Cellular pathology of the rat aorta. Pseudo-vacuoles and myo-endothelial herniae

Effect of inflammatory stimuli on the silver staining pattern of the rat carotid endothelium

Silver nitrate stains the intercellular junctions of the endothelium and other cytoplasmic or membrane components. Two protocols are described for the silver staining of rat carotid endothelium that exclude the use of pressurized fixatives and simplify the technique previously described for rat aorta. The entire surface of the carotid endothelium was examined and several parameters (stigmata, granularity, clustering of anionic sites, transversal lines, weakening of silver lines and leukocyte adhesion) were evaluated. We studied the pattern of silver staining in two situations: (1) endothelial activation and (2) neurogenic inflammation. Endothelial activation was produced by the intravenous administration of a proinflammatory albumin or polyinosinic acid. Both products cause a marked increase in leukocyte adhesion concomitant with a decrease in argyrophilia and a weakness or loss of silver lines. Neurogenic inflammation, which is mediated by substances released from sensory nerves, was induced by the intravenous administration of substance P or capsaicin. Both stimuli produced an increase in argyrophilia and weakness or loss of silver lines. Substance P caused a clustering of anionic sites, whereas this phenomenon was more discrete with capsaicin. Nearly 80% of all examined rats (controls and inflammatory stimuli treated) showed endothelial membrane disruptions formed by clusters of cells often in the shape of streaks aligned with the long axis of the vessel. The detection of these discontinuities is important, as loss of endothelial integrity is central in the initiation of pathological events.

Increased aortic endothelial death and enhanced transendothelial macromolecular transport in streptozotocin-diabetic rats

Hypertension, cigarette smoking and diabetes mellitus are well-known risk factors for atherosclerosis and coronary heart disease. Repeated endothelial cell injury and increased lipid entry have been suggested as initiating events in atherogenesis. Our previous studies have demonstrated that the frequency of endothelial cell death and associated endothelial permeability were significantly increased in the aorta of spontaneously hypertensive rats and chronic oral nicotine-treated rats. In the present investigation, we examined the hypothesis that diabetes also increases the frequency of arterial endothelial cell death and hence transendothelial macromolecular transport, which may have some implications in increasing lipid entry and thus accelerating atherogenesis. Diabetes was induced in 15 male Sprague-Dawley rats by intraperitoneal injection of 60 mg streptozotocin per kg body weight. The duration of diabetes was 6 weeks. A group of 15 age-matched rats, injected only with the buffer and maintained over the same time period, served as the controls. In en face preparations of the thoracic aorta, IgG-containing dead endothelial cells were identified by an indirect immunoperoxidase method, and endothelial leakage to Evans blue-albumin complexes was quantified by fluorescence microscopy. Diabetic rats, compared to control rats, had significantly higher values for the frequency of endothelial cell death (0.77 +/- 0.10% vs 0.38 +/- 0.04%; p < 0.005 by two-tailed, unpaired Student's t-test) and the number density of Evans blue-albumin leaky foci (4.33 +/- 0.48/mm2 vs 2.99 +/- 0.38/mm2; p < 0.05 by two-tailed, unpaired t-test) in the aorta.(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelial cell protrusions in the rat aortic wall. Immunocytochemical evidence for an alternative transendothelial passage of plasma proteins

Focal morphological changes in the endothelial lining were observed in the aortic wall of control rats. They consisted of endothelial cytoplasmic projections and vacuolar structures protruding towards the luminal space and containing electron-dense material. Some of these structures were observed to open into the subendothelial space. Endogenous albumin was detected in these compartments by applying protein A-gold immunocytochemistry to thin tissue sections of glutaraldehyde-fixed, Lowicryl-embedded aortic segments. The labelling was mainly distributed along the plasma membrane of the projections as well as over the dense content of the endothelial protrusions. The presence of endogenous albumin in these endothelial structures, together with their opening into the subendothelial space, suggests a role for these structures in an alternative transendothelial transport of albumin.

Physiological and pathological morphology of the umbilical and placental circulation

Closure mechanisms of fetal vessels are essential after birth. They are physiological. The speedy and characteristic constriction is made possible by the special construction of the fetal vessels. The so-called "folds of Hoboken" in the umbilical artery initially form tapered constrictions which quickly extend to longer sections of the vessel. The contractions continue to include the chorion plate and the villi. Due to a lack of lamina elastica interna, a protrusion of pear shaped, expanded mediamyocytes and myofibroblasts is possible which reduces the cross-sectional area of these vessels. Similar, but lasting reductions in the cross-section of the villous arteries due to a fibrous-muscular media occur in the second half of pregnancy (IVth sign of maturity). Closure of the fetal vessels during intrauterine life leads to placental insufficiency of fetal death in utero. The endarteritis obliterans closes villous vessels by means of a connective tissue plug which starts on one side of the vessel and continues across the whole width of the lumen. The periphery dependent of these villi remains avascular, as the placenta is limited in its resorptive capabilities. In the case of intrauterine asphyxia, intravasal fibrinthrombi form as a sign of subacute insufficiency in the utero-placental circulation. A decompression collapse occurs in the fetal circulation, when fetal death occurs with continuing maternal circulation.

Ultrastructure of hypertensive rat aorta. Increased basement membrane-like material

To determine the effect of elevated blood pressure on the ultrastructure of rat aorta, hypertension (average mean pressure 163 +/- 17 mm Hg) was produced by suprarenal aortic coarctation. After 3 weeks, the subendothelium of the hypertensive thoracic aorta showed significantly increased volume measurements for mononuclear leukocytes and basement membrane-like material compared with the sham-operated control group. Focal areas of rarefaction of the subendothelial extracellular material were associated with the nearby presence of mononuclear leukocytes. None of these alterations were found in the normotensive abdominal aorta. The tunica media of hypertensive thoracic aorta also contained significantly increased basement membrane-like material. This new finding in an animal hypertension model is the direct result of the quantitative morphological approach employed in this study. In some rats, the partially constricting aortic ligature compromised the right renal artery leading to ischemic atrophy of the right kidney and hyperreninemia in addition to hypertension. In this group, excluded from the previous analysis and evaluated separately, subendothelial thickening and accumulation of basement membrane-like material in the thoracic aorta were greatly increased compared with the control group and other hypertensive rats. This result could not be attributed to an effect of blood pressure alone and might have been caused in part by humoral factors. Basement membrane accumulation appears to be an important early response of the arterial wall to hypertension or other factors in this rat model.

Preservation and structural adaptation of endothelium over experimental foam cell lesions. Quantitative ultrastructural study

To assess the extent to which endothelial cell (EC) structure is modified by hyperlipidemia and by the formation of intimal plaques, we undertook a quantitative ultrastructural study of aortic EC of cynomolgus monkeys after 3 or 6 months on an atherogenic diet. We compared EC in lesion-free areas (LFA) with EC overlying focal discrete foam cell accumulations (FDA) or covering multilayered confluent plaques (MCP). There was a 15% increase in cross-sectional lumen surface profile length over FDA or MCP compared to LFA (p less than 0.005) corresponding to the bulging contours of immediately underlying foam cells. There was, however, no increase in the number of EC per unit of surface area (26.2 +/- 4.47 per 10(4) mm2 for LFA and 26.0 +/- 4.22 for FDA) or, on cross-section, per 100 microns length of underlying internal elastic lamina (8.79 +/- 2.42 for LFA, 8.26 +/- 2.01 for MCP). Nor did the number of surrounding cells contacted by each cell over LFA or MCP differ from normolipemic controls (6.56 +/- 0.85 for LFA and 5.58 +/- 0.86 for MCP). Most ECs were markedly attenuated over lesions, and while the extent and complexity of lateral contact regions between adjacent EC was diminished, the number and complexity of basilar projections was greatly increased. These structures extended among the intimal foam cells to insert on the internal elastic lamina or on intimal matrix fibers, resulting in a 2.7-fold increase in the length of the abluminal portion of the EC profile. The perimeter of the transverse EC profiles was thereby increased from 41.4 +/- 2.12 microns in LFA to 82.2 +/- 5.21 microns over MCP (p less than 0.0001). Polarization of EC in the direction of flow diminished as lesions developed. The ratio of length to width, as well as the standard deviation of the ratio, decreased from 3.51 +/- 3.92 in LFA to 2.35 +/- 0.25 over MCP, due mainly to increases in the proportion of the cell perimeter exposed to the lumen. Lesion localization bore no relationship to the orientation of EC in corresponding locations in the normolipemic controls or in LFA immediately adjacent to plaques. Organelles of EC in hyperlipidemic animals showed features suggestive of increased metabolic activity in all regions, and stress filaments were increased in the EC attenuated over lesions. There was no evidence of EC degeneration, necrosis, or sloughing regardless of lesion location, size, or complexity.

Cellular changes during hypertension: a quantitative study of the rat aorta

Using rats made hypertensive by aortic ligation or by the one kidney--one clip method, we searched the aorta for morphologic clues that could explain why hypertension aggravates atherosclerosis. Both atherosclerosis and hypertension are characterized by an increased migration of mononuclear cells into the aortic intima; we therefore quantitated this phenomenon and studied its time course. In the thoracic aorta of hypertensive rats intimal cells (emigrated mononuclear cells) increased up to 15 times 2 weeks after surgery and remained stationary thereafter. In both control and experimental rats, leukocyte emigration was heavier in the thoracic aorta than in the abdominal region. A two- to threefold increase in medial smooth muscle herniae into the intima (myointimal herniae) was also found at 8 weeks, indicating a smooth muscle cell dysfunction. Electron microscopic study of the intima showed that its thickening was due to blood-borne material and also to extracellular matrix synthesized by the endothelium. Heightened secretion reflects cell activation, a condition that (in the endothelium) leads also to leukocyte adhesion. These data suggest that, in renovascular hypertension, the aortic endothelium is in an activated state, possibly through a hormonal stimulus.

Prostacyclin production by the deendothelialized rabbit aorta

The acute effect of in vitro deendothelialization on the production of prostacyclin (PGI2) by the rabbit aorta has been investigated. The effectiveness of removing endothelium by rubbing it against filter paper or scraping it with a scalpel was demonstrated by scanning electron microscopy and en face examination after silver staining. Endothelium removal produced an immediate stimulation of PGI2 release, resulting in 408% of the control after rubbing and 367% of the control after scraping, during the first 30-min period of incubation. This increased production of PGI2 gradually declined over time to reach values similar to the control after 2h. At that time, the deendothelialized aorta was totally unresponsive to the stimuli that increase PGI2 release in the intact aorta (acetylcholine, ADP, ionophore A23187, and arachidonic acid). The enhanced production of PGI2 in the deendothelialized aorta was associated with an increased release of free arachidonic acid (353% of the control): in contrast with PGI2, this stimulation was maintained for at least 150 min. A transient exposure of the deendothelialized aorta to ibuprofen (250 microM) was followed by a rebound of PGI2 production, which was also prolonged by BW-755C (3-10 microM). In conclusion, removal of the endothelium triggered an immediate and sustained mobilization of free arachidonic acid in the rabbit aorta: the resulting increase of PGI2 production was short-lived, probably as a consequence of cyclooxygenase self-inactivation. Our results indicate that the subendothelium has a significant capacity to produce PGI2, but that this capacity is expressed only briefly.

The significance of endothelial stomata and stigmata in the rat aorta. An electron microscopic study

Perfusion of arteries with dilute silver nitrate produces in the endothelium (a) a pattern of pericellular black lines, which we earlier interpreted as a marker of the physiological electrolyte pathway (Zand et al. 1982), and (b) focal black deposits on or between the cells, either ring-shaped (stomata) or solid (stigmata). The purpose of this study was to clarify the nature and significance of these controversial structures. A glutaraldehyde-fixed normal rat aorta was perfused with silver nitrate; 17 typical stomata and stigmata were photographed en face, then studied on ultrathin serial sections. When seen en face, they fell into three groups: (I) 4 stomata in endothelial cells; (II) 6 stigmata in endothelial cells; (III) 7 stigmata on intercellular junctions. By electron microscopy, (I) all the stomata in endothelial cells corresponded to myoendothelial herniae. (II) Of the 6 stigmata in endothelial cells, 4 corresponded again to myoendothelial herniae, 2 corresponded to blebs (it seemed likely that these blebs had existed in vivo, but the possibility of a fixation artefact could not be excluded). (III) Of the 7 stigmata on intercellular junctions, one corresponded to the diapedesis of a mononuclear cell; the other 6 did not correspond to visible endothelial changes and are best interpreted as points of normally higher permeability. We conclude that stomata and stigmata (under the conditions of our experiments) can be explained in at least 4 different ways, depending in part on their location (in cells, on junctions). These ancient terms therefore remain useful for descriptive purposes, as long as it is realized that their significance in any given case must be determined by electron microscopic study.

Aortic endothelium of alloxan diabetic rabbits: a quantitative study using scanning electron microscopy

Scanning electron microscope studies of the aorta and other major arteries have been performed in alloxan-induced diabetic rabbits. After 5 weeks, a variety of structural abnormalities of the endothelial lining were detected including a significant increase in the number of argyrophilic cells and an increased number of craters or openings in the endothelial junctional region. Evidence of more extensive micro-damage was present after 5 months duration of diabetes. These zones with structural changes in the endothelial lining of major vessels seem to be areas of high predilection to atherosclerosis in diabetes.

Characteristic features of the smooth muscle cell migration in vascular wall injury

Characteristic features of the smooth muscles cell migration and ghost body formation observed in vascular lesions induced by intravenous administration of Lipofundin-S (Braun, Melsungen) were studied and compared to lesions seen in other models. Phenomena revealed can be explained by alterations of the cell environment. The cell, producing ghost body looses a part of the cytoplasm and complies with the new environment. Thus ghost bodies play an active part in maintaining the equilibrium between the cell and its environment.

Formation of "ghost" bodies and calcification in experimental atherosclerosis in nonhuman primates. An ultrastructural study

Aortic lesions from African green monkeys fed a cholesterol diet for up to 24 months were studied by electron microscopy. The lesions, grossly classified as fatty dots and fatty streaks consisted of foam cells, increased amounts of interstitial connective tissues and osmiophilic lipid material. In addition, in the interstitial spaces, there were membrane-bound detached cytoplasmic fragments and deeply osmiophilic calcium spherules. The smooth muscle cells had a frayed appearance and bulbous cytoplasmic pseudopodlike processes. Figures suggestive of transition between these cell processes and the detached cytoplasmic fragments were observed. The detached cytoplasmic fragments or ghost bodies often contained lipid droplets, myelin-like figures and calcific material. The process of budding off cytoplasmic fragments was interpreted as a form of clasmatosis enabling smooth muscle cells to eliminate substances which could not be degraded intracellularly. It is proposed that material presented within the ghost bodies may become a nucleation site for calcium salts deposition. Cell necrosis was not a feature observed in this material.

Aortic endothelial and subendothelial cells in experimental hypertension and aging

The endothelial and subendothelial cellular changes occurring as a result of hypertension and aging were characterized in deoxycorticosterone/salt-treated (D/S), spontaneously hypertensive (SHR), and Wistar-Kyoto (WKY) rats. An increase in the number of subendothelial cells occurred with both hypertension and aging and was most dramatic with D/S hypertension. Many of the cells found in the widened subendothelium showed morphological characteristics of mononuclear cells (both macrophages and lymphocytes), and of smooth muscle cells. Normalization of blood pressure by withdrawal of D/S and maintenance of rats on a low salt diet reversed the number of subendothelial cells to levels of control animals of comparable age. Significant alterations were seen in the aortic endothelial cells of D/S animals. Within 2 to 4 weeks of D/S administration, the endothelial cells doubled in number and often assumed bizarre shapes with nuclear folding and bulging toward the lumen. Some similar abnormalities in endothelial cell shape and appearance occurred with increasing age in the SHR and control WKY, although the number of endothelial cells increased only slightly in these groups. These results suggest that profound cellular changes in the aortic intima occur with an increase in blood pressure. These changes are pronounced in the D/S model of hypertension, while virtually absent in SHR. Comparable alterations also may be seen in aged normotensive animals, but to a lesser extent and with slower progression.