Airway vasculature after mycoplasma infection: chronic leakiness and selective hypersensitivity to substance P

Angiogenesis and microvascular remodeling are features of chronic airway inflammation caused by Mycoplasma pulmonis infection in rats. As airway blood vessels undergo remodeling, they become unusually sensitive to substance P-induced plasma leakage. Here we determined whether the remodeled vessels are leaky under baseline conditions, whether their heightened sensitivity is specific to substance P, and whether the leakage is reversible. Four weeks after infection, the amount of baseline leakage of Evans blue in the tracheal mucosa was two to five times the normal level. Gaps < 1 microm in diameter were located between endothelial cells in some remodeled vessels. Substance P, but not platelet-activating factor or 5-hydroxytryptamine, produced an exaggerated leakage response. Inhalation of the beta2-adrenergic receptor agonist salmeterol reduced the leakage by <60%. We conclude that the blood vessel remodeling after M. pulmonis infection is associated with microvascular leakiness due, in part, to the formation of endothelial gaps. This leakage is accompanied by an abnormal sensitivity to substance P but not to platelet-activating factor or 5-hydroxytryptamine and can be reduced by beta2-agonists.     

Vascular abnormalities due to hyperthermia in chick embryos

Intraembryonic vascular abnormalities were studied in chick embryos exposed to temperatures 3 degrees C and 4 degrees C above normal temperature (38 degrees C) from the beginning of incubation. The average duration of hyperthermia was 54 and 53 hours, respectively. Immediately after exposure, the embryos were examined with FITC-Dextran microangiography in vivo. Following hyperthermia various abnormalities in the heart, ventral aortae, aortic arches, omphalomesenteric arteries, and the distal dorsal aortae frequently occurred. There were also significant microvascular changes in the head, in the lateral and caudal parts of the embryos, and in the pellucid area of the yolk sac. In another series incubation at 41 degrees C, hyperthermia of 3 degrees C during the first 3 days of development produced several extraembryonic vascular abnormalities. These included duplication and abnormal branching of the cranial vitelline vein, absence or abnormal course of the omphalomesenteric vessels, aneurysmatic dilatation or abnormal course of the caudal vitelline vein, and aneurysmatic dilatation or occlusion of the abdominal venous sinus. Most frequent findings were blind, congested, and dilated microvascular segments in the pellucid area, commonly associated with an irregular microvascular pattern and perivascular swelling. The abnormalities described are assumed to be caused by the direct effects of hyperthermia upon the developing vessels resulting in microvascular insufficiencies, pathological leakage, and perivascular oedema. Such disturbances may have serious consequences for embryonic vascular development and microcirculation, which in turn may have adverse effects on further embryonic growth and development.     

Adhesive interactions between CD34<Superscript>+</Superscript>-derived dendritic cell precursors and dermal microvascular endothelial cells studied by scanning electron microscopy

Dendritic cells are migratory cells. Before they extravasate from the circulation into the skin across capillary blood vessel walls, they have to interact with endothelial cells. Using a fluorimetric adhesion assay, we have recently shown that CD34⁺-derived dendritic cell precursors are able to bind to resting and stimulated dermal microvascular endothelial cells. In the present study, we attempted to visualize this process at an ultrastructural level. CD34⁺ progenitor cells were purified from human cord blood samples by means of immunomagnetic beads, and dendritic cells were generated by culture in the presence of GM-CSF, TNF- and hSCF for 5 days. Immature CD83^– CD86^low dendritic cells were added to human dermal microvascular endothelial cells grown to confluence on membrane chambers. After 2 h, unbound dendritic cell precursors were removed, and bound cells were prepared for routine scanning electron microscopy. We found that (1) dendritic cell precursors firmly adhere to microvascular endothelial cells, enveloping them with their surface processes; (2) dendritic cell precursors are extremely deformable as they squeeze through the dense network of microvascular endothelial cells; (3) microvascular endothelial cells form, in part, a multi-layered network rather than the typical cobblestone pattern as seen by phase-contrast microscopy. The morphology of dendritic cell precursors and of human dermal microvascular endothelial cells was examined here, for the first time, by scanning electron microscopy. These data further emphasize that CD34⁺-derived dendritic cells efficiently adhere to dermal microvascular endothelial cells.     

Ultrastructure of pericytes in early stages of histamine-induced inflammation

Physiological and ultrastructural assessment of changes in the walls of venules in the rat cremaster muscle after administration of histamine indicates that pericytes have essential roles in the normal functioning of venules during inflammation. Fluorescein-labelled albumin was used to quantitate macromolecular leakage and to select suitable venules for ultrastructural analysis 4 and 7 minutes after addition of histamine. Pericytes were concentrated over endothelial cell junctions and gaps. At 4 minutes, when albumin leakage was becoming detectable, gaps between endothelial cells were observed in the venule wall. In 24 serially sectioned gaps, pericytes formed covers, with contact points to the endothelial cells along the sides of the gaps. At 7 minutes, when albumin leakage was maximal, gaps with pericyte covers were still evident, but more commonly observed were pericyte covers over closed endothelial cell junctions. Spaces between the innermost pericytes and endothelial cells were enlarged by an order of magnitude, from 95 nm in controls to 872 nm at 4 minutes and 958 nm at 7 minutes. Pericytes formed coverings or bridges over inclusions of extravasated cells, fluid, proteins, and the vascular label monastral blue. The data indicate that pericytes protect the endothelial lining of venules during histamine-induced inflammation by forming a cohesive covering across gaps.     

Hyperthermia inhibits angiogenesis

Since in vitro studies have demonstrated that capillary endothelial cells are thermosensitive, experiments were performed to determine the (in vivo) heat sensitivity of blood capillaries and their endothelial cells. Angiogenesis discs were implanted subcutaneously in mice, and vascular growth was stimulated by slow release of epidermal growth factor placed in the center of each disc. After 5 days of growth the discs were subjected to radiofrequency-induced hyperthermia. Heat exposures were 41, 42, 43, and 44 degrees C for 30 min. Control discs were sham treated. Seven days after heating the discs were extracted and paraffin embedded. Centripetal (radial) vessel growth was measured in magnified medial planar sections. An inverse relationship was demonstrated between vessel growth and exposure temperature. The extent of the fibroblastic growth was also inversely proportional to temperature. Thus, at least in this system, the microvasculature shows dose-dependent damage by hyperthermia, consistent with preceding in vitro observations. This inhibition of angiogenesis may result from endothelial cell killing, interference with cell replication, inhibition of cell migration, or a combination of these mechanisms.     

Amyloid β aggregation induces human brain microvascular endothelial cell death with abnormal actin organization

Cerebral amyloid angiopathy (CAA) is a disease in which amyloid β (Aβ) is deposited on the walls of blood vessels in the brain, making those walls brittle and causing cerebral hemorrhage. However, the mechanism underlying its onset is not well understood. The aggregation and accumulation of Aβ cause the occlusion and fragility of blood vessels due to endothelial cell damage, breakdown of the blood-brain barrier, and replacement with elements constituting the blood vessel wall. In this study, we observed the effect of Aβ on human primary brain microvascular endothelial cells (hBMECs) in real-time using quantum dot nanoprobes to elucidate the mechanism of vascular weakening by Aβ. It was observed that Aβ began to aggregate around hBMECs after the start of incubation and that the cells were covered with aggregates. Aβ aggregates firmly anchored the cells on the plate surface, and eventually suppressed cell motility and caused cell death. Furthermore, Aβ aggregation induced the organization of abnormal actin, resulting in a significant increase in intracellular actin dots over 10 μm². These results suggest that the mechanism by which Aβ forms a fragile vessel wall is as follows: Aβ aggregation around vascular endothelial cells anchors them to the substrate, induces abnormal actin organization, and leads to cell death.     

Increased blood–brain barrier permeability and endothelial abnormalities induced by vascular endothelial growth factor

The early effects of intracerebrally infused vascular endothelial growth factor (VEGF) on the blood–brain barrier (BBB) to endogenous albumin were studied using a quantitative immunocytochemical procedure. In addition, transmission electron microscopy was used to observe morphological changes induced in brain vasculature. A solution of VEGF in saline (40 ng/10 μl) was infused into the parieto-occipital cortex of mice, which were killed 10 min, 30 min, and 24 h afterwards. Untreated mice and mice that received infusion of saline only were used as controls. For immunocytochemical evaluation, ultrathin sections of immersion-fixed brain samples embedded in Lowicryl K4M were exposed to anti-albumin antiserum followed by protein A-gold. Simultaneously, other brain samples embedded in Spurr resin were used for ultrastructural examination. Morphometric and statistical analysis indicated that as soon as 10 min after infusion of VEGF, 33% of vascular profiles were leaking albumin, and this value increased at 30 min to 92%. This effect of VEGF appears to be of rather short duration because after 24 h, only 27% of vascular profiles showed signs of leakage. The results of ultrastructural observations indicate that VEGF (30 min post-infusion) induces several changes in microvascular segments located in the area of intracerebral infusion of VEGF. These changes consist of the appearance of interendothelial gaps; fragmentation of the endothelium with formation of segmental, fenestrae-like narrowings; degenerative changes of the vascular basement membrane; and the appearance of fibrin gel in the vessel lumen. At 24 h post-infusion, solitary diaphragmed fenestrae appeared in attenuated segments of the endothelium in a few microvascular profiles. These changes, which are interpreted to be preparatory steps for angiogenesis, affect the structural integrity of the vascular segments, leading to extravasation of blood plasma proteins, including albumin. © 1998 Chapman and Hall     

Identification and isolation of endothelial cells based on their increased uptake of acetylated-low density lipoprotein

Acetylated-low density lipoprotein (Ac-LDL) is taken up by macrophages and endothelial cells via the "scavenger cell pathway" of LDL metabolism. In this report, aortic and microvascular endothelial cells internalized and degraded 7-15 times more [125I]-Ac-LDL than did smooth muscle cells or pericytes. Bound [125I]-Ac-LDL was displaced by unlabeled Ac-LDL, but not unmodified LDL. The ability to identify endothelial cells based on their increased metabolism of Ac-LDL was examined using Ac-LDL labeled with the fluorescent probe 1,1'- dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate (Dil-Ac- LDL). When cells were incubated with 10 micrograms/ml Dil-Ac-LDL for 4 h at 37 degrees C and subsequently examined by fluorescence microscopy, capillary and aortic endothelial cells were brilliantly fluorescent whereas the fluorescent intensity of retinal pericytes and smooth muscle cells was only slightly above background levels. Dil-Ac-LDL at the concentration used for labeling cells had no effect on endothelial cell growth rate. When primary cultures of bovine adrenal capillary cells were labeled with 10 micrograms/ml of Dil-Ac-LDL for 4 h at 37 degrees C, then trypsinized and subjected to fluorescence-activated cell sorting, pure cultures of capillary endothelial cells could be obtained. Utilizing this method, large numbers of early passage microvascular endothelial cells can be obtained in significantly less time than with conventional methods.     

Three-dimensional localization and quantification of PAF-induced gap formation in intact venular microvessels

Combining single-vessel perfusion technique with confocal microscopy, this study presents a new approach that allows three-dimensional visualization and quantification of endothelial gaps under experimental conditions identical to those used to measure permeability coefficients, endothelial calcium concentration, and nitric oxide production in individually perfused intact microvessels. This approach provides an efficient means for defining the transport pathways and cellular mechanisms of increased microvascular permeability during inflammation. Platelet-activating factor (PAF) was used to increase the permeability of individually perfused rat mesenteric venules. Fluorescent microspheres (FMs, 100 nm) were used as leakage markers, and confocal images were acquired at successive focal planes through the perfused microvessel. Perfusion of FMs under control conditions produced a thin, uniform layer of FMs in the vessel lumen, but in PAF-stimulated microvessels significant amounts of FMs accumulated at endothelial junctions. Reconstructed confocal images three-dimensionally delineated the temporal and spatial development of endothelial gaps in PAF-stimulated microvessels. The FM accumulation, quantified as the total fluorescence intensity per square micrometer of vessel wall, was 8.4 +/- 1.8 times the control value within 10 min of PAF perfusion and declined to 5.0 +/- 0.6 and 1.4 +/- 0.2 times the control value when FMs were applied 30 and 60 min after PAF perfusion. The changes in the magnitude of FM accumulation closely correlated with the time course of PAF-induced increases in hydraulic conductivity (L(p)), indicating that the opening and closing of endothelial gaps contributed to the transient increase in L(p) in PAF-stimulated microvessels. Electron microscopic evaluations confirmed PAF-induced gap formation and FM accumulation at endothelial clefts.     

Thermal sensitivity of endothelial cells

Experimental work indicates that one of the mechanisms of tumor control by hyperthermia may be damage to blood vessels, resulting in decreased blood flow to the neoplasms. Among the various elements of the microvasculature, endothelial cells are the most important possible targets of thermal injury. Furthermore, neoplasms have a significantly higher proportion of proliferating endothelial cells than do normal tissues. Thus it is necessary to establish the thermal sensitivity of endothelial cells and to explore possible differences in response between resting and proliferating endothelium. We studied the in vitro thermal sensitivity of murine and human capillary endothelial cells compared to human fibroblasts by following cell survival and growth recovery. Nonstimulated endothelial cells are more sensitive than fibroblasts. Their sensitivity is dose dependent within the range of 42 to 45 degrees C/30 min. Stimulation to proliferate by endothelial cell growth factor (ECGF) renders these cells even more sensitive. Morphologic studies confirm these thermal effects in endothelial cells and fibroblasts. These findings support a direct effect of hyperthermia on endothelial cells, which appears to be more severe in proliferating cells. This may explain the reduced blood flow in heated tumors and may indicate a valuable therapeutic gain for hyperthermia.     

Endothelial Rho signaling is required for monocyte transendothelial migration

Bacterial toxins affecting Rho activity in microvascular endothelial cells were employed to elucidate whether endothelial Rho participates in regulating the migration of monocytes across monolayers of cultured endothelial cells. Inactivation of Rho by the Clostridium C3 exoenzyme resulted in an increased adhesion of peripheral blood monocytes to the endothelium and a decreased rate of transendothelial monocyte migration. Cytotoxic necrotizing factor 1-mediated activation of endothelial Rho also reduced the rate of monocyte transmigration, but did not affect monocyte-endothelium adhesion. Thus, efficient leukocyte extravasation requires Rho signaling not only within the migrating leukocytes but also within the endothelial lining of the vessel wall.     

Salvianolic Acid B Ameliorates Lipopolysaccharide-Induced Albumin Leakage from Rat Mesenteric Venules through Src-Regulated Transcelluar Pathway and Paracellular Pathway

Lipopolysaccharide (LPS) causes microvascular barrier disruption, leading to albumin leakage from microvessels resulting in a range of disastrous sequels. Salvianolic acid B (SalB) is a major water-soluble component derived from Salvia miltiorrhiza. Previous studies showed its potential to attenuate microvascular barrier dysfunction, but the underlying mechanism is not fully understood. The present study was intended to investigate the impact of SalB on endothelial cell barrier in vivo in rat mesenteric venules as well as in vitro in human umbilical vein endothelial cells (HUVECs), aiming at disclosing the mechanism thereof, particularly the role of Src in its action. Male Wistar rats were challenged by infusion of LPS (2 mg/kg/h) through left femoral vein for 90 min. SalB (5 mg/kg/h) was administrated either simultaneously with LPS or 30 min after LPS infusion through the left jugular vein. Vesicles in venular walls were observed by electron microscopy. HUVECs were incubated with LPS with or without SalB. The expression of Zonula occluden-1 (ZO-1), VE-cadherin, caveolin-1 and Src in HUVECs was assessed by Western blot and confocal microscopy, binding of SalB to Src was measured using Surface Plasmon Resonance and BioLayer Interferometry. Treatment with SalB inhibited albumin leakage from rat mesenteric venules and inhibited the increase of vesicle number in venular endothelial cells induced by LPS. In addition, SalB inhibited the degradation of ZO-1, the phosphorylation and redistribution of VE-cadherin, the expression and phosphorylation of caveolin-1, and phosphoirylation of Src in HUVECs exposed to LPS. Furthermore, SalB was found able to bind to Src. This study demonstrates that protection of SalB against microvascular barrier disruption is a process involving both para- and trans-endothelial cell pathway, and highly suggests Src as the key enzyme for SalB to work.     

Involvement of the proteasome in activation of endothelial nitric oxide synthase

Nitric oxide originating from the endothelial cells of the vessel wall is essential for the vascular system. It is produced by the enzyme endothelial nitric oxide synthase (eNOS). Cellular eNOS activity is affected by changes in eNOS synthesis. To address whether degradation also contributes to eNOS activity, the effect of proteasome inhibitors on eNOS-mediated NO synthesis was studied in the microvascular endothelial cell line bEnd.3 and in cultured primary aortic endothelial cells. Surprisingly, agonist-induced increases in eNOS activity were reduced to 42 and 50% in the presence of the proteasome inhibiting drugs MG132 and clasto-lactacystin-beta-lactone, respectively (P < 0.01). The decrease in activity occurred within 1 hour of drug treatment and was not accompanied by a change in intracellular levels of either eNOS or its inhibitor caveolin-1. Taken together, these data may indicate that eNOS is regulated by an interacting protein, different from caveolin-1, that inhibits its activity and is rapidly degraded by the proteasome in the presence of eNOS agonists.     

Diffusion kinetics of tritiated uridine into HeLa cells previously exposed to hyperthermia

Hyperthermic treatment of HeLa cells at 42 degrees C for 60 min depressed the specific activity of these cells when incubated with 3H-uridine both during and post heating compared to cells maintained at 37 degrees C. These changes were unlikely to arise from increased leakage from the cells and may partially be attributed to membrane damage influencing facilitated diffusion. Diffusion kinetic data for incorporation of the radiolabel into the T.C.A. soluble and T.C.A. insoluble fractions of HeLa cells indicated that a significant depression of Vmax and a significant elevation of Km for incorporation of 3H UdR into RNA may possibly result from an isotope dilution effect attributed to degrading pre-ribosomal RNA under the effect of hyperthermia.     

New approach to improving endothelial preservation in cryopreserved arterial substitutes

The endothelial loss provoked by the methods of vascular cryopreservation used at most human vessel banks is one of the main factors leading to the failure of grafting procedures performed using cryopreserved vessel substitutes. This study evaluates the effects of the storage temperature and thawing protocol on the endothelial cell loss suffered by cryopreserved vessels, and optimises the thawing temperature and protocol for cryopreserving arterial grafts in terms of that producing least endothelial loss. Segments of the common iliac artery of the minipig (n = 20) were frozen at a temperature reduction rate of 1 degrees C/min in a biological freezer. After storing the arterial fragments for 30 days, study groups were established according to the storage temperature (-80, -145 or -196 degrees C) and subsequent thawing procedure (slow or rapid thawing). Fresh vessel segments served as the control group. Once thawed, the specimens were examined by light, transmission, and scanning electron microscopy. The covered endothelial surface was determined by image analysis. Data for the different groups were compared by one way ANOVA. When cryopreservation at each of the storage temperatures was followed by slow thawing, the endothelial cells showed improved morphological features and viability over those of specimens subjected to rapid thawing. Rapidly thawed endothelial cells showed irreversible ultrastructural damage such as mitochondrial dilation and rupture, reticular fragmentation, and peripheral nuclear condensation. In contrast, slow thawing gave rise to changes compatible with reversible damage in a large proportion of the endothelial cells: general swelling, reticular dilation, mitochondrial swelling, and nuclear chromatin condensation. Gradually thawed cryopreserved arteries showed a lower proportion of damaged cells identified by the TUNEL method compared to the corresponding rapidly thawed specimens (p < 0.05, for all temperatures). In all the groups in which vessels underwent rapid thawing (except at -145 degrees C), significant differences (p < 0.05) in endothelial cover values were recorded with respect to control groups. Storage of cryopreserved vessels at -80 degrees C followed by rapid thawing led to greatest endothelial cell loss (61.36+/-9.06% covered endothelial surface), while a temperature of -145 degrees C followed by slow thawing was best at preserving the endothelium of the vessel wall (89.38+/-16.67% surface cover). In conclusion, storage at a temperature of -145 degrees C in nitrogen vapour followed by gradual automated thawing seems to be the best way of preserving the endothelial surface of the arterial cryograft. This method gives rise to best endothelial cell viability and cover values, with obvious benefits for subsequent grafting.     

An electron microscopic study of the circulatory system in Nereis japonica

Blood vessels in Nereis japonica were studied by electron microscopy. It was found that blood vessels regardless of location were similar in the basic organization of the basal lamina and the usual presence of collagen fibrils on the vessel wall. Differences arise, depending on whether the outside of the basal lamina is covered by peritoneal cells, by gut epithelium, or by epidermis. These relate to the location of the vessels in mesenteries, gut or epidermis, but do not reflect basic structural differences in the vessels themselves. Furthermore, it was concluded that true endothelial cells do not exist in the circulatory system of Nereis japonica and that, in this respect, the system is essentially different from that of vertebrates, in which endothelial cells line the vessels of a closed circulatory system. These considerations lead to the further conclusion that the vascular lumen in Nereis is essentially interstitial space and that the system, which has been known as a typical “closed” circulatory system in annelids, is actually an open circulatory system. Peritoneal cells covering the walls of internal vessels show various degrees of muscular differentiation and those possessing myofilaments may be called “myomesothelial cells.”     

Angiopoietin-1 decreases plasma leakage by reducing number and size of endothelial gaps in venules

Angiopoietin-1 (Ang-1) is essential for remodeling of the primitive vascular plexus and recruitment of mural cells during embryonic development. In the adult vasculature, Ang-1 can reduce plasma leakage in inflammation, but the mechanism of this action is not well understood. In the present study, we determined the magnitude and cellular mechanism of the antileak effect of Ang-1 in the airways of mice. Intravenous injection of bradykinin resulted in leakage of fluorescent microspheres (diameter 25-1,000 nm) from tracheal venules. The leakage peaked in 3-4 min and resolved by 10 min. High-resolution confocal microscopy revealed the presence of focal gaps at intercellular junctions of leaky venules. Genetically engineered Ang-1*, delivered systemically by adenoviral transduction of the liver, reduced leakage of 500-nm microspheres after bradykinin by 69%. The reduction in leakage coincided with a decrease in number and size of endothelial gaps. The proportion of venular surface occupied by endothelial gaps decreased 61%. Microsphere leakage correlated strongly with gap number and size (r2 = 0.89). Together the results suggest that Ang-1 reduces leakage from inflamed venules by restricting the number and size of gaps that form at endothelial cell junctions through effects on intracellular signaling, cytoskeleton, and junction-related molecules.     

Regulation of trophoblast beta1-integrin expression by contact with endothelial cells

Background  

In human and non-human primates, migratory trophoblasts penetrate the uterine epithelium, invade uterine matrix, and enter the uterine vasculature. Invasive trophoblasts show increased expression of β1 integrin. Since trophoblast migration within the uterine vasculature involves trophoblast attachment to endothelial cells lining the vessel walls, this raises the possibility that cell-cell contact and/or factors released by endothelial cells could regulate trophoblast integrin expression. To test this, we used an in vitro system consisting of early gestation macaque trophoblasts co-cultured on top of uterine microvascular endothelial cells.     

Regulation of endothelial connexin40 expression by shear stress

Endothelial connexin (Cx)40 plays an important role in signal propagation along blood vessel walls, modulating vessel diameter and thereby blood flow. Blood flow, in turn, has been shown to alter endothelial Cx40 expression. However, the timing and shear stress dependence of this relationship have remained unclear, as have the signal transduction pathways involved and the functional implications. Therefore, the aim of this study was to quantify the effects of shear stress on endothelial Cx40 expression, to analyze the role of phosphoinositide 3-kinase (PI3K)/Akt signaling involved, and to assess the possible functional consequences for the adaptation of microvascular networks. First-passage human umbilical vein endothelial cells were exposed to defined shear stress conditions and analyzed for Cx40 using real-time RT-PCR and immunoblot analysis. Shear stress caused long-term induction of Cx40 protein expression, with two short-term mRNA peaks at 4 and 16 h, indicating the dynamic nature of the adaptation process. Maximum shear stress-dependent induction was observed at shear levels between 6 and 10 dyn/cm(2). Simulation of this pattern of shear-dependent Cx expression in a vascular adaptation model of a microvascular network led to an improved fit for the simulated results to experimental measurements. Cx40 expression was greatly reduced by inhibiting PI3K or Akt, with PI3K activity being required for basal Cx40 expression and Akt activity taking part in its shear stress-dependent induction.     

Endothelial nuclear bulging: Morphological evidence for an intraglomerular perfusion regulating mechanism in the axolotl

Kidneys of the Axolotl (Ambystoma mexicanum: Urodela, Amphibia) have been investigated by light and electron microscopy as well as microvascular corrosion casting. Numerous glomerular endothelial nuclei bulge into the glomerular capillary lumen. It is concluded that these cells virtually stop the blood-flow in the respective vessel by blockage of erythrocyte passage. A proposal for an intraglomerular regulation mechanism of capillary perfusion (and thus filtration) by endothelial isometric contraction in the Axolotl is made on the basis of morphological methods.