Fetal lung growth: influence of maternal hypoxia and hyperoxia in rats

Environmental Contaminants Exposure and Preterm Birth: A Systematic Review

Preterm birth is an obstetric condition associated with a high risk of infant mortality and morbidities in both the neonatal period and later in life, which has also a significant public health impact because it carries an important societal economic burden. As in many cases the etiology is unknown, it is important to identify environmental factors that may be involved in the occurrence of this condition. In this review, we report all the studies published in PubMed and Scopus databases from January 1992 to January 2019, accessible as full-text articles, written in English, including clinical studies, original studies, and reviews. We excluded articles not written in English, duplicates, considering inappropriate populations and/or exposures or irrelevant outcomes and patients with known risk factors for preterm birth (PTB). The aim of this article is to identify and summarize the studies that examine environmental toxicants exposure associated with preterm birth. This knowledge will strengthen the possibility to develop strategies to reduce the exposure to these toxicants and apply clinical measures for preterm birth prevention.

Hypoxia and Placental Development

Hemochorial placentation is orchestrated through highly regulated temporal and spatial decisions governing the fate of trophoblast stem/progenitor cells. Trophoblast cell acquisition of specializations facilitating invasion and uterine spiral artery remodeling is a labile process, sensitive to the environment, and represents a process that is vulnerable to dysmorphogenesis in pathologic states. Hypoxia is a signal guiding placental development, and molecular mechanisms directing cellular adaptations to low oxygen tension are integral to trophoblast cell differentiation and placentation. Hypoxia can also be used as an experimental tool to investigate regulatory processes controlling hemochorial placentation. These developmental processes are conserved in mouse, rat, and human placentation. Consequently, elements of these developmental events can be modeled and hypotheses tested in trophoblast stem cells and in genetically manipulated rodents. Hypoxia is also a consequence of a failed placenta, yielding pathologies that can adversely affect maternal adjustments to pregnancy, fetal health, and susceptibility to adult disease. The capacity of the placenta for adaptation to environmental challenges highlights the importance of its plasticity in safeguarding a healthy pregnancy. Birth Defects Research 109:1309-1329, 2017.© 2017 Wiley Periodicals, Inc.

Before the first breath: prenatal exposures to air pollution and lung development

Various environmental contaminants are known to impair the growth trajectories of major organs, indirectly (gestational exposure) or directly (postnatal exposure). Evidence associates pre-gestational and gestational exposure to air pollutants with adverse birth outcomes (e.g., low birth weight, prematurity) and with a wide range of diseases in childhood and later in life. In this review, we explore the way that pre-gestational and gestational exposure to air pollution affects lung development. We present results in topics underlining epidemiological and toxicological evidence. We also provide a summary of the biological mechanisms by which air pollution exposure possibly leads to adverse respiratory outcomes. We conclude that gestational and early life exposure to air pollutants are linked to alterations in lung development and function and to other negative respiratory conditions in childhood (wheezing, asthma) that may last into adulthood. Plausible mechanisms encompass changes in maternal physiology (e.g., hypoxia, oxidative stress and inflammation) and DNA alterations in the fetus. Evidence for pre-gestational and gestational effects on the lung is scarce compared with that on early life exposure and further studies are needed. However, the suggested mechanisms are credible and the evidence of pre-gestational and gestational air pollution exposure is robust for adverse birth outcomes. Air pollutants might change lung developmental trajectories of the unborn child predisposing it to diseases later in life highlighting the urgent need for controls on urban air pollution levels worldwide.

Prenatal hypoxia downregulates the expression of pulmonary vascular endothelial growth factor and its receptors in fetal mice

BACKGROUND

Previous reports showed that prenatal hypoxia delays the process of lung maturation. Vascular endothelial growth factor (VEGF) and its receptors were important for lung development. However, the role of VEGF and VEGF receptors in altered fetal lung development and maturation induced by prenatal hypoxia remains unknown.

OBJECTIVES

To elucidate the role of VEGF and VEGF receptors in altered fetal lung development and maturation induced by prenatal hypoxia.

METHODS

Lung sections of control and maternal hypoxic fetal mice were used for the determination of lung development and total RNA isolated from lung homogenates were used for determination of the expression patterns of VEGF, Flt-1, Flk-1, hypoxia-inducible factor (HIF)-1α, HIF-2α, surfactant protein (SP)-A, SP-B, SP-C, and SP-D by quantitative real-time RT-PCR.

RESULTS

Prenatal hypoxia resulted in fetal mice body weight gain impairment, delayed fetal pulmonary aeration and maturation. Pulmonary SP-A, SP-B, SP-C, and SP-D mRNA were all decreased in the prenatal hypoxia group. In addition, we demonstrated that prenatal hypoxia inhibited the developmental increase of pulmonary HIF-1α and HIF-2α expression and resulted in decreasing VEGF and its receptors (Flt-1 and Flk-1) at the mRNA expression level and VEGF protein level in fetal lungs. These inhibitory effects persisted and progressed even when the dams were returned to air.

CONCLUSIONS

We suggest that prenatal hypoxia insults, at least in late gestation, influence pulmonary VEGF and VEGF receptor expression through the down-regulation of HIF pathways and impair fetal lung growth and maturation.

Developmental and genetic components explain enhanced pulmonary volumes of female Peruvian Quechua

High altitude natives have enlarged vital capacities and residual volumes (RV). Because pulmonary volumes are an indication of functionally relevant traits, such as diffusion capacity, the understanding of the factors (genetic/developmental) that influence lung volumes provides insight into the adaptive responses of highlanders. In order to test for the effect of growth and development at high altitude on lung volumes, we obtained forced vital capacities (FVC), RV, and total lung capacities (TLC) for a sample of 65 Peruvian females of mostly Quechua origins (18-34 years) who were sub-divided into two well-matched groups: 1) sea-level born and raised females (BSL, n = 34) from Lima, Peru (150 m), and 2) high-altitude born and raised females (BHA, n = 31) from Cerro de Pasco, Peru (4,338 m). To determine Quechua origins, Native American ancestry proportion (NAAP) for each individual was assessed using a panel of 70 ancestry informative markers. NAAP was similar between groups (BSL = 91.71%; BHA = 89.93%; P = 0.240), and the analysis confirmed predominantly Quechua origins. After adjusting for body size and NAAP, BHA females had significantly higher FVC (3.79 ± 0.06 l; P < 0.001), RV (0.98 ± 0.03 l; P < 0.001) and TLC (4.80 ± 0.07 l; P < 0.001) compared to BSL females (FVC = 3.33 ± 0.05 l; RV = 0.69 ± 0.03 l; TLC = 4.02 ± 0.06 l). NAAP was not associated with FVC (P = 0.352) or TLC (P = 0.506). However, NAAP was positively associated with RV (P = 0.004). In summary, results indicate that developmental exposure to high altitude in females constitutes an important factor for all lung volumes, whereas both genetic and developmental factors seem to be important for RV.

Respiratory mechanics in 1-day old chicken hatchlings and effects of prenatal hypoxia

This study examined the static and dynamic properties of the respiratory system in avian hatchlings, and the effects of incubation in hypoxia (15% O(2)) on these variables. In 1-day old chicken (Gallus gallus) hatchlings killed by an anesthetic overdose the static compliance of the respiratory system (C(rs)) was measured from the pressure-volume curve, constructed by step changes in lung volume. The dynamic compliance, C(rs)(dyn), resistance, R(rs), and time constant, τ(rs), were measured during mechanical ventilation at rates up to 90 cpm. The results indicated that (1) static C(rs) in hatchlings is several folds higher than in neonatal mammals of similar size, (2) during mechanical ventilation the respiratory system becomes hyperinflated and much stiffer; at 65 cpm (which is the respiratory frequency of hatchlings spontaneously breathing at rest) C(rs)(dyn) was about one tenth of the static value, (3) after prenatal hypoxia static C(rs), C(rs)(dyn), R(rs) and τ(rs) were similar to controls; only the magnitude of the hyperinflation was slightly decreased. It is concluded that in avian hatchlings (a) despite the large respiratory volume of the air sacs, expiration can occur passively because the hyperinflation greatly decreases C(rs)(dyn) and shortens τ(rs), and (b) prenatal hypoxia of the level tested has no major effects on the mechanical properties of the respiratory system.

Organ growth in chicken embryos during hypoxia: implications on organ "sparing" and "catch-up growth"

The primary aim of this study was to establish whether or not embryonic hypoxia selectively affects the growth of specific organs. Chicken embryos were incubated either in normoxia (Nx) or in hypoxia (15% O2 from embryonic day E5, Hx). The length of the beak and third toe (as indexes of skeletal growth) and the weights of internal organs (eyes, brain, heart, lungs, liver, kidneys, stomach, and intestines) were collected at E14, E17, E19, and E20. Hypoxia reduced embryonic body weight (BW). At any given age, the specific weight (organ weight/BW) of some organs in Hx was higher, and that of others was lower, than in Nx. However, almost all differences disappeared when organ weights were compared as function of BW, rather than at fixed chronological ages. The important exception was the chorioallantoic membrane (CAM), the mass of which in Hx developed out of proportion. In a third group of embryos, hypoxic until E14 and normoxic thereafter, there was no post-hypoxic catch-up growth, differently from what known to occur postnatally. A possible interpretation is that catch-up growth does not depend on the age of the embryo but on its BW. In conclusion, at least in the chicken embryo and for the level of hypoxia tested, hypoxia has no selective effects on the growth of specific organs, except for the CAM. Qualitative differences in the weight response to hypoxia among organs observed at any given age can be explained largely by the effects of the blunted growth on the growth trajectory of the individual organs.

Physiological consequences of intrauterine insults

Fetal lung development occurs as a complicated series of interactions between the different cell types in the lung in response to different growth factors and hormones. At birth, the human lung is in the stage of alveolar development in which the gas exchange units (alveoli) are being actively formed. The alveolar growth continues into postnatal life. Different intrauterine insults perturb this sequence of lung development in different ways. The ultimate result of aberrant lung development depends on the type of intrauterine insult, the severity, the duration of the insult and the developmental stage at which the insult occurs. This review article focuses on the common intrauterine insults encountered in clinical practice, such as infections, disorders of amniotic fluid volume, nutrition and maternal smoking. The information derived from clinical studies is juxtaposed with data from animal experiments to discuss the physiological consequences of intrauterine insults on fetal lung growth.

The pulmonary paradox in premature infants: in-utero infected lungs do better than those with accelerated maturation

AIMS

To document, and explain, the pulmonary paradox whereby despite relative lung immaturity, preterm infants exposed to amniotic infection (AI) have better postnatal pulmonary function than those exposed to preeclampsia (PE).

METHODS

Lung maturation was characterized in 65 preterm perinatal deaths [AI (n=40) and PE (n=25)] and postnatal respiratory function in 100 preterm survivors [AI (n=50) and PE (n=50)].

RESULTS

At autopsy, lung architecture was in advance of gestational age in 5% of AI infants versus 40% of PE infants (P<0.001). In survivors, the groups were similar in age and Apgar scores. At birth, 40% of the AI group required continuous positive airway pressure or mechanical ventilation versus 24% of the PE group (NS). However, 24 hours later, only 1 AI infant had deteriorated compared to 40% of PE infants (P<0.05).

CONCLUSIONS

Accelerated morphologic lung maturation in preterm PE infants does not translate into improved postnatal respiratory function. Most likely, this is due to a relative lack of surfactant, ascribable to low stimulant cytokine and high TNF-alpha levels. An intrauterine history supplemented by an antenatal cytokine profile could identify an increased exogeneous surfactant need in preterm infants exposed to PE.

Invited review: Physiological and pathophysiological responses to intermittent hypoxia

This mini-review summarizes the physiological adaptations to and pathophysiological consequences of intermittent hypoxia with special emphasis given to the pathophysiology associated with obstructive sleep apnea. Intermittent hypoxia is an effective stimulus for evoking the respiratory, cardiovascular, and metabolic adaptations normally associated with continuous chronic hypoxia. These adaptations are thought by some to be beneficial in that they may provide protection against disease as well as improve exercise performance in athletes. The long-term consequences of chronic intermittent hypoxia may have detrimental effects, including hypertension, cerebral and coronary vascular problems, developmental and neurocognitive deficits, and neurodegeneration due to the cumulative effects of persistent bouts of hypoxia. Emphasis is placed on reviewing the available data on intermittent hypoxia, making extensions from applicable information from acute and chronic hypoxia studies, and pointing out major gaps in information linking the genomic and cellular responses to intermittent hypoxia with physiological or pathophysiological responses.

Brief, intermittent hypoxia restricts fetal growth in Sprague-Dawley rats

This study was conducted to determine whether brief, intermittent exposure to hypoxia with little change in nutrient intake would affect fetal growth. Pregnant rats were exposed to 1 or 2 h of hypoxia (FiO2 = 0.09-0.095) from days 15 to 19 of gestation. Exposure to 1 h of hypoxia decreased fetal body weight and length, liver weight and increased the brain/liver weight ratio (p < 0.05) as compared to controls. Two hours of hypoxia decreased fetal body weight and length, and heart, lung, kidney, gut, brain and liver weights (p < 0.01), but did not affect the brain/liver weight ratio. Two hours of hypoxia decreased maternal food intake and weight gain (p < 0.05), but fetal growth was not significantly altered in pair-fed controls. These data demonstrate that brief, intermittent periods of intrauterine hypoxia have significant effects on fetal growth.

Lung morphometric changes after exposure to hypobaria and/or hypoxia and undernutrition

Lung morphometry was studied in rats between 4 and 7 weeks of age. The animals were divided into 5 groups: general controls (fed ad libitum), hypobaric normoxic, normobaric hypoxic, hypobaric hypoxic, and weight-matched controls (weight matched to the hypobaric hypoxic group). In both hypobaric and normobaric hypoxia, lung volume, alveolar surface area and total alveolar number increased compared to weight-matched controls. In normobaric hypoxia, mean linear intercept, mean chord length of alveoli increased and number of alveoli/unit volume decreased compared to weight-matched animals. In hypobaric hypoxia, only mean chord length increased. Dysanaptic index decreased in both. In hypobaric normoxia, alveolar size and lung volume diminished compared to general controls. Lung growth was impaired in weight-matched controls without affecting airspace dimensions. Hypobaric and normobaric hypoxia increase lung growth overcoming nutritional effects but is dysanaptic. Lung growth in hypobaric hypoxia is mainly determined by low oxygen but low pressure may also produce subtle structural alterations.

Time course of lung growth following exposure to hypobaria and/or hypoxia in rats

Four-week-old rats were divided into five groups: general controls, weight-matched controls (weight matched to hypobaric hypoxia), hypobaric hypoxia, normobaric hypoxia, and hypobaric normoxia. Lung growth impairment in weight-matched animals occurred by reduction in cell number and size. In both hypoxic groups, lung weight, RNA and protein were significantly higher on day 3, and DNA on day 5 and remained higher thereafter. Maximum 3H-TdR incorporation occurred on day 3 in both hypoxic groups. Hypoxia increased RNA/DNA ratio on day 1 and protein/DNA on day 3. Following 3 days of recovery, DNA synthesis and RNA/DNA ratio of hypoxic groups and controls were identical. DNA synthesis also doubled on day 5 in hypobaric normoxia compared to general controls. Hypoxia up regulates lung growth despite down regulation by undernutrition. Maximum lung growth stimulation occurs during early exposure by cellular hypertrophy followed by hyperplasia. Low pressure by itself also stimulates lung growth. Cellular activity returns immediately to normal levels after removal of hypoxic stimulus.

Southwestern Internal Medicine Conference: pulmonary complications of high-altitude exposure

The primary physiologic disturbance at high altitude is hypoxemia, which leads to a cascade of secondary changes in each step of the oxygen-transport chain. The author, in this review, focuses on the alterations in ventilatory control and alveolar-capillary gas exchange at high altitude and discusses the clinical pulmonary complications associated with these alterations, as well as their prevention and management.

Growth retardation and the development of the respiratory system in fetal sheep

In an experimental model of fetal growth retardation which involves the reduction of placental mass in ewes, we have investigated the effects of intrauterine deprivation on aspects of structural development of the trachea and lungs of fetal sheep (140 days gestation). We have also measured the volume of luminal liquid aspirated from the lungs and the phospholipid content of this liquid as an index of pulmonary surfactant production. The effects of growth retardation are evident in the trachea where the structural development of the mucosal and submucosal layers has been affected. Abnormal aspects of development include the frequent lack of a ciliated border on epithelial cells in the mucosal layer and the reduction in the extent of the folds usually characteristic of this layer in near term fetal sheep. Although the fetal lungs are smaller in growth retardation (P less than 0.01) they are appropriate for fetal weight and their structural development does not appear to have been retarded. In contrast, lung liquid volume is significantly reduced in relation to lung weight in growth retarded fetuses and the concentration of phospholipids in lung liquids is also reduced (P less than 0.01).

Role of hyperventilation in hypoxia on lung growth in rats

This study was conducted in an attempt to differentiate the contribution of hyperventilation, if any, from that of low PO2 on adaptive lung growth in response to hypoxia. Male albino rats were exposed to one of the following: (1) Room air for 7 days, as control; (2) 10% O2 in N2 for 7 days; (3) 10% O2 for 6 h, 1 day or 2 days and air for the remaining of 7 days; (4) 10% O2 for 2 days and 7% CO2 in air for 5 days; (5) air for 2 days and 7% CO2 for 5 days; or (6) 7% CO2 in air for 7 days. Lung growth was assessed by measuring the lung weight, lung air volume, lung DNA content and rate of DNA synthesis in lung explants. Hypoxia stimulated lung DNA synthesis even when administered for only 6 h, and the effects persisted for a few days after discontinuation of hypoxia. Hypercapnia did not stimulate DNA synthesis in lung. In 2 day hypoxic 5 day air rats the lung weight and lung DNA content increased, in 2 day air 5 day hypercapnic rats only the lung volume increased, and in 2 day hypoxic 5 day hypercapnic rats all parameters of lung growth, i.e., lung weight, DNA content and air volume increased as in 7 day hypoxic rats. The results suggest that adaptive or compensatory lung growth in hypoxia is brought about on one hand by the direct effect of low PO2 on lung cells, resulting in lung hyperplasia, and on the other hand by the mechanical stimulation of lung tissue by hyperventilation, causing lung distension.

Fetal lung growth: influence of maternal exposure to cold and exercise in rats

The consequences of maternal exposure to low ambient temperature and exercise on maternal and fetal lung growth and in particular on the relationship between the three gas exchange organs (lungs and placenta) were studied in albino rats. Pregnant rats were subjected to 10 degrees C ambient temperature or to daily 10 min swimming exercise beginning at day 3 or day 11 of pregnancy till day 21 when they were sacrificed. Maternal lung growth was assessed by measuring the lung weight, lung air volume and lung DNA content, and the fetal lung growth by lung DNA content. Comparisons were made in rats with litter sizes of 9-14. The major findings were as follows. Cold increased: (1) the maternal lung, liver, kidney and heart size, and fetal body weight, in both groups, but to a greater degree in rats exposed to cold at day 3; and (2) fetal lung DNA content in rats subjected to cold at late gestation. It abolished the relationship between maternal and fetal lung DNA content which exists in large litter size pregnancies. Exercise did not enlarge the maternal lungs; it decreased the placental weight and fetal lung DNA content and abolished the relationship between maternal and fetal lung DNA content in rats subjected to exercise at early gestation. Neither cold nor exercise had an effect on fetal lung maturation. It is postulated that reduction in fetal lung DNA content with maternal exercise may result from the effects of hypoxemia which may be the consequence of reduced uterine blood flow; and that abolition of normally existing direct relationship between maternal and fetal lung DNA content may be the outcome of the effects of alterations in metabolic and endocrine functions, in both the mother and the fetus, in response to cold temperature and exercise, offsetting the influence the growing maternal lung may have on fetal lung growth.