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The alveolar microvascular network is significant for the lung development of vertebrates, which is consisted of tree-like main veins and interconnected capillaries between the main veins. However, it is still unclear how the meshwork pattern is formed by the multigenerational branches. Based on the reaction-diffusion model, we find that the meshwork pattern is transformed from the branching pattern in a spherical shell domain. Furthermore, we find that it is the branch tip fusion that facilitates the meshwork pattern formation, which is guided by the activator peak fusion, and it is the insufficient space for branch tip growth that promotes the branch tip fusion. In addition, we notice that the consumption rate of substrate regulates the meshwork pattern formation and an asymmetric shell domain composed of prolate and oblate hemi-ellipsoidal shell facilitates the dense meshwork pattern formation. Our model provides a qualitative understanding of how the vascular system remodels from the branching pattern to the meshwork pattern and how to make a dense meshwork pattern. Both researches associated with branching and network morphogenesis can benefit from our work., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Lung morphogenesis is a highly orchestrated process beginning with the appearance of lung buds on approximately embryonic day 9.5 in the mouse. Endodermally derived epithelial cells of the primitive lung buds undergo branching morphogenesis to generate the tree-like network of epithelial-lined tubules. The pulmonary vasculature develops in close proximity to epithelial progenitor cells in a process that is regulated by interactions between the developing epithelium and underlying mesenchyme. Studies in transgenic and knockout mouse models demonstrate that normal lung morphogenesis requires coordinated interactions between cells lining the tubules, which end in peripheral saccules, juxtaposed to an extensive network of capillaries. Multiple growth factors, microRNAs, transcription factors, and their associated signaling cascades regulate cellular proliferation, migration, survival, and differentiation during formation of the peripheral lung. Dysregulation of signaling events caused by gene mutations, teratogens, or premature birth causes severe congenital and acquired lung diseases in which normal alveolar architecture and the pulmonary capillary network are disrupted. Herein, we review scientific progress regarding signaling and transcriptional mechanisms regulating the development of pulmonary vasculature during lung morphogenesis.

The documented data regarding the three-dimensional structure of the air capillaries (ACs), the ultimate sites of gas exchange in the avian lung is contradictory. Further, the mode of gas exchange, described as cross-current has not been clearly elucidated. We studied the temporal and spatial arrangement of the terminal air conduits of the chicken lung and their relationship with the blood capillaries (BCs) in embryos as well as the definitive architecture in adults. Several visualization techniques that included corrosion casting, light microscopy as well as scanning and transmission electron microscopy were used. Two to six infundibulae extend from each atrium and give rise to numerous ACs that spread centrifugally. Majority of the ACs are tubular structures that give off branches, which anastomose with their neighboring cognates. Some ACs have globular shapes and a few are blind-ending tapering tubes. During inauguration, the luminal aspects of the ACs are characterized by numerous microvillus-like microplicae, which are formed during the complex processes of cell attenuation and canalization of the ACs. The parabronchial exchange BCs, initially inaugurated as disorganized meshworks, are reoriented via pillar formation to lie predominantly orthogonal to the long axes of the ACs. The remodeling of the retiform meshworks by intussusceptive angiogenesis essentially accomplishes a cross-current system at the gas exchange interface in the adults, where BCs form ring-like patterns around the ACs, thus establishing a cross-current system. Our findings clarify the mode of gas exchange in the parabronchial mantle and illuminate the basis for the functional efficiency of the avian lung., (Copyright © 2010 Wiley-Liss, Inc.)

The juxtaposition of a dense capillary network to lung epithelial cells is essential for air-blood gas exchange. Defective lung vascular development can result in bronchopulmonary dysplasia and alveolar capillary dysplasia. Although vascular endothelial growth factor A (Vegfa) is required for formation of the lung capillary network, little is known regarding the factors that regulate the density and location of the distal capillary plexus and the expression pattern of Vegfa. Here, we show that fibroblast growth factor 9 (FGF9) and sonic hedgehog (SHH) signaling to lung mesenchyme, but not to endothelial cells, are each necessary and together sufficient for distal capillary development. Furthermore, both gain- and loss-of-function of FGF9 regulates Vegfa expression in lung mesenchyme, and VEGF signaling is required for FGF9-mediated blood vessel formation. FGF9, however, can only partially rescue the reduction in capillary density found in the absence of SHH signaling, and SHH is unable to rescue the vascular phenotype found in Fgf9(-/-) lungs. Thus, both signaling systems regulate distinct aspects of vascular development in distal lung mesenchyme. These data suggest a molecular mechanism through which FGF9 and SHH signaling coordinately control the growth and patterning of the lung capillary plexus, and regulate the temporal and spatial expression of Vegfa.

There is increasing evidence that epithelial-vascular interactions are essential for tissue patterning. Here we identified components of the molecular cross talk between respiratory epithelial cells and pulmonary capillaries necessary for the formation of the gas exchange surface of the lung. Selective inactivation of the Vegf-A gene in respiratory epithelium results in an almost complete absence of pulmonary capillaries, demonstrating the dependence of pulmonary capillary development on epithelium-derived Vegf-A. Deficient capillary formation in Vegf-A deficient lungs is associated with a defect in primary septae formation, a morphogenetic process critical for distal lung morphogenesis, coupled with suppression of epithelial cell proliferation and decreased hepatocyte growth factor (Hgf) expression. Lung endothelial cells express Hgf, and selective deletion of the Hgf receptor gene in respiratory epithelium phenocopies the malformation of septae, confirming the requirement for epithelial Hgf signaling in normal septae formation and suggesting that Hgf serves as an endothelium-derived factor that signals to the epithelium. Our findings support a mechanism for primary septae formation dependent on reciprocal interactions between respiratory epithelium and the underlying vasculature, establishing the dependence of pulmonary capillary development on epithelium-derived Vegf-A, and identify Hgf as a putative endothelium-derived factor that mediates the reciprocal signaling from the vasculature to the respiratory epithelium.

Pelicans produce altricial chicks that develop into some of the largest birds capable of sustained flight. We traced pulmonary morphological development in the Australian pelican, Pelicanus conspicillatus, from third trimester embryos to adults. We described growth and development with allometric relationships between lung components and body mass or lung volume, according to the equation y = ax(b). Pelican lung volume increased faster than body mass (b = 1.07). Relative to lung volume, the airways and vascular spaces increased allometrically (b > 1) in embryos, but isometrically (b approximately 1) after hatching. Parabronchial mantle volume decreased (b < 1) prior to hatching and increased isometrically thereafter. Surface area of air capillaries, blood capillaries and the blood-gas barrier increased relative to lung volume (b > 0.67) before and after hatching. Barrier thickness decreased before hatching, remained constant in juveniles and decreased by adulthood. The anatomical diffusing capacity significantly increased before hatching (b = 4.44) and after hatching (b = 1.26). Although altricial pelicans developed pulmonary complexity later than precocial turkeys, the volume-specific characteristics were similar. However, lungs of volant adult pelicans became significantly larger, with a greater capacity for gas exchange, than lungs of terrestrial turkeys. Exchange characteristics of growing pelican lungs were inferior to those of adult birds of 26 other species, but converged with them at maturity.

In the embryonic lung of the domestic fowl, Gallus gallus variant domesticus, hematogenetic and vasculogenetic cells become ultrastructurally clear from day 4 of development. In the former group of cells, filopodial extensions coalesce, cytoplasm thickens, and accumulating hemoglobin displaces the nucleus peripherally while in the latter, conspicuous filopodial extensions and large nuclei develop as the cells assume a rather stellate appearance. From day 5, erythrocytes and granular leukocytes begin forming from cytoarchitecturally cognate hematogenetic cells. The cells become distinguishable when hemoglobin starts to accumulate in the erythroblasts and electron dense bodies form in the leukoblasts. Vasculogenesis begins from day 7 in different areas of the developing lung: erthrocytes (but not granular leukocytes) appear to attract committed vasculogenetic cells (angioblasts) that form an endothelial lining and vessel wall. Arrangement of angioblasts around forming blood vessels sets the direction along which the vessels sprout (angiogenesis). In some areas of the developing lung, through what seems like an inductive erythropoietic process, arcades of erythrocytes organize. Once endothelial cells surround such continuities, discrete vascular units organize. By day 10, the major parts of the in-built (intrinsic) pulmonary vasculature are assembled. Complete pulmonary circulation (i.e., through the exchange tissue) is not established until after day 18 when the blood capillaries start to develop. Since the precursory erythrocytes do not have a respiratory role, it is imperative that de novo erythropoiesis is essential for vasculogenesis. Diffuse (fragmentary) development and subsequent piecemeal assembly of the pulmonary vascular system may explicate the fabrication of a complex circulatory architecture that grants cross-current, counter-current, and multicapillary serial arterialization designs in the exchange tissue of the avian lung. The exceptional respiratory efficiency of the avian lung is largely attributable to the geometries (physical interfacing) between the bronchial and vascular elements at different levels of morphological organization.

Formation of a thin blood-gas barrier in the respiratory (gas exchange) tissue of the lung of the domestic fowl, Gallus gallus variant domesticus commences on day 18 of embryogenesis. Developing from infundibulae, air capillaries radiate outwards into the surrounding mesenchymal (periparabronchial) tissue, progressively separating and interdigitating with the blood capillaries. Thinning of the blood-gas barrier occurs by growth and extension of the air capillaries and by extensive disintegration of mesenchymal cells that constitute transient septa that divide the lengthening and anastomosing air capillaries. After they contact, the epithelial and endothelial cells deposit intercellular matrix that cements them back-to-back. At hatching (day 21), with a thin blood-gas barrier and a large respiratory surface area, the lung is well prepared for gas exchange. In sites where air capillaries lie adjacent to each other, epithelial cells contact directly: intercellular matrix is lacking.

It has recently been postulated that platelet/endothelial cell adhesion molecule-1 (PECAM-1/CD31) might play a role in vascular tube formation. To evaluate the role of PECAM-1/CD31 in the formation of the capillary network in vivo, we conducted an ultrastructural immunohistochemical evaluation of the localization of PECAM-1/CD31 and its developmentally regulated expression in the periphery of the lungs of fetal, newborn, and adult rats. PECAM-1/CD31 was present mainly on luminal surfaces and at the junctions between endothelial cells. Moreover, in fetal lung, products of the immunoreaction were also found on the abluminal surfaces of endothelial cells. To relate those findings to the developmental changes in the capillary area of the lung, we performed a morphometric study of electronmicrographs. The cross-sectional area of blood vessels at the periphery of the lungs was significantly greater in 15-19-day-old fetuses than in postpartum animals (p<0.0001). Disappearance of the expression of PECAM-1/CD31 on the abluminal endothelial surface paralleled the changes in the cross-sectional area of blood vessels that occurred during the perinatal period. (J Histochem Cytochem 48:1283-1289, 2000)

Recent reports have demostrated a link between expression of members of the family of heterogeneous nuclear ribonucleoproteins (hnRNPs) and cancer. Overexpression of hnRNP A2/B1 correlated with the eventual development of lung cancer in three different clinical cohorts. We have studied the expression of hnRNP A2/B1 messenger RNA (mRNA) and protein during mammalian development. The expression of hnRNP A2/B1 mRNA and protein are parallel but change dynamically during critical periods in mouse pulmonary development. hnRNP A2/B1 is first detected in the lung in the early pseudoglandular period, peaks at the beginning of the canalicular period, and remains high during the saccular (alveolar) period. In mouse and rat, hnRNP A2/B1 expression is first evident in the earliest lung buds. As lung development progresses, the cuboidal epithelial cells of the distal primitive alveoli show high levels of the ribonucleoprotein, which is almost undetectable in the proximal conducting airways. The expression of hnRNP A2/ B1 is restricted in mature lung. Similar dynamic pattern of expression through lung development was also found in rat and human lung. Upregulated expression of hnRNP A2/B1 at critical periods of lung development was comparable to the level of expression found in lung cancers and preneoplastic lesions and is consistent with hnRNP A2/B1 overexpression playing an oncodevelopmental role.

The authors have previously shown that fetal tracheal ligation (TL) reverses the pulmonary hypoplasia in experimental diaphragmatic hernia (DH) by accelerating fetal alveolar growth. The purpose of this study was to determine if growth of the accompanying macroscopic and microscopic pulmonary vasculature is also accelerated. Eighteen fetal lambs were divided into three experimental groups: diaphragmatic hernia (DH), DH and simultaneous tracheal ligation (DH/TL), and sham-operated controls (C). Animals were delivered near term, the lungs retrieved, and pulmonary capillary growth (5 to 50 microns in diameter) evaluated by standard morphometric techniques. Capillary ultrastructure was evaluated by electron microscopy. Nine additional fetal lambs of the same gestational age were equally divided into the same three groups and their lungs analyzed by pulmonary arteriography for evaluation of large vessel growth (< 100-microns diameter). Computer digital analysis of angiogram lung slices showed that the total area of large vessels was increased in DH/TL lungs when compared with DH lungs and decreased in DH lungs when compared with C lungs (P = .003); however, the ratio of large vessel area per unit of lung area was similar in all groups. Microscopic morphometry of the capillary bed showed that the total number of capillaries was increased in DH/TL lungs over both DH and C lungs (P = .0001); however, the number of capillaries per alveolus (cap/alv) was similar in all groups. In DH/TL lungs, electron microscopy showed normal capillary wall structure and normal thickness of the capillary-alveolar interface, whereas in DH lungs, capillary structure was abnormal and the capillary-alveolar interface was thickened.(ABSTRACT TRUNCATED AT 250 WORDS)

Renewal of squmous alveolar epithelium following injury probably occurs by division of type 2 cells with subsequent transformation to type 1 epithelium. The present morphologic and cytodynamic studies of developing rat lung provide further evidence that this mechanism reflects the normal process of cell renewal in the alveolar epithelium. From gestation days 18to 20, the epithelium was composed of glycogen-laden cuboidal cells which by day 20 contained lamellar bodies. In this period of high DNA synthesis in thelung, about 50 per cent of 'H-thymidine-labeled pulmonary cells were type 2, and mitoseswere frquently seen in these cells. On day 22, when many type 1 cells were first seen,over-all labeling was much reduced and was predominate in interstitial cells; type 2 labeling fell to about 10 percent. (he labeling index of type 1 cells, about 1 per cent atthis time, did not increase subsequently and mitotic figures were not seen in this celltype. Cells intermediate in formbetween type 2 and type 1 were observed after day 21, and, postnatally, the normal mixed population of epithelium cells lined the alveoli. The findings of maximal DNAsythesis and cell division when the alveoli were lined exclusively by type 2 cells, low labeling and the absence of mitotic figures in type 1 epithelium, and the observation of intermediate cell forms provide additional evidence that the type 2 cell is the progenitorof type 1 epithelium.