Endothelial cell lineages of the heart

During early gastrulation, vertebrate embryos begin to produce endothelial cells (ECs) from the mesoderm. ECs first form primitive vascular plexus de novo and later differentiate into arterial, venous, capillary, and lymphatic ECs. In the heart, the five distinct EC types (endocardial, coronary arterial, venous, capillary, and lymphatic) have distinct phenotypes. For example, coronary ECs establish a typical vessel network throughout the myocardium, whereas endocardial ECs form a large epithelial sheet with no angiogenic sprouting into the myocardium. Neither coronary arteries, veins, and capillaries, nor lymphatic vessels fuse with the endocardium or open to the heart chamber. The developmental stage during which the specific phenotype of each cardiac EC type is determined remains unclear. The mechanisms involved in EC commitment and diversity can however be more precisely defined by tracking the migratory patterns and lineage decisions of the precursors of cardiac ECs. Work carried out by the authors is supported in part by the NIH.     

The vascularization of the human tela choroidea of the lateral ventricle

The human tela choroidea of the lateral ventricle is vascularized by arteries arising from the two systems which form the arterial circle of the base, i.e. the internal carotid system and the vertebral basilar system. This blood supply is given by one anterior choroidal artery and by several posterior choroidal arteries. These arteries anastomose to form multiple indirect and remote links between the carotid and vertebral basilar systems. The capillary networks of the tela choroidea of the lateral ventricle consists of a velar network and of a choroidal network. This duality is constantly observed in the choroid formations of the human brain. The venous vascularization of the tela is tributary of the venous circle of the base of the brain through choroidal veins that drain either into the internal cerebral veins or into the basal veins.     

The venous territories (venosomes) of the human body: experimental study and clinical implications

The venous architecture of the integument and the underlying deep tissues was studied in six total-body human fresh cadavers and a series of isolated regional studies of the limbs and torso. A radiopaque lead oxide mixture was injected, and the integument and deep tissues were dissected and radiographed. The sites of the venous perforators were plotted and traced to their underlying parent veins that accompany the source (segmental) arteries. A series of cross-sectional studies were made in one subject to illustrate the course of the perforators between the integument and the deep tissues. The veins were dissected under magnification to identify the site and orientation of the valves. Results revealed a large number of valveless (oscillating) veins within the integument and deep tissues that link adjacent valved venous territories and allow equilibration of flow and pressure throughout the tissue. Where choke arteries define the arterial territories, they are matched by boundaries of oscillating veins in the venous studies. The venous architecture is a continuous network of arcades that follow the connective-tissue framework of the body. The veins converge from mobile to fixed areas, and they "hitchhike" with nerves. The venous drainage mirrors the arterial supply in the deep tissues and in most areas of the integument in the head, neck, and torso. In the limbs, the stellate pattern of the venous perforators is modified by longitudinal channels in the subdermal network. However, when an island flap is raised, these longitudinal channels are disconnected, and once again the arterial and venous patterns match. Our venous studies add strength to the angiosome concept. Where source arteries supply a composite block of tissue, we have demonstrated radiologically and by microdissection that the branches of these arteries are accompanied by veins that drain in the opposite direction and return to the same locus. Hence each angiosome consists of matching arteriosomes and venosomes. The clinical implications of these results are discussed with particular reference to the design of flaps, the delay phenomenon, venous free flaps, the pathogenesis of flap necrosis, the "muscle pump," varicose veins, and venous ulceration.     

Vascular anatomy of the anterolateral thigh flap

Arterial and venous anatomy and their relation to the anterolateral thigh flap were examined in 10 specimens of six fresh cadavers in which radiopaque materials were injected into both the arterial and venous systems. Territories and positions of individual perforating arteries were measured, and the venous drainage pathway of the flap was analyzed. All specimens were radiographed stereoscopically to observe the three-dimensional structure of the arteries and veins. The territory of each perforating artery was smaller than expected. Most of the venous blood that had perfused the dermis was considered to pool in a polygonal venous network located in the skin layer and to enter the descending branch of the lateral circumflex femoral artery through large descending veins. The venous territories were considered different from the arterial territories. The findings in this study suggest that the design of the anterolateral thigh flap should be based on the venous architecture rather than on the arterial architecture and that the flap survival rate might be improved if thinning is performed appropriately.     

The arterial and venous blood supply of the septum pellucidum in the rat.

The arterial and venous systems of the rat's septum pellucidum has been studied by means of the perfusion technique. The arteries may be classified into three groups. The branches of each of the three groups originate from the a. hemispherica and enter the septum from the frontal direction. The veins may be divided into an anterior and a posterior group. The anterior veins flow through the v. subcallosa into the sinus cavernosus, the posterior veins into v. cerebri magna. The arterial and venous blood supply of the individual nuclei of the septum are compar independent.     

Plasticity of endothelial cells during arterial-venous differentiation in the avian embryo

Remodeling of the primary vascular system of the embryo into arteries and veins has long been thought to depend largely on the influence of hemodynamic forces. This view was recently challenged by the discovery of several molecules specifically expressed by arterial or venous endothelial cells. We here analysed the expression of neuropilin-1 and TIE2, two transmembrane receptors known to play a role in vascular development. In birds, neuropilin-1 was expressed by arterial endothelium and wall cells, but absent from veins. TIE2 was strongly expressed in embryonic veins, but only weakly transcribed in most arteries. To examine whether endothelial cells are committed to an arterial or venous fate once they express these specific receptors, we constructed quail-chick chimeras. The dorsal aorta, carotid artery and the cardinal and jugular veins were isolated together with the vessel wall from quail embryos between embryonic day 2 to 15 and grafted into the coelom of chick hosts. Until embryonic day 7, all grafts yielded endothelial cells that colonized both host arteries and veins. After embryonic day 7, endothelial plasticity was progressively lost and from embryonic day 11 grafts of arteries yielded endothelial cells that colonized only chick arteries and rarely reached the host veins, while grafts of jugular veins colonized mainly host veins. When isolated from the vessel wall, quail aortic endothelial cells from embryonic day 11 embryos were able to colonize both host arteries and veins. Our results show that despite the expression of arterial or venous markers the endothelium remains plastic with regard to arterial-venous differentiation until late in embryonic development and point to a role for the vessel wall in endothelial plasticity and vessel identity.     

Localization of the sites of pulmonary vasomotion by use of arterial and venous occlusion

In this study, we present a new approach for using the pressure vs. time data obtained after various vascular occlusion maneuvers in pump-perfused lungs to gain insight into the longitudinal distribution of vascular resistance with respect to vascular compliance. Occlusion data were obtained from isolated dog lung lobes under normal control conditions, during hypoxia, and during histamine or serotonin infusion. The data used in the analysis include the slope of the arterial pressure curve and the zero time intercept of the extrapolated venous pressure curve after venous occlusion, the equilibrium pressure after simultaneous occlusion of both the arterial inflow and venous outflow, and the area bounded by equilibrium pressure and the arterial pressure curve after arterial occlusion. We analyzed these data by use of a compartmental model in which the vascular bed is represented by three parallel compliances separated by two series resistances, and each of the three compliances and the two resistances can be identified. To interpret the model parameters, we view the large arteries and veins as mainly compliance vessels and the small arteries and veins as mainly resistance vessels. The capillary bed is viewed as having a high compliance, and any capillary resistance is included in the two series resistances. With this view in mind, the results are consistent with the major response to serotonin infusion being constriction of large and small arteries (a decrease in arterial compliance and an increase in arterial resistance), the major response to histamine infusion being constriction of small and large veins (an increase in venous resistance and a decrease in venous compliance), and the major response to hypoxia being constriction of the small arteries (an increase in arterial resistance). The results suggest that this approach may have utility for evaluation of the sites of action of pulmonary vasomotor stimuli.     

Arterial Levels of Oxygen Stimulate Intimal Hyperplasia in Human Saphenous Veins via a ROS-Dependent Mechanism

Saphenous veins used as arterial grafts are exposed to arterial levels of oxygen partial pressure (pO₂), which are much greater than what they experience in their native environment. The object of this study is to determine the impact of exposing human saphenous veins to arterial pO₂. Saphenous veins and left internal mammary arteries from consenting patients undergoing coronary artery bypass grafting were cultured ex vivo for 2 weeks in the presence of arterial or venous pO₂ using an established organ culture model. Saphenous veins cultured with arterial pO₂ developed intimal hyperplasia as evidenced by 2.8-fold greater intimal area and 5.8-fold increase in cell proliferation compared to those freshly isolated. Saphenous veins cultured at venous pO₂ or internal mammary arteries cultured at arterial pO₂ did not develop intimal hyperplasia. Intimal hyperplasia was accompanied by two markers of elevated reactive oxygen species (ROS): increased dihydroethidium associated fluorescence (4-fold, p<0.05) and increased levels of the lipid peroxidation product, 4-hydroxynonenal (10-fold, p<0.05). A functional role of the increased ROS saphenous veins exposed to arterial pO₂ is suggested by the observation that chronic exposure to tiron, a ROS scavenger, during the two-week culture period, blocked intimal hyperplasia. Electron paramagnetic resonance based oximetry revealed that the pO₂ in the wall of the vessel tracked that of the atmosphere with a ~30 mmHg offset, thus the cells in the vessel wall were directly exposed to variations in pO₂. Monolayer cultures of smooth muscle cells isolated from saphenous veins exhibited increased proliferation when exposed to arterial pO₂ relative to those cultured at venous pO₂. This increased proliferation was blocked by tiron. Taken together, these data suggest that exposure of human SV to arterial pO₂ stimulates IH via a ROS-dependent pathway.     

Subepicardial endothelial cells invade the embryonic ventricle wall to form coronary arteries

Coronary arteries bring blood flow to the heart muscle. Understanding the developmental program of the coronary arteries provides insights into the treatment of coronary artery diseases. Multiple sources have been described as contributing to coronary arteries including the proepicardium, sinus venosus (SV), and endocardium. However, the developmental origins of coronary vessels are still under intense study. We have produced a new genetic tool for studying coronary development, an AplnCreER mouse line, which expresses an inducible Cre recombinase specifically in developing coronary vessels. Quantitative analysis of coronary development and timed induction of AplnCreER fate tracing showed that the progenies of subepicardial endothelial cells (ECs) both invade the compact myocardium to form coronary arteries and remain on the surface to produce veins. We found that these subepicardial ECs are the major sources of intramyocardial coronary vessels in the developing heart. In vitro explant assays indicate that the majority of these subepicardial ECs arise from endocardium of the SV and atrium, but not from ventricular endocardium. Clonal analysis of Apln-positive cells indicates that a single subepicardial EC contributes equally to both coronary arteries and veins. Collectively, these data suggested that subepicardial ECs are the major source of intramyocardial coronary arteries in the ventricle wall, and that coronary arteries and veins have a common origin in the developing heart.     

Spare alpha adrenoceptors in the peripheral circulation: excitation-contraction coupling

Postsynaptic alpha adrenoceptors in arteries and veins represent a mixed population of alpha 1 and alpha 2 adrenoceptors, with both subtypes mediating vasoconstriction. In the peripheral arterial circulation, postsynaptic vascular alpha 1 adrenoceptors are found in the adrenergic neuroeffector junction, whereas postsynaptic vascular alpha 2 adrenoceptors are located extrajunctionally. In the venous circulation, it appears that alpha 2 adrenoceptors may be predominantly junctional, whereas alpha 1 adrenoceptors may be predominantly extrajunctional. In general, alpha 1 adrenoceptors play a more important functional role in arteries than in veins, with the converse being true for postsynaptic vascular alpha 2 adrenoceptors. The relationship between alpha-adrenoceptor occupancy and vasoconstrictor response is more favorable for postsynaptic vascular alpha 1 adrenoceptors than for alpha 2 adrenoceptors in both arteries and veins, and there is evidence for a receptor reserve in alpha 1 adrenoceptors in both the arterial and venous circulation. No reserve in postsynaptic vascular alpha 2 adrenoceptors is seen in the arterial circulation, but in isolated venous preparations, a reserve in alpha 2 adrenoceptors has been observed. It has been suggested that spare alpha 2 adrenoceptors found in veins, but not arteries, may be responsible, at least in part, for the exaggerated alpha 2-adrenoceptor-mediated response of veins relative to arteries.     

Occlusion pressures vs. micropipette pressures in the pulmonary circulation

Because of the discrepancies between the arterial and venous occlusion technique and the micropuncture technique in estimating pulmonary capillary pressure gradient, we compared measurements made with the two techniques in the same preparations (isolated left lower lobe of dog lung). In addition, we also obtained direct and reliable measurements of pressures in 0.9-mm arteries and veins using a retrograde catheterization technique, as well as a microvascular pressure made with the double-occlusion technique. The following conclusions were made from dog lobes perfused with autologous blood at normal flow rate of 500-600 ml/min and pressure gradient of 12 mmHg. 1) The double-occlusion technique measures pressure in the capillaries, 2) a small pressure gradient (0.5 mmHg) exists between 30- to 50-micron arteries and veins, 3) a large pressure gradient occurs in arteries and veins greater than 0.9 mm, 4) the arterial and venous occlusion techniques measure pressures in vessels that are less than 900 microns diam but greater than 50 microns, very likely close to 100 microns, 5) serotonin constricts arteries (larger and smaller than 0.9 mm) whereas histamine constricts veins (larger and smaller than 0.9 mm). Thus three different techniques (small retrograde catheter, arterial and venous occlusion, and micropuncture) show consistent results, confirming the presence of significant resistance in large arteries and veins with minimal resistance in the microcirculation.     

Effect of ageing on the murine venous circulation

The effect of ageing on the morphology of veins, venous valves and arteries was investigated in male wild-type mice using an adapted procedure with injection of a silicone polymer Microfil^? that preserves morphology of the vasculature. Throughout the hind limb the arterial, but not the venous, lumen area and wall thickness were significantly greater in 24-month as compared to 10-week-old C57BL/6 mice. Venous valves were most frequently located at the sapheno-femoral vein junction in the lower extremities, and appeared thicker at the base supported by structurally intact collagen fibers, and thinner towards the proximal end of the valve leaflet, with less organized collagen. Overall, valves were less supported by structurally intact collagen at 24 months as compared to 10 weeks. Endothelial expression of CD31, endothelial protein C receptor or von Willebrand factor (VWF) was not affected by age, while thrombomodulin expression was lower in aged versus young arteries. At both ages, expression of VWF was lower at venous valves versus veins. Evaluation of the blood coagulation profile revealed that aged mice had shortened prothrombin time, elevated plasma levels of factor (F)VII, FVIII and VWF and increased neutrophil and platelet counts. Thus, our data indicate that in mice with ageing, venous valves become more fragile, in association with a procoagulant and inflammatory blood phenotype. Taken together, we found that the procoagulant state in ageing, is accompanied by mild vascular changes.     

Characteristics of myogenic reactivity in isolated rat mesenteric veins

Mechanisms of mechanically induced venous tone and its interaction with the endothelium and key vasoactive neurohormones are not well established. We investigated the contribution of the endothelium, l-type voltage-operated calcium channels (L-VOCCs), and PKC and Rho kinase to myogenic reactivity in mesenteric vessels exposed to increasing transmural pressure. The interaction of myogenic reactivity with norepinephrine (NE) and endothelin-1 (ET-1) was also investigated. Pressure myography was used to study isolated, cannulated, third-order rat mesenteric small veins and arteries. NE and ET-1 concentration response curves were constructed at low, intermediate, and high transmural pressures. Myogenic reactivity was not altered by nitric oxide synthase inhibition with N(ω)-nitro-L-arginine (L-NNA; 100 μM) or endothelium removal in both vessels. L-VOCCs blockade (nifedipine, 1 μM) completely abolished arterial tone, while only partially reducing venous tone. PKC (chelerythrine, 2.5 μM) and Rho kinase (Y27632, 3 μM) inhibitors largely abolished venous and arterial myogenic reactivity. There was no significant difference in the sensitivity of NE or ET-1-induced contractions within vessels. However, veins were more sensitive to NE and ET-1 when compared with corresponding arteries at low, intermediate, and high transmural pressures, respectively. These results suggest that 1) myogenic factors are important contributors to net venous tone in mesenteric veins; 2) PKC and Rho activation are important in myogenic reactivity in both vessels, while l-VOCCs play a limited role in the veins vs. the arteries, and the endothelium does not appear to modulate myogenic reactivity in either vessel type; and 3) mesenteric veins maintain an enhanced sensitivity to NE and ET-1 compared with the arteries when studied under conditions of changing transmural distending pressure.     

Arterial and venous vasculature of the heart of the beluga whale (Delphinapterus leucas)

The arteries and veins of the heart of the beluga whale (Delphinapterus leucas) are described from the dissection of nine specimens. The arterial distribution is composed of the basic mammalian pattern of two major vessels, the left and right coronary arteries, which supply the cardiac tissue. The venous drainage is provided by three major systems which are the great, middle, and small cardiac veins. The vascular characteristics of the heart of the beluga whale are the marked sinuosity of both coronary arteries and their main branches, the numerous large interarterial anastomoses between major vessels, and the duplication of vessels in parallel branches. These characteristics are discussed in functional terms and correlated with the diving ability of the species.     

Choose your fate: artery, vein or lymphatic vessel?

The specification of cell fate is integral to embryonic development. Recent research has identified several molecules that are involved in the development of the embryonic vasculature. Their combined actions are required for the specification and development of the arteries, veins and lymphatic vessels; vascular networks that are vital for embryonic and adult survival, and whose malfunction causes major pathological disorders. Recent discoveries have impacted our understanding of the embryonic origins of arterial, venous and lymphatic endothelial cells and the signals that mediate their navigation into mature, functional circulatory systems.     

The vasculature of the rat embryo: an electron microscope study

The ultrastructure of portions of the arterial and venous systems of the 11.5 day old Wistar rat embryos has been studied by scanning and transmission electron microscopy. The vessels at this stage of development are in the form of capillaries, and the arterial and venous types can be distinguished by the morphology of the endothelial cells by SEM. The endothelial cells of the arterial vessels gave prominent nuclear bulges and numerous microvilli apart from their spindle shape, whilst those of the veins appear flattened, are polygonal in shape, and have few microvilli. Transmission electron microscopy shows that the endothelial cells of the arteries and veins are identical in structure. The ultrastructure of these cells resembles that of endothelial cells at later stages of development including the adult type in that mature forms of cytoplasmic organelles are obtained. In studies on the intercellular junctions and fenestrations with lanthanum nitrate, the impression is formed that the vessels at this stage are impermeable to small molecular size particles, compared with adult capillaries. This suggests that cytoplasmic vesicles must play a major role in the transport of macromolecules in the 11.5 day embryonic vessels.     

Artery and vein size is balanced by Notch and ephrin B2/EphB4 during angiogenesis

A mutual coordination of size between developing arteries and veins is essential for establishing proper connections between these vessels and, ultimately, a functional vasculature; however, the cellular and molecular regulation of this parity is not understood. Here, we demonstrate that the size of the developing dorsal aorta and cardinal vein is reciprocally balanced. Mouse embryos carrying gain-of-function Notch alleles show enlarged aortae and underdeveloped cardinal veins, whereas those with loss-of-function mutations show small aortae and large cardinal veins. Notch does not affect the overall number of endothelial cells but balances the proportion of arterial to venous endothelial cells, thereby modulating the relative sizes of both vessel types. Loss of ephrin B2 or its receptor EphB4 also leads to enlarged aortae and underdeveloped cardinal veins; however, endothelial cells with venous identity are mislocalized in the aorta, suggesting that ephrin B2/EphB4 signaling functions distinctly from Notch by sorting arterial and venous endothelial cells into their respective vessels. Our findings provide mechanistic insight into the processes underlying artery and vein size equilibration during angiogenesis.     

Adrenoceptor function in the bovine pulmonary circulation

The bovine pulmonary vascular response to alpha- and beta-agonists was studied using an awake intact calf model. Pulmonary arterial pressure, pulmonary arterial wedge pressure, left atrial pressure, systemic arterial pressure, and cardiac output were measured in response to 3 min infusions of isoproterenol (beta-agonist; 0.12, 0.24, 0.48, 0.9, and 1.8 micrograms X kg-1 X min-1) and phenylephrine (alpha-agonist, 0.15, 0.30, 0.60, 1.15, and 2.30 micrograms X kg-1 X min-1). Phenylephrine caused an increase in vascular resistance in the pulmonary arterial and venous compartments. The slope of the resistance in response to phenylephrine was greater in the pulmonary arterial than pulmonary venous circulation. Isoproterenol resulted in a dose-dependent decrease in vascular resistance in the pulmonary arteries and veins. The vascular resistance was decreased to the same level in the pulmonary arteries and veins although the arteries showed a greater percent change. In addition, isoproterenol infusion resulted in a transient decrease in arterial pH and increase in values for packed cell volume and haemoglobin.     

Comparative characterization of reactions of skeletal muscle arterial and venous vessels to combined adrenergic effects

In experiments with the constant blood flow perfusion of the cat calf muscle and combined actions of adrenalin and noradrenaline were tested as to the blood flow resistance changes of the arterial and venous blood vessels. Separately applied the catecholamines evoked vascular resistance changes practically similar in value; combined effects of catecholamines realized in greater increase of arterial than venous resistance. In contrast to arterial vessels supramaximal stimuli resulted in much lesser constrictive effect as compared with reaction of intramural veins to separately applied catecholamines. Greater doses of catecholamines being combined, stability of effector system of skeletal muscle veins is decreased as compared to arteries.     

Arterialization of the venous system of the brain

The possibility of reverse perfusion of the brain (in which arterial blood flows to brain tissues through venous vessels, and venous blood is drained by the arteries) was studied in acute and chronic experiments on dogs. Blood pressure in cerebral veins could reach 90--120 mm Hg, in Willisii arteries it was 5--35 mm Hg. Liquor pressure reached 20--35 mmHg. After temporary arterialization of the brain venous system (10, 30 and 60 min) the animals survived without impairment of the brain function and behaviour. In the future reverse perfusion of the brain (in which blood pressure in the arteries falls to the level of venous pressure) could be used as a means of urgent surgical intervention in cases of threatened or beginning intracranial arterial hemorrhage.