for 24 hours increased SP-B and SP-A expression 2- to 4-fold over controls. Cyclic deformation also significantly enhanced 3H-choline incorporation into saturated phosphatidylcholine. Dynamic mechanodeformation may be a critical stimulus for fetal lung development.

The effects of prenatal intraamniotic surfactant or dexamethasone administration on lung development are comparable to changes induced by tracheal ligation in an animal model of congenital diaphragmatic hernia

BACKGROUND/PURPOSE

Lung surfactant deficiency contributes to the pathophysiology of congenital diaphragmatic hernia (CDH) and the high neonatal mortality rate. Acceleration of lung surfactant system maturation by prenatal administration of hormones has been described in animal models of CDH. However, in utero tracheal ligation (TL) is the best method to accelerate lung growth and reverse the pulmonary hypoplasia associated with CDH. Although this method offers promise, its application in humans is limited. The aim of this study was to investigate a new noninvasive therapeutic strategy, that is, the prenatal intraamniotic administration of exogenous porcine surfactant or dexamethasone, and compare it with the effects of TL in an animal model of CDH.

METHODS

Twenty-four pregnant New Zealand rabbits underwent surgery on gestational day 24 or 25 to create CDH in 26 fetuses. Five groups of animals were studied: (1) Control, nonoperated fetuses (n=14), (2) CDH (n=6), (3) CDH plus TL (n 6), (4) CDH plus intraamniotic administration of Curosurf (40 mg; n=6), and (5) CDH plus intraamniotic infusion of dexamethasone (0.4 mg; n=8). On gestational day 30, the fetuses were delivered by cesarean section. Functional studies (lung hysteresis curves and lung distensibility), weight and volume of lungs, histopathologic and histomorphometric analysis of lungs were performed.

RESULTS

The authors demonstrated that the hysteresis curve of CDH animals was shifted downward in comparison with controls. The analyses of curves standardized for lung weight indicated that intraamniotic administration of surfactant or dexamethasone improved lung compliance in comparison with controls and CDH fetuses, but TL had no effect on this parameter. Lung distensibility (maximum lung volume at 32 cm of water pressure per gram of lung) was reduced by CDH, but this parameter was increased by intraamniotic administration of drugs and not by TL (P< .05). CDH decreased the weight and volume of lungs (P< .05), and these changes were reversed only by TL, which prevented the herniation of the liver from the abdomen to the thorax. Histologically, CDH lungs treated with TL or intraamniotic administration of drugs demonstrated structural patterns similar to those of controls. Histomorphometric studies proved that CDH promoted significant thickening of septa walls (P< .05), and all the therapeutic methods could reverse this alteration to control values. The alveolar number per area in control lungs, CDH, and CDH plus TL lungs were similar, but in CDH plus surfactant and CDH plus dexamethasone lungs, the decreased number per area (P< .05) demonstrated that the alveolar airspace was increased.

CONCLUSION

From these data the authors conclude that intraamniotic surfactant or dexamethasone administration is capable of preventing pulmonary hypoplasia in fetuses with CDH, and thus, this method may be a substitute for TL.

The plug-unplug sequence: an important step to achieve type II pneumocyte maturation in the fetal lamb model

PURPOSE

The purpose of this study was to test the hypothesis that tracheal obstruction (plugging) in the fetal lamb model leads to a decrease in the absolute number of type II pneumocytes and that reversing the obstruction before birth (unplugging), allows the type II cells to recover while maintaining the beneficial effect on lung growth.

METHODS

Nine time-dated pregnant ewes (term, 145 days), carrying 17 fetuses, were used in this surgical trial. The fetuses were divided into three experimental groups: group A underwent plugging at 93 days gestation, followed by unplugging at 110 days; group B animals had tracheal ligation at 93 days and group C consisted of unoperated controls. All fetuses were delivered by cesarean section at 136 days' gestation. The fetal trachea was obstructed with the tracheoscopically placed detachable balloon described by our group. Unplugging was performed by needle puncture of the balloon under tracheoscopic vision. Outcome measurements consisted of lung-to-body-weight ratio (LWBR), lung morphometry (mean terminal bronchial density [MTBD] and linear intercept [Lm]), and assessment of the number of type II pneumocytes. The latter was determined by in situ hybridization to the mRNA of surfactant protein-C, which is exclusively produced by type II cells. Statistics were calculated using a two-tailed unpaired t test and P less than .05 is considered significant.

RESULTS

Seventeen animals are included in the results. All of them had lung samples analyzed for lung morphometry, whereas for type II cells analysis, three animals were studied in each group. Morphometric analyses were consistent with pulmonary hyperplasia for group B, whereas group A lungs showed more histological maturity than group C albeit not as marked as group B. In group A, there was a similar number of type II cells to that observed in group C (53.2 +/- 3.9 v 55.9 +/- 4.0, P = .66). However, for group B animals, the number of type II pneumocytes was markedly decreased compared with controls (4.7 +/- 0.1 v 55.9 +/- 4, P = .0003).

CONCLUSIONS

The authors conclude that tracheal ligation until birth, although inducing pulmonary hyperplasia, significantly decreases the number of type II pneumocytes in the alveoli. After a temporary 15-day occlusion initiated at 95 days' gestation, there is complete normalization of the density of type II cells. These results bear importance on the duration of PLUG to treat the pulmonary hypoplasia seen in congenital diaphragmatic hernia. Temporary tracheal obstruction now needs to be tested in a hypoplastic lung model.

The effects of tracheal occlusion and release on type II pneumocytes in fetal lambs

Fetal tracheal occlusion (TO) has been shown to lead to lung hyperplasia in various animal models, and this procedure has already been carried out in human fetuses with congenital diaphragmatic hernia (CDH). However, the authors previously showed that TO caused a decrease in type II pneumocytes.

PURPOSE

The aim of this study is to examine the effects of TO and release on type II pneumocytes.

METHOD

To was carried out with a Swan Ganz or Fogarty catheter in fetal sheep at 116 to 118 days of gestation. TO was maintained for 2 weeks followed by deflation of the balloon for 1 week before delivery, in group 1; in group 2, TO was maintained for 19 days and released 2 days before delivery. Group 3 consisted of previously reported animals who had TO maintained until birth. Unoperated twins served as controls. All specimens were analyzed using the surfactant protein C (SP-C) mRNA as a specific marker for type II pneumocytes. We used Northern Blot and in situ hybridization techniques to quantify total SP-C and the density of type II cells. Electron microscopy (EM) was also used to evaluate and quantitate type II cells.

RESULTS

TO resulted in significant lung growth in all groups. In situ hybridization and Northern Blot analysis showed that there was a complete recovery of type II cells in group 1 versus controls. Quantitative EM analysis confirmed these findings. In group 2 the number of type II cells was decreased but there was an increase in SP-C content per type II cell versus group 3.

CONCLUSION

Lung growth after TO appears to occur at the expense of type II cell differentiation. This effect is reversible with the release of TO before birth in this animal model.

Correction of congenital diaphragmatic hernia in utero VIII: Response of the hypoplastic lung to tracheal occlusion

Most fetuses with congenital diaphragmatic hernia (CDH) diagnosed before 24 weeks' gestation die despite optimal postnatal care. In fetuses with liver herniation into the chest, prenatal repair has not been successful. In the course of exploring the pathophysiology of CDH and its repair in fetal lambs, the authors found that obstructing the normal egress of fetal lung fluid enlarges developing fetal lungs, reduces the herniated viscera, and accelerates lung growth, resulting in improved pulmonary function after birth. They developed and tested experimentally a variety of methods to temporarily occlude the fetal trachea, allow fetal lung growth, and reverse the obstruction at birth. The authors applied this strategy of temporary tracheal occlusion in eight human fetuses with CDH and liver herniation at 25 to 28 weeks' gestation. With ongoing experimental and clinical experience, the technique of tracheal occlusion evolved from an internal plug (two patients) to an external clip (six patients), and a technique was developed for unplugging the trachea at the time of birth (Ex Utero Intrapartum Tracheoplasty [EXIT]). Two fetuses had a foam plug placed inside the trachea. The first showed dramatic lung growth in utero and survived; the second (who had a smaller plug to avoid tracheomalacia) showed no demonstrable lung growth and died at birth. Two fetuses had external spring-loaded aneurysm clips placed on the trachea; one was aborted due to tocolytic failure, and the other showed no lung growth (presumed leak) and died 3 months after birth. Four fetuses had metal clips placed on the trachea. All showed dramatic lung growth in utero, with reversal of pulmonary hypoplasia documented after birth. However, all died of nonpulmonary causes. Temporary occlusion of the fetal trachea accelerates fetal lung growth and ameliorates the often fatal pulmonary hypoplasia associated with severe CDH. Although the strategy is physiologically sound and technically feasible, complications encountered during the evolution of these techniques have limited the survival rate. Further evolution of this technique is required before it can be recommended as therapy for fetal pulmonary hypoplasia.