Ultrastructural Pathology of Atherosclerosis, Calcific Aortic Valve Disease, and Bioprosthetic Heart Valve Degeneration: Commonalities and Differences

Atherosclerosis, calcific aortic valve disease (CAVD), and bioprosthetic heart valve degeneration (alternatively termed structural valve deterioration, SVD) represent three diseases affecting distinct components of the circulatory system and their substitutes, yet sharing multiple risk factors and commonly leading to the extraskeletal calcification. Whereas the histopathology of the mentioned disorders is well-described, their ultrastructural pathology is largely obscure due to the lack of appropriate investigation techniques. Employing an original method for sample preparation and the electron microscopy visualisation of calcified cardiovascular tissues, here we revisited the ultrastructural features of lipid retention, macrophage infiltration, intraplaque/intraleaflet haemorrhage, and calcification which are common or unique for the indicated types of cardiovascular disease. Atherosclerotic plaques were notable for the massive accumulation of lipids in the extracellular matrix (ECM), abundant macrophage content, and pronounced neovascularisation associated with blood leakage and calcium deposition. In contrast, CAVD and SVD generally did not require vasculo- or angiogenesis to occur, instead relying on fatigue-induced ECM degradation and the concurrent migration of immune cells. Unlike native tissues, bioprosthetic heart valves contained numerous specialised macrophages and were not capable of the regeneration that underscores ECM integrity as a pivotal factor for SVD prevention. While atherosclerosis, CAVD, and SVD show similar pathogenesis patterns, these disorders demonstrate considerable ultrastructural differences.

Evaluation of Supercritical CO2-Assisted Protocols in a Model of Ovine Aortic Root Decellularization

One of the leading trends in the modern tissue engineering is the development of new effective methods of decellularization aimed at the removal of cellular components from a donor tissue, reducing its immunogenicity and the risk of rejection. Supercritical CO₂ (scCO₂)-assisted processing has been proposed to improve the outcome of decellularization, reduce contamination and time costs. The resulting products can serve as personalized tools for tissue-engineering therapy of various somatic pathologies. However, the decellularization of heterogeneous 3D structures, such as the aortic root, requires optimization of the parameters, including preconditioning medium composition, the type of co-solvent, values of pressure and temperature inside the scCO₂ reactor, etc. In our work, using an ovine aortic root model, we performed a comparative analysis of the effectiveness of decellularization approaches based on various combinations of these parameters. The protocols were based on the combinations of treatments in alkaline, ethanol or detergent solutions with scCO₂-assisted processing at different modes. Histological analysis demonstrated favorable effects of the preconditioning in a detergent solution. Following processing in scCO₂ medium provided a high decellularization degree, reduced cytotoxicity, and increased ultimate tensile strength and Young’s modulus of the aortic valve leaflets, while the integrity of the extracellular matrix was preserved.

Aortic Valve Stenosis and Mitochondrial Dysfunctions: Clinical and Molecular Perspectives

Calcific aortic stenosis is a disorder that impacts the physiology of heart valves. Fibrocalcific events progress in conjunction with thickening of the valve leaflets. Over the years, these events promote stenosis and obstruction of blood flow. Known and common risk factors are congenital defects, aging and metabolic syndromes linked to high plasma levels of lipoproteins. Inflammation and oxidative stress are the main molecular mediators of the evolution of aortic stenosis in patients and these mediators regulate both the degradation and remodeling processes. Mitochondrial dysfunction and dysregulation of autophagy also contribute to the disease. A better understanding of these cellular impairments might help to develop new ways to treat patients since, at the moment, there is no effective medical treatment to diminish neither the advancement of valve stenosis nor the left ventricular function impairments, and the current approaches are surgical treatment or transcatheter aortic valve replacement with prosthesis.

Microstructural alterations owing to handling of bovine pericardium to manufacture bioprosthetic heart valves: A potential risk for cusp dehiscence

INTRODUCTION

Cross-linking and anti-calcification of prosthetic heart valves have been continuously improved to prevent degeneration and calcification. However, non-calcific structural deteriorations such as cuspal dehiscences along the stent still require further analysis.

MATERIAL AND METHOD

Based upon the previous analysis of an explanted valve after 7 years, a fresh commercial aortic valve was embedded in poly(methyl methacrylate) (PMMA) and cut into slices to ensure the detailed observation of the assembly and material structures. A pericardial patch embossed to provide the adequate shape of the cusps was investigated after paraffin embedding and appropriate staining. The microstructural damages that occurred during manufacturing process were identified and evaluated by light microscopy, polarized microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

RESULTS

The wavy collagen bundles, the key structure of the pericardium patch, were damaged to a great extent at suture sites along the stent and in the compressed areas around the stent post. The fixation of the embossed pericardium patch along the plots of the stent aggravated the microstructural modifications. The damages mainly appeared as the elimination of collagen bundle waviness and delamination between the bundles.

CONCLUSION

Considering the modes of failure of the explant, the damages to the collagen bundles may identify the vulnerable sites that play an important role in the cusp dehiscence of heart valve implants. Such information is important to the manufacturers. Recommendations to prevent in vivo cusp dehiscence can therefore be formulated.

[CALCIFYING NANOPARTICLES IN PATHOMORPHOGENESIS OF STRUCTURAL LESIONS OF HEART VALVES]

Over the last 10 years, calcifying nanoparticles (CNP) have attracted attention as structures detected. together with many other nanostructures in biopsies from patients operated for the correction of aortic valve malformations. The results of the present work performed with the use of high-resolution transmission and scanning electron microscopes agree on the whole with the data of other authors. Some new findings include CNP adhesion to collagen fibers and specifically-shaped, shallow invaginations or craters at their surface. The possible pathophysiological mechanisms that promote involvement of CNP in the development of the disease are considered.

On the bending properties of porcine mitral, tricuspid, aortic, and pulmonary valve leaflets

The atrioventricular valve leaflets (mitral and tricuspid) are different from the semilunar valve leaflets (aortic and pulmonary) in layered structure, ultrastructural constitution and organization, and leaflet thickness. These differences warrant a comparative look at the bending properties of the four types of leaflets. We found that the moment-curvature relationships in atrioventricular valves were stiffer than in semilunar valves, and the moment-curvature relationships of the left-side valve leaflets were stiffer than their morphological analog of the right side. These trends were supported by the moment-curvature curves and the flexural rigidity analysis (EI value decreased from mitral, tricuspid, aortic, to pulmonary leaflets). However, after taking away the geometric effect (moment of inertia I), the instantaneous effective bending modulus E showed a reversed trend. The overall trend of flexural rigidity (EI: mitral > tricuspid > aortic > pulmonary) might be correlated with the thickness variations among the four types of leaflets (thickness: mitral > tricuspid > aortic > pulmonary). The overall trend of the instantaneous effective bending modulus (E: mitral < tricuspid < aortic < pulmonary) might be correlated to the layered fibrous ultrastructures of the four types of leaflets, of which the fibers in mitral and tricuspid leaflets were less aligned, and the fibers in aortic and pulmonary leaflets were highly aligned. We also found that, for all types of leaflets, moment-curvature relationships are stiffer in against-curvature (AC) bending than in with-curvature bending (WC), which implies that leaflets tend to flex toward their natural curvature and comply with blood flow. Lastly, we observed that the leaflets were stiffer in circumferential bending compared with radial bending, likely reflecting the physiological motion of the leaflets, i.e., more bending moment and movement were experienced in radial direction than circumferential direction.

Cryoprotective Effect and Optimal Concentration of Trehalose on Aortic Valve Homografts

BACKGROUND AND AIM OF THE STUDY

Cryopreservation allows for the long-term storage of aortic homografts, but a high risk of calcification and degeneration is often observed following their transplantation. The study aim was to investigate the cryoprotective effect of trehalose on aortic valve homografts preserved in liquid nitrogen, and to determine the optimal trehalose concentration for such purpose.

METHODS

Aortic valve homografts obtained from New Zealand White rabbits were processed using different protectants. Samples were assigned at random to four groups: a control group treated with dimethyl sulfoxide (DMSO; 0.1 mol/l), and test groups A, B and C, which were treated with 0.1 mol/l trehalose + 0.1 mol/l DMSO, 0.2 mol/l trehalose + 0.1 mol/l DMSO, or 0.3 mol/l trehalose + 0.1 mol/l DMSO, respectively, as protectant. Samples in each group were allocated randomly to three subgroups and cryopreserved for 12, 15, and 18 months, respectively. After thawing, apoptosis of the cryopreserved homograft samples was evaluated using immunohistochemistry, semi-quantitative RT-PCR and Western blotting methods. The viability of the tissue cells was assessed by monitoring glucose utilization capacity.

RESULTS

The apoptosis assay showed that, among the four groups and at all time points, the expression of caspase-3 was lowest in test groups A and B and highest in the DMSO group. In comparison, the glucose metabolic rates of test groups A and B were highest, while rates for test group C and the control (DMSO) group ranked second and third.

CONCLUSION

When aortic homografts are preserved in liquid nitrogen, the cryoprotective effect of trehalose combined with DMSO was superior to that of DMSO alone. The optimal trehalose concentration to cryoprotect rabbit aortic homografts was between 0.1 and 0.2 mol/l.

Effect of three decellularisation protocols on the mechanical behaviour and structural properties of sheep aortic valve conduits

PURPOSE

To determine the best method for decellularisation of aortic valve conduits (AVCs) that efficiently removes the cells while preserving the extracellular matrix (ECM) by examining the valvular and conduit sections separately.

MATERIAL/METHODS

Sheep AVCs were decellularised by using three different protocols: detergent-based (1% SDS+1% SDC), detergent and enzyme-based (Triton+EDTA+RNase and DNase), and enzyme-based (Trypsin+RNase and DNase) methods. The efficacy of the decellularisation methods to completely remove the cells while preserving the ECM was evaluated by histological evaluation, scanning electron microscopy (SEM), hydroxyproline analysis, tensile test, and DAPI staining.

RESULTS

The detergent-based method completely removed the cells and left the ECM and collagen content in the valve and conduit sections relatively well preserved. The detergent and enzyme-based protocol did not completely remove the cells, but left the collagen content in both sections well preserved. ECM deterioration was observed in the aortic valves (AVs), but the ultrastructure of the conduits was well preserved, with no media distortion. The enzyme-based protocol removed the cells relatively well; however, mild structural distortion and poor collagen content was observed in the AVs. Incomplete cell removal (better than that observed with the detergent and enzyme-based protocol), poor collagen preservation, and mild structural distortion were observed in conduits treated with the enzyme-based method.

CONCLUSIONS

The results suggested that the detergent-based methods are the most effective protocols for cell removal and ECM preservation of AVCs. The AVCs treated with this detergent-based method may be excellent scaffolds for recellularisation.

An infrared spectroscopic study of aortic valve. A possible mechanism of calcification and the role of magnesium salts

In the present study fourier transform infrared (FT-IR) spectroscopy was used to study the mechanism of pathogenesis of aortic valve calcification. The high intensity bands of vCH3 and vCH2 groups of lipids and phospholipids of membranes, in the spectral region 3000-2800 cm(-1), show the high concentration of lipids and fatty components in aortic valve, resulting from degradation of the main aliphatic chain of the membranes, with a change of their permeability and fluidity. The presence of bands at 3075 and 1744 cm(-1), assigned to olefinic (v=CH) and aldehyde carbonyl groups, respectively, implies that reactive oxygen species are involved in the initiation of peroxidation of the lipids and phospholipids. These latter bands are related to the oxidative stress of the patients. From the shifts of bands to lower frequencies of the characteristic absorption bands of amide I and amide II, it is suggested that the proteins change their secondary structure from α-helix to β-sheets and random coil due to modifications of collagen, associated with the permeability of aortic valve atherosclerosis. From the spectral region 1150-900 cm(-1), where the characteristic stretching vibration bands of the phosphate groups (vPO4(-3)) absorb, the calcified aortic valve was found to contain biological hydroxyapatite (Ca10(PO4)6(OH)2), as well as amorphous hydroxyapatite (Ca5(PO4)xOH) and CaHPO4. These findings are in agreement with scanning electron microscopy energy-dispersive X-ray analysis and X-ray diffraction analyses. SEM micrographs show that the valves are rich in fibrils and that the protein-protein cross-linked chemical bonds seem to be the points of initiation of calcification.

Pro-calcific responses by aortic valve interstitial cells in a novel in vitro model simulating dystrophic calcification

Etiopathogenetic mechanisms in calcific aortic valve stenosis are still poorly understood despite this being the third major cause of heart disease in western world. In prior in vitro cultures simulating metastatic calcification, pro-calcific effects on aortic valve interstitial cells (AVICs) resulted by adding bacterial endotoxin lipopolysaccharide (LPS) at high inorganic phosphate (Pi) levels. Here we accomplished improved in vitro models simulating either metastatic (Pi = 2.6 mM) or dystrophic calcification (Pi = 1.3 mM), in which LPS-stimulated bovine AVICs underwent extra-stimulation with macrophage-cytokine-containing media derived from parallel cultures of allogeneic monocyte/macrophages in turn stimulated with LPS. In dystrophic calcification-like cultures, lower calcium amount was spectrometrically assessed with parallel reduced alkaline phosphatase activity with respect to metastatic calcification-like cultures, with an about three-fold slower progression of mineralization. Hydroxyapatite crystal precipitation was ultrastructurally found to correlate with AVIC degeneration processes culminating with the formation of phthalocyanin-positive lipidic layers (PPLs) at the surface of cells and cell-derived matrix-vesicle-like bodies, acting as calcium nucleators according to a pattern mirroring those we had previously found in in vivo conditions. In conclusion, an in vitro model has been developed enabling reliable simulations of the effects exerted on AVICs by putatively pro- or anti-calcific agents.

Scanning Electron Microscopy of Aortic Medial Changes in Aortic Ascending Dilatation

The study of cystic cavities and collagen fibers fragmentation is useful to for a better knowledge of pathogenesis and surgical therapy of medial ascending aortic degeneration. Thus, the aim of this study was to describe by scanning electron microscopy the surfaces and shape of the cysts, measure their area, and identify microcystic spaces related to this degenerative disease. Scanning electron microscopy analysis was performed in 16 out of 36 patients who underwent surgery for ascending aorta dilatation with associated aortic valve disease. The aortic medial wall showed a cribrose appearance at low magnification (x50-100) and the intima was effuse. At high magnification (x500-2000), small cavities (clefts) lined by normal or fragmented elastic fibers and large cavities (pseudocystes) with anfractuous borders lined by fragmented elastic fibers and smooth muscle cells were observed. Furthermore, in the outer media wall microvessels lined by endothelium were also observed. These changes were lacking or less pronounced in normal aorta. SEM allows one to better identify the pathological cavities and to differentiate them from microvessels. These pathological cavities are more numerous and larger in the convexity than in the concavity of the aorta in according to our previous morphological and morphometric findings in asymmetrical aorta dilatation.

Macromolecular transport in heart valves. III. Experiment and theory for the size distribution of extracellular liposomes in hyperlipidemic rabbits

The heart valve leaflets of 29-day cholesterol-fed rabbits were examined by ultrarapid freezing without conventional chemical fixation/processing, followed by rotary shadow freeze-etching. This procedure images the leaflets' subendothelial extracellular matrix in extraordinary detail, and extracellular lipid liposomes, from 23 to 220 nm in diameter, clearly appear there. These liposomes are linked to matrix filaments and appear in clusters. Their size distribution shows 60.7% with diameters 23–69 nm, 31.7% between 70 and 119 nm, 7.3% between 120 and 169 nm, and 0.3% between 170 and 220 nm (superlarge) and suggests that smaller liposomes can fuse into larger ones. We couple our model from Part II of this series (Zeng Z, Yin Y, Jan KM, Rumschitzki DS. Am J Physiol Heart Circ Physiol 292: H2671–H2686, 2007) for lipid transport into the leaflet to the nucleation-polymerization model hierarchy for liposome formation proposed originally for aortic liposomes to predict liposome formation/growth in heart valves. Simulations show that the simplest such model cannot account for the observed size distribution. However, modifying this model by including liposome fusing/merging, using parameters determined from aortic liposomes, leads to predicted size distributions in excellent agreement with our valve data. Evolutions of both the liposome size distribution and total liposome mass suggest that fusing becomes significant only after 2 wk of high lumen cholesterol. Inclusion of phagocytosis by macrophages limits the otherwise monotonically increasing total liposome mass, while keeping the excellent fit of the liposome size distribution to the data.

Macromolecular transport in heart valves. I. Studies of rat valves with horseradish peroxidase

The present study aims to experimentally elucidate subtle structural features of the rat valve leaflet and the related nature of macromolecular transport across its endothelium and in its subendothelial space, information necessary to construct a rational theoretical model that can explain observation. After intravenous injection of horseradish peroxidase (HRP), we perfusion-fixed the aortic valve of normal Sprague-Dawley rats and found under light microscopy that HRP leaked through the leaflet's endothelium at very few localized brown spots, rather than uniformly. These spots grew nearly as rapidly with HRP circulation time before euthanasia as aortic spots, particularly when the time axis only included the time the valve was closed. These results suggest that macromolecular transport in heart valves depends not only on the direction normal to, but also parallel to, the endothelial surface and that convection, as well as molecular diffusion, plays an important role in macromolecular transport in heart valves. Transmission electron microscopy of traverse leaflet sections after 4-min HRP circulation showed a very thin (∼150 nm), sparse layer immediately beneath the endothelium where the HRP concentration was much higher than that in the matrix below it. Nievelstein-Post et al.'s (Nievelstein-Post P, Mottino G, Fogelman A, Frank J. Arterioscler Thromb 14: 1151–1161, 1994) ultrarapid freezing/rotary shadow etching of the normal rabbit valve's subendothelial space supports the existence of this very thin, very sparse “valvular subendothelial intima,” in analogy to the vascular subendothelial intima.

Ultrastructural characterization of calcification onset and progression in subdermally implanted aortic valves. Histochemical and spectrometric data

Detailed characterization of the subdermal model is a significant tool for better understanding of calcification mechanisms occurring in heart valves. In previous ultrastructural investigation on six-week-implantated aortic valve leaflets, modified pre-embedding glutaraldehyde-cuprolinic-blue reactions (GA-CB) enabled sample decalcification with concurrent retention/staining of lipid-containing polyanionic material, which lined cells and cell-derived matrix-vesicle-like bodies (phthalocyanin-positive layers: PPLs) co-localizing with the earliest apatite nucleation sites. Additional post-embedding silver staining (GA-CB-S) revealed PPLs to contain calcium-binding sites. This investigation concerns valve leaflets subjected to shorter implantation times to shed light on the modifications associated with PPLs generation and calcification onset/progression. Spectrometric estimations revealed time-dependent calcium increase, for unreacted samples, and copper modifications indicating an increase in acidic, non-glycanic material, for GA-CB-reacted samples. Two-day-implant thin sections showed emission and subsequent reabsorption of lamellipodium-like protrusions by cells, originating ECM-containing vacuoles, and/or degeneration stages characterized by the appearance of GA-CB-S-reactive, organule-derived dense bodies and progressive dissolution of all cell membranes. In one-week-implants, the first PPL-lined cells were found to co-exist with cells where GA-CB-S-reactive material accumulated, or exudated towards their edges, or outcropped at the ECM milieu, so acquiring PPL features. PPL-derived material was observed increasingly to affect the ECM on thin sections of one-week- to six-week-implants. These results show an endogenous source for PPLs and reveal that a peculiar cascade of cell degenerative steps is associated with valve mineralization in the subdermal model, providing new useful parameters for more reliable comparison of this experimental calcification process versus the physiological and pathological processes.

Suspension string: a new method of aortic valvuloplasty for aortic insufficiency and ventricular septal defect

In a 4-year-old boy with ventricular septal defect, severe aortic insufficiency, and mild infundibular stenosis, a new method was used to reconstruct the prolapsed aortic cusp. Two ends of a pledged stitch were passed through the aorta at each side of the right, noncoronary commissure and then through another pledget, and were then tied repeatedly in a row. The length of the row of knots was equal to that of the free edge of left coronary or noncoronary leaflet. The remainder of the stitch was passed through a pledget and then the aortic wall at each side of the left and right coronary commissure to the extraaortic wall pledget and were tied. A suspension string was formed by the row of knots and supported by a Teflon (Dupont Teflon, Wilmington, DE) felt pledget sandwich at each of two commissures. The free margin of the prolapsed cusp was attached to the suspension string by a continuous suture. The concomitant anomalies were corrected. The result was satisfactory.

Extracellular Matrix Remodeling and Organization in Developing and Diseased Aortic Valves

Heart valve disease is an important cause of morbidity and mortality worldwide. Little is known about valve disease pathogenesis, but increasing evidence implicates a genetic basis for valve disease, suggesting a developmental origin. Although the cellular and molecular processes involved in early valvulogenesis have been well described, less is known about the regulation of valve extracellular matrix (ECM) organization and valvular interstitial cell (VIC) distribution that characterize the mature valve structure. Histochemistry, immunohistochemistry, and electron microscopy were used to examine ECM organization, VIC distribution, and cell proliferation during late valvulogenesis in chicken and mouse. In mature valves, ECM organization is conserved across species, and developmental studies demonstrate that ECM stratification begins during late embryonic cusp remodeling and continues into postnatal life. Cell proliferation decreases concomitant with ECM stratification and VIC compartmentalization. Explanted, stenotic bicuspid aortic valves (BAVs) from pediatric patients were also examined. The diseased valves exhibited disruption of the highly organized ECM and VIC distribution seen in normal valves. Cusps from diseased valves were thickened with increased and disorganized collagens and proteoglycans, decreased and fragmented elastic fibers, and cellular disarray without calcification or cell proliferation. Taken together, these studies show that normal valve development is characterized by spatiotemporal coordination of ECM organization and VIC compartmentalization and that these developmental processes are disrupted in pediatric patients with diseased BAVs.

Anomalous Origin of the Left Coronary Artery from the Right Side of the Aortic Valve in Syrian Hamsters (Mesocricetus auratus)

This study describes the coronary artery distribution patterns associated with the anomalous origin of the left coronary artery from the right side of the aortic valve in Syrian hamsters. The hearts of 15 affected animals were examined by means of a corrosion-cast technique, histology and scanning electron microscopy. The hamsters belonged to a laboratory inbred colony with a high incidence of coronary artery anomalies and bicuspid aortic valves. The aortic valve was tricuspid in eight hamsters and bicuspid in the other seven. In all cases, the right coronary artery was normal, whereas the left main coronary artery trunk arose from the right aortic sinus or from the right side of the ventral aortic sinus when the aortic valve was bicuspid. In 12 specimens, the left main trunk crossed the infundibular septum and then divided into the left circumflex branch and the obtuse marginal branch. In another specimen, the course of the left main trunk was ventral to the right ventricular outflow tract; in the remaining two, it surrounded the aorta dorsally. In man, some of these distribution patterns may cause myocardial ischaemia and sudden death. The present findings prove that the origin of the left coronary artery from the right aortic sinus occurs in primitive mammals such as the Syrian hamster, suggesting that the defect may occur in other mammalian species. Its possible occurrence should be borne in mind in domestic animals, especially in those with signs of myocardial ischaemia after strenuous activity.

Congenital ostial left main coronary artery stenosis associated with a bicuspid aortic valve in a young woman

We describe a 41-year-old woman with no cardiac risk factors, typical exertional angina and an abnormal noninvasive stress test. Coronary angiography demonstrated an ambiguous left main coronary artery (LMCA) stenosis. Intravascular ultrasound (IVUS) demonstrated no atheroma, but the minimum lumen diameter and area of the ostial LMCA were significantly reduced. Transesophageal echocardiography showed normal left ventricular function with a bicuspid aortic valve. Two-vessel coronary artery bypass grafting was subsequently performed. To our knowledge, this is the first IVUS-documented case of a congenital left main coronary artery stenosis associated with a bicuspid aortic valve.

Characterizing nanoscale topography of the aortic heart valve basement membrane for tissue engineering heart valve scaffold design

A fully effective prosthetic heart valve has not yet been developed. A successful tissue-engineered valve prosthetic must contain a scaffold that fully supports valve endothelial cell function. Recently, topographic features of scaffolds have been shown to influence the behavior of a variety of cell types and should be considered in rational scaffold design and fabrication. The basement membrane of the aortic valve endothelium provides important parameters for tissue engineering scaffold design. This study presents a quantitative characterization of the topographic features of the native aortic valve endothelial basement membrane; topographical features were measured, and quantitative data were generated using scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), and light microscopy. Optimal conditions for basement membrane isolation were established. Histological, immunohistochemical, and TEM analyses following decellularization confirmed basement membrane integrity. SEM and AFM photomicrographs of isolated basement membrane were captured and quantitatively analyzed. The basement membrane of the aortic valve has a rich, felt-like, 3-D nanoscale topography, consisting of pores, fibers, and elevations. All features measured were in the sub-100 nm range. No statistical difference was found between the fibrosal and ventricular surfaces of the cusp. These data provide a rational starting point for the design of extracellular scaffolds with nanoscale topographic features that mimic those found in the native aortic heart valve basement membrane.

Normal Physiological Conditions Maintain the Biological Characteristics of Porcine Aortic Heart Valves: An Ex Vivo Organ Culture Study

The aortic valve functions in a complex mechanical environment which leads to force-dependent cellular and tissue responses. Characterization of these responses provides a fundamental understanding of valve pathogenesis. The aim of this work was to study the biological characteristics of native porcine aortic valves cultured in an ex vivo pulsatile organ culture system capable of maintaining physiological pressures (120/80 mmHg) and cardiac output (4.2 l/min). Collagen, sGAG and elastin contents of the valve leaflets were measured and cusp morphology, cell phenotype, cell proliferation and apoptosis were examined. Presence of endothelial cells (ECs) on the leaflet surface was also evaluated. The differences in collagen, sGAG and elastin contents were not significant (p > 0.05) between the cultured and fresh valve leaflets. The cultured valves maintained the native ECM composition of the leaflets while preserving the morphology and cell phenotype. Cell phenotype in leaflets incubated statically under atmospheric conditions decreased compared to fresh and cultured valve leaflets, indicating the importance of mechanical forces in maintaining the natural biology of the valve leaflets. ECs were retained on the surfaces of cultured leaflets with no remodeling of the leaflets. The number of apoptotic cells in the cultured leaflets was significantly (p < 0.05) less than in the statically incubated leaflets and comparable to fresh leaflets. The sterile ex vivo organ culture system thus maintained the viability and native biological characteristics of the aortic valves that were cultured under dynamic conditions for a period of 48 h.

Scanning electron microscopic surface topography of ablation catheter perforations and calcific tear in an explanted bioprosthetic heart valve

The authors report a triplet of leaflet destruction in a bioprosthetic aortic valve explanted at 12 years after iatrogenic ablation catheter perforation in a patient who underwent coronary artery bypass surgery and multiple ablative procedures in the interim. Lesions were examined topographically by scanning electron microscopy. Calcium content was evaluated by mass spectrometry and Von Kossa staining. Leaflets exhibited little calcification, except at the commissures of the valve. Scanning electron microscopy revealed distinct lesion topography. The authors present the scanning electron microscopic characteristics of these lesions and of an incision into the valve made for comparison using a pair of scissors. This is believed to be the first report of scanning electron micrographs of ablation catheter perforations and a calcific tear in the same explanted valve. The findings provide a source for comparison in the etiological determination of explanted bioprosthetic valve lesions using scanning electron microscopy.

Cyclic loading response of bioprosthetic heart valves: effects of fixation stress state on the collagen fiber architecture

Biologically derived, chemically modified collagenous tissues are being increasingly used to fabricate cardiac valve prostheses and as biomaterials in cardiovascular repair. A stress-free state during chemical modification has been shown to preserve the collagen fiber architecture of the native tissue, potentially preserving native mechanical properties and improving prostheses durability. However, it is not known if the native collagen fiber architecture is stable during long-term in vivo operation. To address this question, we obtained porcine aortic valves chemically treated at (i) 0 mmHg transvalvular pressure (with 40 mmHg aortic pressure) and (ii) 4 mmHg transvalvular pressure, then subjected the valves to 0, 1 x 10(6), 50 x 10(6), and 200 x 10(6) in vitro accelerated wear testing (AWT) cycles. The resulting changes in collagen fiber architecture were quantified using small angle light scattering analysis (SALS). SALS measurements indicated that collagen fibers in the 0 mmHg pressure-fixed leaflets became more aligned between 1 x 10(6) and 50 x 10(6) AWT cycles. In contrast, only minor changes (not statistically significant) in collagen fiber orientation occurred in the 4 mmHg pressure-fixed valvular tissue with cycling. It was also noted that although the 0 mmHg group was fixed without transvalvular pressure, distention of the root induced significant changes in collagen structure of the leaflets. Overall, our observations suggest that the native collagen fiber crimp of the 0 mmHg pressure-fixed leaflets were rapidly lost after only 50 x 10(6) AWT cycles (equivalent to approximately 1.6 patient years) and thus may not be maintained over a sufficient period of time to be clinically beneficial. Further, the collagen structure of the native aortic valve is exquisitely sensitive to dimensional change in the aortic root-independent of the presence of transvalvular pressure. Our findings also suggest that without in vivo remodeling, any collagenous tissue used to fabricate BHV may undergo similar degenerative, irreversible changes in vivo.

Imaging of cardiovascular structures using near-infrared femtosecond multiphoton laser scanning microscopy

Multiphoton imaging represents a novel and very promising medical diagnostic technology for the high-resolution analysis of living biological tissues. We performed multiphoton imaging to analyzed structural features of extracellular matrix (ECM) components, e.g., collagen and elastin, of vital pulmonary and aortic heart valves. High-resolution autofluorescence images of collagenous and elastic fibers were demonstrated using multifluorophore, multiphoton excitation at two different wavelengths and optical sectioning, without the requirement of embedding, fixation, or staining. Collagenous structures were selectively imaged by detection of second harmonic generation (SHG). Additionally, routine histology and electron microscopy were integrated to verify the observed results. In comparison with pulmonary tissues, aortic heart valve specimens show very similar matrix formations. The quality of the resulting three-dimensional (3-D) images enabled the differentiation between collagenous and elastic fibers. These experimental results indicate that multiphoton imaging with near-infrared (NIR) femtosecond laser pulses may prove to be a useful tool for the nondestructive monitoring and characterization of cardiovascular structures.

Comparison of biomechanical and structural properties between human aortic and pulmonary valve*1

OBJECTIVE

Pulmonary valve autografts have been reported as clinically effective for replacement of diseased aortic valve (Ross procedure). Published data about pulmonary valve mechanical and structural suitability as a long-term substitute for aortic valve are limited. The aim of this study was to compare aortic and pulmonary valve properties.

METHODS

Experimental studies of biomechanical properties and structure of aortic and pulmonary valves were carried out on pathologically unchanged human heart valves, collected from 11 cadaveric hearts. Biomechanical properties of 84 specimens (all valve elements: cusps, fibrous ring, commissures, sinotubular junction, sinuses) were investigated using uniaxial tensile tests. Ultrastructure was studied using transmission and scanning electron microscopy.

RESULTS

Ultimate stress in circumferential direction for pulmonary valve cusps is higher than for aortic valve (2.78+/-1.05 and 1.74+/-0.29 MPa, respectively). Ultimate stress in radial direction for pulmonary and aortic cusps is practically the same (0.29+/-0.06 and 0.32+/-0.04 MPa, respectively). In ultrastructural study, different layout and density in each construction element are determined. The aortic and pulmonary valves have common ultrastructural properties.

CONCLUSIONS

Mechanical differences between aortic and pulmonary valve are minimal. Ultrastructural studies show that the aortic and pulmonary valves have similar structural elements and architecture. This investigation suggests that the pulmonary valve can be considered mechanically and structurally suitable for use as an aortic valve replacement.

Isolation of intact aortic valve scaffolds for heart-valve bioprostheses: extracellular matrix structure, prevention from calcification, and cell repopulation features

Extracellular matrix (ECM) scaffolds isolated from valvulated conduits can be useful in developing durable bioprostheses by tissue engineering provided that anatomical shape, architecture, and mechanical properties are preserved. As evidenced by SEM, intact scaffolds were derived from porcine aortic valves by the combined use of Triton X-100 and cholate (TRI-COL) or N-cetylpyridinium (CPC) and subsequent nucleic acid removal by nuclease. Both treatments were effective in removing most cells and all the cytomembranes, with preservation of (1) endothelium basal membranes, (2) ECM texture, including the D-periodical interaction of small proteoglycans with normally D-banded collagen fibrils, and (3) mechanical properties of the treated valves. Ultrastructural features agreed with DNA, hexosamine, and uronic acid biochemical estimations. Calcification potential, assessed by a 6-week rat subdermal model, was significantly reduced by TRI-COL/nuclease treatment. This was not true for CPC only, despite better proteoglycan preservation, suggesting that nucleic acids also are involved in calcification onset. Human fibroblasts, used to repopulate TRI-COL samples, formed mono- or multilayers on surfaces, and groups of cells also were scattered within the valve leaflet framework. A biocompatible scaffolds of this kind holds promise for production of durable valve bioprostheses that will be able to undergo probable turnover and/or remodeling by repopulating recipient cells.

[Plethysmographic assessment of pulse wave may be helpful in the detection of prosthetic valve dysfunction--a case report]

A case of a 63-year-old female with prosthetic mitral and aortic valves is described. The long-term post-operative period was complicated by infective endocarditis, persistent atrial fibrillation requiring pacemaker implantation and total a-v node ablation as well as ischaemic stroke which occurred one year before present hospitalisation. This time the patient was admitted to the hospital due to progressive heart failure. Transesophageal echocardiography showed a cyclic intermittent opening of both prosthetic valves full opening was present during every second cardiac cycle. The same phenomenon was documented using plethysmographic recording of a pulse wave from a finger. The patient underwent prosthetic valve replacement. Intraoperatively, a fibrous tissue ingrowth was detected.

Decellularization protocols of porcine heart valves differ importantly in efficiency of cell removal and susceptibility of the matrix to recellularization with human vascular cells

OBJECTIVE

We compared 3 different decellularization protocols in porcine heart valves for efficiency of complete cell removal and potential for recellularization.

METHODS

Porcine aortic and pulmonary roots were treated with trypsin, sodium-dodecyl-sulphate, or a new method using 0.25% tert-octylphenyl-polyoxyethylen in combination with sodium-deoxycholate. After a subsequent ribonuclease digestion, specimens were seeded with in vitro expanded human saphenous vein endothelial cells and myofibroblasts.

RESULTS

After treatment with trypsin and subsequent ribonuclease digestion, endothelial attachment took place; however, xenogenic cells were still visible within the matrix. Unexpectedly, when human cells were seeded onto specimens that had been decellularized with sodium-dodecyl-sulphate, the matrices were surrounded by nonviable endothelial cell fragments, indicating a toxic influence of the ionic detergent; 0.25% tert-octylphenyl-polyoxyethylen together with sodium-deoxycholate completely removed porcine cells and enabled host recellularization.

CONCLUSION

Compared with trypsin and sodium-dodecyl-sulphate involving decellularization procedures, reported to be effective in cell removal and susceptible to recellularization with human cells, only the porcine matrix treated with a new detergent-based decellularization method using 0.25% tert-octylphenyl-polyoxyethylen/sodium-deoxycholate followed by nuclease digestion presented an excellent scaffold for recellularization with human cells.

Morphological changes of the aortic valve leaflets in non-compliant aortic roots: in-vivo experiments

BACKGROUND AND AIM OF THE STUDY

Age-related loss of elasticity of the naturally compliant aortic root disrupts the coordinated function of the valve leaflets. Morphological changes that developed over time in the aortic valve leaflets of non-compliant aortic roots were studied.

METHODS

Stiffening of the aortic roots was achieved in vivo by applying Super Glue around the sinus of Valsalva in 27 New Zealand White rabbits. In nine animals, glue was applied only partially, and eight untreated rabbits served as controls. Histological evaluation of the aortic valves was performed at 8-11 months after surgery, and included immunohistochemistry and confocal microscopy with quantitative tissue assessment. Levels of collagen I, as a main component of fibrosis, and matrix metalloproteinases (MMP)-1 and MMP-9 and angiotensin-converting enzyme (ACE), as regulators of fibrosis, were analyzed. The morphological structure of the aortic valve leaflets was studied, and the length, thickness and area of leaflets were measured.

RESULTS

Leaflects in all groups were found to be composed of a continuous layer of collagen fibers at the mural side, and loose connective tissue containing fibroblasts and few capillaries on the aortic luminal aspect. In stiffened aortic roots, the length and area of the leaflets were increased. The area occupied by collagen was elevated in non-compliant aortic root leaflets, but collagen fluorescence intensity was decreased, indicating less densely packed collagen fibers. Degradation and synthesis of collagen as reflected by MMP-1, MMP-9 and ACE levels was up-regulated.

CONCLUSION

Loss of compliance in aortic roots leads to elongation of the leaflets which, combined with a decrease in collagen density, may render leaflets more susceptible to mechanical stress. In time, this may promote the development of degenerative changes in the aortic valve.

Impact of processing on surface structure of human cardiac valve allografts

Methods of processing and cryopreservation are believed to be the most important factors of long term clinical performance of biological heart valve prostheses. That is why we decided to cooperate in evaluating the impact of current AHV (allograft heartvalve) bank protocol on valve tissue morphology. AHV harvested from "heart-beating" cadaveric donors, considered as a fresh tissue, were compared with valve samples from non-heart beating donors, samples stored in saline, samples treated with antibiotic solution, and finally with cryopreserved valves, stored in liquid nitrogen for months. All samples were dissected, dried with hexamethyldisilazane (HMDS) method, gold-coated, studied and photographed in scanning electron microscope Tesla BS 301. Different superficial patterns were found on ventricular and vascular surfaces of "fresh" semilunar valves. We were able to detect early changes of endothelium after harvesting, denudation of endothelial covering during preservation with and without freezing. Our alternative method of drying samples by HMDS method proved to be suitable for thin membranes of human semilunar valves. Scanning electron microscopy seems to be helpful for morphological control of processing, cryopreservation and liquid nitrogen storage of AHV. We believe that further confrontation of morphological investigation with other methods helps us to develop more suitable protocol of handling AHV in heart valve banking.

Ultrastructural changes of cardiac valves in bacterial endocarditis

AIM

The principal objective of this study was to document morphological changes in valves with acute endocarditis in order to gain further knowledge of the pathogenesis of these diseases.

METHODS

Scanning and transmission electron microscopic investigations were carried out on explanted human heart valves to reveal ultrastructural changes due to bacterial endocarditis.

RESULTS

Bacterial inflammation endocarditis initially induced metaplasia of the endothelial cells which then lose contact with each other. In the 2nd phase of the disease, the collagen fibres are systematically removed whereby large cavities appear. In the 3rd phase, localised hyperplasia of collagen fibres was observed often resulting in the development of vegetation. The ultrastructural changes are uniform and independent of the bacterial species.

CONCLUSION

Bacterial endocarditis is therefore a set of complex interactions between endothelial cells and bacteria which should be taken into consideration for the development of new therapeutic approaches.

Human aortic valve calcification is associated with an osteoblast phenotype

BACKGROUND

Calcific aortic stenosis is the third most common cardiovascular disease in the United States. We hypothesized that the mechanism for aortic valve calcification is similar to skeletal bone formation and that this process is mediated by an osteoblast-like phenotype.

METHODS AND RESULTS

To test this hypothesis, we examined calcified human aortic valves replaced at surgery (n=22) and normal human valves (n=20) removed at time of cardiac transplantation. Contact microradiography and micro-computerized tomography were used to assess the 2-dimensional and 3-dimensional extent of mineralization. Mineralization borders were identified with von Kossa and Goldner's stains. Electron microscopy and energy-dispersive spectroscopy were performed for identification of bone ultrastructure and CaPO4 composition. To analyze for the osteoblast and bone markers, reverse transcriptase-polymerase chain reaction was performed on calcified versus normal human valves for osteopontin, bone sialoprotein, osteocalcin, alkaline phosphatase, and the osteoblast-specific transcription factor Cbfa1. Microradiography and micro-computerized tomography confirmed the presence of calcification in the valve. Special stains for hydroxyapatite and CaPO4 were positive in calcification margins. Electron microscopy identified mineralization, whereas energy-dispersive spectroscopy confirmed the presence of elemental CaPO4. Reverse transcriptase-polymerase chain reaction revealed increased mRNA levels of osteopontin, bone sialoprotein, osteocalcin, and Cbfa1 in the calcified valves. There was no change in alkaline phosphatase mRNA level but an increase in the protein expression in the diseased valves.

CONCLUSIONS

These findings support the concept that aortic valve calcification is not a random degenerative process but an active regulated process associated with an osteoblast-like phenotype.

Degeneration of bioprosthetic heart valve cusp and wall tissues is initiated during tissue preparation: an ultrastructural study

BACKGROUND AND AIM OF THE STUDY

Chronic tissue degeneration is a major factor in the failure of porcine bioprosthetic heart valves. Stabilization with glutaraldehyde (GA) has become the standard in preparation of bioprosthetic heart valves, but there is increasing evidence that GA does not effectively stabilize all tissue structures, specifically glycosaminoglycans (GAGs). The study aim was to establish the status of GAGs in bioprosthetic heart valves and to ascertain whether degeneration of the extracellular matrix (ECM) is initiated during preparation of porcine tissues for use as bioprosthetic heart valves.

METHODS

Stentless porcine bioprosthetic heart valves were prepared by tissue harvesting, 24 h of storage in cold saline, and 14 days' fixation in buffered 0.6% GA. Tissue samples obtained from fresh and fixed aortic cusps and wall conduit were analyzed for ECM integrity and GAG localization by transmission electron microscopy combined with toluidine blue staining.

RESULTS

Major degenerative changes occurred in the ECM ultrastructure of both porcine cusp and wall during tissue preparation for use as bioprosthetic heart valves. Modifications in the aortic cusp included loss of GAGs from the interfibrillary space and from the surface of the collagen fibers. In the aortic wall, GAGs were lost from the interfibrillary space and from the surface of collagen fibers. In addition, the surface of wall elastic fibers exhibited marked paucity of GAGs and elastin-associated microfibrils.

CONCLUSION

The typical steps involved in the preparation of porcine aortic bioprosthetic heart valves induce, or cannot fully prevent, degeneration of some components of the ECM. Controlling the extent of this pre-implantation deterioration will open new gateways for improvement of the quality and durability of future cardiovascular bioprostheses.

Novel ultrastructural features as revealed by phthalocyanine reactions indicate cell priming for calcification in subdermally implanted aortic valves

The roles played by various determinants in physiological, pathological or experimental calcification are still unclear. In this investigation, new insights were gained into structural changes occurring in porcine aortic valves undergoing mineralization in the rat subdermal model and then subjected to reactions with cationic phthalocyanines (PHTs), at salt-critical electrolyte concentrations (CEC). PHT reactions showed decalcifying effects, depending on both acidic pH in the media employed and mineral substitution by Cuprolinic Blue (CB) itself, as well as specific reactivity which enabled the ultrastructural detection of unusual, PHT-positive layers (PPLs) encircling cells and matrix vesicles, at 0.05 M CEC conditions. Other reactions at different CEC conditions, or subsequent to enzymatical or specific extractive treatments, suggest PPL appearance is due to PHT uptake by clustered anionic phospholipids, which seem to be involved in mineral precipitation. PPLs present as a novel, reliable ultrastructural parameter indicating cell propensity in priming experimental and, possibly, pathological calcification.

Osteopontin inhibits mineral deposition and promotes regression of ectopic calcification

Ectopic calcification, the abnormal calcification of soft tissues, can have severe clinical consequences especially when localized to vital organs such as heart valves, arteries, and kidneys. Recent observations suggest that ectopic calcification, like bone biomineralization, is an actively regulated process. These observations have led a search for molecular determinants of ectopic calcification. A candidate molecule is osteopontin (OPN), a secreted phosphoprotein invariantly associated with both normal and pathological mineral deposits. In the present study, OPN was found to be a natural inhibitor of ectopic calcification in vivo. Glutaraldehyde-fixed aortic valve leaflets showed accelerated and fourfold to fivefold greater calcification after subcutaneous implantation into OPN-null mice compared to wild-type mice. In vitro and in vivo studies suggest that OPN not only inhibits mineral deposition but also actively promotes its dissolution by physically blocking hydroxyapatite crystal growth and inducing expression of carbonic anhydrase II in monocytic cells and promoting acidification of the extracellular milieu. These findings suggest a novel mechanism of OPN action and potential therapeutic approach to the treatment of ectopic calcification.

Floppy aortic valves without aortic root dilatation: clinical, histologic, and ultrastructural studies

Gross anatomic, histologic and ultrastructural studies were made on 32 floppy aortic valves (FAVs) resected at the time of aortic valvular replacement for aortic regurgitation. Patients with the FAVs had relatively long clinical courses and had severe aortic regurgitation with mild symptoms of heart failure. The sizes of the mechanical valves implanted in the patients with FAVs were not large, indicating that the aortic regurgitation in these patients was not worsened by dilatation of the aortic ring. Two types of FAVs were recognized grossly, according to whether they showed abnormal cuspal thickening or thinning. Accumulations of myxoid material in the spongiosa were found in all FAVs, regardless of cuspal gross morphology. Histologically, the collagen fibers were sparse and irregularly arranged and elastic fibers were disrupted and finely granular in the myxomaotus areas of FAVs. Ultrastructurally, the myxomatous material consisted of numerous star-shaped proteoglycan granules associated with spiraling collagen fibrils and abnormal elastic fibers. Numerous spiraling collagen fibrils were observed especially at the border area of myxomatous change that extended from the spongiosa into the fibrosa. Abnormal elastic fibers had either a granular appearance of their amorphous components without microfibrils, or irregularly arranged masses of microfibrils without amorphous components. These abnormalities of connective tissue components, resulting from defective formation and/or increased degradation were similar to those in floppy mitral valves, and were related to the floppiness of cardiac valves.

Localization of caveolin 1 in aortic valve endothelial cells using antigen retrieval

Ultrastructural analysis of aortic valve endothelial cells subjected to growth arrest revealed many vesicles defined as caveolae by the localization of caveolin. Translocation of caveolin after exposure to oxidized LDL suggests that the localization of caveolin may be a valuable tool to study models of early atherogenesis. In this study, several antigen retrieval protocols were tested in osmium-fixed and Spurr-embedded cells to determine the optimal method of antigen retrieval in our model system. SDS produced the most consistent labeling pattern. A quantitative evaluation revealed that SDS significantly increased the labeling density in Spurr-embedded cells. The labeling pattern appeared as clusters of gold particles, 15-40 nm in diameter, that were associated with membranes of a similar size which may represent the neck region of the caveolae.

Cells, rather than extracellular matrix, nucleate apatite in glutaraldehyde-treated vascular tissue

Glutaraldehyde (GA) causes a large increase in [Ca(2+)](i) and [P(i)](i) and calcification of porcine aortic valve fibroblasts. Calcification in GA-treated vascular tissue is likely to begin intracellularly, but the potential role of extracellular matrix has not been taken into account in earlier studies. To compare the role of cells and matrix in calcification, intestinal pouches made of a lipid-extracted rat small intestine were prepared. Lipid-extracted porcine aortic valves, or cells cultured from those same valves, were placed in intestinal pouches, sealed, fixed with GA, and grafted in rat subcutis. Cells in the pouches calcified in 3 weeks whereas the valvular matrix did not calcify for 9 weeks. Cellular calcification spread to the wall of the intestinal pouches and grew heavier after 9 weeks. Similarly, smooth muscle cells calcified exclusively in GA-treated rat aorta grafted in rat subcutis for 3 weeks. Calcification of extracellular matrix was seen after 9 weeks. Cells initiate calcification and extracellular matrix serves as a substrate for the subsequent growth of apatite in GA-treated vascular tissue.

Ultrastructural alterations in acquired aortic and mitral valve disease as revealed by scanning and transmission electron microscopical investigations

Scanning and transmission electron microscopical investigations were carried out on explanted human aortic and mitral valves to study the prevalence of hyperplastic and degenerative lesions in acquired valvular dysfunction. Biopsies were taken from 67 aortic and 23 mitral valves. All of the valves examined showed degenerative lesions including a loose binding of the endothelial cells, a partial denudation of the endothelial cover and areas of fibrous hyperplasia surrounded by calcium deposits. Additionally, the formation of various excrescences was detected by means of scanning electron microscopy. Of all excrescences identified, 90% were localized at the free margin of the leaflet, 3% in the subnodular region and 7% in the nodule of Arantius. The ratio of filiform to lamellar forms of hyperplastic lesions was approximately 80% in most of the samples examined. The results presented demonstrate the complex ultrastructural features of surgically explanted human valves showing both degenerative and hyperplastic lesions in the same valve.

Copper retention, calcium release and ultrastructural evidence indicate specific Cuprolinic Blue uptake and peculiar modifications in mineralizing aortic valves

Previously, reactions with copper phthalocyanines at 0.05 M critical electrolyte concentration were found to cause demineralization in calcifying porcine aortic valves after subdermal implantation in rat, as well as simultaneous visualization of peculiar phthalocyanine-positive layers around cells and cell-derived matrix vesicles. In the present investigation, an appraisal was made of the mechanism and specificity of reactions with Cuprolinic Blue by comparing quantitatively calcium release and copper retention by calcified aortic valves reacted with this phthalocyanine under different critical electrolyte concentration conditions, and the corresponding ultrastructural patterns. It was found that (i) decalcifying properties are inversely proportional to salt molarity; (ii) reactivity to Cuprolinic Blue is critical electrolyte concentration-dependent, since the greatest copper retention occurred in 0.05 M critical electrolyte concentration Cuprolinic Blue-reacted samples, the only ones that also exhibited phthalocyanine-positive layers; (iii) the appearance of phthalocyanine-positive layers depends on Cuprolinic Blue uptake, revealing pericellular clustering of calcium-binding, anionic molecules; and (iv) minor Cuprolinic Blue uptake occurs by residual proteoglycans which still remain in the extracellular matrix after 6-week-long subdermal implantation. The present results indicate that this method is appropriate for the study of mineralized tissues and illustrate peculiar tissue modifications occurring at least in the experimental conditions used here.

Fixation-related autolysis and bioprosthetic aortic wall calcification

BACKGROUND AND AIM OF THE STUDY

It has been established previously that immediate fixation and increased glutaraldehyde (GA) concentrations are required to prevent severe autolytic tissue damage during bioprosthetic aortic root production. The study aim was to verify that structure-preserving fixation also reduces aortic wall calcification.

METHODS

Porcine aortic roots were fixed either instantly or after being kept on ice for 48 h (phosphate-buffered saline, PBS). Two concentrations of GA (0.2% and 3.0%) were chosen (4 degrees C, seven days, PBS). Discs of aortic wall tissue (1.2 cm diameter) were implanted subcutaneously in rats for 60 days (n = 10 per group), while aortic roots were implanted in the distal aortic arch of sheep for six weeks (n = 3 per group) and six months (n = 4 per group). Calcification was assessed by atomic absorption spectrophotometry and light microscopy. Fixation-related tissue damage was determined by transmission electron microscopy, and correlated with calcification.

RESULTS

No significant difference in calcification was found between immediate and delayed fixation if tissue was fixed with 0.2% GA. In the 3.0% GA group, both animal models showed a significantly lower level of calcification if tissue was immediately fixed. In the subcutaneous rat model, immediate fixation reduced calcification by 26% (p <0.0001). In the circulatory sheep model immediate fixation did not affect calcification in the short-term six-week implants, but markedly lowered it by 37% (p = 0.035) after six months. Ultrastructurally, there was a significant correlation between membrane damage, vacuolization and vesicle shedding on the one hand, and calcification on the other.

CONCLUSION

Coincidental fixation-related ultrastructural damage and increased calcification was demonstrated in bioprosthetic aortic wall tissue.

Characterization of ultrastructural damage of valves cryopreserved under standard conditions

Cryopreserved allograft valves are increasingly being used as valvular replacements. This study was conducted to characterize the ultrastructural damage on the allograft valves obtained by a current standard protocol of valve procurement, antibiotic exposure, and cryopreservation, as a basis for future studies on allograft valve preservation. Materials used were seven aortic and seven pulmonary fresh porcine valves, which were cryopreserved according to the requirements of the American and European Associations of Tissue Banks. The samples were randomly assigned into four groups: (1) fresh, untreated; (2) fresh, treated with antibiotics for 24 h.; (3) treated with antibiotics and exposed to dimethyl sulfoxide (without freezing); and (4) treated with antibiotics, exposed to dimethyl sulfoxide, and then cryopreserved and stored until the study. All tissue samples were processed simultaneously for routine light microscopy and transmission electron microscopy. Fresh-untreated, antibiotic-treated, and dimethyl sulfoxide-exposed valves showed adequate preservation of cellular components. However, after cryopreservation significant damage was observed in fibroblasts with signs of apoptotic cellular injury. Our observation suggests that apoptosis occurs during valve processing. This apoptotic process may be related to various factors, including chemical injury or hypoxia.

Experimental hypercholesterolemia induces apoptosis in the aortic valve

BACKGROUND AND AIM OF THE STUDY

Aortic valve disease is presently the number one indication for valve replacement in the United States, yet its molecular mechanisms remain unknown. As apoptosis (programmed cell death) occurs in degenerative disease states, it was postulated that experimental hypercholesterolemia is associated with apoptosis in rabbit aortic valves.

METHODS

New Zealand White rabbits (n = 8) were fed a 1% cholesterol diet for 12 weeks; control rabbits (n = 8) were fed a normal diet. After sacrifice of the animals, the aortic valves were dissected. Apoptosis was identified in the valvular lesion by TdT-mediated dUTP-biotin nick end-labeling (TUNEL) technique, and confirmed with transmission electron microscopy. The number of apoptotic cells was measured by computed morphometry.

RESULTS

Valves from hypercholesterolemic rabbits showed an increase in apoptosis. TUNEL staining was identified in the atherosclerotic layer of hypercholesterolemic valves (0.1% of cells), but not in the cells of controls (p <0.0001).

CONCLUSION

Apoptosis is increased in rabbit aortic valves during experimental hypercholesterolemia. If fatal cellular degeneration occurs in hypercholesterolemic valve disease, these data suggest that apoptosis may play a role in the mechanism of valvular disease.

The influence of stenting on the behavior of amino-oleic acid-treated, glutaraldehyde-fixed porcine aortic valves in a sheep model

BACKGROUND AND AIM OF THE STUDY

The durability of freehand-sewn aortic valve homografts used for valve replacement in humans is greater than for stented aortic homografts. In analogy with this, it is expected that the durability of a stentless heterograft will be superior to that of its stented counterpart. Our objective was to investigate the influence of stenting on amino-oleic acid (AOA)-treated, glutaraldehyde-fixed porcine aortic valve bioprostheses.

METHODS

Twelve young sheep underwent implantation of porcine aortic valves in the pulmonary artery: six porcine aortic stentless valves (Freestyle) and six porcine aortic stented valves (Mosaic). In each series, three valves were explanted after three months, and three after six months. Valves were analyzed by gross inspection, radiography, histology, and transmission electron microscopy. Quantitative determination of calcium content was made with atomic absorption spectrometry.

RESULTS

The porcine aortic stentless valve showed extensive calcification of its aortic wall portion, but had perfectly functioning, pliable cusps without calcification up to six months. The cusps of porcine aortic stented valves were also pliable and functioning without calcification up to six months. Only minimal calcification was seen in the aortic wall of the stented valves. At six months after implantation the cusps of stentless valves contained significantly less calcium than those of stented valves (2.7+/-1.2 microg/mg and 7.9+/-2.3 microg/mg, respectively; p = 0.011). However, the aortic wall from stentless valves contained significantly more calcium than that of stented valves (three-month explants: 39.2+/-14.4 versus 7.2+/-2.8 microg/mg; p <0.05; six-month explants: 49.3+/-14.0 versus 14.1+/-5.9 microg/mg; p <0.05).

CONCLUSION

These data suggest that stenting does influence cuspal calcification of AOA-treated, glutaraldehyde-fixed porcine aortic valves.

Preimplant ultrastructure and calcification tendency of various biological aortic valves

BACKGROUND AND AIM OF THE STUDY

In recent years a number of fixation and anti-calcification methods have been developed, but little is yet known about the calcification process of biological valves. The aims of this study were to: (i) perform a systematic ultrastructural investigation on various biological valves; and (ii) determine the extent of calcification of these valves in a subcutaneous rat model.

METHODS

The following porcine aortic prostheses were investigated: Toronto-SPV, Intact, Freestyle, Mosaic and Hancock-II. Samples taken from the valve leaflets, and in the case of the Freestyle and Toronto-SPV valves also from the aortic wall, were examined ultrastructurally using scanning and transmission electron microscopy. Other samples were implanted subcutaneously in Wistar rats for 12 weeks. The calcium content of the samples was measured using atomic absorption spectrophotometry.

RESULTS

All valves examined showed a considerable loss of the endothelial cover. Significant changes in valve ultrastructure were also detected. With regard to calcium content, two valve groups could be distinguished (p <0.05): (i) those with high calcium content, e.g. Toronto-SPV and Intact (>40 mg/g dry tissue); and (ii) those with low calcium content, e.g. Mosaic, Freestyle and Hancock-II (<5 mg/g).

CONCLUSION

Fixation methods have pronounced effects on the ultrastructural integrity of bioprostheses. The degenerative calcification of bioprostheses can be effectively inhibited by glutaraldehyde-free fixation and anti-calcification treatments.

Kangaroo versus porcine aortic valve tissue--valve geometry morphology, tensile strength and calcification potential

OBJECTIVE

Valve related factors and patient related factors are responsible for calcification of valvular bioprostheses. Recent studies showed different donor and recipient species have different influences on the total calcification rate of bioprostheses. This study was performed to evaluate and compare Kangaroo aortic valve leaflets with porcine aortic valve leaflets. Experimental design. Prospective study. Setting. Cardio-thoracic experimental research of a university department.

MATERIALS AND METHODS

Glutaraldehyde-fixed Kangaroo and porcine valve leaflets were evaluated in vitro according to valve geometry (internal diameter and leaflet thickness), morphology (light and electron microscopy) and tensile strength. In vivo evaluation consisted of implantation in a rat model for 8 weeks, Von Kossa stain for calcium and atomic absorption spectrophotometry for total extractable calcium content.

RESULTS

Kangaroo valves indicated a smaller internal valve diameter as well as a thinner valve leaflet (p<0.01, ANOVA) at corresponding body weight, less proteoglycan spicules in the fibrosa, increased elasticity (p<0.05) and low calcification potential (p<0.01, confidence interval 95%).

CONCLUSIONS

Kangaroo aortic valve leaflets have different valvular qualities compared to porcine valve tissue. Kangaroo valve leaflets are significantly superior to porcine valve leaflets as far as calcification is concerned. These results are encouraging and suggest further in vivo evaluation in a larger animal model before clinical application can be considered.

A novel in vitro assessment of tissue valve calcification by a continuous flow type method

A dynamic flow type testing to study calcification was self-designed to investigate calcification in bioprosthetic heart valves. The apparatus consists of a container into which leaflets from a porcine aortic valve are placed, a chamber that contains calcium solution, and a peristaltic pump that provides a continuous supply of the solution toward the container. Efficacy of the apparatus was compared with the conventional batch type calcification testing at 37 degrees C through measuring the amount of calcium and phosphate deposited by inductively coupled plasma (ICP) and scanning electron microscope (SEM). After 14 days, calcium levels detected from the calcified deposit on leaflets were 470.4 +/- 37.0 microg/cm3 in the flow type testing whereas in the batch type testing levels were 81.0 +/- 6.7 microg/cm3. Though the calcium level on the leaflet increased as the exposure time to calcium solution increased in both testings, the rate and the tendency of calcification could be assessed very rapidly by flow type testing in comparison with batch type testing. [Ca]/[P] molar ratio decreased over time, and after 14 days, the ratio was close to 1.83 +/- 0.18 in the flow type testing. The ratio could not be determined in the batch type testing because the deposit was too small to assess. The descending rate of [Ca]/[P] molar ratio demonstrates that deposited calcium-complex at the earliest stage may interact with inorganic phosphate ions to create a calcified deposit mineral precursor. This in vitro dynamic flow type calcification testing was a favorable tool for rapid investigation of calcification.