Development of the acinus in the human lung

Nitric oxide regulation of fetal and newborn lung development and function

In the developing lung, nitric oxide (NO) and cyclic guanosine monophosphate (cGMP) signaling are essential in regulating lung formation and vascular tone. Animal studies have linked many anatomical and pathophysiological features of newborn lung disease to abnormalities in the NO/cGMP signaling system. They have demonstrated that driving this system with agonists and antagonists alleviates many of them. This research has spurred the rapid clinical development, testing, and application of several NO/cGMP-targeting therapies with the hope of treating and potentially preventing significant pediatric lung diseases. However, there are instances when the therapeutic effectiveness of these agents is limited. Studies indicate that injury-induced disruption of several critical components within the signaling system may hinder the promise of some of these therapies. Recent research has identified basic mechanisms that suppress NO/cGMP signaling in the injured newborn lung. They have also pinpointed biomarkers that offer insight into the activation of these pathogenic mechanisms and their influence on the NO/cGMP signaling system's integrity in vivo. Together, these will guide the development of new therapies to protect NO/cGMP signaling and safeguard newborn lung development and function. This review summarizes the important role of the NO/cGMP signaling system in regulating pulmonary development and function and our evolving understanding of how it is disrupted by newborn lung injury.

Mutual Associations of Exposure to Ambient Air Pollutants in the First 1000 Days of Life With Asthma/Wheezing in Children: Prospective Cohort Study in Guangzhou, China

BACKGROUND

The first 1000 days of life, encompassing pregnancy and the first 2 years after birth, represent a critical period for human health development. Despite this significance, there has been limited research into the associations between mixed exposure to air pollutants during this period and the development of asthma/wheezing in children. Furthermore, the finer sensitivity window of exposure during this crucial developmental phase remains unclear.

OBJECTIVE

This study aims to assess the relationships between prenatal and postnatal exposures to various ambient air pollutants (particulate matter 2.5 [PM2.5], carbon monoxide [CO], sulfur dioxide [SO2], nitrogen dioxide [NO2], and ozone [O3]) and the incidence of childhood asthma/wheezing. In addition, we aimed to pinpoint the potential sensitivity window during which air pollution exerts its effects.

METHODS

We conducted a prospective birth cohort study wherein pregnant women were recruited during early pregnancy and followed up along with their children. Information regarding maternal and child characteristics was collected through questionnaires during each round of investigation. Diagnosis of asthma/wheezing was obtained from children's medical records. In addition, maternal and child exposures to air pollutants (PM2.5 CO, SO2, NO2, and O3) were evaluated using a spatiotemporal land use regression model. To estimate the mutual associations of exposure to mixed air pollutants with the risk of asthma/wheezing in children, we used the quantile g-computation model.

RESULTS

In our study cohort of 3725 children, 392 (10.52%) were diagnosed with asthma/wheezing. After the follow-up period, the mean age of the children was 3.2 (SD 0.8) years, and a total of 14,982 person-years were successfully followed up for all study participants. We found that each quartile increase in exposure to mixed air pollutants (PM2.5, CO, SO2, NO2, and O3) during the second trimester of pregnancy was associated with an adjusted hazard ratio (HR) of 1.24 (95% CI 1.04-1.47). Notably, CO made the largest positive contribution (64.28%) to the mutual effect. After categorizing the exposure according to the embryonic respiratory development stages, we observed that each additional quartile of mixed exposure to air pollutants during the pseudoglandular and canalicular stages was associated with HRs of 1.24 (95% CI 1.03-1.51) and 1.23 (95% CI 1.01-1.51), respectively. Moreover, for the first year and first 2 years after birth, each quartile increment of exposure to mixed air pollutants was associated with HRs of 1.65 (95% CI 1.30-2.10) and 2.53 (95% CI 2.16-2.97), respectively. Notably, SO2 made the largest positive contribution in both phases, accounting for 50.30% and 74.70% of the association, respectively.

CONCLUSIONS

Exposure to elevated levels of mixed air pollutants during the first 1000 days of life appears to elevate the risk of childhood asthma/wheezing. Specifically, the second trimester, especially during the pseudoglandular and canalicular stages, and the initial 2 years after birth emerge as crucial susceptibility windows.

TRIAL REGISTRATION

Chinese Clinical Trial Registry ChiCTR-ROC-17013496; https://tinyurl.com/2ctufw8n.

Atelectasis and lung recruitment in pediatric anesthesia: An educational review

General anesthesia is associated with development of pulmonary atelectasis. Children are more vulnerable to the development and adverse effects of atelectasis. We review the physiology and risk factors for the development of atelectasis in pediatric patients under general anesthesia. We discuss the clinical significance of atelectasis, the use and value of recruitment maneuvers, and other techniques available to minimize lung collapse. This review demonstrates the value of a recruitment maneuver, maintaining positive end-expiratory pressure (PEEP) until extubation and lowering FiO₂ where possible in the daily practice of the pediatric anesthetist.

Developmental respiratory physiology

Various developmental aspects of respiratory physiology put infants and young children at an increased risk of respiratory failure, which is associated with a higher rate of critical incidents during anesthesia. The immaturity of control of breathing in infants is reflected by prolonged central apneas and periodic breathing, and an increased risk of apneas after anesthesia. The physiology of the pediatric upper and lower airways is characterized by a higher flow resistance and airway collapsibility. The increased chest wall compliance and reduced gas exchange surface of the lungs reduce the pulmonary oxygen reserve vis-à-vis a higher metabolic oxygen demand, which causes more rapid oxygen desaturation when ventilation is compromised. This review describes the various developmental aspects of respiratory physiology and summarizes anesthetic implications.

Onset of alveolar recirculation in the developing lungs and its consequence on nanoparticle deposition in the pulmonary acinus

The structure of the gas exchange region of the human lung (the pulmonary acinus) undergoes profound change in the first few years of life. In this paper, we investigate numerically how the change in alveolar shape with time affects the rate of nanoparticle deposition deep in the lung during postnatal development. As human infant data is unavailable, we use a rat model of lung development. The process of postnatal lung development in the rat is remarkably similar to that of the human, and the structure of the rat acinus is indistinguishable from that of the human acinus. The current numerical predictions support our group's recent in vivo findings, which were also obtained by using growing rat lung models, that nanoparticle deposition in infants is strongly affected by the change in the structure of the pulmonary acinus. In humans, this major structural change occurs over the first 2 yr of life. Our current predictions would suggest that human infants at the age of ∼ 2 yr might be most at risk to the harmful effects of air pollution. Our results also suggest that dose estimates for inhalation therapies using nanoparticles, based on fully developed adult lungs with simple body weight scaling, are likely to overestimate deposition by up to 55% for newborns and underestimate deposition by up to 17% for 2-yr-old infants.

Lung development

The development of the human lung starts at 4 weeks of gestation with the appearance of the tracheal outgrowth from the foregut and continues into early childhood. Survival at birth is dependent on adequate development and maturation of the lung in utero. Abnormal bronchopulmonary development results in congenital lung malformations, and inadequate development is thought to contribute to bronchopulmonary dysplasia. Complex processes and factors influencing lung development are beginning to be elucidated, and further knowledge will hopefully lead to improved interventions to enhance outcomes in vulnerable or affected infants.

Epithelial-mesenchymal co-culture model for studying alveolar morphogenesis

Division of large, immature alveolar structures into smaller, more numerous alveoli increases the surface area available for gas exchange. Alveolar division requires precise epithelial-mesenchymal interactions. However, few experimental models exist for studying how these cell-cell interactions produce changes in 3-dimensional structure. Here we report an epithelial-mesenchymal cell co-culture model where 3-dimensional peaks form with similar cellular orientation as alveolar structures in vivo. Co-culturing fetal mouse lung mesenchyme with A549 epithelial cells produced tall peaks of cells covered by epithelia with cores of mesenchymal cells. These structures did not form when using adult lung fibroblasts. Peak formation did not require localized areas of cell proliferation or apoptosis. Mesenchymal cells co-cultured with epithelia adopted an elongated cell morphology closely resembling myofibroblasts within alveolar septa in vivo. Because inflammation inhibits alveolar formation, we tested the effects of E. coli lipopolysaccharide on 3-dimensional peak formation. Confocal and time-lapse imaging demonstrated that lipopolysaccharide reduced mesenchymal cell migration, resulting in fewer, shorter peaks with mesenchymal cells present predominantly at the base. This epithelial-mesenchymal co-culture model may therefore prove useful in future studies of mechanisms regulating alveolar morphogenesis.

Reduced platelet-derived growth factor receptor expression is a primary feature of human bronchopulmonary dysplasia

Animal studies have shown that platelet-derived growth factor (PDGF) signaling is required for normal alveolarization. Changes in PDGF receptor (PDGFR) expression in infants with bronchopulmonary dysplasia (BPD), a disease of hypoalveolarization, have not been examined. We hypothesized that PDGFR expression is reduced in neonatal lung mesenchymal stromal cells (MSCs) from infants who develop BPD. MSCs from tracheal aspirates of premature infants requiring mechanical ventilation in the first week of life were studied. MSC migration was assessed in a Boyden chamber. Human lung tissue was obtained from the University of Rochester Neonatal Lung Biorepository. Neonatal mice were exposed to air or 75% oxygen for 14 days. PDGFR expression was quantified by qPCR, immunoblotting, and stereology. MSCs were isolated from 25 neonates (mean gestational age 27.7 wk); 13 developed BPD and 12 did not. MSCs from infants who develop BPD showed lower PDGFR-α and PDGFR-β mRNA and protein expression and decreased migration to PDGF isoforms. Lungs from infants dying with BPD show thickened alveolar walls and paucity of PDGFR-α-positive cells in the dysmorphic alveolar septa. Similarly, lungs from hyperoxia-exposed neonatal mice showed lower expression of PDGFR-α and PDGFR-β, with significant reductions in the volume of PDGFR-α-positive alveolar tips. In conclusion, MSCs from infants who develop BPD hold stable alterations in PDGFR gene expression that favor hypoalveolarization. These data demonstrate that defective PDGFR signaling is a primary feature of human BPD.

Antenatal hypoxia and pulmonary vascular function and remodeling

This review provides evidence that antenatal hypoxia, which represents a significant and worldwide problem, causes prenatal programming of the lung. A general overview of lung development is provided along with some background regarding transcriptional and signaling systems of the lung. The review illustrates that antenatal hypoxic stress can induce a continuum of responses depending on the species examined. Fetuses and newborns of certain species and specific human populations are well acclimated to antenatal hypoxia. However, antenatal hypoxia causes pulmonary vascular disease in fetuses and newborns of most mammalian species and humans. Disease can range from mild pulmonary hypertension, to severe vascular remodeling and dangerous elevations in pressure. The timing, length, and magnitude of the intrauterine hypoxic stress are important to disease development, however there is also a genetic-environmental relationship that is not yet completely understood. Determining the origins of pulmonary vascular remodeling and pulmonary hypertension and their associated effects is a challenging task, but is necessary in order to develop targeted therapies for pulmonary hypertension in the newborn due to antenatal hypoxia that can both treat the symptoms and curtail or reverse disease progression.

Suppression of inflammatory cell trafficking and alveolar simplification by the heme oxygenase-1 product carbon monoxide

Bronchopulmonary dysplasia (BPD), a lung disease of prematurely born infants, is characterized in part by arrested development of pulmonary alveolae. We hypothesized that heme oxygenase (HO-1) and its byproduct carbon monoxide (CO), which are thought to be cytoprotective against redox stress, mitigate lung injury and alveolar simplification in hyperoxia-exposed neonatal mice, a model of BPD. Three-day-old C57BL/6J mice were exposed to air or hyperoxia (FiO2, 75%) in the presence or absence of inhaled CO (250 ppm for 1 h twice daily) for 21 days. Hyperoxic exposure increased mean linear intercept, a measure of alveolar simplification, whereas CO treatment attenuated hypoalveolarization, yielding a normal-appearing lung. Conversely, HO-1-null mice showed exaggerated hyperoxia-induced hypoalveolarization. CO also inhibited hyperoxia-induced pulmonary accumulation of F4/80+, CD11c+, and CD11b+ monocytes and Gr-1+ neutrophils. Furthermore, CO attenuated lung mRNA and protein expression of proinflammatory cytokines, including the monocyte chemoattractant CCL2 in vivo, and decreased hyperoxia-induced type I alveolar epithelial cell CCL2 production in vitro. Hyperoxia-exposed CCL2-null mice, like CO-treated mice, showed attenuated alveolar simplification and lung infiltration of CD11b+ monocytes, consistent with the notion that CO blocks lung epithelial cell cytokine production. We conclude that, in hyperoxia-exposed neonatal mice, inhalation of CO suppresses inflammation and alveolar simplification.

The neonatal lung--physiology and ventilation

This review article focuses on neonatal respiratory physiology, mechanical ventilation of the neonate and changes induced by anesthesia and surgery. Optimal ventilation techniques for preterm and term neonates are discussed. In summary, neonates are at high risk for respiratory complications during anesthesia, which can be explained by their characteristic respiratory physiology. Especially the delicate balance between closing volume and functional residual capacity can be easily disturbed by anesthetic and surgical interventions resulting in respiratory deterioration. Ventilatory strategies should ideally include application of an 'open lung strategy' as well avoidance of inappropriately high VT and excessive oxygen administration. In critically ill and unstable neonates, for example, extremely low-birthweight infants surgery in the neonatal intensive care unit might be an appropriate alternative to the operating theater. Best respiratory management of neonates during anesthesia is a team effort that should involve a joint multidisciplinary approach of anesthetists, pediatric surgeons, cardiologists, and neonatologists to reduce complications and optimize outcomes in this vulnerable population.

Structure and composition of pulmonary arteries, capillaries, and veins

The pulmonary vasculature comprises three anatomic compartments connected in series: the arterial tree, an extensive capillary bed, and the venular tree. Although, in general, this vasculature is thin-walled, structure is nonetheless complex. Contributions to structure (and thus potentially to function) from cells other than endothelial and smooth muscle cells as well as those from the extracellular matrix should be considered. This review is multifaceted, bringing together information regarding (i) classification of pulmonary vessels, (ii) branching geometry in the pulmonary vascular tree, (iii) a quantitative view of structure based on morphometry of the vascular wall, (iv) the relationship of nerves, a variety of interstitial cells, matrix proteins, and striated myocytes to smooth muscle and endothelium in the vascular wall, (v) heterogeneity within cell populations and between vascular compartments, (vi) homo- and heterotypic cell-cell junctional complexes, and (vii) the relation of the pulmonary vasculature to that of airways. These issues for pulmonary vascular structure are compared, when data is available, across species from human to mouse and shrew. Data from studies utilizing vascular casting, light and electron microscopy, as well as models developed from those data, are discussed. Finally, the need for rigorous quantitative approaches to study of vascular structure in lung is highlighted.

Normal development of the lung and premature birth

The following review focuses on the normal development of the lung from conception to birth. The defined periods of lung development-Embryonic, Pseudoglandular, Canalicular, Saccular and Alveolar-will be explored in detail in relation to gestational age. Cellular differentiation, formation of the conducting airways and respiratory zone and development of the alveoli will be reviewed. Pulmonary vascular development will also be examined within these periods to relate the formation of the blood-air barrier to the lungs for their essential function of gas exchange after birth. The development of the surfactant and cortisol systems will also be discussed as these need to be mature before the lungs are able to take on their role of respiration following birth. It is clear that premature birth interrupts normal lung development so the effect of preterm birth on lung development will be examined and the respiratory consequences of very preterm birth will be briefly explored.

Early amniocentesis

Although amniocentesis was first reported in 1882, the technique was not in widespread clinical practice until the 1970's. The reason for this slow uptake was that there were few indications for performing the procedure until fetal karyotyping from amniotic fluid cells became possible in 1966. Currently fetal karyotyping is the commonest indication for the technique and amniocentesis has become the mainstay of antenatal diagnosis.

Airway geometry models of children's lungs for use in dosimetry modeling

Single-path whole-lung and lobar models of the lungs of 11 children between 3 mo and 21 yr of age were developed based on a combination of cast data and published information on distal airway dimensions. The cast data used to generate these models were taken from one of the largest databases of actual measurements in children. The methods used to develop the children's models were based on techniques that have been used to develop adult single-path airway geometry models. Model dimensions for the conducting airways, as well as the estimated dead space, for all children fell within the range of the limited published information. Thus, the method for estimating airway dimensions in adults may be successfully applied to develop estimates of airway dimensions in children. The predicted total lung capacity (TLC) for the older children (aged 8 to 21 yr) fell within or near the range arising from published scaling equations. The assumptions used to generate the gas exchange region for children 8 yr and older produced results that were reasonably consistent with available physiological data. However, these assumptions do not result in a physiologically consistent gas exchange region for children 3 yr of age and younger; also, to maintain physiologically reasonable relationships between dead space and alveolar volume, the models for children 3 yr of age and younger resulted in predicted TLCs well below those predicted using published scaling equations. These discrepancies may be reflective of dysanaptic growth, in which the alveolar region is growing more rapidly than the airways. The results for children 3 yr of age and under suggest the need for a greater understanding of lung development during this critical period. This is particularly important considering the increasing evidence that exposure to pollutants and other toxicants and allergens during the first 2 yr of life may have long-term consequences on respiratory disease outcomes. Our results suggest that the geometry model airway dimensions for all ages are appropriate for use with dosimetry models, but dosimetry modelers need to assess carefully the reasonableness of TLC and functional residual capacity volumes to which airway dimensions are scaled for children 3 yr of age and under.

Modeling approaches for estimating the dosimetry of inhaled toxicants in children

Risk assessment of inhaled toxicants has typically focused upon adults, with modeling used to extrapolate dosimetry and risks from lab animals to humans. However, behavioral factors such as time spent playing outdoors may lead to more exposure to inhaled toxicants in children. Depending on the inhaled agent and the age and size of the child, children may receive a greater internal dose than adults because of greater ventilation rate per body weight or lung surface area, or metabolic differences may result in different tissue burdens. Thus, modeling techniques need to be adapted to children in order to estimate inhaled dose and risk in this potentially susceptible life stage. This paper summarizes a series of inhalation dosimetry presentations from the U.S. EPA's Workshop on Inhalation Risk Assessment in Children held on June 8-9, 2006 in Washington, DC. These presentations demonstrate how existing default models for particles and gases may be adapted for children, and how more advanced modeling of toxicant deposition and interaction in respiratory airways takes into account children's anatomy and physiology. These modeling efforts identify child-adult dosimetry differences in respiratory tract regions that may have implications for children's vulnerability to inhaled toxicants. A decision framework is discussed that considers these different approaches and modeling structures including assessment of parameter values, supporting data, reliability, and selection of dose metrics.

Airway pressure release ventilation and pediatrics: theory and practice

Airway pressure release ventilation (APRV) facilitates oxygenation and ventilation by maintaining an elevated baseline airway pressure similar to continuous positive airway pressure (CPAP). APRV differs from CPAP only by the addition of regular, brief release of airway pressure to facilitate carbon dioxide removal. The baseline pressure maintains a near continuous airway pressure to facilitate recruitment, improving ventilation and oxygenation. Pediatric patients who have recruitable lung disease may be well suited to the application of APRV.

The fetal lung. 1: Developmental aspects

A literature survey is presented on normal fetal development during the embryonic, pseudoglandular, canalicular, saccular and alveolar stages in the human fetus. Normal anatomical and physiological aspects of fetal lung development including the fetal pulmonary circulation are described. Factors which may influence fetal lung growth and consequently may play a role in the development of pulmonary hypoplasia are discussed, such as intrauterine and intrathoracic space, lung fluid, fetal breathing movements, normal balance of volume and pressure in the lung and interference with the blood supply.

Physiology and pathophysiology of lung development

Over the past few years, there has been a considerable improvement in our understanding of the normal development of the fetal lung and its regulation. These advances have occurred mostly through increased knowledge of molecular biological mechanisms of growth and differentiation. These advances have also resulted in an improvement in our comprehension of the pathological basis of various pulmonary diseases. As a result of this new and improved knowledge, new and innovative therapeutic modalities are being introduced into clinical practice. The introduction of surfactant therapy into the clinical setting was one such milestone in neonatal respiratory management. Pulmonary surfactant is responsible for stabilising alveoli during normal respiration, thereby preventing atelectasis or alveolar flooding. Disease processes which result in an insufficiency in surfactant, such as respiratory distress syndrome (RDS) or congenital diaphragmatic hernia (CDH), generally carry a very high mortality rate. Exogenous surfactant administration reduces both the mortality and morbidity associated with RDS and its sequelae, and its efficacy in the treatment of CDH is now being evaluated clinically. Moreover, laboratory studies suggest that surfactant therapy may be used in combination with other treatments, such as tracheal occlusion to promote lung growth in CDH, in order to achieve a maximal effect in these complex, multifactorial lung diseases.

Intraalveolar bubbles and bubble films. I. formation and development during the first 48 hours of extrauterine life in rabbits

BACKGROUND

Aeration of mature lungs at birth depends on formation of intraalveolar bubbles and bubble films (Scarpelli 1978. Pediatr. Res., 12:1070-1076). Bubbles establish immediately structural stability and pulmonary gas exchange. Given that air spaces are cleared in minutes of fetal liquid (the initial substrate for bubble formation), in formation possible beyond this period? If so, is this related to early development of pulmonary function and structure?

METHODS

Mature, spontaneously breathing rabbit pups at 1-10 min and 1, 3, 8, 24, and 48 h after vaginal birth were anesthetized, trachea was occluded at "resting volume" (approximately functional residual capacity), and lungs were rapidly exposed to preserve in vivo intrapulmonary status. The entire lung was examined by stereomicroscopy. Other determinations included resting volume, lung wet weight, volume-pressure curves, histological sections, lung dry weight, tissue hydroxyproline (OH-Pro), and lavage phospholipids (PL). Bubble mobility in situ was tested. Bubbles were released into bathing liquid by incision of peripheral units and monitored over time.

RESULTS

Pup activity and gross appearance of the lungs, together with septal thinning, secondary septal development, clearance of intraluminal liquid, increasing tissue OH-Pro, and PL distribution indicated normal postnatal development. Each aerated unit examined at resting volume (all lobes, all ages) contained intraalveolar bubbles. Transition to free gas exclusively in conducting airways and bubbles/bubble films in peripheral gas exchange units occurred within 1 h. Bubbles appeared to be exclusively within alveoli at 4 h and thereafter. Bubbles persisted and new bubbles were formed during subsequent inflation to maximal volume and deflation to atmospheric pressure (P0). Volume of intact lungs at P0 was maintained by the counterforce of rigid bubble films against tissue retraction. When bubbles were released either at resting volume or at P0, the bubble-free loci became airless. Constant size and stability of released bubbles support preferential incorporation of surfactants into bubble films and constant "near-zero surface tension" (Scarpelli 1978. Pediatr. Res., 12:1070-1076).

CONCLUSIONS

We show the ubiquitous presence of intraalveolar bubbles and bubble films in vivo throughout the first 48 h of postnatal life. Bubble film rigidity sustains aeration and prevents collapse, while low surface tension of the films facilitate liquid removal from the air spaces. Bubbles in situ are stable and, within apparent limits, mobile; after birth they are quickly restricted to the alveolar spaces, leaving airways bubble free.

Differential expression and immunohistochemical localisation of the phenol and hydroxysteroid sulphotransferase enzyme families in the developing lung

Reversible sulphation, catalysed by sulphotransferases and sulphatases, of biologically active compounds such as androgens and oestrogens is a sensitive mechanism for regulating their bioavailability, and we have previously hypothesised that this process plays a significant role in the regulation of human fetal lung development. Sulphation is also a major detoxification reaction, contributing significantly to the body's chemical defence mechanism. We have used qualitative and semiquantitative immunological studies to determine the temporal expression and localisation of phenol and hydroxysteroid sulphotransferases during human lung development. Our results show that in the early fetal lung, phenol sulphotransferase expression is at its highest, and is most widely distributed throughout the developing respiratory epithelium. With later development, expression levels decrease and become predominantly restricted to the more proximal airways. In contrast, hydroxysteroid sulphotransferase is present only at very low levels in the early-gestation lung but expression increases rapidly through gestation to reach an apparent peak by 1 year postnatal age. The proximal-to-distal gradients of phenol and hydroxysteroid sulphotransferase expression were similar in mature respiratory epithelium, with immunoreactivity in ciliated cells, non-ciliated secretory cells and basal cells, but with no apparent expression in mucus-secreting cells. These studies provide supporting evidence for the hypothesis that hydroxysteroid sulphotransferase, an androgen-inactivating enzyme, contributes to the role of androgens in retarding the maturation of human lung in utero.

In utero gene transfer into the pulmonary epithelium

In early gestation the internal surface of the lung is structurally simple and an ideal target for somatic gene transfer. The transfer of genes into the growing lung would be particularly useful in the prenatal correction of cystic fibrosis, which has devastating pulmonary complications. In addition, in utero gene therapy has the potential to immunotolerize the individual, and thereby to avoid the immune reactions now seen with the current generation of adenoviral vectors. We injected a replication-defective adenoviral vector containing the lacZ reporter gene (Ad5.CMVlacZ) into the amniotic fluid of rat pups on the 16th day of gestation. At 16 days of gestation, rat lungs are equivalent in maturity to those of a 22-week human fetus as their airways are lined with undifferentiated multipotential stem cells. The pups showed high-level reporter gene expression in their airways a week following birth (13 days following infection). The expression was maintained during a time when the lung volume increased approximately 20-fold, alveolarization occurred, and the epithelial cells differentiated. These data establish gene targeting of undifferentiated fetal cells as an effective means of gene therapy.

Development of human fetal lung in organ culture compared with in utero ontogeny

In utero, at around 23 wk gestation, the progenitor epithelium of distal airway differentiates into type I and type II pneumatocytes. Human fetal lung organ cultures, as early as 12 wk gestation, have the competence to self-differentiate. Distal airway epithelial immunoreactivity to cytokeratins CK 7, 8, and 18 decreases with differentiation both in utero and in organ culture, whereas reactivity to epithelial membrane antigen remains constant in both. As distal airways dilate, the mean percentage airspace of fetal lungs in organ culture increases to 58%, equivalent to lung of gestation 26.0 +/- 7.3 wk. In organ culture, capillary blood vessels, visualized by vimentin immunoreactivity, remodel and more closely approximate the epithelium but without direct invasion. In utero, at 23 wk gestation, elastin appears as condensation around airways and forms a basis for secondary crests which, by 29 wk gestation, evolve into alveolar septae. In organ culture, no elastin is deposited, no secondary or alveolar crests form, and the lung retains a simple saccular structure. Differentiation of the terminal airway epithelium and mesodermal maturational events to facilitate gas exchange, such as capillary invasion or secondary-alveolar crest formation, are almost synchronous in human lung in utero but clearly dissociate in organ culture.

Emergency radiology of the pediatric chest

This monograph will present an overview of pediatric emergency room chest radiology. Technique, interpretative approach, normal radiologic appearances, common normal variants, and unique features of the pediatric lung are discussed. The radiologic features of common pediatric chest emergencies (infection, airway foreign body, asthma, hydrocarbon aspiration, near-drowning pneumothorax, trauma, hemosiderosis and upper airway obstruction) are described and illustrated.

Relationship between infant lung mechanics and childhood lung function in children of very low birthweight

Twenty-seven children of very low birthweight (less than or equal to 1,500 g) whose lung function had been measured on several occasions during the first year were studied at the age of about 9 years. Fifteen of the children had received neonatal intermittent positive pressure ventilation, mostly for respiratory distress syndrome. Ten of the ventilated children were still oxygen dependent at 30 days of age. Compared to the remainder of the group, mechanically ventilated children had reduced lung compliance in early infancy and increased thoracic gas volume in the middle of their first year. These changes correlated with the level of neonatal respiratory therapy as indicated by the oxygen score. Lung compliance in early infancy, but not thoracic gas volume, correlated with forced expiratory volume at 1 second recorded at 9 years. On the other hand, reduced airway conductance showed no significant correlation with the neonatal oxygen score, but there was a strong correlation between airway conductance late in infancy and lung function at 9 years. This relationship was independent of neonatal mechanical ventilation. We conclude that perinatal factors, which may be associated with disturbed lung mechanics early in infancy, are only weak and indirect predictors of childhood lung function. Airway conductance late in infancy, determined by constitutional factors, prematurity itself or other undetermined factors, is a good predictor of airway function at 9 years.

Postnatal growth of pulmonary acini and alveoli in normal and oxygen-exposed rats studied by serial section reconstructions

Three-dimensional reconstructions from serial sections were used to examine postnatal lung development of rats reared in air (control) or oxygen. From birth to age 21 days, control lung volume increased ninefold, and the average volume of each ventilatory unit (all airspaces distal to a single respiratory bronchiole) increased seven times. There were approximately 5,000 ventilatory units at birth and on day 21, indicating that the lung grew by enlargement and subdivision of ventilatory units and not by their multiplication. Growth in hyperoxia (greater than 97%) for 7 days had no effect on the number of ventilatory units but, compared to controls, total lung volume and ventilatory unit volume were reduced 32% and 16%, respectively. At birth there were 0.6 x 10(6) alveoli, and at age 7 days in controls alveolar number increased 16-fold while the average volume of a single alveolus fell to one-sixth that at birth. Exposure to hyperoxia for 7 days stopped alveolarization; the surface area to volume ratio (Sa/V) of the ventilatory unit was lower, alveolar number was the same as at birth, and the alveoli present were large. At age 21 days, after 14 days of recovery in air, lung volume and ventilatory unit volume were greater than in controls but the Sa/V of the ventilatory unit was still depressed 20%. Alveoli from oxygen-exposed lungs were larger than in controls, and a greater size distribution coefficient showed them to be more variable. A shape coefficient for alveoli did not change as a function of the animal's age or oxygen treatment; it demonstrated proportional growth of alveolar height and diameter.

The pulmonary vasculature in meconium aspiration

Some studies have suggested that pulmonary hypertension in the newborn with meconium aspiration can be attributed to a primary prenatal increase in pulmonary arterial musculature; but this concept has been controversial. To examine this question, we reviewed 62 infants autopsied at The Johns Hopkins Hospital, 24 of whom demonstrated meconium aspiration, 20 with meconium staining but no aspiration, and 18 with abruptio placentae without meconium aspiration or staining. Clinical and pathologic features were evaluated and cross-sectional arterial medial area was determined at the junction of the conducting and respiratory airways in nondistended lungs. No significant difference in arterial medial area was found between infants with meconium aspiration and those with meconium staining only or abruptio placentae. In addition, circumferentially muscularized intraacinar arteries were present in all infants with meconium aspiration and abruptio placentae, and all but one infant with meconium staining alone. Comparison of lungs with and without arterial injection and fixation in distention showed that injection does not uniformly distend vessels and that formalin distention may remove or mask meconium. The study suggests that meconium aspiration and its complications, not primary structural arterial changes, account for pulmonary hypertension in infants with meconium aspiration.

Morphogenesis of the respiratory bronchiole in rhesus monkey lungs

The epithelium of the respiratory bronchiole in the adult rhesus monkey consists of two populations: a pseudostratified epithelium with basal, mucous goblet, and ciliated cells located near the pulmonary artery (PA); and a simple cuboidal epithelium composed only of nonciliated bronchiolar epithelial (or Clara) cells in areas away from the PA. This study describes the pattern of differentiation of these two epithelial populations, and their relationship to the PA and to the time of appearance of alveoli in the respiratory bronchiole of the rhesus monkey during the period of 90-125 days gestational age (DGA). These events were related to changes in the adjacent parenchyma. Dissected airways of infusion-fixed, critical-point-dried lungs were evaluated by scanning microscopy followed by light microscopy of the same airways. At 54% of gestation (90 DGA), the distal airway was lined by a mixture of ciliated and nonciliated cells. By 67% of gestation (110 DGA), the ciliated cells were confined to the epithelium over the PA. The underlying connective tissue initially was cellular containing few fibers but was fibrous by 76% of gestation (125 DGA). Alveolarization began near the most distal cartilage at 57% of gestation (95 DGA), the same period at which secondary septation occurred in the distal acinus. Thus, alveolarization occurred simultaneously in two centers: 1) the proximal centriacinar region in the vicinity of the most distal cartilage and 2) the distal lung parenchyma. The duration of centriacinar alveolarization was short, approximately 5 days.

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

Three fetal lambs underwent phrenic nerve section between Days 99 and 104 of gestation, and 2 twins of the experimental animals underwent sham operation at the same time. When they were killed at 135 to 137 days of gestation, the experimental animals had lower specific lung weights (g/kg) and lung volumes (ml/kg) and had delayed lung development by subjective microscopy. Light microscopic morphometry showed significantly less volume proportion of potential gas-exchanging air spaces, less parenchyma, and more gas-exchanging wall. Scanning electron microscopy confirmed these findings and also showed that the transition zone between conducting and gas-exchanging areas was less sharp in the experimental animals, attributed to diminished alveolarization of distal conducting airways. Transmission electron microscopy, together with morphometry, showed a diminished maturation of alveolar Type II cells, with fewer osmiophilic lamellar bodies and more glycogen. The number of mesenchymal-Type II cell interconnections was not altered. Maturation of bronchiolar epithelium was not affected, and mesenchymal-epithelial connections were not observed. We conclude that bilateral phrenic nerve section not only diminishes lung growth, but also diminishes intrauterine maturation of the alveolar well. Maturation of bronchiolar epithelium may not be affected by fetal respiration.

Trial of an intervention to reduce passive smoking in infancy

We tested a health education intervention program to reduce passive smoking in infancy. The aim was to develop an instrument for study of tobacco smoke exposure and childhood respiratory illness. One hundred and eighty-four women who had smoked during pregnancy were allocated by month of delivery to an intervention group, to a minimal contact group, or to a follow-up only comparison group. Exposure to smoke was assessed 3 months later by questionnaire and by measurement of cotinine in samples of maternal and infant urine. There was a reduction in maternal smoking associated with contact with research staff, but this was not statistically significant. There were no differences between the groups in the exposure of infants to tobacco smoke. Reasons for this finding may include the timing of the intervention, the heterogeneity of the target group, and the manner in which information was presented on health risks caused by parental smoking.

Morphometric analysis of the growth of the normal fetal guinea pig lung

The structural development of the fetal guinea pig lung is described and quantified morphometrically in this report. At 35 days gestation the lung is in the pseudoglandular phase of growth, by 40 days it is in the canalicular phase, and at 50 days the saccular growth phase has begun. At term (67 days), the fetal guinea pig lung appears mature. From the beginning of the canalicular to the end of the saccular phases, the correlation coefficient between lung volume and gestational age is +.98, between internal surface area and gestational age is +.94 and between total number of saccules and gestational age is +.97. Internal surface area (ISA) correlates closely with lung volume (r = +.99) and the correlation coefficient between total number of saccules and lung volume is +.98. At term, lung volume is 4.22 ml. ISA is 0.5 M2, and total number of saccules is 253 million. Parenchymal growth is achieved by increases in both number and size of airspaces in the canalicular phase, primarily by increases in number during the early saccular phase and largely by increases in airspace size near term. The total length of parenchymal elastic tissue increases from 223 M at 45 days gestation to 5,253 M at term. Elastic tissue fibers first appear in the parenchyma of the fetal guinea pig lung during the canalicular phase, when the rate of saccule formation is high. The quantitative increase in elastic tissue correlates closely with the increase in the total number of saccules from day 45 to day 60 of gestation (r = +.99). The rate of elastic tissue growth increases sharply in the late saccular phase, coinciding with the period of greatest saccular expansion. These data suggest an interdependent relationship between saccular growth, i.e., proliferation and expansion, and the development of lung parenchymal elastic tissue.

The structural composition of the pulmonary acinus in the mouse. A scanning electron microscopical and developmental-biological analysis

For analysis of the structural composition of the pulmonary acinus in the mouse, fixed lungs of adult mice were examined by scanning and transmission electron microscopy. The murine acinus proved to consist of one or two generations of rather short respiratory bronchioles and about three generations of alveolar ducts opening into alveolar sacs. The epithelial cells lining the respiratory bronchioles (except for a sharply demarcated zone in the initial portion of the first-order branch) have a cuboidal or squamous shape and are very probably of the same type as those lining the alveolar ducts and sacs, i.e., small and great alveolar cells and/or their precursor cells. Therefore, this respiratory bronchiole portion may be considered part of the respiratory unit or pulmonary acinus. The initial zone of the first-order branch is lined by columnar epithelium, i.e., bronchial epithelium (mainly Clara cells), and should be assigned to the bronchial system of the lung.

Effect of ozone on mean linear intercept in the lung of young beagles

Although photochemical air pollutants are believed to be associated with respiratory illness, there is also a need to consider their possible effects on postnatal lung maturation. The purpose of this study was to determine whether the maturation of lungs of young beagle dogs might be altered by an inhalation exposure to ozone that represents a severe 5-d episode of photochemical oxidant air pollution. Exposures were at 6 wk of age to purified air, 1 or 2 ppm ozone for 4 h/d on 5 consecutive days. After holding for 6 wk in clean air, lungs were removed and weighed, and the left lung was fixed both by inflation at 30 cm pressure and immersion using buffered formalin. Histologic sections were used for morphometric measurements. Statistical analysis showed that the mean linear intercept (inversely related to lung surface area) was greater than controls (up to about 5%) in the 1 ppm ozone-exposed group. This effect was not seen at 2 ppm ozone, apparently due to large variations in mean linear intercept. No significant differences were seen in body weight, chest girth, lung weight, or volumes of the fixed, inflated lungs. It is concluded that if anatomic maturation of the lung was retarded by this brief regimen of ozone exposure, the effect was small and not likely to have major health consequences.

Morphometry of postnatal development in the porcine lung

Alveolar regions of normal pig lungs (newborn to 60-day-old) were characterized morphometrically to provide a basis for comparison in future investigations of porcine respiratory diseases. Endotracheal installation of fixative was done to expand the lungs uniformly at total capacity. Differential effects of lobar variations were determined by stratified random sampling of lung lobes. A stereologic study was done by point and intersection counts on electron micrographs. At birth, the lungs were remarkably well developed. Relative alveolar and capillary surface densities and air-blood tissue barrier thicknesses were at adult levels. In allometric regressions, volumes and surfaces of lung components regressed directly to lung volume, but monoexponentially (to the 3/4 power) with body weight. In the first postnatal week, however, relative volume densities of cellular interstitium in septal tissue and of capillary lumina in parenchyma increased at statistically significant levels. Composition of lung parenchyma and septa was changed, although without statistically significant direct impact on parameters related to gas exchange. Type II pneumocytes had increased nuclear to cytoplasmic volume ratios in 7- to 14-day-old pigs, probably reflecting cell activation and increased surfactant production. Age (postnatal lung growth) created the most substantial variance of results; interanimal variation in pigs of the same age was less important and no consistent lobar variations were seen.

Scanning electron microscope study of the development of the human respiratory acinus

Plastic corrosion casts were made of lungs from fetuses aged from 19 weeks' gestation to term and of lungs from a child and two adults to study the development of the respiratory acinus. To achieve reliable infusion of the most peripheral airspaces a high viscosity plastic was used that is not known to have been employed previously for corrosion casting of lungs. The casts were examined in the scanning electron microscope and showed the increase both in number and in length of the airways distal to the terminal bronchiole and also the change in shape and complexity of the most peripheral airspaces as the lung matures. The terminal airspaces change from short, simple, tubular endings at 19 weeks' gestation to short, shallow saccules from around 30 weeks' gestation to full term and contrast with the deep cup shaped alveoli in the adult. Measurements of the size of the terminal airspace at various stages of development are presented. This new approach, allowing three dimensional study of the peripheral airspaces of the developing lung, will be useful for investigating the lung pathology of neonates.

The neonatal porcine lung: ultrastructural morphology and postnatal development of the terminal airways and alveolar region

Morphology and postnatal development of the porcine lung are described in animals ranging in age from newborn through 60 days. Standardized fixation was accomplished by intratracheal instillation of glutaraldehyde under constant pressure. Light microscopic, scanning, and transmission electron microscopic investigations revealed that the porcine lung follows the common architecture of mammalian lungs, but has certain peculiarities as well: intravascular macrophages, ultrastructurally similar to Kupffer cells, are attached to endothelial cells in pulmonary capillaries and are involved in erythrophagocytosis during the first postnatal weeks. Type II pneumocytes of newborn pigs exhibit signs of cell activation, mainly complex nuclear bodies in the cell nuclei. At the same time high levels of 17-hydroxycorticosteroids are observed in the newborn blood plasma. Terminal airways of the porcine lung are nonalveolarized and are, therefore, of purely conductive function. At birth the porcine lung exhibits a high degree of maturity, and thick-walled primary saccules, as described in newborn rodents, are not seen. Septa appear straight and smooth, owing to rare ramification. Septal buds are discernible, and two capillary networks visible on both sides of septal cross sections are seen. Further subdivision of the airspaces occurs in the first two postnatal weeks. Precociousness and fast postnatal growth of the porcine species are assumed to be the reason of this advanced degree of lung maturity at birth and the following rapid pulmonary development.

Pulmonary vascular and alveolar development in tetralogy of Fallot: a recommendation for early correction

Using quantitative morphometric techniques, we analysed pulmonary arterial and alveolar development in the lungs of seven children aged 1.2-12 years who died during or soon after repair of tetralogy of Fallot. One child had a residual ventricular septal defect and survived for five months. One other child had had a previous Waterston-Cooley anastomosis (Waterston shunt). Postmortem lung volume in relation to body surface area was generally below normal for age, the alveoli were small, and the total alveolar number was below normal in five of the seven cases. Microscopically, airway and alveolar structure appeared normal. The preacinar arteries were larger and the intra-acinar arteries were smaller than normal for age. The preacinar elastic pulmonary arteries appeared to contain less elastin and in both preacinar and intra-acinar muscular arteries the media was thinner than normal, although muscle was normally distributed along the arterial pathway. Eccentric areas of intimal fibrosis were small and uncommon. The bronchial arteries were generally more prominent than usual both macroscopically and microscopically, but no abnormal bronchopulmonary connections were present. After corrective surgery a residual ventricular septal defect and pulmonary hypertension were associated with arterial medial hypertrophy, and this change was also found in the right lung of a normotensive patient who had had a Waterston shunt. This group probably represents the most favourable clinical picture of tetralogy in patients who usually survive but, even so, pulmonary arterial and alveolar development was abnormal. The structural findings are discussed in relation to the functional outcome in patients with tetralogy who have survived. Repair of the abnormality during the first two to three years of life is recommended.

Ventilation-perfusion relationships in children

Ventilation-perfusion relationships (Va/Q) were studied in nine lung-healthy children, 7-15 years old, in whom a diagnostic heart catheterization was performed. VA/Q was assessed by a multiple inert-gas elimination technique. In subjects above 10 years of age, single, narrow modes of ventilation and perfusion were seen centred upon a VA/Q ratio of one. In three of the four youngest children, 10 years old or less, additional; 'high' VA/Q regions were observed. Neither shunt, nor 'low' VA/Q regions were seen in any child. Arterial PO2 was above 12 kPa in all except one and no age dependence was observed.

Advanced pulmonary development in newborn guinea pigs (Cavia porcellus)

Morphological and morphometric evidence is presented to support the hypothesis that lung growth is advanced in mammals born at a relatively mature stage. The lungs from fetal and postnatal guinea pigs ranging in age from gestational age 56 days (normal gestation is 68 days in this species) through 16 days postpartum were fixed in situ by intratracheal glutaraldehyde. Morphometry included measurements of lung volume (VL), tissue and air-space volumes, fraction of respiratory parenchyma, alveolar (SA) and capillary (SC) surface areas, and the arithmetic mean thickness of the tissue barrier (tau t). VL, SA, and SC all increased monoexponentially versus body weight (W) from birth to adulthood; the lungs appeared to be in the equilibrated growth phase, with no postnatal period of pronounced tissue proliferation as reported in the newborn rat and mouse. The prepartum value of tau t was 1.96 micrometers; this value decreased by parturition of 1.27 micrometers and did not change significantly with additional age. At the light-microscopical level, respiratory bronchioles could be visualized giving rise to alveolar ducts by a gestational age of 58 days (10 days preterm) with well-developed alveolar septal partitioning evident. Structures resembling the primitive pre-alveolar saccules of newborn rats were never seen in even the youngest fetal animals. Elastin fibers were also evident at this age, both in bronchiolar and duct walls, as well as in alveolar septa. Using electron microscopy, the air-blood barrier appeared mature by a gestational age of 61 days and thereafter, double capillary layers were only rarely seen in septal walls.

Developmental trends of liver parenchyma in the terminal period of gestation: a stereological approach

The aim of the presented study was to establish the baseline data concerning the volumes density of basic architectonic components of human fetal liver parenchyma. The stereological measuring principles of the point-counting method were used. The livers of 38 appropriate for gestational age fetuses, and 12 small for gestational age fetuses were measured. All fetuses investigated were stillborns. A lower volume density of hepatocytic population was found in full-term mature fetuses, when compared with the adults. A lower volume density of hepatocytes has been also disclosed in intra-uterine growth retarded fetuses, when compared with their physiologically developed counterparts of the same gestational age.