Evidence and structural mechanism for late lung alveolarization

According to the current view, the formation of new alveolar septa from preexisting ones ceases due to the reduction of a double- to a single-layered capillaries network inside the alveolar septa (microvasculature maturation postnatal days 14-21 in rats). We challenged this view by measuring stereologically the appearance of new alveolar septa and by studying the alveolar capillary network in three-dimensional (3-D) visualizations obtained by high-resolution synchrotron radiation X-ray tomographic microscopy. We observed that new septa are formed at least until young adulthood (rats, days 4-60) and that roughly half of the new septa are lifted off of mature septa containing single-layered capillary networks. At the basis of newly forming septa, we detected a local duplication of the capillary network. We conclude that new alveoli may be formed in principle at any time and at any location inside the lung parenchyma and that lung development continues into young adulthood. We define two phases during developmental alveolarization. Phase one (days 4-21), lifting off of new septa from immature preexisting septa, and phase two (day 14 through young adulthood), formation of septa from mature preexisting septa. Clinically, our results ask for precautions using drugs influencing structural lung development during both phases of alveolarization.     

Influence of lung volume and left atrial pressure on reverse pulmonary venous blood flow

Infarction of the lung is uncommon even when both the pulmonary and the bronchial blood supplies are interrupted. We studied the possibility that a tidal reverse pulmonary venous flow is driven by the alternating distension and compression of alveolar and extra-alveolar vessels with the lung volume changes of breathing and also that a pulsatile reverse flow is caused by left atrial pressure transients. We infused SF6, a relatively insoluble inert gas, into the left atrium of anesthetized goats in which we had interrupted the left pulmonary artery and the bronchial circulation. SF6 was measured in the left lung exhalate as a reflection of the reverse pulmonary venous flow. No SF6 was exhaled when the pulmonary veins were occluded. SF6 was exhaled in increasing amounts as left atrial pressure, tidal volume, and ventilatory rates rose during mechanical ventilation. SF6 was not excreted when we increased left atrial pressure transients by causing mitral insufficiency in the absence of lung volume changes (continuous flow ventilation). Markers injected into the left atrial blood reached the alveolar capillaries. We conclude that reverse pulmonary venous flow is driven by tidal ventilation but not by left atrial pressure transients. It reaches the alveoli and could nourish the alveolar tissues when there is no inflow of arterial blood.     

Heterogeneous capillary recruitment among adjoining alveoli

Pulmonary capillaries recruit when microvascular pressure is raised. The details of the relationship between recruitment and pressure, however, are controversial. There are data supporting 1). gradual homogeneous recruitment, 2). sudden and complete recruitment, and 3). heterogeneous recruitment. The present study was designed to determine whether alveolar capillary networks recruit in a variety of ways or whether one model predominates. In isolated, pump-perfused canine lung lobes, fields of six neighboring alveoli were recorded with video microscopy as pulmonary venous pressure was raised from 0 to 40 mmHg in 5-mmHg increments. The largest group of alveoli (42%) recruited gradually. Another group (33%) recruited suddenly (sheet flow). Half of the neighborhoods had at least one alveolus that paradoxically derecruited when pressure was increased, even though neighboring alveoli continued to recruit capillaries. At pulmonary venous pressures of 40 mmHg, 86% of the alveolar-capillary networks were not fully recruited. We conclude that the pattern of recruitment among neighboring alveoli is complex, is not homogeneous, and may not reach full recruitment, even under extreme pressures.     

Red blood cell orientation in pulmonary capillaries and its effect on gas diffusion.

When alveoli are inflated, the stretched alveolar walls draw their capillaries into oval cross sections. This causes the disk-shaped red blood cells to be oriented near alveolar gas, thereby minimizing diffusion distance. We tested these ideas by measuring red blood cell orientation in histological slides from rapidly frozen rat lungs. High lung inflation did cause the capillaries to have oval cross sections, which constrained the red blood cells within them to flow with their broad sides facing alveolar gas. Low lung inflation stretched alveolar walls less and allowed the capillaries to assume a circular cross section. The circular luminal profile permitted the red blood cells to have their edges facing alveolar gas, which increased the diffusion distance. Using a finite-element method to calculate the diffusing capacity of red blood cells in the broad-side and edge-on orientations, we found that edge-on red blood cells had a 40% lower diffusing capacity. This suggests that, when capillary cross sections become circular, whether through low-alveolar volume or through increased microvascular pressure, the red blood cells are likely to be less favorably oriented for gas exchange.     

Temporal expression of alpha-smooth muscle actin and drebrin in septal interstitial cells during alveolar maturation.

In rat lung, the definitive alveoli are established during development by the outgrowth of secondary septa from the primary septa present in newborn; however, the mechanism of alveolar formation has not yet been fully clarified. In this study, we characterize the septal interstitial cells in developing alveoli. During the perinatal period, alpha-SMA-containing slender cells were found in the primitive alveolar septa. Alpha-SMA-containing cells were detected at the tips of the septa until postnatal day 21, when the alveolar formation was almost completed, but disappeared in adult. Immunoelectron microscopy demonstrated that alpha-SMA is localized mainly in the cellular protrusions, which are connected with the elastic fibers around the interstitial cells. Developmentally regulated brain protein (drebrin) is also located in the cell extensions containing alpha-SMA in immature alveolar interstitial cells. In adult lung, alpha-SMA-positive cells are located only at the alveolar ducts but are not found in the secondary septa. Desmin is expressed only in alpha-SMA-containing cells at the alveolar ducts but not in those at the tip of alveolar septa. These results suggest that a part of the septal interstitial cells are temporarily alpha-SMA- and drebrin-positive during maturation. Alpha-SMA- and drebrin-containing septal interstitial cells (termed septal myofibroblast-like cells) may play an important role in alveolar formation.     

Status of the capillary endothelium of the lung in normovolemic perfusion of the organ through the system of the pulmonary artery

Pulmonary perfusion for 30 min to the dog under conditions of normovolemia is not accompanied with any essential changes in parameters of alveolar capillaries endothelium. Just the opposite, transformation of endothelial lining of the peribronchial capillaries demonstrates possible disturbances of the liquor transport across the walls of these vessels. The volumetric part of the interstitial space near these capillaries increases, while in the alveolar septa it does not change. In lymph formation, flowing out of the lung, together with bronchial capillaries, blood capillaries of the alveoli must take part.     

A scanning and transmission electron microscopic study of the bat lung

The lungs of two adult species of bat Epomophorus wahlbergi and Miniopterus minor fixed with 2.3% glutaraldehyde were processed for SEM (scanning electron microscope) and TEM (transmission electron microscope) examination by the standard procedures. The bat lung comprised a blood and air conducting zone (consisting of bronchi, bronchioles and large blood vessels), the intermediate zone (made up of alveolar ducts), and the respiratory zone, which consisted of alveoli and blood capillaries. The interalveolar septa comprised basically granular pneumocytes (type II cells), squamous pneumocytes (type I cells), endothelial cells, and, in the interstitium, collagen and elastic fibres with occasional fibrocytes. Blood capillaries were interposed in the interalveolar septa, thus bulging into adjacent alveoli. It was noted that grossly, architecturally and structurally, the bat lung was similar to that of a terrestrial mammal. However, in previous morphometric and physiological studies it has been found that bats have a large lung, a thin pulmonary blood-gas barrier, a large pulmonary capillary blood volume, and high haematocrit and haemoglobin concentration. The bat lung, while retaining the basic mammalian pulmonary design, is well adapted to provide the large amount of oxygen demanded by flight. The avian pulmonary design (the lung-air sac system) is thus not a prerequisite to flight.     

Effect of alveolar liquid on distribution of blood flow in dog lungs.

Previous studies have shown that a shift in blood flow away from edematous regions does not occur until the alveoli contain liquid. The present experiments were designed to examine the separate effect of air space liquid, air space plus interstitial liquid, and reduced lung volume on blood flow. We found that reduced lung volume was not associated with significant changes in blood flow and that no systematic change in blood flow occurred when alveoli were filled with isosmotic liquid (autologous plasma). However, when hyposmotic liquid (dilute plasma) was instilled so that both the air space and the alveolar wall interstitial space were filled, blood flow was systematically reduced. This suggested that interstitial liquid was responsible raising vascular resistance in these experiments and that it might also be important in raising local vascular resistance in pulmonary edema. This latter hypothesis was tested in isolated perfused lobes where rapid freezing and quantitative histology showed that the number of open capillaries was significantly reduced in the liquid-filled alveoli (P less than 0.001). These observations suggest that interstitial pressure rises in pulmonary edema with the result that the transmural pressure of the alveolar vessels falls and vascular resistance is increased.     

Theoretical modeling of the interaction between alveoli during inflation and deflation in normal and diseased lungs

Alveolar recruitment is a central strategy in the ventilation of patients with acute lung injury and other lung diseases associated with alveolar collapse and atelectasis. However, biomechanical insights into the opening and collapse of individual alveoli are still limited. A better understanding of alveolar recruitment and the interaction between alveoli in intact and injured lungs is of crucial relevance for the evaluation of the potential efficacy of ventilation strategies. We simulated human alveolar biomechanics in normal and injured lungs. We used a basic simulation model for the biomechanical behavior of virtual single alveoli to compute parameterized pressure–volume curves. Based on these curves, we analyzed the interaction and stability in a system composed of two alveoli. We introduced different values for surface tension and tissue properties to simulate different forms of lung injury. The data obtained predict that alveoli with identical properties can coexist with both different volumes and with equal volumes depending on the pressure. Alveoli in injured lungs with increased surface tension will collapse at normal breathing pressures. However, recruitment maneuvers and positive endexpiratory pressure can stabilize those alveoli, but coexisting unaffected alveoli might be overdistended. In injured alveoli with reduced compliance collapse is less likely, alveoli are expected to remain open, but with a smaller volume. Expanding them to normal size would overdistend coexisting unaffected alveoli. The present simulation model yields novel insights into the interaction between alveoli and may thus increase our understanding of the prospects of recruitment maneuvers in different forms of lung injury.     

Flow through zone 1 lungs utilizes alveolar corner vessels

We have previously observed flows equivalent to 15% of the resting cardiac output of rabbits occurring through isolated lungs that were completely in zone 1. To distinguish between alveolar corner vessels and alveolar septal vessels as a possible zone 1 pathway, we made in vivo microscopic observations of the subpleural alveolar capillaries in five anesthetized dogs. Videomicroscopic recordings were made via a transparent thoracic window with the animal in the right lateral position. From recordings of the uppermost surface of the left lung, alveolar septal and corner vessels were classified depending on whether they were located within or between alveoli, respectively. Observations were made with various levels of positive end-expiratory pressure (PEEP) applied only to the left lung via a double-lumen endotracheal tube. Consistent with convention, flow through septal vessels stopped when PEEP was raised to the mean pulmonary arterial pressure (the zone 1-zone 2 border). However, flow through alveolar corner vessels continued until PEEP was 8-16 cmH2O greater than mean pulmonary arterial pressure (8-16 cm into zone 1). These direct observations support the idea that alveolar corner vessels rather than patent septal vessels provide the pathway for blood flow under zone 1 conditions.     

Morphometric changes in the human pulmonary acinus during inflation

Despite decades of research into the mechanisms of lung inflation and deflation, there is little consensus about whether lung inflation occurs due to the recruitment of new alveoli or by changes in the size and/or shape of alveoli and alveolar ducts. In this study we use in vivo (3)He lung morphometry via MRI to measure the average alveolar depth and alveolar duct radius at three levels of inspiration in five healthy human subjects and calculate the average alveolar volume, surface area, and the total number of alveoli at each level of inflation. Our results indicate that during a 143 ± 18% increase in lung gas volume, the average alveolar depth decreases 21 ±5%, the average alveolar duct radius increases 7 ± 3%, and the total number of alveoli increases by 96 ± 9% (results are means ± SD between subjects; P < 0.001, P < 0.01, and P < 0.00001, respectively, via paired t-tests). Thus our results indicate that in healthy human subjects the lung inflates primarily by alveolar recruitment and, to a lesser extent, by anisotropic expansion of alveolar ducts.     

Morphological and pharmacological basis for pulmonary ventilation in Amphiuma tridactylum

Summary The lung of the giant salamander, Amphiuma tridactylum, is divided into respiratory alveoli by muscular septa that increase the surface area of the lung as well as provide a mechanism for its almost complete collapse during exhalation. The epithelium of the internal surface is of two types: respiratory, composed of a single layer of pneumocytes overlying anastomosing capillaries, and non-respiratory, composed of ciliated cells and mucus-secreting goblet cells. Non-respiratory epithelium covers the apical edges of the septa, whereas the respiratory epithelium lines the alveoli. The smooth muscle of the septa and walls of the lung was studied in preparations of uninflated and acetylcholine-contracted lung. The muscle cells are ultrastructurally similar to other types of smooth muscle but are surrounded by extraordinary amounts of extracellular matrix, containing collagen and elastic fibers and numerous fine fibrils of unknown composition. Smooth muscle in isolated lung strips contracted in a dose-dependent manner when treated with acetylcholine or methacholine; contraction was blocked by atropine. Responses of lung strips to adrenergic agents were limited; only high doses of adrenalin caused slight relaxation of previously contracted muscle. These observations support the hypothesis that contraction of pulmonary smooth muscle is responsible for the ventilatory efficiency of the lung.     

Arguments against and alternatives for an extracellular surfactant layer in the alveoli of mammalian lung

It is generally believed that lung alveoli contain an extracellular aqueous layer of surfactant material, which is allegedly required to prevent alveolar collapse at small lung volume; the surfactant's major constituent is a fully saturated phospholipid, referred to as dipalmitoyl lecithin or DPL. I herein demonstrate that the surfactant hypothesis of alveolar stability is fundamentally wrong. Although DPL is synthesized inside type II epithelial cells and stored in the typical inclusion bodies therein and lowers surface tension to zero in the surface balance, there is no evidence to the effect that type II cells secrete the DPL surfactant into the aqueous intra-alveolar layer which is shown by electron microscopy in support of the surfactant theory. To the contrary, all the evidence indicates that, when seen, such an extracellular layer is an artifact. This is probably upon the damage glutaraldehyde inflicts onto alveolar structures during fixation of air-inflated lung tissue. Furthermore, several cogent arguments invalidate the belief that an extracellular layer of DPL and serum proteins is present in the alveoli of normal lung. In light of these arguments, a surface tension role of DPL in alveolar stability is excluded. Three hypotheses for an alternative role of DPL in respiration mechanics are proposed. They are: (a) alveolar clearance by viscolytic and surfactant action (bubble or foam formation) on the aqueous systems which are present in lung alveoli during edema and in prenatal life and which would otherwise be impervious to air; (b) homeostasis of blood palmitate in normal lung; (c) modulation of the elasticity of terminal lung tissue by the intact inclusion bodies and parts thereof inside type II cells in normal lung.     

Differential and specific labeling of epithelial and vascular endothelial cells of the rat lung by Lycopersicon esculentum and Griffonia simplicifolia I lectins

In the rat lung, we found that the Lycopersicon esculentum (LEA) lectin specifically binds to the epithelium of bronchioles and alveoli whereas Griffonia simplicifolia I (GS-I) binds to the endothelium of alveolar capillaries. The differential binding affinity of these lectins was examined on semithin (approximately 0.5 microns) and thin (less than 0.1 (microns) frozen sections of rat lung lavaged to remove alveolar macrophages. On semithin frozen sections, LEA bound to epithelial cells lining bronchioles and the alveoli (type I, but not type II epithelial cells). On thin frozen sections, biotinylated Lycopersicon esculentum (bLEA)-streptavidin-gold conjugates were confined primarily to the luminal plasmalemma of type I cells. bGS-I-streptavidin-Texas Red was detected on the endothelial cells of alveolar capillaries and postcapillary venules but not on those of larger venules, veins or arterioles. By electron microscopy, GS-I-streptavidin-gold complexes were localized primarily to the luminal plasmalemma of thick and thin regions of the capillary endothelium. Neither lectin labeled type II alveolar cells, but both lectins labeled macrophages in the interstitia and in incompletely lavaged alveoli.     

A scanning and transmission electron microscopic study of the lung of a caecilian Boulengerula taitanus

The lung of an apodan amphibian Bouiengerula taitanus has been investigated by scanning and transmission electron microscopy. This caecilian has only a single, long tubular lung that tapers towards the caudal end of the body. The lung has a central air duct which radially opens into a single stratum of alveoli lined by well developed septa that attach to two diametrically opposite trabeculae. The trabeculae carry the pulmonary artery and vein. The septa have blood capillaries on both surfaces and supportive and contractile elements like collagen, smooth muscle, elastic tissue and fibrocytes. The alveolar surface has only a single population of pneumocytes that combine the morphological features of the mammalian type 1 and 2 cells, i.e. they contain the osmiophilic, lamellated bodies and are squamous in form. Through subepithelial cytoplasmic invaginations, the pneumocytes, together with their basement lamina, were observed to be firmly attached to the septa1 tissue elements, presumably to avoid mechanical detachment during the rapid respiratory movements. The compartmentation of the whole lung in this species is viewed as a means of increasing the surface area available for gas exchange which, coupled with other already established cardiovascular adaptations in this species, may be of significance in its fossorial mode of life, an environment that is usually hypoxic and hypercarbic.     

Electron microscopy of lung rapidly frozen under controlled physiological conditions

Light microscopy of lung rapidly frozen under controlled physiological conditions has been very successful in correlating pulmonary structure and function. However, to study some aspects of pulmonary capillary morphology, the higher resolution of electron microscopy (EM) is necessary. To date, most EM of lung has involed the instillation of a fixative through the airways or vascular system, techniques that probably alter the normal pressure relationships of the capillaries and therefore their morphology. We describe here a technique for rapidly freezing lung to a depth of 1--2 mm below the pleural surface and preparing sections for EM. Lungs from open-chest rats were frozen at various transpulmonary pressures with cold (--80 degrees C) 70% ethylene glycol. Small pieces were then fixed with a solution containing glutaraldehyde and paraformaldehyde for 24 h at --50 degrees C. Staining was with osmium tetroxide and uranyl acetate. Lung frozen at high volumes showed marked stretching of the alveolar septa with severe deformation of the capillaries. Lung frozen at low inflation pressures revealed open capillaries containing numerous red blood cells; in addition, infolding of the alveolar wall was frequently seen. We conclude that this technique gives a level of preservation of rapidly frozen lung suitable for electron microscopy.     

Acellular hemoglobin solution enters compressed lung capillaries more readily than red blood cells.

High lung inflation pressures compress alveolar septal capillaries, impede red cell transit, and interfere with oxygenation. However, recently introduced acellular hemoglobin solutions may enter compressed lung capillaries more easily than red blood cells. To test this hypothesis, we perfused isolated rat lungs with fluorescently labeled diaspirin cross-linked hemoglobin (DCLHb; 10%) and/ or autologous red cells (hematocrit, 20). Septal capillaries were compressed by setting lung inflation pressure above vascular pressures (zone 1). Examination by confocal microscopy showed that DCLHb was distributed throughout alveolar septa. Furthermore, this distribution was not affected by adding red blood cells to the perfusate. We estimated the maximum acellular hemoglobin mass within septa to be equivalent to that of 15 red blood cells. By comparison, we found an average of 2.7 +/- 4.6 red cells per septum in zone 1. These values increased to 30.4 +/- 25.8 and 50.4 +/- 22.1 cells per septum in zones 2 and 3, respectively. We conclude that perfusion in zone 1 with a 10% acellular hemoglobin solution may increase the hemoglobin concentration per septum up to fivefold compared with red cell perfusion.     

The morphology of the lung of the African lungfish,Protopterus aethiopicus

Summary The lung of the African lungfish (Protopterus aethiopicus) is paired, long and cylindrical. It is situated on the dorsal aspect of the coelomic cavity ventral to the ribs. Much of the gas exchange tissue is found in the proximal aspect of the lung with the caudal part largely taken up by a centrally situated air-duct with a few large peripherally located alveoli. Interalveolar septa, arranged at differing hierarchical levels from the air-duct, subdivide the lung into alveoli, the gas exchange compartments. The alveolar surface is covered by some cells characterized by microvilli on their free surface, while others are devoid of such structures. The general organization of the lung of Protopterus aethiopicus is similar to that of the other genera of Dipnoi, Neoceratodus and Lepidosiren, with the interalveolar septa increasing the surface area for gas exchange through pulmonary compartmentation. The abundant septal smooth muscle fibres and elastic tissue may contribute to the physiomechanical compliance of the lung. The undifferentiated alveolar pneumocytes and the double capillary system, observed in Protopterus, in general appear to characterize the very primitive lungs of the lower air-breathing vertebrates.     

Size distribution of recruited alveolar volumes in airway reopening.

In 11 isolated dog lung lobes, we studied the size distribution of recruited alveolar volumes that become available for gas exchange during inflation from the collapsed state. Three catheters were wedged into 2-mm-diameter airways at total lung capacity. Small-amplitude pseudorandom pressure oscillations between 1 and 47 Hz were led into the catheters, and the input impedances of the regions subtended by the catheters were continuously recorded using a wave tube technique during inflation from -5 cm H(2)O transpulmonary pressure to total lung capacity. The impedance data were fit with a model to obtain regional tissue elastance (Eti) as a function of inflation. First, Eti was high and decreased in discrete jumps as more groups of alveoli were recruited. By assuming that the number of opened alveoli is inversely proportional to Eti, we calculated from the jumps in Eti the distribution of the discrete increments in the number of opened alveoli. This distribution was in good agreement with model simulations in which airways open in cascade or avalanches. Implications for mechanical ventilation may be found in these results.     

Three-dimensional convective alveolar flow induced by rhythmic breathing motion of the pulmonary acinus

Low Reynolds number flows (Re<1) in the human pulmonary acinus are often difficult to assess due to the submillimeter dimensions and accessibility of the region. In the present computational study, we simulated three-dimensional alveolar flows in an alveolated duct at each generation of the pulmonary acinar tree using recent morphometric data. Rhythmic lung expansion and contraction motion was modeled using moving wall boundary conditions to simulate realistic sedentary tidal breathing. The resulting alveolar flow patterns are largely time independent and governed by the ratio of the alveolar to ductal flow rates, Qa/Qd. This ratio depends uniquely on geometrical configuration such that alveolar flow patterns may be entirely determined by the location of the alveoli along the acinar tree. Although flows within alveoli travel very slowly relative to those in acinar ducts, 0.021%相似文献