A new model for in vitro clot formation that considers the mode of the fibrin(ogen) contacts to platelets and the arrangement of the platelet cytoskeleton

The constitution of platelet-fibrin(ogen) contacts, the separation of the platelets initially aggregated, and the rearrangement of the platelet cytoskeleton during clot formation (0.5 to 60 minutes after thrombin stimulation) were investigated using ultrastructural and immunocytochemical techniques. After aggregation, fibrin polymerizing within focal contacts and from degranulating secretory granules contributed to the fibers. The initially formed focal contacts with fibrin obviously persisted during clot formation. The physiological branching of the fibers enabled separation of platelets. The contact associated cytoskeleton formed a constricting and fiber internalizing sphere, but later stress fiber like bundles. As retraction progressed, the cytoskeleton changed to stress fiber connecting focal contacts with fibers. A model of clot formation in vitro is presented that reflects both the contributions of platelets (fibrin fiber internalization and retraction) and of fibers (branching) enabling the retraction.

Localization of protein kinase A and vitronectin in resting platelets and their translocation onto fibrin fibers during clot formation

Physiological stimulation of platelets with thrombin brings about the release of protein kinase A (PKA) into the plasma. In human blood, this kinase singles out and phosphorylates vitronectin (Vn), a multifunctional regulatory protein, which was proposed to play an important role in the control of fibrinolysis. Here we present immuno-cytochemical evidence to show: (i) that intact platelets possess on their surface an ecto-PKA which can preferentially phosphorylate Vn; (ii) that in the resting platelet, both the catalytic and the regulatory subunits of PKA are present on the platelet surface, in the surface-connected canalicular system, and within the alpha-granules of the platelets; (iii) that the process initiated upon platelet activation, which leads to the formation of fibrin fibers and consequently forms the fibrin net, is accompanied by a translocation of PKA, of Vn, and of PAI-1 onto the fibrin fibers. We propose that the localization and the translocation of these proteins in the fibrin net, together with our finding that PKA phosphorylation of Vn reduces its grip of PAI-1, can unleash PAI-1 in its free form. The free PAI-1 can then assume its latent (non inhibitory) conformation, allow plasminogen activators to trigger the formation of active plasmin, and to initiate fibrinolysis.

Vitamin E reduces platelet adhesion to human endothelial cells in vitro

Although it has been reported that vitamin E (alpha-tocopherol) can reduce platelet adhesiveness and aggregation in vivo, the mechanism is still unknown. Therefore, the aim of the present study was to determine whether incubations of platelet-rich plasma (PRP) with vitamin E influence platelet adhesion to cultured endothelial cells. To exclude blood plasma involvement, also washed platelets were pretreated with alpha-tocopherol. Vitamin E (0.5-1.0 mM) was added to PRP or washed platelets. Endothelial cells in monolayer were incubated with thrombin-activated platelets (1 or 2 U/ml). After 1 hr of incubation, non-adhered platelets were removed and counted. Treating of PRP with alpha-tocopherol inhibited platelet adhesion to endothelial cell monolayer. This effect was dose dependent on concentrations of alpha-tocopherol and thrombin. In our experiments PRP was treated with alpha-tocopherol and endothelial cell monolayer was used as test surface. These findings agree with previous observations on the adhesivity of platelets to synthetic surfaces after dietary vitamin E in healthy volunteers. When washed platelets were incubated with alpha-tocopherol, no significant reduction of adhesion was detectable. As preincubation of washed platelets with alpha-tocopherol does not inhibit platelet adhesion, it may be supposed that the effect of vitamin E does not occur in a directly cellular mechanism. The data suggest that alpha-tocopherol may reduce platelet adhesiveness probably after incorporation by plasma lipoproteins.

Oxidized low-density lipoprotein inhibits the binding of monoclonal antibody to platelet glycoprotein IIB-IIIA

Previous studies have shown that oxidized low-density lipoprotein (LDL) induces platelet activation more effectively than native LDL. To achieve a better understanding of the mechanism underlying the activation of human platelets by oxidized LDL, the present study relates the effect of oxidized LDL to changes of binding characteristics for glycoprotein (GP) IIb-IIIa. Washed human platelets were treated by monoclonal antibody against GP IIb-IIIa, and the ligand-receptor complexes were revealed by immunocytochemical techniques on the ultrastructural level. The localization of the antiglycoprotein IIb-IIIa was time-dependent. After binding to the platelet surface membrane and open canalicular system, the surface-membrane labeling decreased during longer incubation periods. Preincubation with oxidized LDL inhibited the binding of antiglycoprotein IIb-IIIa. Our findings suggest that GP IIb-IIIa acts as a receptor for oxidized LDL. The binding of oxidized LDL to the GP IIb-IIIa might be the first step in platelet activation by plasma lipoproteins.

Oxidized LDL damages endothelial cell monolayer and promotes thrombocyte adhesion

The influence of oxidized low density lipoprotein (LDL) on a human endothelial cell monolayer was examined. The resulting contraction of the oxidized LDL-damaged endothelial cells lets intercellular spaces become enlarged and therefore visible via light microscopy. Electron microscopy reveals that the structural damage facilitates thrombocyte adhesion and formation of microthrombi. Oxidized LDL appears to play a pivotal role in initiating and deteriorating thromboembolic complications.

Oxidized low-density lipoprotein increases endothelial intracellular calcium and alters cytoskeletal f-actin distribution

Central to the pathogenesis of atherosclerosis is an abnormally functioning endothelium and a consequent loss of vascular integrity. These abnormalities may be induced by haemodynamic factors, biochemical substances, and also by oxidatively modified low-density lipoprotein (LDL). To understand the mechanism by which oxidized LDL causes endothelial dysfunction, human umbilical vein endothelial cells (HUVECs) were loaded with FURA-2, and intracellular calcium mobilization was studied in acute (seconds after LDL was injected) or chronic (24 h after LDL was injected) preparations. Our results demonstrate that 100 microg mL(-1) oxidized LDL increases HUVEC intracellular calcium. In contrast, native LDL at this same concentration had no effect. In addition, chronic exposure (24 h) of HUVECs to oxidized LDL significantly increases HUVEC intracellular calcium. Fluorescent photomicrographs of HUVECs stained with BODIPY-phalloidin f-actin indicates that oxidized LDL causes a reorganization of microfilaments. The results of this study demonstrate that the mechanism by which oxidized LDL causes a loss of vascular integrity could be through activation of endothelial cells to increase cytosolic calcium, which alters the endothelial barrier by reorganizing the cytoskeleton.

Influence of purified apoprotein E on platelet activation induced by serotonin

Serotonin induces platelet activation. Purified apoprotein E of 300 micrograms/ml prevented morphological alterations of blood platelets stimulated with serotonin (5 microM). Lower concentrated apoprotein E showed no such clear effects. These findings suggest that apoprotein E may liter atherosclerosis by suppressing agonist-induced platelet activation.

Oxidized low density lipoprotein inhibits platelet plasma membrane Ca(2+)-ATPase

Oxidized low density lipoprotein (LDL) has been shown to enhance platelet activation. Since platelet activation is accompanied by an increase in cytosolic calcium, the effects of oxidized LDL on plasma membrane Ca(2+)-ATPase, plasma membrane fluidity and cytoplasmic calcium were studied in human platelets and purified platelet plasma membranes. Our results demonstrate that oxidized LDL, but not native LDL, inhibits the activity of Ca(2+)-ATPase in purified platelet plasma membranes (P < 0.01). Addition of the free radical scavenger alpha-tocopherol had no effect on the ability of oxidized LDL to inhibit the Ca(2+)-ATPase. An increased cytoplasmic calcium level in whole platelets was induced by oxidized LDL (P < 0.01), indicating that the plasma membrane Ca(2+)-extrusion pump may also be inhibited in vivo by oxidized LDL, although other mechanisms for the increase in cytoplasmic calcium are possible. Since no change in membrane fluidity was observed in platelet plasma membranes exposed to oxidized or native LDL as estimated by steady state trimethylammonium diphenylhexatriene (TMA-DPH) anisotropy, oxidized LDL does not affect the Ca(2+)-ATPase by grossly changing the membrane environment. The present results suggest that exposure of platelets to oxidized LDL causes inhibition of the plasma membrane Ca(2+)-ATPase which contributes to the observed increase in cytoplasmic calcium and increased sensitivity to agonists.

High-density lipoprotein fails to inhibit serotonin-induced activation of blood platelets

High-density lipoprotein (HDL) of 100-400 micrograms/ml did not prevent morphological alterations of human blood platelets treated with serotonin (1-5 microM). Highly concentrated HDL (1,200 micrograms/ml) appeared to activate platelets in vitro. These findings indicate that whole HDL may not inhibit agonist-induced platelet activation.

Adhesion of washed blood platelets in vitro is advanced, accelerated, and enlarged by oxidized low-density lipoprotein

In order to study the influence of oxidized low-density lipoprotein (Ox-LDL) on platelet functional morphology at an early activation stage, washed human blood platelets were stimulated by 100 micrograms/ml Ox-LDL at 37 degrees C. The settling and spreading process of stimulated and unstimulated platelets on Formvar-coated glass was observed for approximately 20 min by reflection contrast microscopy (RCM) and quantified by image analysis. Each group consisted of at least 250 platelets. The results show that incubation with Ox-LDL causes platelet shape change and pseudopodia formation. The sedimentation of stimulated platelets precedes that of unstimulated platelets by approximately 3 min. The increase of the total adhesion area of all Ox-LDL treated platelets is significantly accelerated in comparison to normal platelets (20.45 microns2/min vs. 15.45 microns2/min; P < 0.01). The mean total adhesion area of Ox-LDL-treated platelets was generally larger than that of untreated platelets (189.7 microns2 vs. 144.7 microns2; P < 0.01). The disappearance of intracellular granules after platelet activation, observed by RCM, is supported by transmission electron microscopy. Our results suggest that Ox-LDL activates platelets and advances and accelerates their adhesion and thereby may contribute to pathological thrombosis and arteriosclerosis.

Endothelial cells injured by oxidized low density lipoprotein

Cultured endothelial cells from bovine aorta were exposed to oxidized low density lipoprotein and examined by electron microscopy. The endothelial cells contracted slightly and the intercellular junctions became unclear. Some osmiophilic material increased in the cytoplasm. The oxidized low density lipoprotein appears to injure endothelial cells and thereby plays a causative role in atherogenesis and thrombogenesis.

Outgrowing nerves in the foreleg of a mouse embryo as viewed by three-dimensional reconstruction from electron micrographs

Electron micrographs from serial cross-sections of 12-day-old mouse forelegs were digitized and three-dimensional reconstruction of the data was carried out on an Apple Macintosh Quadra 700 computer using a program especially designed for this purpose. Two nerve endings of the palmar net of the median nerve were visualized together with their accompanying Schwann cells and the surrounding processes of fibroblasts. Naked axons invade straightly into the embryonic connective tissue and serve as contact guidance for the Schwann cells to follow. Fibroblasts with long processes extend around the axons with a parallel orientation. Contacts between axons and fibroblasts are occasionally observed. It is unclear whether the parallel orientation of nerve endings and fibroblast extensions have any biological significance.

Oxidized LDL induces serotonin release from blood platelets

Oxidized low-density lipoprotein (LDL) induces a release of serotonin from morphologically resting platelets and shape changed platelets. This suggests that oxidized LDL, a newly reported weak agonist, contributes to atherogenesis and thrombogenesis by stimulating platelets.

Decreased adhesion of oxidized LDL-stimulated platelets caused by cytochalasin D

The adhesion of human blood platelets is studied with an in vitro model using reflection contrast microscopy and an image analysis system. The adhesive feature is promoted by oxidatively modified low density lipoprotein, which also induces functional morphological changes of platelets. However, when washed platelets are pretreated with 0.05 mM cytochalasin D, oxidized low density lipoprotein (100 micrograms/ml) causes a slower increase of the adhesion area (11.6 microns 2/min) compared to untreated platelets (15.7 microns 2/min) or platelets treated by oxidized low density lipoprotein alone (20.5 microns 2/min, P < 0.01). These results are supported by light transmission analysis and by transmission electron microscopy. Our experiments suggest that cytochalasin D inhibits the change of platelets in shape induced by oxidized low density lipoprotein, hinders the adhesion, but does not prevent the adhesion entirely.

Functional morphological alterations of human blood platelets induced by oxidized low density lipoprotein

The effect of oxidized low density lipoprotein (LDL) on the functional morphology of human platelets in vitro was studied by means of transmission electron microscopy. The washed platelets, stimulated by oxidized LDL (50-300 micrograms protein/ml), showed disc-sphere transformation, centralization of granules and complete degranulation in a dose- and time-dependent manner. A cytodamage in platelet membrane was induced by oxidized LDL leading to a lower electron density of cytoplasm compared to control. The morphological observations were supported by an analysis of the platelet shape-change parameter. Since the shape change, induced by oxidized LDL (50 micrograms/ml), was inhibited by a preincubation of platelets with staurosporine (10 nM), the protein kinase C was probably involved in the platelet activation initiated by oxidized LDL. The present results suggest that oxidized LDL could contribute to pathological thrombosis and atherogenesis by activating platelets.

Sertoli cell nuclear changes in human testicular biopsies as revealed by three dimensional reconstruction

Serial semithin sections of human testicular biopsy material were used for three-dimensional reconstruction in order to obtain information about Sertoli cell nuclei in normal and pathologically altered seminiferous epithelia. The three dimensional reconstruction program is based on the triangulation of image point series. It includes a calculation modus for determining surfaces and an approximation formula for the estimation of volumes. Nuclei from the following specimens were reconstructed and for each the volume (v), surface (s), and a quotient (v/s), which is regarded as a marker for the degree of membrane infoldings, were calculated as follows: (1) For normal spermatogenesis (stage 1, 2, 3, 5; n = 18) v = 409.7 +/- 33.0 microns 3, s = 429.6 +/- 40.5 microns 2, v/s = 0.96 +/- 0.05.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of low density lipoproteins on cytosolic free Ca2+ concentration and dense tubular system in human platelets

The cytosolic free Ca2+ concentration [Ca2+]i and dense tubular system (DTS) of washed human platelets were affected by low density lipoproteins (LDL) of 25 micrograms/ml at 37 degrees C for 10 minutes. After the incubation with LDL, the [Ca2+]i increased from 115 +/- 29 nM to 141 +/- 24 nM. LDL promoted the increase of [Ca2+]i (471 +/- 31 nM) induced by thrombin (0.03 U/ml) as compared to that which thrombin did alone (240 +/- 11 nM) (p < 0.05). The increased Ca2+ influx from the extracellular space is thought to be the reason of the increase in [Ca2+]i, since the effect of LDL was abolished by removal of external Ca2+ by EGTA. The DTS changed primarily from thin elongated forms to rounded vesicles. No evidence was noticed that LDL caused a mobilization of Ca2+ from the DTS.

Effect of intravenously injected collagenase on the concentration of circulating platelets in rats

An enzymatically induced irritation of the vessel wall by intravenous injection of collagenase (1.5 U per kg body weight) into rats resulted in a transient decrease of the platelet concentration in the flowing blood. This may be caused by an occurrence of inter-endothelial gaps distributed throughout the total area of the circulatory system, as could be observed by electron-microscopic studies. Subendothelial structures, at the sites of those gaps, induce a formation of multilocal microthrombi of platelets. At some gaps within the endothelial layer a deposit of fibrin could also be observed hinting at a participation of the blood coagulation system in the sealing processes.

Cationized ferritin as a platelet-stimulating surface probe. Binding to platelets and effects on platelet function

Polycationic derivatives of ferritin containing primary amino groups (CFah) or tertiary amino groups (CFdmp) were potent platelet agonists inducing shape change, aggregation and secretion, but also agglutination in the presence of EDTA. Pretreatment of platelets with neuraminidase, PGE1, indomethacin, or creatine kinase/creatine phosphate inhibited CF-induced activation. In contrast, neuraminidase and PGE1 increased the agglutination by CF, indicating an inverse relationship between activation and CF-induced agglutination. At pH 7.4, the cationic charges of CFdmp exceeded those of CFah by a factor of 1.5 and the platelets bound approximately 1.5 times more CFah than CFdmp, suggesting the same number of anionic surface sites for both CF preparations. The capacity of the platelets to bind CF was diminished by 55% at 0 degree C or by 62% after aldehyde fixation and by 13% with PGE1. This suggests that the binding capacity depends on the mobility of the binding sites in the plane of the membrane but is only slightly increased by platelet activation. Binding to fixed or cold platelets approached equilibrium within a few seconds whereas saturation required several minutes at 37 degrees C. Neuraminidase preferentially reduced the slow binding and much less the rapid binding. Since activation by CF developed during seconds, suppressible by a brief treatment with neuraminidase 25 mU/ml, a small portion of neuraminidase-sensitive sites appears to be necessary for CF-induced platelet activation. Full activation and agglutination occurred at CF concentrations far below saturating concentrations. The results show that neither CF-induced activation nor agglutination depend on a simple neutralization of the negative surface charge.

Extramembraneous particles and structural variations of tubular myelin figures in rat lung surfactant

Tubular myelin figures of pulmonary surfactant were examined by electron microscopy after fixation in glutaraldehyde and postfixation in an osmium tetroxide-ferrocyanide mixture. Bilayered membranes were seen as parallel arrays or as lattices with spacings varying from about 36 to 50 nm. This method also produced good visualization of drumstick-like particles, 5 nm in diameter and about 15 nm in length. The particles were regularly spaced at intervals of 16 nm in rows along the rectangular angles of myelin membranes. Depending on the size of the tubules the particles contacted each other in the center of the tubules at low diameters (tubular diameter less than 40 nm) and formed a continuous filamentous central core, or they were separated from one another (tubular diameter greater than 40 nm). In the latter case the central core had a hollow appearance. Based on further findings employing tannic acid, lipid extraction with 2,2-dimethoxypropane, and a ruthenium red-osmium tetroxide technique for the demonstration of polyanionic proteins it is suggested that these particles are protein in nature and that they are involved in the formation and maintenance of the structure of tubular myelin. A new concept of the ultrastructure of tubular myelin figures is proposed.

Postnatal formation of the blood-testis barrier in the rat with special reference to the initiation of meiosis

Postnatal formation of the Blood-Testis Barrier (BTB) in the rat was studied by either fixation in hypertonic fixative or employing lanthanum tracer. After 15 days of age, meiosis has reached different stages of spermatogenesis in different zones of the seminiferous cords. Only in those parts where germ cells are in the pachytene stage of meiosis do Sertoli cells form an effective barrier or tight compartment. Between 16 and 19 days of age, final formation of the BTB, which is to be found in the adult rat testis, occurs by zygotene and then leptotene stages successively entering the tight compartment. Thus, formation of a BTB by Sertoli cells does not occur synchronously along the length of the seminiferous cord but in accordance with the stage of meiosis of the associated germ cells.

The alveolar-lining layer in the lung of the axolotl, Ambystoma mexicanum. An electron-microscopic study using heavy metal complexes

Lungs of neotenic larvae of Ambystoma mexicanum were prepared for maintaining the air-tissue boundary during aldehyde fixation. Four methods of postfixation were applied: 1) osmium tetroxide followed by en-bloc staining with uranyl acetate and phosphotungstic acid, 2) ruthenium redosmium tetroxide, 3) osmium tetroxide-ferrocyanide, and 4) tannic acid-osmium tetroxide. Three types of cells line the inner surface of the axolotl lung: 1) pneumocytes, covering the capillaries with flat cellular extensions and containing two types of granules: the osmiophilic lamellar bodies, precursors of extracellular membranous material, and apical granules of unknown significance; 2) ciliated cells, also containing osmiophilic lamellar bodies; and 3) goblet cells filled with secretory granules as well as osmiophilic bodies. The extracellular material forms membranous whorls as well as tubular myelin figures, consisting of membranous "backbones" combined with an intensely stained substance. This material strikingly resembles the surfactant of amphibian lungs.

Lipid extracting properties of 2,2-dimethoxypropane as revealed by electron microscopy and thin layer chromatography

Dehydration with 2,2-dimethoxypropane acidified with either 1 N HCL or 0.05 N HCL in an 1:100 ratio has been found to extract various biologically active lipids. The effect is enhanced by an increased proton concentration and seems to occur only after complete H2O transformation. It is suggested that lipid extraction is caused by a reoxidation of lower osmium oxides with subsequent attack on the unfixed lipids by the organic solvent DMP.

Problems of the use of 2,2-dimethoxypropane as dehydrating agent in preparing single cells for transmission electron microscopy

Dehydration with 2,2-dimethoxypropane acidified with 1 n HCl in an 1:100 volume ratio has been found to be unsuitable for the preservation of cell membranes in single cell preparations. Reducing the HCl concentration to 0.1 or 0.01 n results in an improvement of membrane structures. This effect is rather independent of different fixation modes, buffer concentrations, or embedding media. It is suggested that membrane destruction is caused by a proton concentration exceeding a suitable level.

Ruthenium red as a stain for electron microscopy. Some new aspects of its application and mode of action

Commercial ruthenium red has been tested for its purity by spectrophotometry. Impurities detected by this method could be abolished by nitric acid-precipitation of ruthenium brown. This substance has no effect on cell surface staining and converts almost completely to ruthenium red under the conditions used in electron microscopy. It was found, by photometric analysis, that in the ruthenium red-osmium tetroxide-cacodylate combination, generally used for cell surface staining, chemical reactions between ruthenium red and osmium tetroxide occur. As aerial oxidation of hexammineruthenium2+ leads to a product with some surface staining capability, it is suggested that an oxidized product of ruthenium red is responsible for binding to cellular components, and that a reduced product of osmium tetroxide gives an additional contrast enhancement. In ruthenium red-osmium dioxide combinations ruthenium red seems to bind to cell surfaces without any molecular alteration, and contrast is gained by the model proposed by Blanquet (1976b). The latter method could open a way for investigating the binding of ruthenium red to certain natural compounds involved in calcium transport, as postulated by a number of authors. Both ruthenium-osmium combinations differ in their cell surface staining ability. The ruthenium red-osmium dioxide combination tends to form distinct subunits, whereas the osmium tetroxide variety stains homogeneously. In combination with osmium dioxide, the surface staining is affected by EDTA, and, in contrast to osmium tetroxide, a successive application of ruthenium red and osmium dioxide as possible.

Methods of heavy metal electron microscopic histochemistry applied to frog lung surfactant

Four heavy metal staining methods have been applied to frog lung surfactant. Among them, the iodoplatinate method is the only one that almost exclusively visualizes the phospholipid moiety being produced in the lamellated bodies of the pulmonary epithelial cells and forming the backbone of organized structures within the extracellular lining layer. The other three techniques-ruthenium red-osmium tetroxide, osmium tetroxide-ferrocyanide, acidic phosphotungstic acid in chromatic (Rambourg technique)--more or less give electron contrast to glycoproteins and to a lesser extent to the hydrophilic parts of phospholipids. They all show the extracellular lining layer to be a two component system: the content of the lamellar bodies form--when released--membranous configurations, similar to those observed in mammalian lungs; they unfold in an amorphous hypophase, which is apparently secreted by goblet cells of the pulmonary epithelium.

Ultrahistochemical investigations of dog lung surfactant with ruthenium red and iodoplatinate reactions

Dog lungs have been fixed by immersion and submitted to two histochemical procedures. An iodoplatinate reaction technique to demontrate choline phospholipids stains cell membranes, inclusion bodies of type II alveolar epithelial cells and tubular myelin figures of pulmonary surfactant, the latter as electron-dense lines measuring 5 nm. The ruthenium red procedure gives rise to an intense contrast of the free surface of alveolar epithelium. The 5 nm-lines of the pulmonary surfactant are seen as electron-lucent lines, but bordered by electron-dense rims. Though both techniques have limitations in their interpretation, which are discussed in this paper, they demonstrate the tubular myelin figures to be a highly organized mixture of phospholipids and glycoproteins.

Electron microscopic studies of the lung of the frog. II. Topography of the inner surface by scanning and transmission electron microscopy

Scanning and transmission electron microscopy were used to study the inner architecture of the frog lung. In some specimens the alveolar surface mucus layer was removed to permit the examination of underlying features. The inner surface of the frog's lung is covered by a layer of microvilli belonging to only one type of epithelial cells. The boundaries of these epithelial cells are demarcated by small ridges. Different degrees of lung expansion cause variations of the surface topography. The morphology of certain surface features is examined in detail. Several methods of drying the specimens are compared.