The mechanisms of reparative regeneration of venous endothelium

The reaction of endothelial cells of the inferior vena cava in response to freezing-induced lesions has been analysed in the experiments on 34 young adult Kyoto-Wistar normotensive rats. First the de-endothelialized surface is covered with flattened platelets and then, three days after surgery, the endothelium is restored as a result of migration and proliferation of endotheliocytes. The migrating endothelial cells removed the adhered platelets from de-endothelialized surface. The young endothelium was presented by a single layer of strongly elongated endothelial cells whose axis was parallel to the flow of blood. An immature endothelium is characterized by an increased number of endotheliocytes. No essential differences in the reaction of venous and aortic endothelium have been revealed in response to freezing-induced lesions.     

Damage and regeneration of the endothelium of the aorta in experimental hypercholesterolemia

In dynamics of the experimental hypercholesterolemia in rabbits, peculiarities of endothelial regeneration have been studied. Comparison of proliferative activity level in endotheliocytes with structural-functional state of the endothelial monolayer at atherogenesis makes it possible to consider, that the lesion of the endothelium cannot be regarded as an initiating factor for formation of atherosclerotic lesions. Formation of the lesions in the internal lining of the arteries is preceded by certain disorders in permeability of the endothelial barrier at increasing concentration of cholesterin in blood plasma, accompanying with a sharp activation of the cell proliferative activity. When lipid plates and atherosclerotic plaques are already formed, the processes of the endothelial damage and regeneration occur in parallel. The regeneration is ensured with an intensive proliferation and growth of endotheliocytes onto deendotheliolized areas of the damaged intima.     

Stages of endotheliocyte differentiation in the genesis of capillaries in human prenatal morphogenesis

Blood microvessels of the functionally different human organs were studied by electron microscopy to reveal regularities of cytodifferentiation of endotheliocytes in prenatal morphogenesis. Such stages of differentiation of endotheliocytes were determined: mesenchymal cells----endotheliocytes of pre-existing capillaries ("marginal" cells, primordial endotheliocytes, mature endotheliocytes of pre-existing capillaries)----specialized types of endothelium (somatic, fenestrated, sinusoidal, sinusal types of endothelium, high endothelium of post-capillary venules).     

Endothelial reparative regeneration--a self-regulating process?

When investigating dynamics of reparative regeneration of the rat aorta endothelium after injuring a considerable area by means of cryogenic lesion, a certain tendency to cluster arrangement of proliferating endothelial cells (EC) has been revealed at the edge of the defect both at the beginning and at the end of the reparative process. This fact together with the data on mechanisms of migration and proliferation of EC makes it possible to suppose certain scheme of the mitotic reaction development in endothelium. It consists of centropetal movement of waves of interdependent acts of contraction and flattening of endotheliocytes. Some characteristics of the regenerative proliferation of endothelium make it possible to formulate a hypothesis on autowave character of its regulation.     

Microrelief of the internal surface of the thoracic duct in dogs (based on scanning electron microscopy data)

The investigation has been performed on 17 mature dogs. With the aim to investigate the internal surface of the thoracic duct, the methods of scanning electron microscopy of native, impregnated and corrosive preparations have been used. The analysis of the scannograms obtained demonstrates that the valves of the thoracic ducts are mainly bicuspid, have a typical infundibular form, their cuspides are fused, forming a mesentery-like fold on the duct wall. It plays the main role in ensuring unilateral transport of lymph. The microrelief of the thoracic duct endothelium possesses a rather complex organization. The following relief formations are distinguished in endotheliocytes: 1. nuclei-containing eminence, 2. marginal processes, 3. separate microvilli. The dependence of the microrelief on arrangement of endothelial cells in the valvular area or intervalvular fragments is noted. This is also concerned orientation of endotheliocytes; therefore, predominance of stretched endotheliocytes of the postvalvular fragment can be connected with demonstration of viscous friction force in the lymph stream, that produces deforming and orienting influence on the endotheliocytes.     

Reorganization of endothelial cells cytoskeleton during formation of functional monolayer in vitro

Endothelium lining the inner surface of vessels regulates permeability of vascular wall by providing exchange between blood circulation in vessels and tissue fluid and therefore performs a barrier function. Endothelial cells (ECs) in culture are able to maintain the barrier function peculiar to cells of vascular endothelium in vivo. The endothelial monolayer in vitro is a unique model system that allows studying interaction of cytoskeletal and adhesive structures of endotheliocytes from the earliest stages of its formation. In the present work, we described and quantitatively characterized the changes of EC cytoskeleton from the moment of spreading of endotheliocytes on glass and the formation of the first contacts between neighbor cells until formation of a functional confluent monolayer. The main type of intermediate filaments of ECs are vimentin filaments. At different stages of endothelial monolayer formation, disposition of vimentin filaments and their amount do not change essentially, they occupy more than 80% of the cell area. Actin filaments system of endotheliocytes is represented by cortical actin at the cell periphery and by bundles of actin stress fibers organized in parallel. With formation of contacts between cells in native endothelial cells, the number of actin filaments rises and thickness of their bundles increases. With formation of endothelial monolayer, there are also changes in the microtubules system—their number increases at the cell edge. At all stages of EC monolayer formation, the number of microtubules in the region of the already formed intercellular contacts exceeds the number of microtubules in the free lamella region of the cell.     

Vascular endothelium (review). I. General morphology. 2B: phylogenesis of the vascular endothelium

The phylogenetic descent of vascular endothelium from mesenchyme--derived precursors is described related to the development of a vessel--bound microcirculation. Endothelial precursors in primitive animals may have migrated into tissue clefts gradually forming vascular tubes. True microcirculatory vessels at first appear in the nemertines, a closed vascular system is present in some annelids whereas in arthropods an open lacunar system predominates. The first appearance of true endotheliocytes is under discussion; the author gives some evidence that it is present already in some annelids. Precursor of the endothelial wall of vessels may be the so called "Leydig's membrane", covered with amoebocytes and other mesenchymal cells. The molluscs exhibit many variants of endothelium. In the fishes, the vascular system begins to split into a blood and a lymphatic system. Obviously the specialization of endothelium correlates with the level of evolution. Despite the complicated course, the evolution of endothelium may be regarded as monophyletic.     

Changes in the venular epithelium of the mesentery in rats after application of hydrogen peroxide

The change of the venular permeability and endothelial structure was studied with the aid of fluorometry and electron microscopy after application of H2O2. Transmural transfer rate of FITC-albumin++ and permeable fraction of the vessel walls were increased. There were formed the "leaks" and transendothelial canals in the venular endothelium. Numerous local membrane injuries of the endotheliocytes and pericytes, degranulation of the mast cells and destruction of thrombocytes into the vessel lumen were detected.     

Regional characteristics of the organization of the aortic endothelium (a quantitative analysis)

By means of SEM-analysis in 110 microphotos (from 14 aortas of white rats) with the aim to estimate heteromorphism of the thoracic part endothelium, quantitative characteristics of endotheliocytes have been studied in the ventral and dorsal surfaces, and also around the ostia of the intercostal arteries. For the quantitative analysis organization of the lines of the interendotheliocyte borders is taken into account. Endotheliocytes around the ostium are more elongated but occupy less area and have less straight contours in comparison with cells in other regions; they have also greater variability. Essential differences in arranging the endothelial cells into the layer on the ventral and dorsal surfaces of the aorta are revealed.     

Mechanisms of neovasculogenesis and its regulation in the adult organism

Under regeneration of organs, wound healing, tumour growth, inflammatory processes, under many compensatory and adaptive reactions in the organism of mature persons and animals, an inevitable formation of new blood vessels (neovasculogenesis) takes place. Modern notions on mechanisms of neovasculogenesis are based on the fact that new formation of vessels in a mature organism includes processes of migration and replication of endothelial cells according to the principle: "endothelium from endothelium". The literature data on neovasculogenesis in the mature organism are summarized and compared with the authors' investigations. Characterization of new blood vessels growth is presented; ultrastructural organization of endotheliocytes in growing capillaries, formation of barrier-transport properties in the newly formed vessels, role of inductors and inhibitors of neovasculogenesis in creation of new vascular formations are considered.     

Condition of the endothelium of the aorta during hypodynamia

The state of the aortic internal lining under conditions of hypodynamia has been studied on 47 white male rats morphometrically and stereologically in film preparations of endothelium. The results of measurements have been treated by means of the moving average method. Significance of differences has been estimated using nonparametric criterion of Wilcoxon--Mann--Whitney. For objective periodization of reactions centring of approximated curves with successive calculation of the variation coefficient of the centered ordinates in every time point has been calculated. Hypokinesia produces manifested changes of phasic character in endothelium. Restoration of the disturbed homeostasis in endothelium is ensured on the bases of both intracellular and cellular regeneration. At late stages (beginning from the 20th day) density in arrangement of endotheliocytes in the layer decreases, as well as the degree of tissue regularity. Polymorphism of endothelium is essentially manifested. Parameters demonstrating destruction and death of cells are rather high. Under such conditions, the response reaction of the arterial wall could acquire the character of developing atherosclerotic lesion.     

The mechanism of the elimination of dead cells in the myocardium of the fetal rat]

At investigation of the intact and damaged myocardium of the rat fetuses (age 17-21 days) several ways of elimination of the perished cells have been revealed: a) exfoliation of the endocardial perished cells into the ventricular cavity; b) phagocytosis by phagocytes of the fetus; c) bordering of the perished cells by endotheliocytes with a subsequent joining of the endothelium to the blood stream. When the fetal myocardium is damaged, elimination of the perished cells is not ensured, nevertheless, during physiological death of the myocardial cells this method is probably effective. The restrictive function of the endothelium plays an essential role in morphogenesis of the heart and is ensured by certain changes in composition of the intercellular substance.     

State of the endothelium of the aorta during chronic stress

In 50 white male rats (18-control and 32-experimental) using morphometrical and stereological methods for film preparations of endothelium, the state of the internal aortal lining at various stages of chronic stress developing during muscular activity has been studied. The data obtained are statistically treated by the sliding averages method, reliability of the differences is estimated according to the value of nonparametrical criterion of Wilcoxon-Mann-Whitney. For objective periodization of the endothelial reaction to stress, centered moving averages curves are performed with subsequent calculation of the coefficient of variation for the aligned ordinates of the approximated values in every temporal point. The aortal endothelium reaction to the stress is of phasic character. During the alarm reaction, destructive changes predominate; they are accompanied with an increased mitotic activity of endotheliocytes up to 2.2%. The resistance stage increases compensatory-adaptive rearrangements, which are manifested not only as cellular hyperplasia, but as hyperthrophy of nuclei, increasing number of strangulated nuclei, nucleoli, as well as binuclear and trinucler cells. To the exhaustion period the increase of alternative and restorative transformations of the tissue corresponds.     

Specific endothelial bodies in growing capillaries of the rabbit cornea

Contents of Veibel-Palade corpuscles have been studied in endotheliocytes of vessels of the rabbit eye limb (control) and in the corneal capillaries, growing after burn effect and after a combined effect of burn and colchycin. In the growing capillaries of the cornea the specific endothelial bodies are constant components of the cells and make effect to processes of the vascular new formations.     

Differentiation of hepatic sinusoid vessel cells in rats in the embryonal and postnatal periods of ontogenesis

Differentiation of the hepatic sinusoidal capillary cells has been studied in the embryonal and postnatal periods of the rat ontogenesis. In the embryonal liver cellular elements are differentiated divergently with formation of endotheliocytes, Kupffer cells and fat-accumulating cells. Differentiation of endotheliocytes is accompanied with their flattening, formation of fenestrae and pores in peripheral processes, increasing amount of micro- and macropinocytic vesicles, and also development of intercellular contacts, presented at first as simple, and then as tight junctions. Formation of intercellular clefts occurs by means of parting of simple junctions; this ensures migration of blood cells across the endothelial lining. Kupffer cells develop out of immature macrophagal elements of the perisinusoidal space. During differentiation they migrate into the endothelial lining. Simultaneously in their cytoplasm lysosomes accumulate and acid phosphatase activity increases. The fat-accumulating cells are formed out of poorly differentiating fibroblast-like elements of the perisinusoidal space. Their differentiation is accompanied with a progressive development of the granular endoplasmic reticulum and with accumulation of reticular fibers in the perisinusoidal space, and in cytoplasm--lipid inclusions.     

Dose-dependent effect of nocodazole on endothelial cell cytoskeleton

The endothelium lining the inner surface of blood vessels fulfils an important barrier function and specifically, it controls vascular membrane permeability as well as nutrient and metabolite exchange in circulating blood and tissue fluids. Disturbances in vascular endothelium barrier function (vascular endothelium dysfunction) are coupled to cytoskeleton rearrangements, actomyosin contractility, and as a consequence, formation of paracellular gaps between endothelial cells. Microtubules constitute the first effector link in the reaction cascade resulting in vascular endothelium dysfunction. Increased vascular permeability associated with many human diseases is also manifested as a side effect in anticancer mitosis-blocking therapy. The aim of this study was to examine the possibility of preventing side effects of mitostatic drugs in patients with vascular endothelium dysfunction and to establish effective doses able to disrupt the microtubular network without interfering with the endothelial barrier function. Previously, it was found that the population of endothelial cell microtubules is heterogeneous. Along with dynamic microtubules, cell cytoplasm contains a certain amount of post-translationally modified microtubules that are less active and less susceptible to external influences than dynamic microtubules. We have shown that the area occupied with stable microtubules is relatively large (approx. one third of the total cell area). We assume that it can account for a higher resistance of the endothelial monolayer to factors responsible for vascular endothelium dysfunction. This hypothesis was validated in this study, in which nocodazole was used to induce vascular endothelium dysfunction in lung endothelial cells. The effect of nocodazole on endothelial cell cytoskeleton was found to be dose-dependent. Nocodazole in micromolar concentrations not only irreversibly changed the barrier function, but also upset the viability of endothelial cells and induced their death. Nanomolar concentrations of nocodazole also increased the permeability of the endothelial monolayer; this effect was reversible at the drug concentration ranging from 100 to 200 nM. At 100 nM, nocodazole induced partial disruption of the microtubule network near the cell margin without any appreciable effect on acetylated microtubules and actin filaments. At 200 nM, nocodazole exerted a pronounced effect on the system of dynamic (but not acetylated) microtubules and increased the population of actin filaments in the central region of the cell. Our data suggest that disruption of peripheral microtubules triggers a cascade of reactions culminating in endothelial barrier dysfunction; however, the existence of a large population of microtubules resistant to nanomolar concentrations of the drug provides higher viability of endothelial cells and restores their functional activity.     

Characterization of the enhanced adhesion of neutrophil leukocytes to thrombin-stimulated endothelial cells.

We have characterized the mechanisms by which thrombin enhances neutrophil leukocyte (PMN) adhesion to human endothelial cells in vitro. Thrombin rapidly and transiently increased PMN adhesion by an action on the endothelial cells. The transience of the response was due to at least two factors: desensitization of the endothelial cell responsiveness to thrombin in the continued presence of the agonist; and the lability (t1/2 less than 15 min) of the effector molecules expressed by the endothelium. Experiments with exogenous platelet-activating factor (PAF) and with PAF antagonists demonstrated that PAF production, although it may facilitate the enhanced PMN adhesion seen in response to thrombin, is not sufficient to explain the reaction. By using a variety of antibodies directed against cell surface ligands, and comparing adhesion of PMN to endothelium and to protein-coated surfaces, we deduce that several endothelial ligands not previously reported as playing a role in PMN adhesion are involved in these interactions. Of particular interest was the finding that antibodies recognizing two thrombin-regulated endothelial cell surface ligands, GMP-140 and the CD63-related Ag, both inhibited adhesion of PMN to thrombin- or LPS-pretreated endothelium. We conclude that thrombin acts to enhance PMN adhesion to endothelium at least in part by transiently altering the conformation or level of expression of these ligands.     

Comparative histochemical and electron microscopic studies of the sinus and venous walls of the human spleen with special reference to the sinus-venous connections

Sinus and venous walls of normal human spleens were studied with enzyme histochemical and electron microscopic methods. Particular attention was paid to the connections between sinuses and veins. Histochemically the sinus lining cells revealed a distinct naphthol-AS-acetate-esterase activity but no reaction for alkaline phosphatase. Venous endothelial cells were positive for the latter but negative for the former enzyme. In the sinus-venous junctional area there were no endothelial cells with reactivity for both enzymes. Electron microscopically both the sinus lining cells and the venous endothelial cells could be clearly characterized and therefore easily distinguished from one another on morphological grounds. There were no clear ultrastrural indications of transitional forms between sinus lining cells and venous endothelial cells in the sinus-venous area. According to these findings, sinus lining cells represent a specialized endothelium, but one with practically no morphological similarities to the venous endothelium.     

Upregulation of the low density lipoprotein receptor at the blood-brain barrier: intercommunications between brain capillary endothelial cells and astrocytes

In contrast to the endothelial cells in large vessels where LDL receptors are downregulated, brain capillary endothelial cells in vivo express an LDL receptor. Using a cell culture model of the blood-brain barrier consisting of a coculture of brain capillary endothelial cells and astrocytes, we observed that the capacity of endothelial cells to bind LDL is enhanced threefold when cocultured with astrocytes. We next investigated the ability of astrocytes to modulate endothelial cell LDL receptor expression. We have shown that the lipid requirement of astrocytes increases the expression of endothelial cell LDL receptors. Experiments with dialysis membranes of different pore size showed that this effect is mediated by a soluble factor(s) with relative molecular mass somewhere between 3,500 and 14,000. Substituting astrocytes with smooth muscle cells or brain endothelium with endothelium from the aorta or the adrenal cortex did not enhance the luminal LDL receptor expression on endothelial cells, demonstrating the specificity of the interactions. This factor(s) is exclusively secreted by astrocytes cocultured with brain capillary endothelial cells, but it also upregulates the LDL receptor on other cell types. This study confirms the notion that the final fine tuning of cell differentiation is under local control.     

Effect of inflammatory cytokines on the adherence of tumor cells to endothelium in a murine model

We have demonstrated that pretreatment of mouse brain microvascular endothelial cells (MBE) with tumor necrosis factor-alpha (TNF), IL-1, or LPS augmented the binding of P815 mastocytoma cells in vitro. The effect of these agents was dose and time dependent. PMA was able to mimic the influence of these factors to a limited degree. The effect of TNF on endothelium was accompanied by the appearance of changes in the expression of proteins isolated from endothelial cell membranes. The adherence of tumor cells to endothelium was not inhibited by RGD-containing peptides but could be decreased by preincubation of endothelium with high concentrations of FCS. Our data suggest that cytokines regulate the synthesis of endothelial adhesion proteins which may be involved in tumor cell adherence leading to metastasis. These results raise the possibility that cytokines may exert paradoxical effects in vivo, i.e., a cytotoxic effect that reduces tumor mass accompanied by a metastasis-enhancing effect that actually promotes dissemination of the remaining tumor cells. Definition of the molecular events involved in tumor cell-endothelial cell interactions may lead to strategies for minimizing the latter effect in therapeutic settings.