Preparation of vascular endothelium for scanning electron microscopy: a comparison of the effects of perfusion and immersion fixation

Rescuing Perishable Neuroanatomical Information from a Threatened Biodiversity Hotspot: Remote Field Methods for Brain Tissue Preservation Validated by Cytoarchitectonic Analysis, Immunohistochemistry, and X-Ray Microcomputed Tomography

Biodiversity hotspots, which harbor more endemic species than elsewhere on Earth, are increasingly threatened. There is a need to accelerate collection efforts in these regions before threatened or endangered species become extinct. The diverse geographical, ecological, genetic, morphological, and behavioral data generated from the on-site collection of an individual specimen are useful for many scientific purposes. However, traditional methods for specimen preparation in the field do not permit researchers to retrieve neuroanatomical data, disregarding potentially useful data for increasing our understanding of brain diversity. These data have helped clarify brain evolution, deciphered relationships between structure and function, and revealed constraints and selective pressures that provide context about the evolution of complex behavior. Here, we report our field-testing of two commonly used laboratory-based techniques for brain preservation while on a collecting expedition in the Congo Basin and Albertine Rift, two poorly known regions associated with the Eastern Afromontane biodiversity hotspot. First, we found that transcardial perfusion fixation and long-term brain storage, conducted in remote field conditions with no access to cold storage laboratory equipment, had no observable impact on cytoarchitectural features of lizard brain tissue when compared to lizard brain tissue processed under laboratory conditions. Second, field-perfused brain tissue subjected to prolonged post-fixation remained readily compatible with subsequent immunohistochemical detection of neural antigens, with immunostaining that was comparable to that of laboratory-perfused brain tissue. Third, immersion-fixation of lizard brains, prepared under identical environmental conditions, was readily compatible with subsequent iodine-enhanced X-ray microcomputed tomography, which facilitated the non-destructive imaging of the intact brain within its skull. In summary, we have validated multiple approaches to preserving intact lizard brains in remote field conditions with limited access to supplies and a high degree of environmental exposure. This protocol should serve as a malleable framework for researchers attempting to rescue perishable and irreplaceable morphological and molecular data from regions of disappearing biodiversity. Our approach can be harnessed to extend the numbers of species being actively studied by the neuroscience community, by reducing some of the difficulty associated with acquiring brains of animal species that are not readily available in captivity.

A novel application of microsphere perfusion and scanning electron microscopy to the identification of pulmonary arterioles in guinea-pig and rabbit lungs

In arterioles of the lung the intravascular blood pressures are lower than in comparable vessels in the systemic circulation and the arteriole walls are thinner. Therefore, it is very difficult to distinguish between arterioles and venules of the same size using scanning electron microscopy. This study describes a novel application of latex microsphere perfusion and scanning electron microscopy which distinguishes between pulmonary arterioles and venules on the basis of endothelial cell morphology. Microspheres, 90 and 45 microns in diameter, were perfused into the arterial side of the pulmonary circulation of guinea-pig and rabbit lungs. Scanning electron microscopy of the arterioles on both sides of the lodged microspheres indicated that the endothelial cells are spindle shaped. In contrast, the endothelial cells of equal diameter venules are polygonal. Furthermore, the nuclei of the arteriolar endothelial cells were significantly (P = 0.019) narrower than those of endothelial cells in venules of equal diameter. Finally, it was observed that the differences between arteriole and venule endothelial cells persisted distally to the capillaries.

Tridimensional ultrastructure of perfusion fixed gastrointestinal epithelial cells by high resolution scanning electron microscopy

Improvements in the design of modern scanning electron microscopes (SEM) and new methods of specimen preparation incorporating chemical removal of the cytosol and cytoskeleton, now make it possible to view cells and their organelles in three dimensions (3D) at high magnification. In this experiment, high resolution SEM (HRSEM) utilizing new methods of tissue preparation was used to study the intracellular structures of the mouse ileum. In addition, in vivo intestinal perfusion was used to further enhance cellular preservation. Using these modifications it was possible to visualize, in 3D, the fine structure of intestinal epithelial cells and intracellular organelles such as the nucleus, mitochondria, endoplasmic reticulum, and Golgi complex, as well as microvilli and cell membrane. Whole mitochondria appeared as irregularly shaped organelles which contained tubular cristae. Plate-like cristae were not observed. The brush border was found to be closely packed array of cylindrical projections. The extensive folding and structural intricacy of lateral cell membranes between absorptive cells could only be appreciated by viewing this tissue with 3D HRSEM. The use of HRSEM to study 3D ultrastructure of cells and their organelles will improve our understanding of the structure-function relationships in both the healthy and diseased gastrointestinal tract.

Aneurysm of the rabbit common carotid artery induced by periarterial application of calcium chloride in vivo

Experimental aneurysmatic dilatation of the rabbit common carotid artery was induced by a single, periarterial application of calcium chloride in vivo. Vessels were fixed in situ after 3 d, 1 wk, 3 wk, 6 wk, and 12 wk by intracardiac perfusion of glutaraldehyde and tissues prepared for light, scanning, and transmission electron microscopy. Progressive focal aneurysmal dilatation was seen limited to the site of calcium application with endothelial damage and thrombus formation in areas of irregular luminal contour. Disruption of the elastic network of the intima and media was seen with varying degrees of intimal fibromuscular hyperplasia and medial disorganization. The calcium-elastic tissue complex was the focus of the inflammatory, arteriosclerotic reaction and subsequent aneurysm formation. The inflammatory cell infiltration initially included primarily neutrophils followed by lymphocytes, plasma cells, monocytes, and multinucleated giant cells. These studies support the hypothesis that disruption of the elastic tissue network of the vascular wall represents an important pathogenetic factor in the initiation of aneurysmal dilatation. In addition, the results of these studies suggest that interaction of calcium with the elastica of the arterial wall may represent an important pathogenetic factor in the initiation of giant cell arteritis.

Effect of hypertension on small-vessel healing after microvascular anastomosis

Artery-to-artery microvascular anastomosis (MVA) has become an established therapeutic alternative for patients with giant intracranial aneurysms and other forms of cerebrovascular disease. Many patients afflicted with cerebrovascular disease also suffer from hypertension. To evaluate the effect of hypertension on healing of small arteries following MVA, 36 spontaneously hypertensive rats were subjected to end-to-side MVA of their common carotid arteries. At specific times after surgery the rats were sacrificed; the anastomotic site was removed and examined with light microscopy and scanning electron microscopy. All MVA's were patent, and there was no evidence of thrombus formation or stenosis at the operative site in any group. Also, there was no difference between the spontaneously hypertensive rats and the control normotensive rat group in the rate of endothelial coverage of the MVA. The presence of hypertension appears to have no effect on healing of small arteries in the rat.

Freeze-replica study of lipid droplets in the Ito cells of rat liver

The structure of lipid droplets in the Ito cells was studied by the freeze-replica method in vitamin A-treated rats. Most lipid droplets were fractured in planes through their lipid contents. Some of them were surrounded by a membrane structure that had intramembranous particles (IMP), which were more numerous on the protoplasmic face than the exoplasmic face. Those with the membrane containing IMP correspond to the membrane-bounded lipid droplets (lipid droplets surrounded by lysosomes) seen in the electron microscopic observation of conventional thin section preparations. There were no lipid droplets showing concave or convex multilayered structures in the Ito cells from properly fixed livers. Since these multilayered structures were observed only in the materials fixed by immersion with glutaraldehyde or fixed 40 minutes after the death of animals, it is likely that they represent an artifact, showing an autolytic change, caused by poor fixation or postmortem change.

Morphologic quantitation of pseudointima and effects of antiplatelet drugs on vascular prostheses in goats

The natural process of endothelialization, pseudointimal formation, and connective tissue incorporation of the expanded PTFE grafts in the goat was documented through histologic examination of specimens harvested at 2, 4, 6, and 8 weeks. The goats demonstrated a progressive increase in pseudointimal pannus ingrowth from the anastomoses at a rate of 11.3 mm over a 12 week period. Histologic changes according to time of vascular graft incorporation in the goat model were found to be comparable to those of the dog, pig, and calf models reported in the literature. Platelet-inhibiting drugs, aspirin, dipyridamole, nifedipine, and ibuprofen were administered to goats after replacement of their infrarenal aorta with 5 cm of 8 mm diameter expanded PTFE grafts. The effects of the drugs on graft endothelialization and anastomotic pseudointimal formation was compared with those in the untreated control group after 12 weeks. Aspirin and dipyridamole had no detrimental effect on the healing process compared with the untreated control group. Studies with nifedipine and ibuprofen did not demonstrate a decrease in pseudointimal hyperplasia. Antiplatelet treatment resulted in no significant change in the rate of endothelialization of expanded PTFE grafts.

Intimal changes associated with arterial spasm induced by periarterial application of calcium chloride

Arterial spasm was induced by application of calcium chloride to the adventitial surface of the rabbit common carotid artery in vivo. Sodium chloride (NaCl) was applied to the contralateral vessel as control. Vessels were fixed in situ by intravascular perfusion after 15 min, 1 hr, or 24 hr and prepared for light and scanning electron microscopy (SEM). With SEM, the luminal surface at the site of calcium application showed severe longitudinal folding accompanied by endothelial desquamation with extensive platelet deposition on exposed subendothelium. The luminal cross-sectional area was reduced by 53 +/- 19.5% after 15 min and by 44 +/- 12% after 1 hr as compared with the contralateral control. Furthermore, the luminal area at the site of calcium application was found to be reduced by 42 +/- 8% after 1 hr when compared with segments of the same vessel distal to the site of calcium application. Blood flow rate, as measured by electromagnetic flow probe, was not reduced. Vessels examined after 24 hr showed a significant increase in luminal cross-sectional area as compared with contralateral control vessels (136 +/- 70%). Control vessels (NaCl) showed no significant change in luminal cross-sectional area and no endothelial desquamation or platelet deposition after 15 min, 1 hr, or 24 hr. Examination of histologic sections showed calcium precipitation within the attached thrombus after 15 min with calcium deposits also adherent to the adjacent luminal aspect of the internal elastic lamina (IEL). By 24 hr, this precipitation extended throughout the media. Marked deposition of leukocytes was seen after 24 hr which showed a preferential attachment for areas of endothelial damage and discontinuity of IEL.

Coronary artery spasm in intact dogs induced by potassium and serotonin

Although coronary artery spasm has been implicated as an important cause of myocardial ischemia in humans, an animal model of reversible segmental coronary constriction has not been described. To provoke coronary spasm in open-chest dogs, selected vasoconstricting agents adsorbed to viscous ion exchange gels were applied topically to the surface of epicardial coronary arteries. The procedure provided a sustained localized release of drug, and minimized effects on contiguous myocardium or on the systemic circulation. Segmental arterial constrictor responses were evaluated by sonomicrometry, arteriography, and electromagnetic flow measurements. Potassium evoked sustained constrictions or spasms, and concomitantly reduced flow by -42 +/- 4% (SE; n = 34). Serotonin likewise produced sustained decreases in flow of -22 +/- 6% (SE; n = 5). Other constrictors, including norepinephrine and angiotensin, failed to evoke sustained constrictions. Spasms nearly abolished reactive hyperemic responses elicited by temporary complete occlusion of the artery. Intravenous nitroglycerin and dihydropyridine calcium antagonists promptly relieved the spasms. Scanning electronmicroscopic examination of the intimal surface of arteries undergoing sustained spasm revealed no platelet thrombi. Thus, nonthrombotic, vasodilator-sensitive segmental coronary spasms were elicited by endogenous constrictors which may play a role in regulating flow to ischemic myocardium.

The thermic vascular anastomosis (TVA). IV. Analysis by scanning electron microscopy

In the present study 48 thermic vascular anastomoses performed in Sprague Dawley Wistar rats were subjected to analysis by scanning electron microscopy. Depending on the period of postoperative survival (up to 18 months) normal smooth endothelium, dissected cuffs and aneurysms were seen. Endothelial damage through heat application following bipolar coagulation was only slight. In one 18 months old anastomosis an aneurysm with a delicate network of endothelialized filaments was seen leaving a free passage for the blood flow centrally. A comparison of the literature describing the healing processes of intima following various traumas is made.

Scanning electron-microscopic study of the inner surface of experimental aneurysms in rabbits

A scanning electron-microscopic study of the inner surface of experimental saccular aneurysms in rabbits was conducted to demonstrate the effects of haemodynamics on the aneurysmal wall. Animals with aneurysms were killed from 2 to 83 weeks postoperatively, and the controls from 7 to 51 weeks. In both series, re-endothelialization was complete in approximately 2 weeks. In control tissue endothelium was normal, and sutures caused mild distortion of the luminal surface. In the aneurysms, there were tracts of spindle-shaped cells, often arranged in whorl-like patterns and interspersed with regions of polyhedral cells with thickened intercellular borders, prominent stomata and numerous microvilli. In some areas, long filamentous intercellular processes were prominent. The few jet lesions seen were covered with polyhedral cells and were less complex than those in arteriovenous fistulae. Ridges, caused by elevated subendothelial connective tissues were crescentic or circular, outlining depressions and craters, as if delineating vortices. The carotid artery opposite the aneurysm exhibited endothelium similar to normal tissue, except in older aneurysms where corrugations caused by the internal elastic lamina were completely or partially absent. The experiments demonstrate the effect of intra-aneurysmal haemodynamics on the topography of the endothelium and subendothelial tissues of the sac wall, and support the concept that endothelial topography in the vascular system under physiological conditions is haemodynamically determined.

Morphology of arteries, veins, and capillaries in cancer of the larynx: scanning electron-microscopical study on microcorrosion casts

Morphology of blood vessels in cancer of the larynx, which as other solid tumors has great neoangiogenic abilities, was studied on microcorrosion casts in SEM. Most evident changes of the endothelial patterns were seen on the casts of capillaries, venules, and veins. Capillaries, especially the newly formed, are the most numerous constituent of the vascular bed in all zones of cancer of the larynx. These vessels exhibit great morphological differentiation varying from the single, blind-ended pipes via relatively long hairpin loops spirally twisted in the long axis, to strongly spiralled and convoluted loops, resembling pseudoglomeruli. The newly formed capillaries deriving predominantly from the host's capillaries, venules, and veins have a embryonal character. It seems reasonable to presume that the neoangiogenesis process triggered by active influence of the tumor angiogenesis factor, is in principle, a repetition of the mechanism of the embryonal angiogenesis.

Endothelial cell damage and thrombus formation after partial arterial constriction: relevance to the role of coronary artery spasm in the pathogenesis of myocardial infarction

The left anterior descending coronary artery of four dogs and the right common carotid artery of 15 rabbits were subjected to 40-60% reduction in transluminal diameter for 1 hour by partial ligation with suture thread. Scanning electron microscopic examination of the luminal surface of these vessels revealed endothelial craters and balloons, fragmentation and desquamation on the proximal slope of the constriction. Platelet attachment to exposed subendothelial tissues was clearly evident, and microthrombi were seen at the point of maximum constriction. Blood flow, as measured by electromagnetic flow probe, was virtually unchanged upon partial ligation. In control studies, where a second ligature was placed proximal to and before the first to reduce blood to the distal constriction site substantially or totally, endothelial desquamation was found in only one of 14 animals, and the number of craters and balloons was significantly reduced. We suggest that endothelial damage and thrombus formation may occur at the site of focal arterial constriction even when the reduction in transluminal diameter is insufficient to alter substantially the rate of flow.

Thrombogenicity of different suture materials as revealed by scanning electron microscopy

The thrombogenicity of five suture materials--polypropylene (Prolene), polyester (Mersilene), polyglactin 910 (Vicryl), nylon (Tthilon), and silk--was examined in vivo by scanning electron microscopy. The most blood-compatible material appeared to be Prolene. A rather satisfactory antithrombogenciity was shown by Mersilene, in comparison with which Ethilon and Vicryl provided less thromboresistance. The possible connection between the chemical changes which occur during the hydrolysis of Vicryl and its blood compatibility is discussed. Of the tested materials, silk proved to be the most thrombogenic. This study shows that the thrombogenicity of a suture material could be easily and conveniently tested in an experimental model by scanning electron microscopy, thus enabling the first choice of a suitable suture for the cardiovascular surgery, before one continues with other more sophisticated methods for the evaluation of thromboresistance.

Re-endothelialization of microvascular carotid end-to-side anastomosis in the rat

A carotid end-to-side anastomosis was performed on 25 male and female Wistar rats (mean weight 197.8 gm). The animals were sacrificed at time intervals varying from 0 to 21 days after the operation. The anastomosis was exposed, the aorta cannulated, and the animals perfused with a 2.5% buffered glutaraldehyde solution at a constant pressure of 80 mm Hg. The anastomoses were removed for scanning electron microscopic (SEM) and light microscopic studies. The SEM results indicate that after the acute platelet-fibrin reaction in the first 48 hours, the suture line itself becomes re-endothelialized after 4 days. On the stitches, however, a cellular population consisting of leukocytes transforming into flattened cells was seen after 2 days. The morphology of these cells and their role in the regeneration of endothelium is discussed. This study presents evidence supporting a blood-borne genesis of endothelial cells in vivo.

Morphologic effects of pressure changes on canine carotid artery endothelium as observed by scanning electron microscopy

By using scanning electron microscopy (SEM) we were able to follow the sequence of ultrastructural alterations of the luminal surface which occurred when specimens of canine carotid artery were subjected to controlled transmural pressures. Specimens of carotid artery were removed from dogs following fixation at experimental pressures ranging from 0 mm Hg to 100 mm Hg. The endothelium of specimens fixed at 0 mm Hg has parallel longitudinal ridges formed by the contraction of the underlying internal elastic lamina. With increasing transmural pressure, the luminal surface undergoes a gradual flattening of the endothelial ridges so that at 100 mm Hg, these ridges have completely disappeared. The observed morphologic changes of the arterial endothelium indicate that SEM can provide good ultrastructural information on blood vessels subjected to controlled transmural pressure and that the pressure-dependent alterations must be considered in studies on vascular structure and function.

Endothelial cell damage and thrombus formation following temporary arterial occlusion. Effects of pretreatment with aspirin or heparin

✓ The effects of restoration of blood flow on endothelial damage that occurs distal to the site of temporary arterial occlusion with surgical clips, and the effects of heparin and aspirin on thrombus formation at the site of the clip, were studied by scanning electron microscopy (SEM). The right carotid artery of 45 rabbits was occluded with a Heifetz clip for 30 minutes. The clips were then removed and blood flow resumed for periods of 30 minutes, 1 hour, 2 hours, and 24 hours. A second group was preteated intravenously with heparin (1400 units/kg), and a third group was pretreated orally with aspirin (50 mg/kg). The SEM analysis of arterial segments distal and proximal to the clip indicated that the frequency of occurrence of endothelial crater- and balloon-like vesicular defects decreased to zero within 24 hours of restoration of blood flow. Examination of arterial segments compressed by the clip revealed endothelial desquamation and thrombus formation within 30 minutes of resumption of flow. The maximum degree of thrombus formation occurred within 1 to 2 hours of resumption of flow, with a subsequent decrease in the extent of deposition of platelets, fibrin, erythrocytes, and leukocytes within 24 hours. No change was found in composition or quantity of thrombus formation after pretreatment with heparin although, in the latter specimens, fibrin deposition appeared considerably less. However, pretreatment with aspirin resulted in marked reduction in the overall quantity of thrombus formation at the site of the clip following temporary occlusion.

A scanning electron microscopic (SEM) study of the re-endothelialization of the carotid end-to-side anastomosis in the rat

A carotid end-to-side anastomosis was made on 25 male Wistar rats (mean weight 197.8 g). At different time intervals, from 0 to 21 days after the operation, the animals were sacrificed. The anastomosis was exposed, the aorta was cannulated, and the animals were perfused at a constant pressure of 80 mm Hg with a 2 1/2% glutaraldehyde solution. The anastomoses were taken out for further SEM and light microscopic (LM) study. The SEM results indicate that after the acute platelet-fibrin reaction in the first 48 hours the suture line itself becomes reendothelialized after 4 days. On the stitches, however, (EthilonR 11 X 0, 2871 G, BV7 needle) after two days a cellular population was seen, consisting of leucocytes transforming into flattened cells. The morphology of these cells and their role in the regeneration of endothelium is discussed. This study gives evidence to the possibility of a blood-borne genesis of endothelial cells in vivo.

Endothelial lesions associated with vascular clamping--surface micropathology by scanning electron microscopy

The endothelial cell pavement in normal blood vessels is very fragile. In cardiovascular surgery, the need for secure clamping of the aorta results in trauma which may be followed by thrombosis or occlusion. Various types of vascular clamps and other related devices offered commercially were clinically investigated in canine models and their effects on the vascular endothelium were ascertained with the aid of scanning electron microscopy. On the basis of these studies, the Fogarty silicone filled clamp appeared to be the least traumatic. However, further technical developments in this area are needed in order to obtain a completely atraumatic method of temporarily closing major blood vessels.

Ultrastructural changes in feline arterial endothelium following subarachnoid hemorrhage

Scanning electron microscopy of feline basilar arterial endothelium 4 hours, and 1, 3,5, and 7 days after subarachnoid hemorrhage (SAH) showed longitudinal furrows that correlated with angiographically demonstrated vasospasm. These ridges persisted after fixation at physiological pressure, and probably reflected medial contraction with undulation of the underlying elastic lamina. No change in endothelial cell morphology or thrombogenesis was observed as long as 7 days after SAH. There is no evidence from this study to suggest that ischemia from vasospasm is a product of thromboembolism from damaged endothelial surfaces.

Endothelial cell damage by temporary arterial occlusion with surgical clips. Study of the clip site by scanning and transmission electron microscopy

The effects of temporary vascular occlusion with surgical clips on the underlying endothelial lining were studied with scanning (SEM) and transmission (TEM) electron microscopy. Twenty-five rabbits were anesthetized and both common carotid arteries exposed. A Heifetz clip was used to occlude the right carotid artery for 5, 15, and 30 minutes, and 2 hours in five animals each. The clips were removed and the vessels immediately perfused with glutaraldehyde. In five remaining animals, the right carotid arteries were occluded for 30 minutes followed by removal of the clip and resumption of blood flow for 30 minutes prior to fixation. Combined SEM and TEM examination of the endothelium of compressed segments revealed "craters" and "balloons", blebs and vacuoles, swollen mitochondria, dilated granular endoplasmic reticulum, and subendothelial edema. There were also areas of endothelial cell flattening, discontinuity, and desquamation exposing the subendothelial tissues. Following restoration of flow, platelets and fibrin were found adherent to altered endothelial cells and to exposed subendothelial tissues. Endothelial craters and balloons were also found distal and, significantly less frequently, proximal to the site of occlusion. It is suggested that antiplatelet aggregating agents may prove beneficial for the prevention of thrombus formation at the site of the clip as well as craters and balloons distal to the clip following procedures requiring temporary vascular occlusion.