Massive hemorrhage causes changes in morphometric parameters of lung capillaries and concentration of leukocytes in microvasculature

Implications of microscale lung damage for COVID-19 pulmonary ventilation dynamics: A narrative review

The COVID-19 pandemic surges on as vast research is produced to study the novel SARS-CoV-2 virus and the disease state it induces. Still, little is known about the impact of COVID-19-induced microscale damage in the lung on global lung dynamics. This review summarizes the key histological features of SARS-CoV-2 infected alveoli and links the findings to structural tissue changes and surfactant dysfunction affecting tissue mechanical behavior similar to changes seen in other lung injury. Along with typical findings of diffuse alveolar damage affecting the interstitium of the alveolar walls and blood-gas barrier in the alveolar airspace, COVID-19 can cause extensive microangiopathy in alveolar capillaries that further contribute to mechanical changes in the tissues and may differentiate it from previously studied infectious lung injury. Understanding microlevel damage impact on tissue mechanics allows for better understanding of macroscale respiratory dynamics. Knowledge gained from studies into the relationship between microscale and macroscale lung mechanics can allow for optimized treatments to improve patient outcomes in case of COVID-19 and future respiratory-spread pandemics.

Assessing the Effects of Fibrosis on Lung Function by Light Microscopy-Coupled Stereology

Pulmonary diseases such as fibrosis are characterized by structural abnormalities that lead to impairment of proper lung function. Stereological analysis of serial tissue sections allows detection and quantitation of subtle changes in lung architecture. Here, we describe a stereology-based method of assessing pathology-induced changes in lung structure.

Lung morphometry: the link between structure and function

The study of the structural basis of gas exchange function in the lung depends on the availability of quantitative information that concerns the structures establishing contact between the air in the alveoli and the blood in the alveolar capillaries, which can be entered into physiological equations for predicting oxygen uptake. This information is provided by morphometric studies involving stereological methods and allows estimates of the pulmonary diffusing capacity of the human lung that agree, in experimental studies, with the maximal oxygen consumption. The basis for this "machine lung" structure lies in the complex design of the cells building an extensive air-blood barrier with minimal cell mass.

Lung Structure and the Intrinsic Challenges of Gas Exchange

Structural and functional complexities of the mammalian lung evolved to meet a unique set of challenges, namely, the provision of efficient delivery of inspired air to all lung units within a confined thoracic space, to build a large gas exchange surface associated with minimal barrier thickness and a microvascular network to accommodate the entire right ventricular cardiac output while withstanding cyclic mechanical stresses that increase several folds from rest to exercise. Intricate regulatory mechanisms at every level ensure that the dynamic capacities of ventilation, perfusion, diffusion, and chemical binding to hemoglobin are commensurate with usual metabolic demands and periodic extreme needs for activity and survival. This article reviews the structural design of mammalian and human lung, its functional challenges, limitations, and potential for adaptation. We discuss (i) the evolutionary origin of alveolar lungs and its advantages and compromises, (ii) structural determinants of alveolar gas exchange, including architecture of conducting bronchovascular trees that converge in gas exchange units, (iii) the challenges of matching ventilation, perfusion, and diffusion and tissue-erythrocyte and thoracopulmonary interactions. The notion of erythrocytes as an integral component of the gas exchanger is emphasized. We further discuss the signals, sources, and limits of structural plasticity of the lung in alveolar hypoxia and following a loss of lung units, and the promise and caveats of interventions aimed at augmenting endogenous adaptive responses. Our objective is to understand how individual components are matched at multiple levels to optimize organ function in the face of physiological demands or pathological constraints.

How common is the lipid body-containing interstitial cell in the mammalian lung?

Pulmonary lipofibroblasts are thought to be involved in lung development, regeneration, vitamin A storage, and surfactant synthesis. Most of the evidence for these important functions relies on mouse or rat studies. Therefore, the present study was designed to investigate the presence of lipofibroblasts in a variety of early postnatal and adult mammalian species (including humans) to evaluate the ability to generalize functions of this cell type for other species. For this purpose, lung samples from 14 adult mammalian species as well as from postnatal mice, rats, and humans were investigated using light and electron microscopic stereology to obtain the volume fraction and the total volume of lipid bodies. In adult animals, lipid bodies were observed only, but not in all rodents. In all other species, no lipofibroblasts were observed. In rodents, lipid body volume scaled with body mass with an exponent b = 0.73 in the power law equation. Lipid bodies were not observed in postnatal human lungs but showed a characteristic postnatal increase in mice and rats and persisted at a lower level in the adult animals. Among 14 mammalian species, lipofibroblasts were only observed in rodents. The great increase in lipid body volume during early postnatal development of the mouse lung confirms the special role of lipofibroblasts during rodent lung development. It is evident that the cellular functions of pulmonary lipofibroblasts cannot be transferred easily from rodents to other species, in particular humans.

A first principles calculation of the oxygen uptake in the human pulmonary acinus at maximal exercise

It has recently been shown that the acinus can have a reduced efficiency due to a "screening effect" governed by the ratio of oxygen diffusivity to membrane permeability, the gas flow velocity, as well as the size and configuration of the acinus. We present here a top to bottom calculation of the functioning of a machine acinus at exercise that takes this screening effect into account. It shows that, given the geometry and the breathing dynamics of real acini, respiration can be correlated to a single equivalent parameter that we call the integrative permeability. In particular we find that both V(O(2,max)) and PA(O(2)) depend on this permeability in a non-linear manner. Numerical solutions of dynamic convection-diffusion equations indicate that only a narrow range of permeability values is compatible with the experimental measurements of PA(O(2)) and V(O(2,max)). These permeability values are significantly smaller than those found in the literature. In a second step, we present a new type of evaluation of the diffusive permeability, yielding values compatible with the top to bottom approach, but smaller than the usual morphometric value.

Imaging of the mouse lung with scanning laser optical tomography (SLOT)

The current study focuses on the use of scanning laser optical tomography (SLOT) in imaging of the mouse lung ex vivo. SLOT is a highly efficient fluorescence microscopy technique allowing rapid scanning of samples of a size of several millimeters, thus enabling volumetric visualization by using intrinsic contrast mechanisms of previously fixed lung lobes. Here, we demonstrate the imaging of airways, blood vessels, and parenchyma from whole, optically cleared mouse lung lobes with a resolution down to the level of single alveoli using absorption and autofluorescence scan modes. The internal structure of the lung can then be analyzed nondestructively and quantitatively in three-dimensional datasets in any preferred planar orientation. Moreover, the procedure preserves the microscopic structure of the lung and allows for subsequent correlative histologic studies. In summary, the current study has shown that SLOT is a valuable technique to study the internal structure of the mouse lung.

Optimized murine lung preparation for detailed structural evaluation via micro-computed tomography

Utilizing micro-X-ray CT (μCT) imaging, we sought to generate an atlas of in vivo and intact/ex vivo lungs from normal murine strains. In vivo imaging allows visualization of parenchymal density and small airways (15-28 μm/voxel). Ex vivo imaging of the intact lung via μCT allows for improved understanding of the three-dimensional lung architecture at the alveolar level with voxel dimensions of 1-2 μm. μCT requires that air spaces remain air-filled to detect alveolar architecture while in vivo structural geometry of the lungs is maintained. To achieve these requirements, a fixation and imaging methodology that permits nondestructive whole lung ex vivo μCT imaging has been implemented and tested. After in vivo imaging, lungs from supine anesthetized C57Bl/6 mice, at 15, 20, and 25 cmH(2)O airway pressure, were fixed in situ via vascular perfusion using a two-stage flushing system while held at 20 cmH(2)O airway pressure. Extracted fixed lungs were air-dried. Whole lung volume was acquired at 1, 7, 21, and >70 days after the lungs were dried and served as validation for fixation stability. No significant shrinkage was observed: +8.95% change from in vivo to fixed lung (P = 0.12), -1.47% change from day 1 to day 7 (P = 0.07), -2.51% change from day 1 to day 21 (P = 0.05), and -4.90% change from day 1 to day 70 and thereafter (P = 0.04). μCT evaluation showed well-fixed alveoli and capillary beds correlating with histological analysis. A fixation and imaging method has been established for μCT imaging of the murine lung that allows for ex vivo morphometric analysis, representative of the in vivo lung.

Allometry of the mammalian intracellular pulmonary surfactant system

Alveolar epithelial (AE) surface area is closely correlated with body mass (BM) in mammals. The AE is covered by a surfactant layer produced by alveolar epithelial type II (AE2) cells. We hypothesized that the total number of AE2 cells and the volume of intracellular surfactant-storing lamellar bodies (Lb) are correlated with BM with a similar slope as AE surface area. We used light and electron microscopic stereology to estimate the number and mean volume of AE2 cells and the total volume of Lb in 12 mammalian species ranging from 2 to 3 g (Etruscan shrew) to 400–500 kg (horse) BM. The mean size of Lb was evaluated using the volume-weighted mean volume and the volume-to-surface ratio of Lb. The mean volume of AE2 cells was 500–600 μm3 in most species, but was higher in Etruscan shrew, guinea pig, and human lung. The mean volume of Lb per AE2 cell was 80–100 μm3 in most species, with the same exceptions as above. However, the total number of AE2 cells and the total volume of Lb were closely correlated with BM and exhibited an allometric relationship similar to the slope of AE surface area. The mean size of Lb was similar in all investigated species. In conclusion, the mean volume of AE2 cells and their Lb are independent of BM but show some interspecific variations. The adaptation of the intracellular surfactant pool size to BM is obtained by the variation of the number of AE2 cells in the lung.

Is length an appropriate estimator to characterize pulmonary alveolar capillaries? A critical evaluation in the human lung

Stereological estimations of total capillary length have been used to characterize changes in the alveolar capillary network (ACN) during developmental processes or pathophysiological conditions. Here, we analyzed whether length estimations are appropriate to describe the 3D nature of the ACN. Semi-thin sections of five human lungs, previously investigated by Gehr et al. (Respir Physiol 1978; 32:121-140), were used to estimate alveolar capillary length using a "design-based" or a "model-based" stereological approach. The design-based approach involves counting of capillary profiles related to a defined area of the reference space. The model-based approach bases on the assumption that capillaries are round tubes and length was calculated from capillary volume and surface area. The model-based approach provided a mean of 6,950 km (SD: 3,108 km) for total capillary length, the design-based approach resulted in a mean of 2,746 km (SD: 722 km). Because of the geometry of the ACN both approaches carry an unpredictable bias. The bias incurred by the design-based approach is proportional to the ratio between radius and length of the capillary segments in the ACN, the number of branching points and the winding of the capillaries. The model-based approach is biased because of the real noncylindrical shape of capillaries and the network structure. In conclusion, the estimation of the total length of capillaries in the ACN cannot be recommended as the geometry of the ACN does not fulfill the requirements for stereological length estimation. Until new methods are being developed, the unbiased estimates of capillary volume, and surface area should be preferred.

Morphometry and allometry of the postnatal marsupial lung development: an ultrastructural study

An utrastructural morphometric study of the postnatally remodelling lungs of the quokka wallaby (Setonix brachyurus) was undertaken. Allometric scaling of the volumes of the parenchymal components against body mass was performed. Most parameters showed a positive correlation with body mass in all the developmental stages, except the volume of type II pneumocytes during the alveolar stage. The interstitial tissue and type II cell volumes increased slightly faster than body mass in the saccular stage, their growth rates declining in the alveolar stage. Conversely, type I pneumocyte volumes increased markedly in both the saccular and alveolar stages. Both capillary and endothelial volumes as well as the capillary and airspace surface areas showed highest rates of increase during the alveolar stage, at which time the rate was notably higher than that of the body mass. The pulmonary diffusion capacity increased gradually, the rate being highest in the alveolar stage and the adult values attained were comparable to those of eutherians.

Chronic hypoxia causes angiogenesis in addition to remodelling in the adult rat pulmonary circulation

Chronic hypoxia caused by migration of native sea-level dwellers to high altitude or chronic lung disease leads to the development of increased pulmonary vascular resistance and pulmonary hypertension. This altitude-induced hypertension offers no obvious benefit and may indeed be maladaptive. A major mechanism thought to contribute to the development of pulmonary hypertension is hypoxia-induced loss of small blood vessels, sometimes termed rarefaction or pruning. More recent evidence caused us to question this widely accepted concept including the potent angiogenic effect of chronic hypoxia in all other vascular beds and the demonstration that new vessels can form in the pulmonary circulation when stimulated by chronic infection and lung resection. We tested the hypothesis that chronic environmental hypoxia causes angiogenesis in the adult pulmonary circulation by using stereological techniques combined with confocal microscopy to examine the resultant changes in pulmonary vascular structure in rats. We found that chronic hypoxia resulted in increased total pulmonary vessel length, volume, endothelial surface area and number of endothelial cells in vivo. This is the first reported demonstration of hypoxia-induced angiogenesis in the mature pulmonary circulation, a structural adaptation that may have important beneficial consequences for gas exchange. These findings imply that we must revise the widely accepted paradigm that hypoxia-induced loss of small vessels is a key structural change contributing to the development of pulmonary hypertension in high altitude adaptation and chronic lung disease.

Vascular immunotargeting to endothelial surface in a specific macrodomain in alveolar capillaries

A novel 85 kD glycoprotein (gp85) is a marker of the avesicular zone, a thin part of pulmonary endothelial cells separating alveolar and vascular compartments and lacking vesicles. This report presents the first evaluation whether mAb 30B3, a monoclonal antibody to gp85, can be used for targeting of drugs to the surface of lung endothelium. 125I-mAb 30B3 accumulated in isolated perfused lungs (IPL) (22.8 +/- 1.1 versus 0.5 +/- 0.1 %ID/g for 125I-IgG) and accumulated preferentially in the lungs after intravenous or intraarterial injection (10.9 +/- 0.7 and 11.0 +/- 1.5 versus 0.9 +/- 0.2 %ID/g for 125I-IgG). 125I-mAb 30B3 uptake in IPL was rapid (T1/2 15 min), saturable (Bmax appr. 10(5) molecules/cell), specific (inhibited by nonlabeled mAb 30B3) and temperature independent (26.3 +/- 2.1 versus 22.8 +/- 1.1 %ID/g at 6 degrees C versus 37 degrees C). Biotinylated mAb 30B3 permitted subsequent accumulation of perfused avidin derivative in IPL. Because these data indicated that mAb 30B3 binds to an accessible, poorly internalizable antigen in the lung, we conjugated mAb 30B3 with a plasminogen activator, 125I-tPA. After intravenous injection in rats, lung-to-blood ratio was 8.4 +/- 0.9 for mAb 30B3/125I-tPA versus 0.4 +/- 0.1 for IgG/125I-tPA, indicating that mAb 30B3 may deliver drugs, which was supposed to exert therapeutic action in the vascular lumen (e.g., antithrombotic proteins), to the surface of pulmonary endothelium.

Lung uptake of antibodies to endothelial antigens: key determinants of vascular immunotargeting

Vascular immunotargeting is a mean for a site-selective delivery of drugs and genes to endothelium. In this study, we compared recognition of pulmonary and systemic vessels in rats by candidate carrier monoclonal antibodies (MAbs) to endothelial antigens platelet endothelial cell adhesion molecule (PECAM)-1 (CD31), intercellular adhesion molecule (ICAM)-1 (CD54), Thy-1.1 (CD90.1), angiotensin-converting enzyme (ACE; CD143), and OX-43. Tissue immunostaining showed that endothelial cells were Thy-1.1 positive in capillaries but negative in large vessels. In the lung, anti-ACE MAb provided a positive staining in 100% capillaries vs. 5–20% capillaries in other organs. Other MAbs did not discriminate between pulmonary and systemic vessels. We determined tissue uptake after infusion of 1 μg of 125I-labeled MAbs in isolated perfused lungs (IPL) or intravenously in intact rats. Uptake in IPL attained 46% of the injected dose (ID) of anti-Thy-1.1 and 20–25% ID of anti-ACE, anti-ICAM-1, and anti-OX-43 (vs. 0.5% ID of control IgG). However, after systemic injection at this dose, only anti-ACE MAb 9B9 displayed selective pulmonary uptake (16 vs. 1% ID/g in other organs). Anti-OX-43 displayed low pulmonary (0.5% ID/g) but significant splenic and cardiac uptake (7 and 2% ID/g). Anti-Thy-1.1 and anti-ICAM-1 displayed moderate pulmonary (4 and 6% ID/g, respectively) and high splenic and hepatic uptake (e.g., 18% ID/g of anti-Thy-1.1 in spleen). The lung-to-blood ratio was 5, 10, and 15 for anti-Thy-1.1, anti-ACE, and anti-ICAM-1, respectively. PECAM antibodies displayed low pulmonary uptake in perfusion (2% ID) and in vivo (3–4% ID/g). However, conjugation with streptavidin (SA) markedly augmented pulmonary uptake of anti-PECAM in perfusion (10–54% ID, depending on an antibody clone) and in vivo (up to 15% ID/g). Therefore, ACE-, Thy-1.1-, ICAM-1-, and SA-conjugated PECAM MAbs are candidate carriers for pulmonary targeting. ACE MAb offers a high selectivity of pulmonary targeting in vivo, likely because of a high content of ACE-positive capillaries in the lungs.

The lung of the marmoset (Callithrix jacchus): ultrastructure and morphometric data

Owing to its small size (body weight 300-400 g), its modest demands on animal husbandry, and in particular its relatively long life-span (up to 12 years) the common marmoset (cotton ear marmoset: Callithrix jacchus (Cj)) might be a useful animal model to study the adaptive capacity to different energetic demands, adverse environmental influences such as air pollution, and aging of the lung. In order to describe the gas exchange apparatus of healthy marmosets as a basis for further pulmonary research, the lungs of three young adult animals have been analysed both qualitatively and quantitatively (by morphometry) at the light and electronmicroscopic level. Qualitatively, there is a general similarity in the architecture and structure of lung parenchyma between marmosets and other mammals. Quantitatively, the alveolar surface area was found to be 7662+/-1647 cm(2). Capillary surface area and volume were 6000+/-1549 cm(2), and 1.01+/-0.34 ml, respectively. The harmonic mean thickness of the air-blood barrier was 0.517+/-0.117 microm. These morphometric parameters allowed to estimate the diffusing capacity for oxygen at 0.0299+/-0.0134 ml O(2) (sec mmHg)(-1). In comparison with mammals of similar body size (rats, quinea pigs) it appears that the marmoset has a higher gas exchanging capacity of the lung, which might reflect the 'athletic' activity of this small primate. An incidental finding worth mentioning is the individual variability of septal structures due to variations in capillary blood volume and hematocrit. The distinction between such functional variations and subtle pathologic alterations of lung tissue requires a morphometric analysis at the electron-microscopic level.

Adaptive hypoxic tolerance in the subterranean mole rat Spalax ehrenbergi: the role of vascular endothelial growth factor

Spalax ehrenbergi has evolved adaptations that allow it to survive and carry out normal activities in a highly hypoxic environment. A key component of this adaptation is a higher capillary density in some Spalax tissues resulting in a shorter diffusion distance for oxygen. Vascular endothelial growth factor (VEGF) is an angiogenic factor that is critical for angiogenesis during development and in response to tissue ischemia. We demonstrate here that VEGF expression is markedly increased in those Spalax tissues with a higher capillary density relative to the normal laboratory rat Rattus norvegicus. Upregulation of VEGF thus appears to be an additional mechanism by which Spalax has adapted to its hypoxic environment.

Red cell distortion and conceptual basis of diffusing capacity estimates: finite element analysis

To understand the effects of dynamic shape distortion of red blood cells (RBCs) as it develops under high-flow conditions on the standard physiological and morphometric methods of estimating pulmonary diffusing capacity, we computed the uptake of CO across a two-dimensional geometric capillary model containing a variable number of equally spaced RBCs. RBCs are circular or parachute shaped, with the same perimeter length. Total CO diffusing capacity (DLCO) and membrane diffusing capacity (DMCO) were calculated by a finite element method. DLCO calculated at two levels of alveolar PO2 were used to estimate DMCO by the Roughton-Forster (RF) technique. The same capillary model was subjected to morphometric analysis by the random linear intercept method to obtain morphometric estimates of DMCO. Results show that shape distortion of RBCs significantly reduces capillary diffusive gas uptake. Shape distortion exaggerates the conceptual errors inherent in the RF technique (J. Appl. Physiol. 79: 1039-1047, 1995); errors are exaggerated at a high capillary hematocrit. Shape distortion also introduces additional error in morphometric estimates of DMCO caused by a biased sampling distribution of random linear intercepts; errors are exaggerated at a low capillary hematocrit.

Working underground: respiratory adaptations in the blind mole rat

Mole rats (Spalax ehrenbergi superspecies) perform the heavy work of digging their subterranean burrows in Israel under highly hypoxic/hypercapnic conditions. Unlike most other mammals, they can achieve high levels of metabolic rate under these conditions, while their metabolic rate at low work rates is depressed. We explored, by comparing mole rats with white rats, whether and how this is related to adaptations in the design of the respiratory system, which determines the transfer of O2 from the lung to muscle mitochondria. At the same body mass, mole rats were found to have a significantly smaller total skeletal muscle mass than ordinary white rats (-22%). In contrast, the fractional volume of muscle mitochondria was larger by 46%. As a consequence, both species had the same total amount of mitochondria and achieved, under normoxia, the same V(O2max). Whereas the O2 transport capacity of the blood was not different, we found a larger capillary density (+31%) in the mole rat muscle, resulting in a reduced diffusion distance to mitochondria. The structural pulmonary diffusing capacity for O2 was greater in the mole rat (+44%), thus facilitating O2 uptake in hypoxia. We conclude that structural adaptations in lung and muscle tissue improve O2 diffusion conditions and serve to maintain high metabolic rates in hypoxia but have no consequences for achieving V(O2max) under normoxic conditions.

Morphometric model for pulmonary diffusing capacity. I. Membrane diffusing capacity

The pulmonary diffusing capacity is related to the quantitative design characteristics of the pulmonary gas exchanger. The current model for estimating DLO2 from morphometric data breaks the diffusion path for O2 into four steps, three of which represent the membrane part of DLO2. A critique of this model on the basis of newer evidence leads to a modification of the model where the path from the alveolar surface to the erythrocyte membrane is considered as a single step. The structural determinant of this model for DMO2 is the ratio of effective diffusion surface to effective total barrier thickness. The effective surface is formulated as a fraction of the alveolar surface area, the most robust measure of lung design, whereas the effective barrier thickness is the harmonic mean distance--or mean proximity--between alveolar surface and erythrocyte surface. The methods for obtaining the morphometric measurements are discussed. The results show that the new morphometric estimates of DMO2 are 33% lower than those obtained with the old model, resulting in a reduction of the estimates of DLO2 by 10-20%.