Pulmonary hypoplasia and fetal breathing in preterm premature rupture of membranes

Mechanics of Lung Development

We summarize how skeletal muscle and lung developmental biology fields have been bridged to benefit from mouse genetic engineering technologies and to explore the role of fetal breathing-like movements (FBMs) in lung development, by using skeletal muscle-specific mutant mice. It has been known for a long time that FBMs are essential for the lung to develop properly. However, the cellular and molecular mechanisms transducing the mechanical forces of muscular activity into specific genetic programs that propel lung morphogenesis (development of the shape, form and size of the lung, its airways, and gas exchange surface) as well as its differentiation (acquisition of specialized cell structural and functional features from their progenitor cells) are only starting to be revealed. This chapter is a brief synopsis of the cumulative findings from that ongoing quest. An update on and the rationale for our recent International Mouse Phenotyping Consortium (IMPC) search is also provided.

Knockout of insulin-like growth factor-1 receptor impairs distal lung morphogenesis

Background

Insulin-like growth factors (IGF-I and -II) are pleiotropic regulators of somatic growth and development in vertebrate species. Endocrine and paracrine effects of both hormones are mediated by a common IGF type 1 receptor (IGF-1R). Lethal respiratory failure in neonatal IGF-1R knockout mice suggested a particular role for this receptor in pulmonary development, and we therefore investigated the consequences of IGF-1R inactivation in lung tissue.

Methods and Findings

We first generated compound heterozygous mutant mice harboring a hypomorphic (Igf1r^neo) and a null (Igf1r⁻) allele. These IGF-1R^neo/− mice express only 22% of normal IGF-1R levels and are viable. In adult IGF-1R^neo/− mice, we assessed lung morphology and respiratory physiology and found normal histomorphometric characteristics and normal breathing response to hypercapnia. We then generated homozygous IGF-1R knockout mutants (IGF-1R^−/−) and analyzed their lung development during late gestation using histomorphometric and immunohistochemical methods. IGF-1R^−/− embryos displayed severe lung hypoplasia and markedly underdeveloped diaphragms, leading to lethal neonatal respiratory distress. Importantly, IGF-1R^−/− lungs from late gestation embryos were four times smaller than control lungs and showed markedly thickened intersaccular mesenchyme, indicating strongly delayed lung maturation. Cell proliferation and apoptosis were significantly increased in IGF-1R^−/− lung tissue as compared with IGF-1R^+/+ controls. Immunohistochemistry using pro-SP-C, NKX2-1, CD31 and vWF as markers revealed a delay in cell differentiation and arrest in the canalicular stage of prenatal respiratory organ development in IGF-1R^−/− mutant mice.

Conclusions/Significance

We found that low levels of IGF-1R were sufficient to ensure normal lung development in mice. In contrast, complete absence of IGF-1R significantly delayed end-gestational lung maturation. Results indicate that IGF-1R plays essential roles in cell proliferation and timing of cell differentiation during fetal lung development.

Current technology in the diagnosis of developmentally related lung disorders

Respiratory disorders that present in the newborn period may result from structural, functional, or acquired mechanisms that limit gas exchange between the airspace and vascular bed. Exciting new imaging, gene sequencing, mass spectrometry, and molecular and cell-based techniques are enhancing our understanding of mechanisms of disease; highlighting the complexity of interactions between genes, development, and environment in the manifestation of health and disease; and becoming part of the clinical armamentarium for the care of patients. Some of these technologies and their clinical potential are briefly reviewed in this paper.

Abnormal development of the intercostal muscles and the rib cage in Myf5-/- embryos leads to pulmonary hypoplasia

The aim of our study was to investigate the importance of pulmonary distension and fetal breathing-like movements executed by the contractile activity of the intercostal respiratory muscles for proper lung growth and maturation. Lung development in Myf5-/- embryos, lacking the rib cage and functional intercostal musculature, was compared with wild-type controls at embryonic days 14.5, 16.5, and 18.5. Our data revealed that Myf5-/- embryos suffered from pulmonary hypoplasia in part due to the decreased number of proliferating lung cells and in part due to the increased number of terminal deoxynucleotidyl transferase mediated dUTP nick end labeling (TUNEL) -positive cells. In addition, the proximal-to-distal expression gradient of thyroid transcription factor-1 observed in wild-type embryos was not maintained in Myf5-/- embryos. The number of lung cells expressing platelet-derived growth factor-BB, its receptor and insulin growth factor-I was significantly decreased in the hypoplastic lung. By contrast, no difference in the expression pattern of surfactant associated proteins or Clara cells marker was detected between wild-type and Myf5-/- embryos. Collectively, our data suggest that the mechanochemical signal transduction pathway used in vitro is also effective in vivo influencing lung growth but not lung cell maturation and resulting in lung hypoplasia. These data affirm the role of fetal breathing-like movements in lung organogenesis.

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.

Congenital anomalies of the fetal/neonatal chest

The ability to identify and confidently diagnose a wide range of congenital fetal thoracic lesions has increased tremendously in the past 2 decades with the emergence of high-resolution sonography and ultrafast MRI sequences. Imaging studies constitute a vital component in the diagnosis of these lesions, whether in the fetal, neonatal or childhood periods. In addition to providing a road map for potential intervention, imaging techniques have provided important information about normal development, natural history, and prognosis. In the prenatal stage, these features aid in family counseling, pregnancy management, and the identification of a subgroup of patients who may benefit from fetal intervention. In the neonatal and childhood periods, imaging studies facilitate timely diagnosis and institution of appropriate therapeutic strategies.

Relation between oligohydramnios and spinal flexion in the human fetus

BACKGROUND

Oligohydramnios, a severe reduction in the volume of amniotic fluid, is associated with fetal lung hypoplasia but underlying processes are unclear. Studies in sheep suggest that oligohydramnios may lead to lung hypoplasia by causing increased flexion of the fetal spine, but this has not been demonstrated in the human, which has a different uterine anatomy.

AIMS

Our aims were to quantify spinal flexion in the human fetus and to determine the relationship between oligohydramnios and the degree of spinal flexion.

SUBJECTS AND METHODS

In 35 pregnancies, we used ultrasonography to assess amniotic fluid volume and to image the fetal spine so as to provide an index of mean spinal radius between the upper thoracic and lumbar spine. In 17 pregnancies with evidence of oligohydramnios resulting from premature rupture of membranes (mean +/- SEM gestation 28.8 +/-1.4 weeks), the mean radius of fetal spinal curvature between the upper thorax and sacrum was compared with values from 18 control fetuses with normal amounts of amniotic fluid (at 28.9 +/- 1.6 weeks).

RESULTS

In each fetus, the spine could be imaged and the mean radius of curvature calculated. Oligohydramnios was associated with a significantly increased degree of fetal spinal flexion compared to controls.

CONCLUSION

In human pregnancy, oligohydramnios is associated with increased flexion of the fetal spine, which is likely due to reduced uterine volume. This could contribute to the development of fetal lung hypoplasia, fetal immobility and other fetal anomalies.

The fetal lung. 2: Pulmonary hypoplasia

This review describes the pathogenesis of pulmonary hypoplasia and highlights its clinical, radiological and pathologic features, with emphasis on oligohydramnios-related pulmonary hypoplasia. Since pulmonary hypoplasia may lead to severe respiratory distress immediately after birth and even to neonatal death, an accurate and patient-friendly prenatal test for early detection and distinction between lethal and non-lethal pulmonary hypoplasia is still highly desirable. An extended overview of the proposed methods for the prenatal prediction of pulmonary hypoplasia is presented.

Mechanical strain-induced proliferation and signaling in pulmonary epithelial H441 cells

Pulmonary epithelial cells are exposed to mechanical strain during physiological breathing and mechanical ventilation. Strain regulates pulmonary growth and development and is implicated in volutrauma-induced fibrosis. The mechanisms of strain-induced effects are not well understood. It was hypothesized that mechanical strain induces proliferation of pulmonary epithelial cells and that this is mediated by signals initiated within seconds of strain. To test this hypothesis, human pulmonary adenocarcinoma H441 cells were strained in vitro. Cyclic as well as tonic strain resulted in increased cellular proliferation. Western blot analysis of strained cells demonstrated three newly phosphorylated tyrosine residues within 30 s of strain. Phosphorylation of mitogen-activated protein kinases p42/44 increased, electrophoretic mobility shift assay demonstrated activation of transcription factor activating protein-1, and immunohistochemistry demonstrated increased phosphorylation of c-jun in response to strain. The tyrosine kinase inhibitor genistein blocked the strain-induced proliferation. We conclude that strain induces proliferation in pulmonary epithelial cells and that tyrosine kinase activity is necessary to signal the proliferative response to mechanical strain.

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.

Midtrimester premature rupture of the membranes

Midtrimester premature rupture of the membranes is an uncommon adverse complication of pregnancy with an occurrence of approximately 0.65%. Significant perinatal and maternal morbidity includes pulmonary hypoplasia, restriction deformities, and sequelae of prematurity as well as maternal chorioamnionitis and endometritis. Moreover, 31% of survivors are affected by long-term complications such as chronic lung disease, neurological and developmental abnormalities. Fetal survival is evident with increasing latency and good residual amniotic fluid (largest vertical pocket > or = 2 cm). In this article, we review the relevant literature regarding prognosis, counseling, and management of the patient with membrane rupture in the midtrimester.