Spherical alveolar shapes in live mouse lungs.

Understanding how the alveolar mechanics work in live lungs is essential for comprehending how the lung behaves during breathing. Due to the lack of appropriate imaging tools, previous research has suggested that alveolar morphologies are polyhedral rather than spherical based on a 2D examination of alveoli in fixed lungs. Here, we directly observe high-resolution 3D alveoli in live mice lungs utilizing synchrotron x-ray microtomography to show spherical alveolar morphologies from the live lungs. Our measurements from x-ray microtomography show high sphericity, low packing density, big alveolar size, and low osmotic pressure, indicating that spherical alveolar morphologies are natural in living lungs. The alveolar packing fraction is quite low in live lungs, where the spherical alveoli would behave like free bubbles, while the confinement of alveolar clusters in fixed lungs would lead to significant morphological deformations of the alveoli appearing polyhedral. Direct observations of the spherical alveolar shapes will help understand and treat lung disease and ventilation.

Characteristics of pulmonary microvascular structure in postnatal yaks.

Yaks are typical plateau-adapted animals, however the microvascular changes and characteristics in their lungs after birth are still unclear. Pulmonary microvasculature characteristics and changes across age groups were analysed using morphological observation and molecular biology detection in yaks aged 1, 30 and 180 days old in addition to adults. Results: Our experiments demonstrated that yaks have fully developed pulmonary alveolar at birth but that interalveolar thickness increased with age. Immunofluorescence observations showed that microvessel density within the interalveolar septum in the yak gradually increased with age. In addition, transmission electron microscopy (TEM) results showed that the blood-air barrier of 1-day old and 30-days old yaks was significantly thicker than that observed at 180-days old and in adults (P < 0.05), which was caused by the thinning of the membrane of alveolar epithelial cells. Furthermore, Vegfa and Epas1 expression levels in 30-day old yaks were the highest in comparison to the other age groups (P < 0.05), whilst levels in adult yaks were the lowest (P < 0.05). The gradual increase in lung microvessel density can effectively satisfy the oxygen requirements of ageing yaks. In addition, these results suggest that the key period of yak lung development is from 30 to 180 days.

Sleeve gastrectomy ameliorates alveolar structures and surfactant protein expression in lungs of obese and diabetic rats.

BACKGROUND: Bariatric surgeries have been shown to be effective in reversing damaged pulmonary function in individuals suffering from obesity and type 2 diabetes mellitus, whereas its underlying mechanisms remain largely unknown. METHODS: Sleeve gastrectomy (SG) was performed on obese and diabetic Wistar rats, and their pulmonary function and lung tissues were compared to sham-operated (SH) obese and diabetic rats, and age-matched healthy controls (C) to explore the improvements in microstructures and expression of surfactant protein (SP)-A and -C at postoperative 4th, 8th, and 12th week. RESULT: Apart from the profound metabolic changes and improvement in pulmonary function, lung volume was restored along with an improved diffusion capacity noted by thinned capillary basement membrane and decreased harmonic mean length of diffusion barrier in SG rats. The digital slices of light microscope showed the general changes brought on by the SG, including normalized basic structures, ameliorated inflammatory status, as well as reduced lipid deposition, where the hydroxyproline (HYP), triglyceride (TG) assays, and electron microscope further suggested that the improvement in alveolar structures lies in reduced collagen fibers, lipids and septal tissues, increased capillary blood, and normalized alveolar type 2 (AT2) cells. Besides, disrupted SP-A and SP-C expression were also normalized after SG. CONCLUSION: The improvement of lung function after SG is related to the ameliorated alveolar structures, and surface protein expression induced by weight loss and improved glucose metabolism.

Comprehensive anatomic ontologies for lung development: A comparison of alveolar formation and maturation within mouse and human lung.

BACKGROUND: Although the mouse is widely used to model human lung development, function, and disease, our understanding of the molecular mechanisms involved in alveolarization of the peripheral lung is incomplete. Recently, the Molecular Atlas of Lung Development Program (LungMAP) was funded by the National Heart, Lung, and Blood Institute to develop an integrated open access database (known as BREATH) to characterize the molecular and cellular anatomy of the developing lung. To support this effort, we designed detailed anatomic and cellular ontologies describing alveolar formation and maturation in both mouse and human lung. DESCRIPTION: While the general anatomic organization of the lung is similar for these two species, there are significant variations in the lung's architectural organization, distribution of connective tissue, and cellular composition along the respiratory tract. Anatomic ontologies for both species were constructed as partonomic hierarchies and organized along the lung's proximal-distal axis into respiratory, vascular, neural, and immunologic components. Terms for developmental and adult lung structures, tissues, and cells were included, providing comprehensive ontologies for application at varying levels of resolution. Using established scientific resources, multiple rounds of comparison were performed to identify common, analogous, and unique terms that describe the lungs of these two species. Existing biological and biomedical ontologies were examined and cross-referenced to facilitate integration at a later time, while additional terms were drawn from the scientific literature as needed. This comparative approach eliminated redundancy and inconsistent terminology, enabling us to differentiate true anatomic variations between mouse and human lungs. As a result, approximately 300 terms for fetal and postnatal lung structures, tissues, and cells were identified for each species. CONCLUSION: These ontologies standardize and expand current terminology for fetal and adult lungs, providing a qualitative framework for data annotation, retrieval, and integration across a wide variety of datasets in the BREATH database. To our knowledge, these are the first ontologies designed to include terminology specific for developmental structures in the lung, as well as to compare common anatomic features and variations between mouse and human lungs. These ontologies provide a unique resource for the LungMAP, as well as for the broader scientific community.

Ventilación colateral: una propuesta para terminologia histologica / Collateral ventilation: a terminologia histologica proposal

La correcta utilización de los términos morfológicos está estandarizada por las terminologías, una de ellas es la Terminologia Histologica (TH). Éstas sugieren la exclusión de los epónimos. Pese a esto, existen estructuras que continúan en esta condición. Específicamente, "Poros de Kohn, Canales de Martin y Canales de Lambert" son términos que componen la ventilación colateral (VC) y son ejemplo de esta situación. Así, el objetivo del presente estudio fue identificar en TH los términos asociados a la VC a fin de proponer denominaciones siguiendo las recomendaciones de la Federación Internacional de Programas de Terminologías Anatómicas (FIPAT). Se buscaron los términos relacionados a la VC en TH, posteriormente, se efectuó el mismo ejercicio en textos de histología, además de esto, en base de datos MedLine a través de su buscador PudMed con el siguiente algoritmo: (lung) AND (alveoli pulmonary) AND (airway) AND (collateral) AND (ventilation). En TH se encontró el término Porus septalis (H3. 05.02.0.00036) para referirse al término Poros de Kohn, en seis textos de histología se menciona el término Poros de Kohn, en 21 artículos revisados se menciona la VC, de estos, en diez se utiliza el epónimo Poro de Kohn, para referirse a los poros septales, el epónimo Canales de Lambert fue utilizado en seis artículos y el epónimo Canales de Martin, apareció en cinco artículos. A partir de la información encontrada, su desarrollo histórico, sumado a los lineamientos de la FIPAT, proponemos complementar e incluir en TH los términos Porus septalis alveolaris para los poros de Kohn, Ductus bronchiolaris alveolaris para los Canales de Lambert y Ductus interbronquiolaris para los canales de Martin, respectivamente.

The correct use of morphological terms is standardized by the Terminologies, one of them is the Histological Terminology (HT) For these Terminologies, the exclusion of eponyms is recommended. Despite this, there are structures that remain as eponyms. Three in particular: Pores of Kohn, Martin Channels and Lambert Channels are terms that make up collateral ventilation (CV) and are an example of this. Thus, the objective of the present study was to identify in the HT the terms associated with the CV in order to propose denominations following the recommendations of the Federative International Programme on Anatomical Terminologies (FIPAT). The terms related to CV in the TH were researched, and subsequently, the same exercise was carried out in histology texts. The MedLine database was also used through its PudMed search engine with the following algorithm: (lung) AND (alveoli pulmonary) AND (airway) AND (collateral) AND (ventilation). In HT the term Porus Septalis" (H3.05.0.0.036) was found to refer to the term "Pores of Kohn, in six histology texts the term Pores of Kohn is mentioned, in 21 reviewed articles the CV is mentioned, of these, in ten the eponymous Pores of Kohn is used, to refer to the Septal Pores, the eponymous Lambert Channels was used in six articles and the eponymous Martin Channels appeared in five articles. From the information found, its historical development, added to the guidelines of the FIPAT, we propose complementing and including in the HT the terms "Porus septalis alveolaris" for pores of Kohn, "Ductus bronchiolaris alveolaris" for the Lambert Channels and "Ductus interbronquiolaris" for the Martin Channels, respectively.

Pores of Kohn: forgotten alveolar structures and potential source of aerosols in exhaled breath.

Analysis of human and animal exhaled breath has identified numerous compounds including proteins and surfactant constituents from the deep lung. Some mechanisms such as coughing, breaking of surfactant/mucus plugs, or 'bronchiole film bursting' have been proposed to explain the presence of these proteins from the deep lung but do not include possible contributions from Pores of Kohn. A re-examination of the change in diameter as well as forces exerted by surfactant film in the Pores of Kohn during normal inspiration, demonstrates that these channels should open following rupture of the surfactant film; which could generate aerosols of surfactant film constituents. Generation, of such deep-lung aerosols, is predicted to begin during inhalation when lung tissue surface area has increased by at least a factor of 2.

Optimal diameter reduction ratio of acinar airways in human lungs.

In the airway network of a human lung, the airway diameter gradually decreases through multiple branching. The diameter reduction ratio of the conducting airways that transport gases without gas exchange is 0.79, but this reduction ratio changes to 0.94 in acinar airways beyond transitional bronchioles. While the reduction in the conducting airways was previously rationalized on the basis of Murray's law, our understanding of the design principle behind the acinar airways has been far from clear. Here we elucidate that the change in gas transfer mode is responsible for the transition in the diameter reduction ratio. The oxygen transfer rate per unit surface area is maximized at the observed geometry of acinar airways, which suggests the minimum cost for the construction and maintenance of the acinar airways. The results revitalize and extend the framework of Murray's law over an entire human lung.

Modeling Airflow and Particle Deposition in a Human Acinar Region.

The alveolar region, encompassing millions of alveoli, is the most vital part of the lung. However, airflow behavior and particle deposition in that region are not fully understood because of the complex geometrical structure and intricate wall movement. Although recent investigations using 3D computer simulations have provided some valuable information, a realistic analysis of the air-particle dynamics in the acinar region is still lacking. So, to gain better physical insight, a physiologically inspired whole acinar model has been developed. Specifically, air sacs (i.e., alveoli) were attached as partial spheroids to the bifurcating airway ducts, while breathing-related wall deformation was included to simulate actual alveolar expansion and contraction. Current model predictions confirm previous notions that the location of the alveoli greatly influences the alveolar flow pattern, with recirculating flow dominant in the proximal lung region. In the midalveolar lung generations, the intensity of the recirculating flow inside alveoli decreases while radial flow increases. In the distal alveolar region, the flow pattern is completely radial. The micron/submicron particle simulation results, employing the Euler-Lagrange modeling approach, indicate that deposition depends on the inhalation conditions and particle size. Specifically, the particle deposition rate in the alveolar region increases with higher inhalation tidal volume and particle diameter. Compared to previous acinar models, the present system takes into account the entire acinar region, including both partially alveolated respiratory bronchioles as well the fully alveolated distal airways and alveolar sacs. In addition, the alveolar expansion and contraction have been calculated based on physiological breathing conditions which make it easy to compare and validate model results with in vivo lung deposition measurements. Thus, the current work can be readily incorporated into human whole-lung airway models to simulate/predict the flow dynamics of toxic or therapeutic aerosols.

Functional morphometry for the estimation of the alveolar surface area in prematurely-born infants.

Conventionally, the alveolar surface area (SA) has been measured by using post-mortem morphometry. Such studies have highlighted that SA in prematurely-born infants is markedly smaller when compared to term-born infants as a result of postnatal impairment or arrest of alveolar development. We herein explore how, non-invasive measurements of the ventilation/perfusion ratio (VA/Q) can be used to estimate SA in prematurely-born surviving, convalescent infants. We also compare SA in prematurely-born infants measured at term-corrected age, to term-born infants using previously published datasets of VA/Q. Fick's first law of diffusion is employed for the conversion of VA/Q measurements to SA values after correcting for differences in pulmonary perfusion, thickness of the respiratory membrane and alveolar-arterial gradient. We report that SA is fivefold smaller in prematurely-born compared to term-born infants. We conclude that non-invasive measurements of VA/Q can be used for the functional estimation of SA which could, in turn, be used as a future outcome measure in respiratory studies of prematurely-born infants.

Stereological analysis of individual lung lobes during normal and aberrant mouse lung alveolarisation.

The quantitative assessment of the lung architecture forms the foundation of many studies on lung development and lung diseases, where parameters such as alveoli number, alveolar size, and septal thickness are quantitatively influenced by developmental or pathological processes. Given the pressing need for robust data that describe the lung structure, there is currently much enthusiasm for the development and refinement of methodological approaches for the unbiased assessment of lung structure with improved precision. The advent of stereological methods highlights one such approach. However, design-based stereology is both expensive and time-demanding. The objective of this study was to examine whether 'limited' stereological analysis, such as the stereological analysis of a single mouse lung lobe, may serve as a surrogate for studies on whole, intact mouse lungs; both in healthy lungs and in diseased lungs, using an experimental animal model of bronchopulmonary dysplasia (BPD). This served the dual-function of exploring BPD pathobiology, asking whether there are regional (lobar) differences in the responses of developing mouse lungs to oxygen injury, by examining each mouse lung lobe separately in the BPD model. Hyperoxia exposure resulted in decreased alveolar density, alveoli number, and gas-exchange surface area in all five mouse lung lobes, and increased the arithmetic mean septal thickness in all mouse lung lobes except the lobus cardialis. The data presented here suggest that - in healthy developing mice - a single mouse lung lobe might serve as a surrogate for studies on whole, intact mouse lungs. This is not the case for oxygen-injured developing mouse lungs, where a single lobe would not be suitable as a surrogate for the whole, intact lung. Furthermore, as the total number of alveoli can only be determined by an analysis of the entire lung, and given regional differences in lung structure, particularly under pathological conditions, the stereological assessment of the whole, intact lung remains desirable.

Immunohistochemical study on the distribution of ß-defensin 1 and ß-defensin 2 throughout the respiratory tract of healthy rats.

The distributions of ß-defensin 1 and 2 in secretory host defense system throughout respiratory tract of healthy rats were immunohistochemically investigated. In the nasal epithelium, a large number of non-ciliated and non-microvillous cells (NCs) were immunopositive for both ß-defensin 1 and 2, whereas a small number of goblet cells (GCs) were immunopositive only for ß-defensin 1. Beta-defensin 2-immunopositive GCs were few. In the nasal glands, a small number of acinar cells and a large number of ductal epithelial cells were immunopositive for both ß-defensins. In the laryngeal and tracheal epithelia, a very few NCs and GCs were immunopositive for both ß-defensins. In laryngeal and tracheal glands, a very few acinar cells and a large number of ductal epithelial cells were immunopositive for both ß-defensins. In the extra-pulmonary bronchus, a small number of NCs were immunopositive for both ß-defensins. A small number of GCs were immunopositive for ß-defensin 1, whereas few GCs were immunopositive for ß-defensin 2. From the intra-pulmonary bronchus to alveoli, a very few or no epithelial cells were immunopositive for both ß-defensins. In the mucus and periciliary layers, ß-defensin 1 was detected from the nose to the extra-pulmonary bronchus, whereas ß-defensin 2 was weakly detected only in the nose and the larynx. These findings suggest that the secretory sources of ß-defensin 1 and 2 are mainly distributed in the nasal mucosa and gradually decrease toward the caudal airway in healthy rats.

Alveoli, teeth, and tooth loss: Understanding the homology of internal mandibular structures in mysticete cetaceans.

The evolution of filter feeding in baleen whales (Mysticeti) facilitated a wide range of ecological diversity and extreme gigantism. The innovation of filter feeding evolved in a shift from a mineralized upper and lower dentition in stem mysticetes to keratinous baleen plates that hang only from the roof of the mouth in extant species, which are all edentulous as adults. While all extant mysticetes are born with a mandible lacking a specialized feeding structure (i.e., baleen), the bony surface retains small foramina with elongated sulci that often merge together in what has been termed the alveolar gutter. Because mysticete embryos develop tooth buds that resorb in utero, these foramina have been interpreted as homologous to tooth alveoli in other mammals. Here, we test this homology by creating 3D models of the internal mandibular morphology from terrestrial artiodactyls and fossil and extant cetaceans, including stem cetaceans, odontocetes and mysticetes. We demonstrate that dorsal foramina on the mandible communicate with the mandibular canal via smaller canals, which we explain within the context of known mechanical models of bone resorption. We suggest that these dorsal foramina represent distinct branches of the inferior alveolar nerve (or artery), rather than alveoli homologous with those of other mammals. As a functional explanation, we propose that these branches provide sensation to the dorsal margin of the mandible to facilitate placement and occlusion of the baleen plates during filer feeding.

Evidence for age-dependent air-space enlargement contributing to loss of lung tissue elastic recoil pressure and increased shear modulus in older age.

As a normal part of mature aging, lung tissue undergoes microstructural changes such as alveolar air-space enlargement and redistribution of collagen and elastin away from the alveolar duct. The older lung also experiences an associated decrease in elastic recoil pressure and an increase in specific tissue elastic moduli, but how this relates mechanistically to microstructural remodeling is not well-understood. In this study, we use a structure-based mechanics analysis to elucidate the contributions of age-related air-space enlargement and redistribution of elastin and collagen to loss of lung elastic recoil pressure and increase in tissue elastic moduli. Our results show that age-related geometric changes can result in reduction of elastic recoil pressure and increase in shear and bulk moduli, which is consistent with published experimental data. All elastic moduli were sensitive to the distribution of stiffness (representing elastic fiber density) in the alveolar wall, with homogenous stiffness near the duct and through the septae resulting in a more compliant tissue. The preferential distribution of elastic proteins around the alveolar duct in the healthy young adult lung therefore provides for a more elastic tissue.NEW & NOTEWORTHY We use a structure-based mechanics analysis to correlate air-space enlargement and redistribution of elastin and collagen to age-related changes in the mechanical behavior of lung parenchyma. Our study highlights that both the cause (redistribution of elastin and collagen) and the structural effect (alveolar air-space enlargement) contribute to decline in lung tissue elastic recoil with age; these results are consistent with published data and provide a new avenue for understanding the mechanics of the older lung.

Stereological monitoring of mouse lung alveolarization from the early postnatal period to adulthood.

Postnatal lung maturation generates a large number of small alveoli, with concomitant thinning of alveolar septal walls, generating a large gas exchange surface area but minimizing the distance traversed by the gases. This demand for a large and thin gas exchange surface area is not met in disorders of lung development, such as bronchopulmonary dysplasia (BPD) histopathologically characterized by fewer, larger alveoli and thickened alveolar septal walls. Diseases such as BPD are often modeled in the laboratory mouse to better understand disease pathogenesis or to develop new interventional approaches. To date, there have been no stereology-based longitudinal studies on postnatal mouse lung development that report dynamic changes in alveoli number or alveolar septal wall thickness during lung maturation. To this end, changes in lung structure were quantified over the first 22 mo of postnatal life of C57BL/6J mice. Alveolar density peaked at postnatal day (P)39 and remained unchanged at 9 mo (P274) but was reduced by 22 mo (P669). Alveoli continued to be generated, initially at an accelerated rate between P5 and P14, and at a slower rate thereafter. Between P274 and P669, loss of alveoli was noted, without any reduction in lung volume. A progressive thinning of the alveolar septal wall was noted between P5 and P28. Pronounced sex differences were observed in alveoli number in adult (but not juvenile) mice, when comparing male and female mouse lungs. This sex difference was attributed exclusively to the larger volume of male mouse lungs.

Dephasing and diffusion on the alveolar surface.

We propose a surface model of spin dephasing in lung tissue that includes both susceptibility and diffusion effects to provide a closed-form solution of the Bloch-Torrey equation on the alveolar surface. The nonlocal susceptibility effects of the model are validated against numerical simulations of spin dephasing in a realistic lung tissue geometry acquired from synchotron-based µCT data sets of mouse lung tissue, and against simulations in the well-known Wigner-Seitz model geometry. The free induction decay is obtained in dependence on microscopic tissue parameters and agrees very well with in vivo lung measurements at 1.5 Tesla to allow a quantification of the local mean alveolar radius. Our results are therefore potentially relevant for the clinical diagnosis and therapy of pulmonary diseases.

Reduced-Dimension Modeling Approach for Simulating Recruitment/De-recruitment Dynamics in the Lung.

Acute respiratory distress syndrome is a pulmonary disease that requires the use of mechanical ventilation for patient recovery. However, this can lead to development of ventilator-induced lung injury caused by the over-distension of alveolar tissue and by the repetitive closure (de-recruitment) and reopening (recruitment) of airways. In this study, we developed a multi-scale model of the lung from a reduced-dimension approach to investigate the dynamics of ventilation in the lung during airway collapse and reopening. The model consisted of an asymmetric network geometry with 16 generations of liquid-lined airways with airflow driven by a variable pleural pressure. During the respiratory cycle changes in airway radii and film thickness yield the formation of liquid plugs that propagate and rupture throughout the airway network. Simulations were conducted with constant surface tension values [Formula: see text] dyn/cm. It was observed that the time onset of plug creation and rupture depended on the surface tension, as well as the plug aggregation/splitting behavior at bifurcations. Additionally, the plug propagation behavior was significantly influenced by presence of plugs in adjacent airways (i.e. parent and daughters) that affected the driving pressure distribution locally at bifurcations and resulted in complex aggregation and splitting behavior. This model provides an approach that has the ability to simulate normal and pathophysiological lung conditions with the potential to be used in personalized clinical medicine.

Toward a comprehensive interpretation of intravital microscopy images in studies of lung tissue dynamics.

Intravital microscopy (IVM) is a well-established imaging technique for real-time monitoring of microscale lung tissue dynamics. Although accepted as a gold standard in respiratory research, its characteristic image features are scarcely understood, especially when trying to determine the actual position of alveolar walls. To allow correct interpretation of these images with respect to the true geometry of the lung parenchyma, we analyzed IVM data of alveoli in a mouse model in comparison with simultaneously acquired optical coherence tomography images. Several IVM characteristics, such as double ring structures or disappearing alveoli in regions of liquid filling, could be identified and related to the position of alveoli relative to each other. Utilizing a ray tracing approach based on an idealized geometry of the mouse lung parenchyma, two major reflection processes could be attributed to the IVM image formation: partial reflection and total internal reflection between adjacent alveoli. Considering the origin of the reflexes, a model was developed to determine the true position of alveolar walls within IVM images. These results allow thorough understanding of IVM data and may serve as a basis for the correction of alveolar sizes for more accurate quantitative analysis within future studies of lung tissue dynamics.

Effect of Posture on Regional Deposition of Coarse Particles in the Healthy Human Lung.

BACKGROUND: Understanding the regional partition of deposition of inhaled particles within the lung is important for improving targeted delivery of inhaled aerosolized drugs. One factor affecting regional deposition is gravity. As the lung deforms under its own weight, changes in lung volume, in airway geometries, and in spatial patterns of ventilation distribution between postures have the potential to alter the regional distribution of deposited particles. METHODS: Using gamma-scintigraphy, we measured regional deposition and clearance of (99m)Tc labeled particles (5 µm) in 6 healthy subjects, with aerosol inhalation occurring both in the supine and seated postures at constant flow (0.5 L/sec) and breathing rate (15 breaths/min). After aerosol deposition, mucociliary clearance data were collected in the seated posture, immediately post-particle administration, 1 h 30 min, 4 h, and 22 h post-inhalation. Relative regional deposition was computed using retention (R) at the different time points, with (1-R(1h30min)), (R(1h30min)- R(4h)), and (R(4h)- R(22h)) corresponding to deposition in the large, intermediate, and small airways, respectively. Alveolar deposition was estimated as the relative retention at 22 h (R(22h)). RESULTS: Relative deposition of coarse particles in the alveolar region decreased from 60±8% seated to 34±16% supine (p=0.04). This change was accompanied by an increase in relative deposition in the intermediate (7±3% seated to 16±17% supine, P=0.09) and small airways (19±6% seated to 34±13% supine, p=0.06) when inhalation occurred in the supine posture. No change was observed in central to peripheral deposition (C/P ratio), the skew of the deposition distribution, or the apex-to-base ratio of deposition between seated and supine postures. CONCLUSIONS: Inhalation of coarse particles in the supine posture shifts relative deposition from the alveolar to the bronchial airways, when compared to the seated posture, likely driven by changes in functional residual capacity, and airway size, as well as changes in the regional distribution of ventilation between postures.

Regional differences in alveolar density in the human lung are related to lung height.

The gravity-dependent pleural pressure gradient within the thorax produces regional differences in lung inflation that have a profound effect on the distribution of ventilation within the lung. This study examines the hypothesis that gravitationally induced differences in stress within the thorax also influence alveolar density in terms of the number of alveoli contained per unit volume of lung. To test this hypothesis, we measured the number of alveoli within known volumes of lung located at regular intervals between the apex and base of four normal adult human lungs that were rapidly frozen at a constant transpulmonary pressure, and used microcomputed tomographic imaging to measure alveolar density (number alveoli/mm3) at regular intervals between the lung apex and base. These results show that at total lung capacity, alveolar density in the lung apex is 31.6 ± 3.4 alveoli/mm3, with 15 ± 6% of parenchymal tissue consisting of alveolar duct. The base of the lung had an alveolar density of 21.2 ± 1.6 alveoli/mm3 and alveolar duct volume fraction of 29 ± 6%. The difference in alveolar density can be negated by factoring in the effects of alveolar compression due to the pleural pressure gradient at the base of the lung in vivo and at functional residual capacity.

Synchrotron X-ray imaging of pulmonary alveoli in respiration in live intact mice.

Despite nearly a half century of studies, it has not been fully understood how pulmonary alveoli, the elementary gas exchange units in mammalian lungs, inflate and deflate during respiration. Understanding alveolar dynamics is crucial for treating patients with pulmonary diseases. In-vivo, real-time visualization of the alveoli during respiration has been hampered by active lung movement. Previous studies have been therefore limited to alveoli at lung apices or subpleural alveoli under open thorax conditions. Here we report direct and real-time visualization of alveoli of live intact mice during respiration using tracking X-ray microscopy. Our studies, for the first time, determine the alveolar size of normal mice in respiration without positive end expiratory pressure as 58 ± 14 (mean ± s.d.) µm on average, accurately measured in the lung bases as well as the apices. Individual alveoli of normal lungs clearly show heterogeneous inflation from zero to ~25% (6.7 ± 4.7% (mean ± s.d.)) in size. The degree of inflation is higher in the lung bases (8.7 ± 4.3% (mean ± s.d.)) than in the apices (5.7 ± 3.2% (mean ± s.d.)). The fraction of the total tidal volume allocated for alveolar inflation is 34 ± 3.8% (mean ± s.e.m). This study contributes to the better understanding of alveolar dynamics and helps to develop potential treatment options for pulmonary diseases.