Scanning electron microscope study of the developing microvasculature in the postnatal rat lung

Parabronchial angioarchitecture in developing and adult chickens

The avian lung has a highly sophisticated morphology with a complex vascular system. Extant data regarding avian pulmonary angioarchitecture are few and contradictory. We used corrosion casting techniques, light microscopy, as well as scanning and transmission electron microscopy to study the development, topography, and distribution of the parabronchial vasculature in the chicken lung. The arterial system was divisible into three hierarchical generations, all formed external to the parabronchial capillary meshwork. These included the interparabronchial arteries (A1) that ran parallel to the long axes of parabronchi and gave rise to orthogonal parabronchial arteries (A2) that formed arterioles (A3). The arterioles formed capillaries that participated in the formation of the parabronchial mantle. The venous system comprised six hierarchical generations originating from the luminal aspect of the parabronchi, where capillaries converged to form occasional tiny infundibular venules (V6) around infundibulae, or septal venules (V5) between conterminous atria. The confluence of the latter venules formed atrial veins (V4), which gave rise to intraparabronchial veins (V3) that traversed the capillary meshwork to join the interparabronchial veins (V1) directly or via parabronchial veins (V2). The primitive networks inaugurated through sprouting, migration, and fusion of vessels and the basic vascular pattern was already established by the 20th embryonic day, with the arterial system preceding the venous system. Segregation and remodeling of the fine vascular entities occurred through intussusceptive angiogenesis, a process that probably progressed well into the posthatch period. Apposition of endothelial cells to the attenuating epithelial cells of the air capillaries resulted in establishment of the thin blood-gas barrier. Fusion of blood capillaries proceeded through apposition of the anastomosing sprouts, with subsequent thinning of the abutting boundaries and ultimate communication of the lumens. Orthogonal reorientation of the blood capillaries at the air capillary level resulted in a cross-current system at the gas exchange interface.

Effect of intraluminal pillars on particle motion in bifurcated microchannels

A central feature of intussusceptive angiogenesis is the development of an intravascular pillar that bridges the opposing sides of the microvessel lumen. In this report, we created polydimethyl siloxane (PDMS) microchannels with geometric proportions based on corrosion casts of the colon microcirculation. The structure of the PDMS microchannels was a bifurcated channel with an intraluminal pillar in the geometric center of the bifurcation. The effect of the intraluminal pillar on particle flow paths was investigated using an in vitro perfusion system. The microchannels were perfused with fluorescent particles, and the particle movements were recorded using fluorescence videomicroscopy. We found that the presence of an intravascular pillar significantly decreased particle velocity in the bifurcation system (p < 0.05). In addition, the pillar altered the trajectory of particles in the center line of the flow stream. The particle trajectory resulted in prolonged pillar contact as well as increased residence time within the bifurcation system (p < 0.001). Our results suggest that the intravascular pillar not only provides a mechanism of increasing resistance to blood flow but may also participate in spatial redistribution of cells within the flow stream.

Tumour vascularization: sprouting angiogenesis and beyond

Tumour angiogenesis is a fast growing domain in tumour biology. Many growth factors and mechanisms have been unravelled. For almost 30 years, the sprouting of new vessels out of existing ones was considered as an exclusive way of tumour vascularisation. However, over the last years several additional mechanisms have been identified. With the discovery of the contribution of intussusceptive angiogenesis, recruitment of endothelial progenitor cells, vessel co-option, vasculogenic mimicry and lymphangiogenesis to tumour growth, anti-tumour targeting strategies will be more complex than initially thought. This review highlights these processes and intervention as a potential application in cancer therapy. It is concluded that future anti-vascular therapies might be most beneficial when based on multimodal anti-angiogenic, anti-vasculogenic mimicry and anti-lymphangiogenic strategies.

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.

Vascular sprout formation entails tissue deformations and VE-cadherin-dependent cell-autonomous motility

Embryonic and fetal vascular sprouts form within constantly expanding tissues. Nevertheless, most biological assays of vascular spouting are conducted in a static mechanical milieu. Here we study embryonic mouse allantoides, which normally give raise to an umbilical artery and vein. However, when placed in culture, allantoides assemble a primary vascular network. Unlike other in vitro assays, allantoic primordial vascular cells are situated on the upper surface of a cellular layer that is engaged in robust spreading motion. Time-lapse imaging allows quantification of primordial vascular cell motility as well as the underlying mesothelial tissue motion. Specifically, we calculate endothelial cell-autonomous motion by subtracting the tissue-level mesothelial motion from the total endothelial cell displacements. Formation of new vascular polygons is hindered by administration of function-blocking VE-cadherin antibodies. Time-lapse recordings reveal that (1) cells at the base of sprouts normally move distally "over" existing sprout cells to form new tip-cells; and (2) loss of VE-cadherin activity prevents this motile behavior. Thus, endothelial cell-cell-adhesion-based motility is required for the advancement of vascular sprouts within a moving tissue environment. To the best of our knowledge, this is the first study that couples endogenous tissue dynamics to assembly of vascular networks in a mammalian system.

Structural aspects of postnatal lung development - alveolar formation and growth

The human lung is born with a fraction of the adult complement of alveoli. The postnatal stages of human lung development comprise an alveolar stage, a stage of microvascular maturation, and very likely a stage of late alveolarization. The characteristic structural features of the alveolar stage are well known; they are very alike in human and rat lungs. The bases for alveolar formation are represented by immature inter-airspace walls with two capillary layers with a central sheet of connective tissue. Interalveolar septa are formed by folding up of one of the two capillary layers. In the alveolar stage, alveolar formation occurs rapidly and is typically very conspicuous in both species; it has therefore been termed 'bulk alveolarization'. During and after alveolarization the septa with double capillary networks are restructured to the mature form with a single network. This happens in the stage of microvascular maturation. After these steps the lung proceeds to a phase of growth during which capillary growth by intussusception plays an important role in supporting gas exchange. In view of reports that alveoli are added after the stage of microvascular maturation, the question arises whether the present concept of alveolar formation needs revision. On the basis of morphological and experimental findings we can state that mature lungs contain all the features needed for 'late alveolarization' by the classical septation process. Because of the high plasticity of the lung tissues, late alveolarization or some forms of compensatory alveolar formation may be considered for the human lung.

Fluorescent microangiography (FMA): an improved tool to visualize the pulmonary microvasculature

Visualization of the complex lung microvasculature and resolution of its three-dimensional architecture remains a difficult experimental challenge. We present a novel fluorescent microscopy technique to visualize both the normal and diseased pulmonary microvasculature. Physiologically relevant pulmonary perfusion conditions were applied using a low-viscosity perfusate infused under continuous airway ventilation. Intensely fluorescent polystyrene microspheres, confined to the vascular space, were imaged through confocal optical sectioning of 200 microm-thick lung sections. We applied this technique to rat lungs and the markedly enhanced depth of field in projected images allowed us to follow vascular branching patterns in both normal lungs and lungs from animals with experimentally induced pulmonary arterial hypertension. In addition, this method allowed complementary immunostaining and identification of cellular components surrounding the blood vessels. Fluorescent microangiography is a widely applicable and quantitative tool for the study of vascular changes in animal models of pulmonary disease.

A Model for Lymphatic Regeneration in Tissue Repair of the Intestinal Muscle Coat

The gastrointestinal lymphatic system, which comprises a network of thin-walled vessels, is essential for the regulation of tissue fluid volume, immune function, and transport of fatty nutrients. The identification of specific lymphatic endothelial markers has facilitated analyses of lymphatic organization and lymphangiogenesis during individual development and tissue repair. The intestinal muscle coat producing motor activity develops a dense maze-like lymphatic network by vascular sprouting consisting of thin lymphatic endothelial projections and splitting of the vessels. The lymphatic regeneration in the tissue repair of the intestinal muscle coat is essentially attributable to sprouting from preexisting lymphatics, and it progresses vigorously with vascular maturation. The regrowing lymphatic endothelial cells exhibit structural changes indicating a high migratory potential and a close association with regenerating stromal cells. The upregulation of VEGF-C, a specific lymphangiogenic molecule, in a subpopulation of the stromal cells probably contributes to lymphatic regeneration by activating its receptor, VEGFR-3, on the regrowing lymphatic endothelial cells.

Chick chorioallantoic membrane as an in vivo model to study vasoreactivity: characterization of development-dependent hyperemia induced by epoxyeicosatrienoic acids (EETs)

Shell-less culture of chick chorioallantoic membrane (CAM) of developing chicken embryos is a useful model to evaluate the effects of vascular agents. We assessed the response of CAM vessels to epoxyeicosatrienoic acids (EETs), derivatives of the essential fatty acid arachidonic acid, that have a number of important biological functions, including dilation of microvessels in the coronary, cerebral, renal, and mesenteric circulations. Three of four regioisomers of EETs, 14,15-, 11,12-, and 8,9-EET, induced a characteristic dose-dependent acute hyperemia within 4 min after application on 10-day-old CAMs. This response was marked in early stages of development (between days 8 and 10), but the frequency and intensity of the response were reduced after 11 days of development. Histological examination demonstrated that the hyperemia was not due to extravasation of erythrocytes. However, many capillaries were distended and contained densely packed erythrocytes as compared to uniformly arranged vessels and erythrocytes in untreated CAMs. Transmission electron microscopy showed the basal laminae surrounding capillaries remained intact, similar to those in vehicle-treated or untreated CAM tissue. The hyperemia was specific to EETs since we did not observe it to be induced by other vasodilators such as nitric oxide or prostacyclin. In conclusion, we report a novel vascular response to EETs using the CAM as an in vivo model. These lipids specifically distend a subset of capillaries in a dose- and development-dependent manner.

The influence of fluid shear stress on the remodeling of the embryonic primary capillary plexus

The primary capillary plexus in early yolk sacs is remodeled into matured vitelline vessels aligned in the direction of blood flow at the onset of cardiac contraction. We hypothesized that the influence of fluid shear stress on cellular behaviors may be an underlying mechanism by which some existing capillary channels remain open while others are closed during remodeling. Using a recently developed E-Tmod knock-out/lacZ knock-in mouse model, we showed that erythroblasts exhibited rheological properties similar to those of a viscous cell suspension. In contrast, the non-erythroblast (NE) cells, which attach among themselves within the yolk sac, are capable of lamellipodia extension and cell migration. Isolated NE cells in a parallel-plate flow chamber exposed to fluid shear stress, however, ceased lamellipodia extension. Such response may minimize NE cell migration into domains exposed to fluid shear stress. A two-dimensional mathematical model incorporating these cellular behaviors demonstrated that shear stress created by the blood flow initiated by the embryonic heart contraction might be needed for the remodeling of primary capillary plexus.

Microvascular growth, development, and remodeling in the embryonic avian kidney: the interplay between sprouting and intussusceptive angiogenic mechanisms

Embryonic development is associated with extensive vascular growth and remodeling. We used immunohistochemical, light and electron microscopical techniques, as well as vascular casting methods to study the developing chick embryo kidney with special attention to the interplay between sprouting and intussusceptive vascular growth modes. During inauguration at embryonic day 5 (E5), the early mesonephros was characterised by extensive microvascular sprouting. By E7, the vascular growth mode switched to intussusception, which contributed to rapid kidney vasculature growth up to E11, when the first obvious signs of vascular degeneration were evident. The metanephros underwent similar phases of vascular development inaugurating at E8 with numerous capillary sprouts and changing at E13 to intussusceptive growth, which was responsible for vascular amplification and remodeling. A phenomenal finding was that future renal lobules arose as large glomerular tufts, supplied by large vessels, which were split into smaller intralobular feeding and draining vessels with subsequent formation of solitary glomeruli. This glomerular duplication was achieved by intussusception, i.e., by formation of pillars in rows and their successive merging to delineate the vascular entities. Ultimately, the maturation of the vasculature was achieved by intussusceptive pruning and branching remodeling. An interesting finding was that strong VEGF expression was associated with the sprouting phase of angiogenesis while bFGF was upregulated during the phase of intussusceptive microvascular growth. We conclude that microvascular growth and remodeling in avian kidney follows an adroitly crafted pattern, which entails a precise spaciotemporal interplay between sprouting and intussusceptive angiogenic growth modes supported partly by VEGF and bFGF.

beta-Catenin expression during vascular development and degeneration of avian mesonephros

beta-Catenin is a structural component of adherens junctions, a regulator of the Wnt signalling pathway and a transcriptional co-activator with a key role in vascular patterning. The avian mesonephros is a transitory embryonic kidney that is used in the study of vascular development and degeneration. Here we examine beta-catenin expression in this model during vascular development and degeneration. Quail embryos with developing or degenerating mesonephros were studied, on day 6 (30HH) or day 11 of incubation (40HH), respectively. QH1 whole mounts of developing mesonephros revealed numerous angioblast-like cells situated in the paramesonephric duct that seem to invade the mesonephros. Although these cells did not express beta-catenin, the surrounding periductal mesenchymal cells translocated high levels of beta-catenin into the nucleus. In contrast, degenerating mesonephros were devoid of angioblast-like cells and beta-catenin was lower than in the developing mesonephros. beta-Catenin was significantly reduced in the glomerular capillary tuffs, indicating that it was particularly down-regulated in the vascular system. No sex-related differences in beta-catenin expression were observed in degenerating mesonephros. Furthermore, two special populations of glomerular and peritubular endothelial cells were observed in degenerating mesonephros: one translocating beta-catenin into the nucleus and the other in apoptosis that did not translocate it. In conclusion, our results indicate that the paramesonephric duct is a potential new vasculogenetic pathway, and suggest that beta-catenin plays a role in the fate of mesonephric endothelial cells.

Intussusceptive angiogenesis: its emergence, its characteristics, and its significance

This review shall familiarize the reader with the various aspects of intussusceptive angiogenesis (IA). The basic event in IA is the formation of transvascular tissue pillars. Depending on location, timing, and frequency of pillar emergence, the IA process has different outcomes. In capillaries, a primary IA function is to expand the capillary bed in size and complexity (intussusceptive microvascular growth). It represents an alternative to capillary sprouting. Highly ordered pillar formation in a developing capillary network leads to the formation of vascular trees (intussusceptive arborization). In small arteries and veins, pillar formation at the vessels' branching angles leads either to remodeling of the branching geometry or even to vascular pruning (intussusceptive branching remodeling). It appears essential that future angiogenic research considers always both phenomena, sprouting and intussusception. Vascularization of tissues, organs, and tumors rely heavily on both mechanisms; neglecting one or the other would obscure our understanding of the angiogenesis process.

Vascular endothelial growth factor reduces mural cell coverage of endothelial cells and induces sprouting rather than luminal division in an HT1080 tumour angiogenesis model

Vascular endothelial growth factor (VEGF) plays a central role in tumour angiogenesis. In a mouse intramuscular tumour model using VEGF-transfected HT1080 human fibrosarcoma, we investigated the morphological features and patterns of remodelling in size-matched tumours. Compared with the control tumours (C group), the VEGF-transfected tumours (V group) showed vigorous neovascularization with larger vessels. Fenestrations and disruptions of endothelia were specific to the V group. Three types of vascular remodelling, i.e. sprouting, luminal division and intussusceptive microvascular growth, were present in both groups. Morphometric analyses revealed that mural cell coverage of the endothelial cells was significantly smaller in the V group compared with that in the C group (V group, 28.2 +/- 18.6%; C group, 41.6 +/- 21.1%; P < 0.0001). To determine the prevalence of remodelling patterns, the occurrences of abluminal and luminal processes on endothelial cell surfaces were quantified. Abluminal processes are defined as cytoplasmic protrusions of the abluminal membrane of endothelial cells, which can vary from tiny spurs to solid sprouts of the cell. On the other hand, luminal processes are defined as intraluminal protrusions of the endothelial cell membrane, including various membranous changes from filiform processes to rather thick cytoplasmic bulges. An abluminal process is thought to represent an initial morphological change in sprouting type angiogenesis, and a luminal process to be a sign of implementation of luminal division. The frequency of abluminal processes was significantly higher in the V group than in the C group (V group, 0.243 +/- 0.138/microm; C group, 0.114 +/- 0.101/microm; P < 0.0001). In contrast, the number of luminal processes on the endothelial cells per micrometre was statistically comparable between the groups (V group, 0.285 +/- 0.252/microm; C group, 0.309 +/- 0.236/microm, P = 0.381). These results indicate that sprouting is the main mode of VEGF-induced tumour angiogenesis.

Neonatal dexamethasone induces premature microvascular maturation of the alveolar capillary network

Postnatal glucocorticoid treatment of preterm infants was mimicked by treating newborn rats with dexamethasone (0.1-0.01 microg/g, days 1-4). This regimen has been shown to cause delayed alveolarization. Knowing that microvascular maturation (transformation of double- to single-layered capillary networks in alveolar septa) and septal thinning prevent further alveolarization, we measured septal maturation on electron photomicrographs in treated and control animals. In treated rats and before day 10, we observed a premature nonreversing microvascular maturation and a transient septal thinning, which both appeared focally. In vascular casts of both groups, we observed contacts between the two capillary layers of immature alveolar septa, which were predictive for capillary fusions. Studying serial electron microscopic sections of human lungs, we were able to confirm the postulated fusion process for the first time. We conclude that alveolar microvascular maturation indeed occurs by capillary fusion and that the dexamethasone-induced impairment of alveolarization is associated with focal premature capillary fusion.

A three-dimensional study of endometrial microvessels in women using the contraceptive subdermal levonorgestrel implant system, norplant®

Recent evidence points towards a substantial disturbance of the process of angiogenesis within the endometrium in women who are exposed continuously to low dose contraceptive progestogens. This results in the patchy appearance of abnormally small and abnormally large, thin-walled vessels in the superficial regions of exposed endometrium. Three-dimensional pictures were developed from digitised images of serial tissue sections of endometrium in which microvascular endothelial cells were labelled with antibodies to the endothelial cell surface antigen, CD34 and their basement membranes labelled with anti-Collagen IV antibodies. Microvessels from endometrium exposed to continuous low-dose levonorgestrel from a subdermal implant system (Norplant) displayed considerable variations in size and shape. No spiral arterioles were identified. Some microvessels showed considerable dilatation, distortion and variability in the presence of surrounding basement membrane components. Other endothelial structures included narrow, solid cords of endothelial cells, without basement membranes, which often connected with normal or abnormal vessels containing lumens. Some areas, especially deeper in the tissue, contained microvessels of normal size and shape surrounded by basement membrane. These images have revealed an overall picture of great variability in superficial endometrial vascular structures in some women using a low-dose levonorgestrel implant system which appears substantially different from that seen in normal endometrium.

Therapeutic targeting of the tumor vasculature

A functional tumor vasculature is essential for tumor growth and metastasis and makes an attractive target for therapy. Both antiangiogenic and antivascular approaches are being developed for this purpose. In this article, the current antiangiogenic and antivascular approaches to cancer therapy, potential for their combination with radiotherapy, methods for identifying new targets on the tumor vasculature, and methods for evaluating new vascular-targeted strategies in in vivo model systems are reviewed.

Vasculogenesis and angiogenesis

Two distinct mechanisms, vasculogenesis and angiogenesis implement the formation of the vascular network in the embryo. Vasculogenesis gives rise to the heart and the first primitive vascular plexus inside the embryo and in its surrounding membranes, as the yolk sac circulation. Angiogenesis is responsible for the remodeling and expansion of this network. While vasculogenesis refers to in situ differentiation and growth of blood vessels from mesodermal derived hemangioblasts, angiogenesis comprises two different mechanisms: endothelial sprouting and intussusceptive microvascular growth (IMG). The sprouting process is based on endothelial cell migration, proliferation and tube formation. IMG divides existing vessel lumens by formation and insertion of tissue folds and columns of interstitial tissue into the vessel lumen. The latter are termed interstitial or intervascular tissue structures (ITSs) and tissue pillars or posts. Intussusception also includes the establishment of new vessels by in situ loop formation in the wall of large veins. The molecular regulation of these distinct mechanisms is discussed in respect to the most important positive regulators, VEGF and its receptors flk-1 (KDR) and flt-1, the Angiopoietin/tie system and the ephrin-B/EpH-B system. The cellular mechanisms and the molecular regulation of angiogenesis in the pathological state are summarized and the differences of physiological and pathological angiogenesis elaborated.

Lessons from a canine model of compensatory lung growth

For over a century, canines have been used to study adaptation to surgical lung resection or pneumonectomy (PNX) that results in a quantifiable and reproducible loss of lung units. As reviewed by Schilling (1965), the first successful experimental pneumonectomies were performed in dogs and rabbits in 1881. By the early 1920s, it was appreciated that dogs can function normally with one remaining lung that increases in volume to fill the thoracic cavity (Andrus, 1923; Heuer and Andrus, 1922; Heuer and Dunn, 1920); these pioneering observations paved the way for surgeons to perform major lung resection in patients. Reports in the 1950s (Schilling et al., 1956) detail surprisingly well-preserved work performance in dogs following staged resection of up to 70% of lung mass. Since then, the bulk of the literature on post-PNX adaptation has shifted to rodents, especially for defining molecular mediators of compensatory lung growth. Because rodents are smaller and easier to handle, more animals can be studied over a shorter duration, resulting in time and cost savings. On the other hand, key aspects of lung anatomy, development, and time course of response in the rodent do not mimic those in the human subject, and few rodent studies have related structural adaptation to functional consequences. In larger mammals, anatomical lung development more closely resembles that in humans, and physiological function can be readily measured. Because dogs are natural athletes, functional limits of compensation can be characterized relatively easily by stressing oxygen transport at peak exercise. Thus, the canine model remains useful for relating structure to function, defining sources and limits of adaptation as well as evaluating therapeutic manipulation. This chapter summarizes key concepts of compensatory lung growth that have been consolidated from canine studies: (i) structure-function relationships during adaptation, (ii) dysanaptic (unequal) nature of compensation, and (iii) signals for initiation of cellular growth.

Microvasculature in the Terminal Air Spaces of the Lungs of the Baird's Beaked Whale (Berardius bairdii)

Lungs were obtained from five adult Baird's beaked whales (Berardius bairdii) and examined by means of light microscopy and scanning electron microscopy of corrosion casts. The alveolar septa of these whales are thick with a connective tissue core and a bi-layer capillary bed. A double capillary network is regularly found in the alveolar duct and alveolar septa. Occasionally, septa adjacent to alveoli and alveoli themselves show only a single capillary layer. The distance between the two capillary layers has a tendency to decrease toward the end of airspaces, suggesting an end result of capillary fusion. Vascular replicas of venous vessels have annular furrows at regular intervals of 50 to 100 microm, which are caused by focal aggregations of collagen fibers circularly oriented and located immediately underneath the endothelium. The first valves appear in the collecting venules gathering alveolar capillaries. These valves are quite characteristic of flap-, funnel-and/or chimney like structures.

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.

Morphogenesis of blood vessels in the head muscles of avian embryo: spatial, temporal, and VEGF expression analyses

Adult skeletal muscle is a highly vascularized tissue, but the development of intramuscular endothelial networks has not been well studied. In quail embryos, QH1-positive angioblasts are present and moving throughout myogenic head mesoderm before the onset of primary myotube formation. On day 5 of incubation, concurrent with early myotube formation and aggregation, angioblasts establish a transient vascular plexus surrounding the myogenic condensations. Between days 5 and 9, the intramuscular vessels form an irregular network of endothelial cords and patent channels and only later are the parallel arrays of capillaries characteristic of adult muscles established. Microinjections using India ink, QH1, and Mercox resin reveal that these intramuscular capillaries are typically not connected to systemic vessels of the head until day 10, which is near the end of primary myogenesis and corresponds to the onset of muscular function. Morphometric analyses performed during primary myogenesis stages show a decrease in muscle cell density but no significant changes in intramuscular vascular density between days 5 and 9. This finding was surprising, as it is generally assumed that muscle growth requires elevated oxygen and nutrient levels. Moreover, there are no significant morphometric differences in vascular supply to embryonic fast and slow muscles. Endothelial tissue density is similar in slow muscles (oculorotatory, e.g., lateral rectus), fast muscles (mandibular depressor), and mixed muscles, in which the fiber types can be interspersed (jaw adductors) or segregated (branchiomandibular). Vascular endothelial growth factor (VEGF) protein is abundant in myotubes but not endothelial cells within both fast and slow head muscles at days 7 and 9. However, in some mixed muscles, only a minority of myotubes, which do not correspond to one specific fiber type, express VEGF. These results document a dynamic set of intramuscular and perimuscular angiogenic reorganizations during avian head myogenesis. Thus far, no vasculogenic distinctions between fast and slow muscles have been observed, although muscle heterogeneity in VEGF expression is evident.

Thickness of the blood-gas barrier in premature and 1-day-old newborn rabbit lungs

The pulmonary capillaries of neonatal lungs are potentially vulnerable to stress failure because of the complex changes in the pulmonary circulation that occur at birth. We studied the ultrastructure of the blood-gas barrier (BGB) in premature and 1-day-old rabbit lungs and compared it with the ultrastructure of adult lungs. Normal gestation of rabbits is 30 days. After extensive pilot measurements, three premature (27 days gestation) and three newborn (1 day old) rabbit lungs were perfusion-fixed at arterial, venous, and airway pressures of 25, 0, and 10 cmH2O, respectively, and the measurements were compared with those of three adult lungs. The thickness of the capillary endothelium, alveolar epithelium, and interstitium of the BGB was measured at right angles to the barrier at random points. A striking finding was the large number of measurements of the interstitial thickness in 1-day-old lungs that were very thin (0-0.1 microm). The percentages of occurrence of very thin interstitium in premature, 1-day-old, and adult lungs were 35.3 +/- 9.4, 71.7 +/- 5.2, and 43.0 +/- 2.6, respectively (P < 0.02 for 1 day old vs. premature and adult). Given the previously found relationship between stress failure and interstitial thickness, this large proportion of very thin interstitial layers in the capillaries of 1-day-old lungs is a reasonable explanation for their previously demonstrated vulnerability to stress failure.

Intussusceptive capillary growth is required for glomerular repair in rat Thy-1.1 nephritis

BACKGROUND

Injection of anti-Thy-1.1 antibodies to rats causes mesangiolysis, with subsequent capillary loss. This dramatic event is followed by almost complete recovery of glomerular architecture. However, the precise cellular mechanisms of revascularization are not fully understood.

METHODS

Glomerulonephritis was induced by the injection of monoclonal anti-Thy-1.1 antibody to rats. Structural changes in the glomerular vasculature, with special emphasis on the repair phase, were studied with corrosion casting technique, light microscopy (LM), transmission electron microscopy (TEM), and scanning electron microscopy (SEM).

RESULTS

Specifically during the extensive revascularization period, numerous tiny holes of about 1.5 microm in diameter were found on the glomerular cast. The presence of transluminal tissue pillar was confirmed with LM and SEM. These findings indicate the involvement of intussusceptive capillary growth. TEM study demonstrated that some tissue pillars were composed of endothelial cells only and others had cytoplasmic process of mesangial cells in their cores sandwiched both sides by endothelial walls.

CONCLUSION

Intussusceptive capillary growth, or nonsprouting angiogenesis is involved in the postinjury angiogenesis in the glomeruli, in which process mesangial cells as well as endothelial cells may play important roles.

Human uterine vascular structures in normal and diseased states

Menstrual bleeding disorders are one of the most common problems presenting to the gynaecologist. There is increasing evidence that alterations in human uterine vascular structures are associated with spontaneous and sex steroid-induced changes in menstrual bleeding patterns. This article will discuss the normal anatomy and physiology of uterine vascular growth, breakdown, and repair and will indicate where altered anatomy and function may contribute to menstrual disorder. In particular, impact of low-dose progestogens on the endometrial vasculature and endometrial vascular fragility will be discussed. Disturbances of myometrial vascular growth and remodeling, including fibroids, adenomyosis, implantation, and arterio-venous malformations will also be addressed.

Microvascular assembly and cell invasion in chick mesonephros grafted onto chorioallantoic membrane

Embryonic tissues, in common with other tissues, including tumours, tend to develop a substantial vasculature when transplanted onto the chorioallantoic membrane (CAM). Studies conducted to date have not examined in any detail the identity of vessels that supply these grafts, although it is known that the survival of transplanted tissues depends on their ability to connect with CAM vessels supplying oxygen and nutrients. We grafted the mesonephros, a challenging model for studies in vascular development, when it was fully developed (HH35). We used reciprocal chick-quail transplantations in order to study the arterial and venous connections and to analyse the cell invasion from the CAM to the organ, whose degeneration in normal conditions is rapid. The revascularization of the grafted mesonephros was produced by the formation of peripheral anastomoses between the graft and previous host vasculatures. The assembly of graft and CAM blood vessels occurred between relatively large arteries or veins, resulting in chimeric vessels of varying morphology depending on their arterial or venous status. Grafts showed an increased angiogenesis from their original vasculature, suggesting that the normal vascular degeneration of the mesonephros was partially inhibited. Three types of isolated host haemangioblast were identified in the mesonephros: migrating angioblast-like cells, indicating vasculogenesis, undifferentiated haematopoietic cells and macrophages, which might have been involved in the angiogenesis. Tomato lectin was found to bind activated macrophages in avian embryos.

Effects of neonatal high-dose short-term glucocorticoid treatment on the lung: a morphologic and morphometric study in the rat

Glucocorticoids are often applied in neonatology and perinatology to fight the problems of respiratory distress and chronic lung disease. There are, however, many controversies regarding the adverse side effects and long-term clinical benefits of this therapeutic approach. In rats, glucocorticoids are known to seriously impair the formation of alveoli when applied during the first two postnatal weeks even at very low dosage. The current study investigates short-term and long-term glucocorticoid effects on the rat lung by means of morphologic and morphometric observations at light and electron microscopic levels. Application of a high-dosage protocol for only few days resulted in a marked acceleration of lung development with a precocious microvascular maturation resulting in single capillary network septa in the first 4 postnatal days. By postnatal d 10, the lung morphologic phenotype showed a step back in the maturational state, with an increased number of septa with double capillary layer, followed by an exceptional second round of the alveolarization process. As a result of this process, there was an almost complete recovery in the parenchymal lung structure by postnatal d 36, and by d 60, there were virtually no qualitative or quantitative differences between experimental and control rats. These findings indicate that both dosage and duration of glucocorticoid therapy in the early postnatal period are very critical with respect to lung development and maturation and that a careful therapeutic strategy can minimize late sequelae of treatment.

Intussusceptive angiogenesis--the alternative to capillary sprouting

In contrast to sprouting angiogenesis, which is a well established mode of new blood vessel formation, intussusceptive angiogenesis (IA) is a relatively new concept in vascular biology. It was first discovered in the lung as a means of capillary network growth (intussusceptive microvascular growth). The mechanism consists in the repeated insertion of new slender transcapillary tissue pillars, which subsequently increase in size, thus allowing the capillary network to grow in itself (i.e., by intussusception). It could be shown that IA was present in all organs and species investigated so far, so that it appears to be an ubiquitous phenomenon in vertebrates at least. It was not a surprise therefore to find that IA also played a role in tumour vascularisation. Morphological analysis has yet brought evidence for 6 different modes of pillar formation. They all have in common that, at one time, two endothelial leaflets (e.g. of opposite capillary walls) come into close contact, form new junctional complexes, then thin out to finally give way to the invading interstitial tissue, particularly to fibroblasts, myofibroblasts and pericytes. Once such a transcapillary pillar is formed, it can subsequently grow to the size of a normal intercapillary mesh. The addition of collagen fibrils to the pillar core will stabilize the pillar mechanically. Recent observations allowed to extend the IA concept further: The same structural mechanism of intussusceptive pillar formation was shown to contribute also to the formation of vascular trees (arborisation) and to be involved in vascular remodeling. Although numerous growth factors and receptors have already been suggested as being active in IA, very few hard facts are at present available which would allow to get a comprehensive view of IA regulation.

How a highly complex three-dimensional network of blood vessels regresses: the gill blood vascular system of tadpoles of Xenopus during metamorphosis. A SEM study on microvascular corrosion casts

The morphology and vasculature of filter plates and filter rows of tadpoles of Xenopus laevis Daudin were studied from prometamorphosis (stage 56) to metamorphic climax (stage 62) by scanning electron microscopy (SEM) of (i) critical point-dried specimens and (ii) microvascular corrosion casts. The number and frequencies of the highest branching orders of venous filter row vessels, which reach their maximum (6) in stage 58, as well as vessel diameters, interbranching distances, and branching angles were measured. Branching orders continuously decrease until stage 62, so that only two are left. Vessel (luminal) diameters increased from 16.9 +/- 0.65 microm (SEM) (stage 56) to 19.3 +/- 0.61 microm (stage 58) (branching order 0) and then decreased to 16.7 +/- 0.91 microm (stage 62). Interbranching distances were largest in stage 59 (57.5 +/- 2.07 microm (branching order 0). Branching angles were highest in stage 62 (91.4 degrees +/- 8.82 degrees; branching order 2) and lowest in stage 56 (76.0 degrees +/- 4.64 degrees; branching order 4). From the morphology of cast vessels we conclude that vascular regression is organized by progressive "fusion" of opposite vessels in the highest branching orders ("miniaturization") until only a single capillary loop connects the terminal arteriole and the main filter fold venule. With obliteration of the single capillary loop, the regression of the gill vasculature is terminated. It is noted that the "fusion" of gill vessels is reminiscent of "reversed" intussusceptive microvascular growth.

Optimality in the developing vascular system: branching remodeling by means of intussusception as an efficient adaptation mechanism

The theory of bifurcating vascular systems predicts vessel diameters that are related to optimality criteria like minimization of pumping energy or of building material. However, mechanisms for producing the postulated optimality have not been described so far, and quantitative data on bifurcation diameters during development are scarce. We used an embryonic vascular bed that rapidly grows and adapts to changing hemodynamic conditions, the chicken chorioallantoic membrane (CAM), and correlated vascular cast and tissue section morphology with in vivo time-lapse video monitoring. The bifurcation exponent delta and associated parameters were quantitatively assessed in arterial and venous microvessels ranging in diameter from 30 to 100 microm. We observed emergence of optimality by means of intussusception, i.e., formation of transvascular tissue pillars. In addition to intussusceptive microvascular growth (IMG = expansion of capillary networks) and intussusceptive arborization (IAR = formation of feeding vessels from capillaries) the observed intussusception at bifurcations represents a third variant of nonsprouting angiogenesis. We call it intussusceptive branching remodeling (IBR). IBR occurred in vessels of considerable diameter by means of two alternative mechanisms: either through pillars arising close to a bifurcation, which increased in girth until they merged with the connective tissue in the bifurcation angle; or through pillars arising at some distance from the bifurcation point, which then expanded by formation of ingrowing tissue folds until they became connected to the tissue of the bifurcation angle. Morphologic evidence suggests that IBR is a wide-spread phenomenon, taking place also in lung, intestinal, kidney, eye, etc., vasculature. Irrespective of the mode followed, IBR led to a branching pattern close to the predicted optimum, delta = 3.0. Significant differences were observed between delta at arterial bifurcations (2.70 to 2.90) and delta at venous bifurcations (2.93 to 3.75). IBR, by means of eccentric pillar formation and fusion, was also involved in vascular pruning. Experimental changes in CAM hemodynamics (by locally increasing blood flow) induced onset of IBR within less than 1 hr. Our study provides morphologic and quantitative evidence that a similar cellular machinery is used for all three variants of vascular intussusception, IMG, IAR, and IBR. It thus provides a mechanism of efficiently generating complex blood transport systems from limited genetic information. Differential quantitative outcome of IBR in arteries and veins, and the experimental induction of IBR strongly suggest that hemodynamic factors can instruct embryonic vascular remodeling toward optimality.

Suppression of cell proliferation and programmed cell death by dexamethasone during postnatal lung development

Prematurely born babies are often treated with glucocorticoids. We studied the consequences of an early postnatal and short dexamethasone treatment (0.1-0.01 microg/g, days 1-4) on lung development in rats, focusing on its influence on peaks of cell proliferation around day 4 and of programmed cell death at days 19-21. By morphological criteria, we observed a dexamethasone-induced premature maturation of the septa (day 4), followed by a transient septal immatureness and delayed alveolarization leading to complete rescue of the structural changes. The numbers of proliferating (anti-Ki67) and dying cells (TdT-mediated dUTP nick end labeling) were determined and compared with controls. In dexamethasone-treated animals, both the peak of cell proliferation and the peak of programmed cell death were reduced to baseline, whereas the expression of tissue transglutaminase (transglutaminase-C), another marker for postnatal lung maturation, was not significantly altered. We hypothesize that a short neonatal course of dexamethasone leads to severe but transient structural changes of the lung parenchyma and influences the balance between cell proliferation and cell death even in later stages of lung maturation.

Impairment of rat postnatal lung alveolar development by glucocorticoids: involvement of the p21CIP1 and p27KIP1 cyclin-dependent kinase inhibitors

It has been shown that glucocorticoids accelerate lung development by limiting alveolar formation resulting from a premature maturation of the alveolar septa. Based on these data, the aim of the present work was to analyze the influence of dexamethasone on cell cycle control mechanisms during postnatal lung development. Cell proliferation is regulated by a network of signaling pathways that converge to the key regulator of cell cycle machinery: the cyclin-dependent kinase (CDK) system. The activity of the various cyclin/CDK complexes can be modulated by the levels of the cyclins and their CDKs, and by expression of specific CDK inhibitors (CKIs). In the present study, newborn rats were given a 4-d treatment with dexamethasone (0.1-0.01 microg/g body weight dexamethasone sodium phosphate daily on d 1-4), or saline. Morphologically, the treatment caused a significant thinning of the septa and an acceleration of lung maturation on d 4. Study of cyclin/CDK system at d 1-36 documented a transient down-regulation of cyclin/CDK complex activities at d 4 in the dexamethasone-treated animals. Analysis of the mechanisms involved suggested a role for the CKIs p21CIP1 and p27KIP1. Indeed, we observed an increase in p21CIP1 and p27KIP1 protein levels on d 4 in the dexamethasone-treated animals. By contrast, no variations in either cyclin and CDK expression, or cyclin/CDK complex formation could be documented. We conclude that glucocorticoids may accelerate lung maturation by influencing cell cycle control mechanisms, mainly through impairment of G1 cyclin/CDK complex activation.

Development of the lymphatic network in the muscle coat of the rat jejunum as revealed by enzyme-histochemistry

The process of lymphangiogenesis was studied in the muscle coat of the rat small intestine by light and scanning and transmission electron microscopy; identification of lymphatic vessels was made by 5'-nucleotidase staining. Light and scanning electron microscopy demonstrated that the intramuscular lymphatic network formation, which started only postnatally, was attributable to the vascular sprouting of slender lymphatic endothelial projections and to a splitting of the vessels, causing intervascular meshes of various sizes. The growing lymphatics were consistently closed by the endothelial cells, which were characterized by an abundance of cell organelles and prominent cytoplasmic processes. The cells often revealed close contacts with the processes of developing smooth muscle cells in the jejunal muscle coat, suggesting a possible role for the latter cells in the guidance of the lymphatic extension. The present study is the first to suggest the closed nature of lymphatics persisting throughout their development, even at the initial stage of lymphangiogenesis.

Chorioallantoic membrane capillary bed: a useful target for studying angiogenesis and anti-angiogenesis in vivo

The chick embryo chorioallantoic membrane (CAM) is an extraembryonic membrane that is commonly used in vivo to study both angiogenesis and anti-angiogenesis. This review 1) summarizes the current knowledge about the structure of the CAM's capillary bed; 2) discusses the controversy about the existence of a single blood sinus or a capillary plexus underlying the chorionic epithelium; 3) describes a new model of the CAM vascular growth, namely the intussusceptive mode; 4) reports findings regarding the role played by endogenous fibroblast growth factor-2 in CAM vascularization; and 5) addresses the use and limitations of the CAM as a model for studying angiogenesis and anti-angiogenesis.

Vascular morphogenesis and remodeling in a human tumor xenograft: blood vessel formation and growth after ovariectomy and tumor implantation

To determine mechanisms of blood vessel formation and growth in solid tumors, we used a model in which LS174T human colon adenocarcinomas are grown in the isolated ovarian pedicle of nude mice. Reconstruction of 3500 histological serial sections demonstrated that a new vascular network composed of venous-venous loops of varying sizes grows inside the tumor from the wall of the adjacent main vein. Loops elongate and remodel to establish complex loop systems. The mechanisms of loop formation and remodeling correspond to intussusceptive microvascular growth (IMG). In the tissue surrounding the tumor segmentation, another mechanism of IMG is prevalent in venous vessels. Comparison to vascular morphogenesis in the ovariectomized pedicle not only confirms the existence of corresponding mechanisms in both systems, but also reveals numerous sprouts that are superimposed onto loop systems and pathological deviations of loop formation, remodeling, and segmentation in the tumor. These pathological mechanisms interfere with vessel patency that likely cause heterogenous perfusion and hypoxia thus perpetuating angiogenesis. Blood vessel formation based on IMG was also detected in a large thrombus that completely occluded a part of an ovarian artery branch.

Vascular morphogenesis and remodeling in a model of tissue repair: blood vessel formation and growth in the ovarian pedicle after ovariectomy

To investigate mechanisms of vascular morphogenesis in tissue repair, we performed ovariectomy with resection of the corresponding branches of the ovarian vessels in nude mice. This induces a vascular network remodeling response in the healing ovarian pedicle. Reconstruction of 2000 histological serial sections demonstrated that a new vascular network composed of venous-venous loops forms in the wall of the dilated ovarian vein. Preexisting veins of all sizes, including a branch of the main artery, are subjected to segmentation. Loop formation and segmentation are based on intussusceptive microvascular growth. Loop formation is followed by elongation. Loop remodeling occurs also by intussusception and results in the formation of compound loop systems. All loop systems observed were completely patent. Blind-ending sprouts were extremely rare. Anastomoses between the preexisting vessels subjected to segmentation and the loop systems were established to include the newly formed vessels into the preexisting vascular network. The formation of an increasing number of patent loop systems likely decreases hypoxia and subsequently arrests angiogenesis with transformation of the granulation tissue into a scar. Loop formation also occurred inside a large thrombus that occluded a part of the lumen of the main vein.

Morphological analysis of the postnatally developing marsupial lung: The quokka wallaby

We investigated the events that take place during the postnatal morphogenesis of the lung of the quokka wallaby, Setonix brachyurus, using the light microscope and both the scanning and transmission electron microscopes. The lung of term, newborn babies (joeys) at 3-days of postnatal life was at late canalicular stage and comprised large airways and tubules separated by thick mesenchymal interstitium. The tubules were lined by a low cuboidal epithelium but had few portions with true gas exchange barrier where capillaries came into close contact with squamous type of epithelium. By the fifth day postpartum, the lung entered the early saccular stage characterised by large air sacs, thinner septa, a better developed double capillary system and conversion of the cuboidal epithelium into a squamous one of type I cells interrupted by groups of cuboidal type II cells with lamellar bodies. Transitory respiratory bronchioles were recognisable toward the end of this stage. Formation of secondary septa started by Day 15, dividing the saccules into several generations of smaller air spaces. There were alternating and concurrent periods of tissue proliferation and air space expansion, followed by septal thinning. Alveolization started from about 125 days postpartum when the first burst of small sized air spaces bounded by septa with a single capillary layer were encountered. By Day 180 the process of alveolization was completed with only occasional septa showing a double capillary system and by Day 210 postnatally, the lung resembled that of an adult. For the first time in a mammal, the canalicular stage was encountered postnatally during lung development.

Vascular remodelling during the normal and malignant life cycle of the mammary gland

The mammary gland life cycle is exemplified by massive, physiologically dictated changes in cell number and composition, architecture, and functionality. These drastic upheavals, by necessity, also involve the mammary endothelium, which undergoes angiogenic expansion during pregnancy and lactation followed by ordered regression during involution. In this review, we summarise data obtained using the Mercox methyl methacrylate corrosion cast technique to analyse the mammary gland vasculature during normal development and carcinogenesis. Concomitant with epithelial cell expansion, the mammary vasculature grows during the first half of pregnancy by sprouting angiogenesis whereas the last half of pregnancy and lactation are characterised by the non-proliferative intussusceptive angiogenesis. The vasculature of the lactating gland is composed of a well-developed capillary meshwork enveloping the secretory alveoli with basket-like honeycomb structures. During involution, regression of the vasculature is achieved by regional collapse of the honeycomb structures, capillary retraction, and endothelial attenuation. This process appears partly to involve apoptosis. However, an additional mechanism involving remodelling without cell death, which we have termed angiomeiosis, must exist to explain the morphological observations. Interestingly, in mammary tumours of neuT transgenic mice, both sprouting and intussusceptive angiogenesis was observed simultaneously in the same nodules, a finding with potential implications for cancer therapy. The underlying molecular mechanisms controlling angiogenic modulation in the mammary gland, particularly angiogenic regression and the endothelial:parenchymal interplay, are poorly understood. However, the data summarised in this review indicate that precisely these molecular mechanisms offer novel alternatives for specific and effective treatment of breast cancer.

Vasculogenesis and angiogenesis as mechanisms of vascular network formation, growth and remodeling

Two distinct mechanisms, vasculogenesis and angiogenesis implement the formation of the vascular network in the embryo. Vasculogenesis gives rise to the heart and the first primitive vascular plexus inside the embryo and in its surrounding membranes, as the yolk sac circulation. Angiogenesis is responsible for the remodeling and expansion of this network. While vasculogenesis refers to in situ differentiation and growth of blood vessels from mesodermal derived hemangioblasts, angiogenesis comprises two different mechanisms: endothelial sprouting and intussusceptive microvascular growth (IMG). The sprouting process is based on endothelial cell migration, proliferation and tube formation. IMG divides existing vessel lumens by formation and insertion of tissue folds and columns of interstitial tissue into the vessel lumen. The latter are termed interstitial or inter-vascular tissue structures (ITSs) and tissue pillars or posts. Intussusception also includes the establishment of new vessels by in situ loop formation in the wall of large veins. The molecular regulation of these distinct mechanisms is discussed in respect to the most important positive regulators, vascular endothelial growth factor (VEGF) and its receptors flk-1 (KDR) and flt-1, the Angiopoietin/tie system and the ephrin-B/EpH-B system. The cellular mechanisms and the molecular regulation of angiogenesis in the pathological state are summarized and the differences of physiological and pathological angiogenesis elaborated.

Is the sheet-flow design a 'frozen core' (a Bauplan) of the gas exchangers? Comparative functional morphology of the respiratory microvascular systems: illustration of the geometry and rationalization of the fractal properties

The sheet-flow design is ubiquitous in the respiratory microvascular systems of the modern gas exchangers. The blood percolates through a maze of narrow microvascular channels spreading out into a thin film, a "sheet". The design has been convergently conceived through remarkably different evolutionary strategies. Endothelial cells, e.g. connect parallel epithelial cells in the fish gills and reptilian lungs; epithelial cells divide the gill filaments in the crustacean gills, the amphibian lungs, and vascular channels on the lung of pneumonate gastropods; connective tissue elements weave between the blood capillaries of the mammalian lungs; and in birds, the blood capillaries attach directly and in some areas connect by short extensions of the epithelial cells. In the gills, skin, and most lungs, the blood in the capillary meshwork geometrically lies parallel to the respiratory surface. In the avian lung, where the blood capillaries anastomose intensely and interdigitate closely with the air capillaries, the blood occasions a 'volume' rather than a 'sheet.' The sheet-flow design and the intrinsic fractal properties of the respiratory microvascular systems have produced a highly tractable low-pressure low-resistance region that facilitates optimal perfusion. In complex animals, the sheet-flow design is a prescriptive evolutionary construction for efficient gas exchange by diffusion. The design facilitates the internal and external respiratory media to be exposed to each other over an extensive surface area across a thin tissue barrier. This comprehensive design is a classic paradigm of evolutionary convergence motivated by common enterprise to develop corresponding functionally efficient structures. With appropriate corrections for any relevant intertaxa differences, use of similar morphofunctional models in determining the diffusing capacities of various gas exchangers is warranted.

Intussusceptive angiogenesis: its role in embryonic vascular network formation

Intussusceptive angiogenesis is a novel mode of blood vessel formation and remodeling, which occurs by internal division of the preexisting capillary plexus without sprouting. In this study, the process is demonstrated in developing chicken eye vasculature and in the chorioallantoic membrane by methylmethacrylate (Mercox) casting, transmission electron microscopy, and in vivo observation. In a first step of intussusceptive angiogenesis, the capillary plexus expands by insertion of numerous transcapillary tissue pillars, ie, by intussusceptive microvascular growth. In a subsequent step, a vascular tree arises from the primitive capillary plexus as a result of intussusceptive pillar formation and pillar fusions, a process we termed "intussusceptive arborization." On the basis of the morphological observations, a 4-step model for intussusceptive arborization is proposed, as follows: phase I, numerous circular pillars are formed in rows, thus demarcating future vessels; phase II, formation of narrow tissue septa by pillar reshaping and pillar fusions; phase III, delineation, segregation, growth, and extraction of the new vascular entity by merging of septa; and phase IV, formation of new branching generations by successively repeating the process, complemented by growth and maturation of all components. In contrast to sprouting, intussusceptive angiogenesis does not require intense local endothelial cell proliferation; it is implemented primarily by rearrangement and attenuation of the endothelial cell plates. In summary, transcapillary pillar formation, ie, intussusception, is a central and probably widespread process, which plays a role not only in capillary network growth and expansion (intussusceptive microvascular growth), but also in vascular plexus remodeling and tree formation (intussusceptive arborization).