Inhibition of the calcification of porcine valve tissue by selective lipid removal

HLA Awareness in tissue decellularization: A paradigm shift for enhanced biocompatibility, studied on the model of the human fascia lata graft

Last twenties, tissue engineering has rapidly advanced to address the shortage of organ donors. Decellularization techniques have been developed to mitigate immune rejection and alloresponse in transplantation. However, a clear definition of effective decellularization remains elusive. This study compares various decellularization protocols using the human fascia lata model. Morphological, structural and cytotoxicity/viability analyses indicated that all the five tested protocols were equivalent and met Crapo's criteria for successful decellularization. Interestingly, only the in vivo immunization test on rats revealed differences. Only one protocol exhibited Human Leucocyte Antigen (HLA) content below 1% residual threshold, the only criterion preventing rat immunization with an absence of rat anti-human IgG switch after one month (N=4 donors for each of the 7 groups, added by negative and positive controls, n=28). By respecting a refined set of criteria, i.e. lack of visible nuclear material, <50ng DNA/mg dry weight of extracellular matrix, and <1% residual HLA content, the potential for adverse host reactions can be drastically reduced. In conclusion, this study emphasizes the importance of considering not only nuclear components but also major histocompatibility complex in decellularization protocols and proposes new guidelines to promote safer clinical development and use of bioengineered scaffolds.

Ultrastructural and Immunohistochemical Detection of Hydroxyapatite Nucleating Role by rRNA and Nuclear Chromatin Derivatives in Aortic Valve Calcification: In Vitro and In Vivo Pro-Calcific Animal Models and Actual Calcific Disease in Humans

Calcification starts with hydroxyapatite (HA) crystallization on cell membranous components, as with aortic valve interstitial cells (AVICs), wherein a cell-membrane-derived substance containing acidic phospholipids (PPM/PPLs) acts as major crystal nucleator. Since nucleic acid removal is recommended to prevent calcification in valve biosubstitutes derived from decellularized valve scaffolds, the involvement of ribosomal RNA (rRNA) and nuclear chromatin (NC) was here explored in three distinct contexts: (i) bovine AVIC pro-calcific cultures; (ii) porcine aortic valve leaflets that had undergone accelerated calcification after xenogeneic subdermal implantation; and (iii) human aortic valve leaflets affected by calcific stenosis. Ultrastructurally, shared AVIC degenerative patterns included (i) the melting of ribosomes with PPM/PPLs, and the same for apparently well-featured NC; (ii) selective precipitation of silver particles on all three components after adapted von Kossa reactions; and (iii) labelling by anti-rRNA immunogold particles. Shared features were also provided by parallel light microscopy. In conclusion, the present results indicate that rRNA and NC contribute to AVIC mineralization in vitro and in vivo, with their anionic charges enhancing the HA nucleation capacity exerted by PPM/PPL substrates, supporting the concept that nucleic acid removal is needed for valve pre-implantation treatments, besides better elucidating the modality of pro-calcific cell death.

Optimal delipidation solvent to secure extracellular matrix from human perirenal adipose tissue

The objective of this study was to select the optimal delipidation solvent for preparation of human perirenal adipose tissue-derived extracellular matrix (ECM). Human perirenal adipose tissue can be obtained in large amounts during surgery, and it can be an alternative source of human ECM. Delipidation is an essential procedure for the ECM preparation, because lipid strongly inhibits regeneration of target tissue. Isopropanol has been widely used as a delipidation solvent for adipose tissue. However, because adipose tissue is mostly composed of nonpolar lipid, a nonpolar solvent might be more effective for delipidation. We evaluated the delipidation efficiency of acetone, chloroform, methanol, ether, ethanol, isopropanol, water, chloroform/methanol, ethanol/heptane, ether/methanol, hexane/ethanol, and butanol/methanol solvents for ECM extraction from human perirenal adipose tissue. Among them, acetone-treated adipose tissue showed the greatest delipidation efficiency (93.05%), significantly lower residual DNA content, and the greatest residual collagen concentration (42.49 ± 0.05 μg/g). In addition, acetone-treated tissue also had well-preserved ultrastructure with high porosity and significantly low in vitro cytotoxicity. These results suggested that acetone may be an optimal delipidation solvent for extraction of ECM from human perirenal adipose tissue.

Modification of decellularized vascular xenografts with 8-arm polyethylene glycol suppresses macrophage infiltration but maintains graft degradability

Because acellular vascular xenografts induce an immunological reaction through macrophage infiltration, they are conventionally crosslinked with glutaraldehyde (GA). However, the GA crosslinking reaction inhibits not only the host immune reaction around the graft but also the graft's enzymatic degradability, which is one of the key characteristics of acellular grafts that allow them to be replaced by host tissue. In this study, we used an 8-arm polyethylene glycol (PEG) to successfully suppress macrophage infiltration, without eliminating graft degradation. Decellularized ostrich carotid arteries were modified with GA or N-hydroxysuccinimide-activated 8-arm PEG (8-arm PEG-NHS), which has a molecular weight of 17 kDa. To evaluate the enzymatic degradation in vitro, the graft was immersed in a collagenase solution for 12 hr. The 8-arm PEG-modified graft was degraded to the same extent as the unmodified graft, but the GA-modified graft was not degraded. The graft was transplanted into rat subcutaneous tissue for up to 8 weeks. Although CD68-positive cells accumulated in the unmodified graft, they did not infiltrate into either modified graft. However, the GA-modified grafts calcified, but the 8-arm PEG-modified graft did not calcify after transplantation. These data suggested that 8-arm PEG-NHS is a promising modification agent for biodegradable vascular xenografts, to suppress acute macrophage infiltration only.

Antigen removal process preserves function of small diameter venous valved conduits, whereas SDS-decellularization results in significant valvular insufficiency

Chronic venous disease (CVD) is the most common reported chronic condition in the United States, affecting more than 25 million Americans. Regardless of its high occurrence, current therapeutic options are far from ideal due to their palliative nature. For best treatment outcomes, challenging cases of chronic venous insufficiency (CVI) are treated by repair or replacement of venous valves. Regrettably, the success of venous valve transplant is dependent on the availability of autologous venous valves and hindered by the possibility of donor site complications and increased patient morbidity. Therefore, the use of alternative tissue sources to provide off-the-shelf venous valve replacements has potential to be extremely beneficial to the field of CVI. This manuscript demonstrates the capability of producing off-the-shelf fully functional venous valved extracellular matrix (ECM) scaffold conduits from bovine saphenous vein (SV), using an antigen removal (AR) method. AR ECM scaffolds maintained native SV structure-function relationships and associated venous valves function. Conversely, SDS decellularization caused significant changes to the collagen and elastin macromolecular structures, resulting in collagen fibril merging, elimination of fibril crimp, amalgaming collagen fibers and fragmentation of the inner elastic lamina. ECM changes induced by SDS decellularization resulted in significant venous valve dysfunction. Venous valved conduits generated using the AR approach have potential to serve as off-the-shelf venous valve replacements for CVI. STATEMENT OF SIGNIFICANCE: Retention of the structure and composition of extracellular matrix (ECM) proteins within xenogeneic scaffolds for tissue engineering is of crucial importance, due to the undeniable effect ECM proteins can impose on repopulating cells and function of the resultant biomaterial. This manuscript demonstrates that alteration or elimination of ECM proteins via commonly utilized decellularization approach results in complete disruption of venous valve function. Conversely, retention of the delicate ECM structure and composition of native venous tissue, using an antigen removal tissue processing method, results in preservation of native venous valve function.

Small Diameter Xenogeneic Extracellular Matrix Scaffolds for Vascular Applications

Currently, despite the success of percutaneous coronary intervention (PCI), coronary artery bypass graft (CABG) remains among the most commonly performed cardiac surgical procedures in the United States. Unfortunately, the use of autologous grafts in CABG presents a major clinical challenge as complications due to autologous vessel harvest and limited vessel availability pose a significant setback in the success rate of CABG surgeries. Acellular extracellular matrix (ECM) scaffolds derived from xenogeneic vascular tissues have the potential to overcome these challenges, as they offer unlimited availability and sufficient length to serve as "off-the-shelf" CABGs. Unfortunately, regardless of numerous efforts to produce a fully functional small diameter xenogeneic ECM scaffold, the combination of factors required to overcome all failure mechanisms in a single graft remains elusive. This article covers the major failure mechanisms of current xenogeneic small diameter vessel ECM scaffolds, and reviews the recent advances in the field to overcome these failure mechanisms and ultimately develop a small diameter ECM xenogeneic scaffold for CABG. Impact Statement Currently, the use of autologous vessel in coronary artery bypass graft (CABG) is common practice. However, the use of autologous tissue poses significant complications due to tissue harvest and limited availability. Developing an alternative vessel for use in CABG can potentially increase the success rate of CABG surgery by eliminating complications related to the use of autologous vessel. However, this development has been hindered by an array of failure mechanisms that currently have not been overcome. This article describes the currently identified failure mechanisms of small diameter vascular xenogeneic extracellular matrix scaffolds and reviews current research targeted to overcoming these failure mechanisms toward ensuring long-term graft patency.

A comparison study of different decellularization treatments on bovine articular cartilage

Previous researches have emphasized on suitability of decellularized tissues for regenerative applications. The decellularization of cartilage tissue has always been a challenge as the final product must be balanced in both immunogenic residue and mechanical properties. This study was designed to compare and optimize the efficacy of the most common chemical decellularization treatments on articular cartilage. Freeze/thaw cycles, trypsin, ethylenediaminetetraacetic acid (EDTA), sodium dodecyl sulfate (SDS), and Triton-X 100 were used at various concentrations and time durations for decellularization of bovine distal femoral joint cartilage samples. Histological staining, scanning electron microscopy, DNA quantification, compressive strength test, and Fourier-transform infrared spectroscopy were performed for evaluation of the decellularized cartilage samples. Treatment with 0.05% trypsin/EDTA for 1 day followed by 3% SDS for 2 days and 3% Triton X-100 for another 2 days resulted in significant reduction in DNA content and simultaneous maintenance of mechanical properties. Seeding the human adipose-derived stem cells onto the decellularized cartilage confirmed its biocompatibility. According to our findings, an optimized physiochemical decellularization method can yield in a nonimmunogenic biomechanically compatible decellularized tissue for cartilage regeneration application.

Ectopic mineralization in heart valves: new insights from in vivo and in vitro procalcific models and promising perspectives on noncalcifiable bioengineered valves

Ectopic calcification of native and bioprosthetic heart valves represents a major public health problem causing severe morbidity and mortality worldwide. Valve procalcific degeneration is known to be caused mainly by calcium salt precipitation onto membranes of suffering non-scavenged cells and dead-cell-derived products acting as major hydroxyapatite nucleators. Although etiopathogenesis of calcification in native valves is still far from being exhaustively elucidated, it is well known that bioprosthesis mineralization may be primed by glutaraldehyde-mediated toxicity for xenografts, cryopreservation-related damage for allografts and graft immune rejection for both. Instead, mechanical valves, which are free from calcification, are extremely thrombogenic, requiring chronic anticoagulation therapies for transplanted patients. Since surgical substitution of failed valves is still the leading therapeutic option, progressive improvements in tissue engineering techniques are crucial to attain readily available valve implants with good biocompatibility, proper functionality and long-term durability in order to meet the considerable clinical demand for valve substitutes. Bioengineered valves obtained from acellular non-valvular scaffolds or decellularized native valves are proving to be a compelling alternative to mechanical and bioprosthetic valve implants, as they appear to permit repopulation by the host's own cells with associated tissue remodelling, growth and repair, besides showing less propensity to calcification and adequate hemodynamic performances. In this review, insights into valve calcification onset as revealed by in vivo and in vitro procalcific models are updated as well as advances in the field of valve bioengineering.

Approaches to improve integration and regeneration of an ex vivo derived temporomandibular joint disc scaffold with variable matrix composition

Due to their natural biochemical and biomechanical characteristics, using ex vivo tissues as platforms for guided tissue regeneration has become widely accepted, however subsequent attachment and integration of these constructs in vivo is often overlooked. A decellularized porcine temporomandibular joint (TMJ) disc has shown promise as a scaffold to guide disc regeneration and preliminary work has shown the efficacy of surfactant (SDS) treatment within the fibrocartilaginous disc to remove cellular components. The majority of studies focus on the intermediate region of the disc (or disc proper). Using this approach, inherent attachment tissues can be maintained to improve construct stability and integration within the joint. Unlike human disc attachment tissue, the porcine attachment tissues have high lipid content which would require a different processing approach to remove immunogenic components. In order to examine the effect of delipidation on the attachment tissue properties, SDS and two organic solvent mixtures (acetone/ethanol and chloroform/methanol) were compared. Lipid and cellular solubilization, ECM alteration, and seeded human mesenchymal stem cell (MSC) morphology and viability were assessed. Quantitative analysis showed SDS treatments did not effectively delipidate the attachment tissues and cytotoxicity was noted toward MSC in these regions. Acetone/ethanol removed cellular material but not all lipids, while chloroform/methanol removed all visible lipid deposits but residual porcine cells were observed in histological sections. When a combination of approaches was used, no residual lipid or cytotoxicity was noted. Preparing a whole TMJ graft with a combined approach has the potential to improve disc integration within the native joint environment.

Porcine pulmonary valve decellularization with NaOH-based vs detergent process: preliminary in vitro and in vivo assessments

Background

Glutaraldehyde fixed xenogeneic heart valve prosthesis are hindered by calcification and lack of growth potential. The aim of tissue decellularization is to remove tissue antigenicity, avoiding the use of glutaraldehyde and improve valve integration with low inflammation and host cell recolonization. In this preliminary study, we investigated the efficacy of a NaOH-based process for decellularization and biocompatibility improvement of porcine pulmonary heart valves in comparison to a detergent-based process (SDS-SDC0, 5%).

Methods

Native cryopreserved porcine pulmonary heart valves were treated with detergent and NaOH-based processes.

Decellularization was assessed by Hematoxylin and eosin/DAPI/alpha-gal/SLA-I staining and DNA quantification of native and processed leaflets, walls and muscles.

Elongation stress test investigated mechanical integrity of leaflets and walls (n = 3 tests/valve component) of valves in the native and treated groups (n = 4/group).

Biochemical integrity (collagen/elastin/glycosaminoglycans content) of leaflet-wall and muscle of the valves (n = 4/group) was assessed and compared between groups with trichrome staining (Sirius Red/Miller/Alcian blue).

Secondly, a preliminary in vivo study assessed biocompatibility (CD3 and CD68 immunostaining) and remodeling (Hematoxylin and eosin/CD31 and ASMA immunofluorescent staining) of NaOH processed valves implanted in orthotopic position in young Landrace pigs, at 1 (n = 1) and 3 months (n = 2).

Results

Decellularization was better achieved with the NaOH-based process (92% vs 69% DNA reduction in the wall). Both treatments did not significantly alter mechanical properties. The detergent-based process induced a significant loss of glycosaminoglycans (p < 0,05).

In vivo, explanted valves exhibited normal morphology without any sign of graft dilatation, degeneration or rejection. Low inflammation was noticed at one and three months follow-up (1,8 +/− 3,03 and 0,9836 +/− 1,3605 CD3 cells/0,12 mm² in the leaflets). In one animal, at three months we documented minimal calcification in the area of sinus leaflet and in one, microthrombi formation on the leaflet surface at 1 month. The endoluminal side of the valves showed partial reendothelialization.

Conclusions

NaOH-based process offers better porcine pulmonary valve decellularization than the detergent process. In vivo, the NaOH processed valves showed low inflammatory response at 3 months and partial recellularization. Regarding additional property of securing, this treatment should be considered for the new generation of heart valves prosthesis.

Graphical abstract

Graphical abstract of the study

A novel tissue treatment to reduce mineralization of bovine pericardial heart valves

OBJECTIVE

With the increasing use of bioprostheses worldwide, continuous efforts have been made to improve tissue durability. We introduce a new treatment for bovine pericardium combining octanediol-ethanol based phospholipid removal with taurine-based glutaraldehyde neutralization and storage in an aldehyde-free solution (FREE).

METHODS

Treated tissues were evaluated by mechanical and biochemical characterization, phospholipid content, aldehyde levels, cell cultures on pericardial samples (L929 fibroblasts and human umbilical vein endothelial cells), rat subcutaneous implantations, and long-term juvenile sheep mitral valve implantations (n = 3). Comparisons were made to glutaraldehyde-fixed bovine pericardium or to samples from commercially available biological valves (ie, Trifecta [St Jude Medical, Saint Paul, Minn] and Perimount Magna Ease [Edwards Lifesciences, Irvine, Calif]).

RESULTS

FREE-treated pericardium had similar mechanical strength and biochemical properties as commercially available valves. Compared with glutaraldehyde-only samples, FREE-treated samples showed lower phospholipid levels (P < .01), significantly better growth of L929 fibroblasts, and lower calcification levels in rat subcutaneous implants (P < .01). Compared with samples from Linx- (Trifecta) and ThermaFix-treated (Perimount Magna Ease) valves, similar low levels of phospholipids were observed as were similar low calcification levels in subcutaneous implants, but tissue extractions from FREE-treated samples showed the lowest levels of extracted aldehydes (P < .01). Mitral implants of FREE-treated valves in juvenile sheep had excellent hemodynamic behavior without any sign of degeneration or calcification at 5 months.

CONCLUSIONS

The new FREE treatment combines an adequate phospholipid reduction and aldehyde neutralization with storage in an aldehyde-free solution. This combination enhances the anticalcification properties and may thereby improve long-term durability of the tissue.

Decellularized bovine intervertebral disc as a natural scaffold for xenogenic cell studies

Low back pain that is associated with disc degeneration contributes to a huge economic burden in the worldwide healthcare system. Traditional methods, such as spinal fusion, have been adopted to relieve mechanical back pain, but this is compromised by decreased spinal motion. Tissue engineering has attracted much attention, and aims to correct the changes fundamentally occurring in the discs by a combination of cell biology, molecular biology and engineering. Synthetic materials including poly(l-lactic acid) or poly(glycolic acid) and biomolecules like hyaluronic acid or collagen have been adopted in the development of disc scaffolds for studying therapeutic approaches. Nevertheless, the complex biological and mechanical environment of the intervertebral disc (IVD) makes the synthesis of an artificial IVD with biomaterials a difficult task. Thus the aim of this study was to develop a natural disc scaffold for culturing disc cells for future development of biological disc constructs. We adopted a combination of currently used decellularization techniques to decellularize bovine IVD to create a complete endplate-to-endplate IVD scaffold. By altering the chemical and physical decellularization parameters, we reported the removal of up to 70% of the endogenous cells, and were able to preserve the glycosaminoglycan content, collagen fibril architecture and mechanical properties of the discs. The reintroduction of nucleus pulposus cells into the scaffold indicated a high survival rate over 7days, with cell penetration. We have shown here that conventional methods used for decellularizing thin tissues can also be applied to large organs, such as IVD. Our findings suggest the potential of using decellularized IVD as a scaffold for IVD bioengineering and culturing of cells in the context of the IVD niche.

Immune response in patients receiving a bioprosthetic heart valve: lack of response with decellularized valves

Conventional biological heart valves treated with glutaraldehyde (GA) reveal a limited lifespan due to calcification. This is assumed to be an immune response initiated process, which is not seen with decellularized valves. However, their immunological potential is still a matter of debate. Therefore, serum samples from patients undergoing heart valve surgery were obtained before (Pre), after (Post), and 9-12 months after operation (Follow Up). Immunoglobulin G (IgG) and M (IgM) antibodies against porcine collagen I and α-Gal (Gal-alpha1,3-Gal-beta1,4-GlcNac-R) were determined for decellularized and GA treated valves. Antibody titers for collagen type I revealed no significant alteration for both types of valves. However, a considerable anti-α-Gal antibody response was observed in patients with GA-treated porcine valves. In detail, IgM antibodies were increased during follow up (p<0.05), whereas decellularized valves revealed a minor decrease in the IgM response (p<0.001). IgG antibodies were considerably increased with GA-treated porcine (p<0.05) and bovine (p<0.01) xenografts, whereas there was lack of response with decellularized valves. This indicates that GA treatment is not sufficient to eliminate immune response to the α-Gal epitope completely. Future investigations will have to verify whether immune response to α-Gal can be linked to the limited durability of conventional valves.

An overview of tissue and whole organ decellularization processes

Biologic scaffold materials composed of extracellular matrix (ECM) are typically derived by processes that involve decellularization of tissues or organs. Preservation of the complex composition and three-dimensional ultrastructure of the ECM is highly desirable but it is recognized that all methods of decellularization result in disruption of the architecture and potential loss of surface structure and composition. Physical methods and chemical and biologic agents are used in combination to lyse cells, followed by rinsing to remove cell remnants. Effective decellularization methodology is dictated by factors such as tissue density and organization, geometric and biologic properties desired for the end product, and the targeted clinical application. Tissue decellularization with preservation of ECM integrity and bioactivity can be optimized by making educated decisions regarding the agents and techniques utilized during processing. An overview of decellularization methods, their effect upon resulting ECM structure and composition, and recently described perfusion techniques for whole organ decellularization techniques are presented herein.

Physiological variables involved in heart valve substitute calcification

Biochemical, histological and genetic studies using in vitro/in vivo models have demonstrated that pathological calcification of bioprosthetic heart valves (BHV) is regulated by various mechanisms associated with physiological variables. The major objective of this review is to characterize physiological variables involved in BHV calcification. This review examines our understanding of the systemic cellular behavior and physiological regulation processes behind BHV calcification and its clinical applications.

Decellularized Xenogenic Heart Valves Reveal Remodeling and Growth Potentialin Vivo

We have developed an advanced tissue processing technique on porcine pulmonary heart valves for pulmonary valve replacement and its initial clinical application during the autograft operation according to Ross. The novel concept consists of a cell-free matrix achieved by deoxycholic acid treatment that is repopulated by host cells in vivo. Molecular biology, radioligand binding, and electron microscopy consistently showed that these valves are almost free of cellular components. Animal experiments and clinical investigations revealed excellent hemodynamic properties of the valves, no need for antithrombotic therapy, and repopulation by host cells without any signs of calcification. In juvenile sheep the internal diameter of the implanted valves significantly increased in growing animals by approximately 10 mm. The repopulation of the decellularized heart valves was found not only in sheep but also in humans, which indicates that the underlying mechanisms, presumably repair mechanisms, might be common in mammals. If these findings can be confirmed by others, they will lead to new concepts in the field of cardiovascular tissue engineering that will eliminate the need for in vitro construction of autologous heart valves.

The potential calcification of octacalcium phosphate on intraocular lens surfaces

Recently, calcification was observed on implanted intraocular lens (IOL) surfaces when viscoelastic substances were applied during surgery. To elucidate the mechanisms of mineral formation, the crystallization of calcium phosphates on IOL surfaces was studied in vitro with nanomolar sensitivity using a constant composition method. Three different commercial viscoelastic materials (Viscoat, OcuCoat, and Amvisc Plus) were investigated and it was found that some IOLs treated with Viscoat or Amvisc Plus induced the nucleation and growth of octacalcium phosphate crystallites under biological conditions. After treatments, the IOL surfaces became more hydrophilic probably because of the high viscoelastic phosphate and carboxylate contents. In contrast to Viscoat, the use of OcuCoat during surgery resulted in virtually no octacalcium phosphate nucleations. Calcification studies of IOL surfaces treated with fatty acids, which are present in human aqueous humor, suggest that hydrophobic cyclic silicones adsorbed on the IOL surfaces interact strongly with hydrophobic hydrocarbon chains of the fatty acids, creating a layer of amphiphiles oriented with functional carboxylate groups exposed to the aqueous solution and serving as active calcification sites.

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.

Calcification of intraocular implant lens surfaces

Calcification of octacalcium phosphate [Ca8H2(PO4)6 x 5H2O, OCP] on differently packaged "Ultem" and "Surefold" intraocular implant lens surfaces has been studied in vitro in solutions supersaturated with respect to OCP at pH = 7.10 and 37 degrees C. No mineral deposition was observed on the lenses packaged in Ultem vials even after treatment with behenic acid, one of the fatty acids identified on explanted lenses. Following treatment with behenic acid, nucleation of OCP occurred on the lenses from Surefold vials, which incorporate silicone gaskets; induction periods preceding calcification were about 6 h. No mineralization was found on the lenses in vials with other gasket materials, including polytetrafluoroethylene, fluorocarbon elastomer, and polypropylene. The results of this study indicate that both silicone and fatty acids such as behenic acid play important roles in inducing the in vivo calcification of OCP on IOL lenses; all of the lens treatment steps were necessary for nucleation induction.

Gelatinases in soft tissue biomaterials. Analysis of different crosslinking agents

Chemical modification of pericardium-based cardiac valves tends to reduce the relatively high degree of biodegradation and calcification of the implanted bioprostheses. We analysed the tissue properties of pericardium from young calves and pigs after crosslinking with different agents (glutaraldehyde. diphenylphosphorylazide (DDPA), 1-ethyl-3,3-dimethyl-aminopropyl-carbodiimide (EDAC)) and when exposed to anticalcification treatments (chloroform/methanol or ethanol) prior to glutaraldehyde (GA) crosslinking. Protein extraction after tissue homogenisation in the presence of detergents showed that crosslinking using GA or DPPA was much more effective. The amounts of protein extracted from these two groups of chemically modified pericardium were significantly lower: the other modified tissues presented only a slight reduction when compared with untreated tissue. Matrix metalloproteinases- (MMP) 2 and 9 were detected in native pericardium from calf and pig by zymography. While the MMP-9/MMP-2 activity ratio was close to 1 in pig pericardium, it was 8.5-fold higher in bovine tissue. Crosslinking with GA and with DPPA almost completely abolished gelatinase activities, even when equal amounts of solubilised protein were loaded onto the zymograms. Anticalcification treatments followed by GA crosslinking or treatment with EDAC were not as effective in reducing gelatinase activities; but, interestingly, a relative reduction of MMP-9 versus MMP-2 was detected. The presence of these gelatinase activities in pericardium may contribute to the in vivo degradability of pericardium-based cardiac valves.

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.

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.

Chemical treatment and tissue selection: factors that influence the mechanical behaviour of porcine pericardium

Calcification and mechanical failure are the major causes of the loss of cardiac bioprostheses. The chemical treatments used to stabilize the tissue employed are considered to play a fundamental role in the development of these two phenomena, although the problem is multifactorial and the underlying causes are yet to be fully identified. Currently, there is an ongoing search for chemical treatments capable of reducing or eliminating the process of calcification while preserving the mechanoelastic characteristics of the tissue. One of the approaches to this effort is the elimination of the phospholipid component from the biological tissue employed in prosthesis construction. There is evidence that this component may be responsible for the precipitation of calcium salts. The present study compares two delipidating chemical treatments involving chloroform/methanol and sodium dodecyl sulfate (SDS) with the use of glutaraldehyde (GA) alone. For this purpose, porcine pericardial tissue was subjected to tensile strength testing employing a hydraulic simulator. A total of 234 samples were studied 90 treated with GA, 72 treated with chloroform/methanol and 72 treated with SDS. The mean breaking strength was significantly higher in the samples treated with GA (between 43.29 and 63.01 MPa) when compared with those of tissue treated with chloroform/methanol (29.92-42.30 MPa) or with SDS (13.49-19.06 MPa). In a second phase of the study, selection criteria based on morphological and mechanical factors were applied to the pericardial membranes employing a system of paired samples. The mathematical analysis of the findings in one fragment will aid in determining the mechanical behavior of its adjacent twin sample. In conclusion, the anticalcification chemical treatments tested in the experimental model conferred a lesser mechanical resistance than that obtained with GA. On the other hand, the utilization of paired samples was found to be useful in the prediction of the mechanical behavior of porcine pericardial tissue. Nevertheless, in order for our method of selection to be considered the most adequate approach, it will be necessary to validate these findings in dynamic studies involving a real, functional model.

Calcification characteristics of porcine stented valves in a juvenile sheep model

BACKGROUND

Different antimineralization treatments of stented porcine bioprostheses were evaluated: ethanol (Epic), alpha-amino-oleic acid (AOA) (Mosaic), and sodium dodecyl sulfate (SDS) (Hancock II). A nontreated, glutaraldehyde-fixed valve (Labcor) served as control.

METHODS

For each treatment, six valves were implanted in juvenile sheep in the pulmonary position. Valves were explanted after 3 and 6 months and examined macroscopically, by roentgenogram and light and transmission electron microscopy. Calcium content (microg/mg) was determined by atomic absorption spectrometry.

RESULTS

The Labcor valves revealed small calcium deposits in the cusps, although calcium content remained low (median value 0.4+/-0.8 microg/mg). SDS did not prevent cusp calcification as assessed by histology and calcium content measurement, which was higher than in all other valves: 1.9+/-4.6 microg/mg (p < 0.05). Cusp retraction and rupture were occasionally found in the Hancock. The Mosaic and Epic valves showed no cusp calcification and had low calcium contents (0.3+/-2.4 microg/mg and 0.7+/-0.6 microg/mg, respectively). Epic showed less pannus formation, but had hematoma or iron staining in the cusps.

CONCLUSIONS

SDS is inefficient as an antimineralization treatment, in contrast to ethanol or AOA. Cusp hematoma after ethanol treatment needs further investigation.

The foreign body reaction to a biodegradable biomaterial differs between rats and mice

Before a biomaterial can be applied in the clinic, biocompatibility must be tested in in vivo models, by monitoring the foreign body reaction. In this study, we compared the foreign body reaction (FBR) to the biodegradable biomaterial hexamethylenediisocyanate crosslinked dermal sheep collagen (HDSC) between several strains of rats and mice. HDSC disks were implanted subcutaneously on the backs of AO, BN, F344, LEW, and PVG rats and on the backs of 129 SVEV, BALB/c, and C57BL/6 mice. Materials were explanted after 7, 14, 21, and 28 days and processed for (immuno) light and transmission electron microscopic evaluation. In all rat strains, giant cell formation and phagocytosis of HDSC bundles were comparable. In addition, in the PVG rat, many plasma cells infiltrated the HDSC disks. Only a few T cells were present in AO and PVG rats, whereas, in F344 and LEW rats, the presence of T cells was more pronounced. BN rats showed an intermediate T-cell infiltration. In mice, the FBR to HDSC was comparable between the different strains. Compared with rats, giant cell formation was limited, whereas stroma formation was more abundant. Phagocytosis of HDSC bundles rarely occurred in mice, whereas calcification was observed more often. It is concluded that the FBR to HDSC clearly differs between rats and mice. This has consequences for assessment studies on biocompatibility and also on fundamental biomaterial research.

Mineralization of glutaraldehyde-fixed porcine aortic valve cusps in the subcutaneous rat model: analysis of variations in implant site and cuspal quadrants

When evaluating the efficacy of new antimineralization treatments for bioprosthetic heart valves, subcutaneous implantation in a rat model often is used as an initial test. Although this model is widely used, there still are many aspects of its implementation that have not been investigated. To further investigate several parameters that may affect mineralization in the subcutaneous rat model, portions of glutaraldehyde-treated porcine aortic valve cusps were implanted both ventrally and dorsally into 21-day-old male Sprague-Dawley rats. Cusp quadrants were explanted at 1,2, and 3 weeks postimplantation and the calcium levels determined by atomic absorption spectroscopy. The objective of this study was to determine whether or not different implant locations and/or regions of the cusps affect the degree to which tissue mineralizes in the subcutaneous rat model. A total of 270 tissue specimens were examined. While the specific portion of the cusp implanted did not significantly affect the degree of mineralization, dorsal implantation resulted in significantly more mineralization than abdominal implantation (p = 0.007). As expected, longer implantation time was associated with greater calcification (p = 0.0001). The results of this study indicate that inconsistent placement of tissues in the rat subcutaneous implant model can result in significant differences in the degree of mineralization.

Assessment of pericardium in cardiac bioprostheses. A review

Cardiac valve bioprostheses are assessed in terms of their present and future clinical utility. The problems concerning durability basically involve early failure due to tears in the valve leaflets and late failure mainly associated with calcification of the biological tissue. New strategies for selection and chemical treatment of the biomaterials employed are analyzed, and the available knowledge regarding their mechanical behavior is reviewed. It is concluded that the durability of these devices, and thus their successful use in the future, depends on the knowledge of the interactions among the different biomaterials of which they are composed, the development of new materials, and the engineering design applied in their construction.

Role of glutaraldehyde in calcification of porcine aortic valve fibroblasts

Glutaraldehyde-treated porcine aortic valve xenografts frequently fail due to calcification. Calcification in the prostheses begins intracellularly. In a previous study, various types of cell injury to canine valvular fibroblasts, including glutaraldehyde treatment, led to calcification. An influx of extracellular Ca2+ into the phosphate-rich cytosol was theorized to be the mechanism of calcification. To test the Ca2+ influx theory, cytosolic Ca2+ and Pi concentrations were assessed in glutaraldehyde-treated porcine aortic valve fibroblasts, and their relationship to a subsequent calcification was studied. Glutaraldehyde caused an immediate and sustained massive cytosolic Ca2+ increase that was dose dependent and a several-fold increase in Pi. Calcification of cells followed within a week. The earliest calcification was observed in blebs formed on glutaraldehyde-treated cells. Live control cells or cells fixed with glutaraldehyde in Ca2+-free solution did not calcify under the same conditions. Concomitant increases in Ca2+ and Pi in glutaraldehyde-treated cells appear to underlie the mechanism of calcification, and the presence of extracellular Ca2+ during glutaraldehyde fixation promotes calcification.

In vitro testing of bioprostheses: influence of mechanical stresses and lipids on calcification

BACKGROUND

Structural valve deterioration of bioprostheses is mainly caused by the progressive development of calcification. Mechanical stresses or lipid deposits in porcine aortic leaflets have been proposed as major factors contributing to the calcification process.

METHODS

A new test protocol consisting of nondestructive holographic interferometry, which allows a quantitative deformation analysis of heart valves, and accelerated dynamic in vitro calcification was used. The rapid calcification fluid contained a final combined calcium and phosphorus concentration of 130 (mg/dL)2 in barbital buffer solution. The calcification of 32 bioprostheses donated by different manufacturers (SJM Bioimplant, Biocor standard, Biocor No-React, Carpentier-Edwards SAV, Bravo, pericardial prototype) was assessed after up to 25 x 10(6) cycles by microradiography and the areas of calcification were compared with the holographic interferograms. The distribution of lipid droplets of four porcine prostheses were visualized by Sudan III stain before the calcification process.

RESULTS

Most of the tested bioprostheses had areas presenting with stress concentrations, and the dynamic in vitro testing resulted in leaflet calcification corresponding to the holographic irregularities. A strong correlation between calcification and stress distribution or lipid accumulation was found (r = 0.72; r = 0.81, respectively). After 19 x 10(6) cycles, the Carpentier-Edwards SAV and the pericardial valves had significantly less calcification than other prostheses tested (p = 0.003), but the variation among individual prostheses from the same manufacturer was even more pronounced.

CONCLUSIONS

Mechanical stresses or lipid accumulation seems to play an important role in the calcification process of bioprostheses. Quality control of bioprosthetic valves using holographic interferometry has the potential to predict calcification before implantation.

Calcification potential of small intestinal submucosa in a rat subcutaneous model

Glutaraldehyde treatment of collagen biomaterials promotes calcification, poor host-tissue incorporation, and ultimately mechanical failure of bioprotheses. Porcine small-intestinal submucosa (SIS) is a biomaterial which has been investigated for several applications including arterial and venous grafts and repair of tendon, ligament, body wall, and urinary bladder defects. The calcification potential of peracetic acid (PAA)-sterilized SIS was studied. Four test samples, (1) native (cleaned, untreated) SIS, (2) SIS sterilized with 0.1% PAA, (3) SIS treated with 0.25% glutaraldehyde for 20 min, and (4) commercially available glutaraldehyde-preserved porcine bioprosthetic heart valve cusp segments (GPV), were each implanted subcutaneously in each of 24 weanling rats. Six rats were euthanatized at 1, 2, 4, and 8 weeks. Evaluation of calcium concentration by atomic absorption spectroscopy and extent of mineralization and fibrosis by light microscopy were performed. Atomic absorption revealed no calcification in native or peracetic acid-treated SIS at any time point compared with preimplant calcium concentration. Statistically significant (P < 0.0001) calcification occurred in glutaraldehyde-treated materials (SIS and GPV) at each evaluation as compared to native and peracetic acid-treated samples. Histopathology indicated native and peracetic acid-treated SIS showed no implant mineralization (P < 0.0001) and little peri-implant fibrosis (P < 0.0001). Results suggested that native and peracetic acid-treated SIS have a low calcification potential and further study of this biomaterial is warranted.

Factors which affect the calcification of tissue-derived bioprostheses

Mineralization of implanted bioprostheses poses a major clinical problem. Crosslinking of collagenous matrices, a process used to render tissues relatively inert and nonbiodegradable, seems to encourage calcification. Residual, noncovalently bound glutaraldehyde, as well as glutaraldehyde crosslinks which can be degraded with time, seem to play a role in this connection. Our findings demonstrate the need to carefully remove noncovalently or labile-associated glutaraldehyde by thorough rinsing or neutralization before implantation. Components of a valve prosthesis such as cusps and aortic wall, which are known to vary in their proportions of collagen, elastin, and noncollagenous proteins and to calcify to different extents, can both be prevented from calcifying if treated with a biphosphonate before implantation. Calcification can also be reduced by selective enzymatic removal of noncollagenous materials. In addition to the age of rats, animals usually used to evaluate calcification, the strain of animal can markedly affect the response. The Fischer-344 rat, a highly inbred animal, will not calcify exhaustively rinsed implants. Our findings suggest that multifactorial approaches may have to be combined to generate the most ideal bioprostheses. These should include careful removal of noncovalently bound glutaraldehyde, neutralization of the nonbifunctionally reacted residues, removal of lipids and noncollagenous proteins (and possibly the more antigenic nonhelical collagen telopeptides), as well as inclusion of agents such as biphosphohates, which by interfering with crystal growth prevent the accumulation of mineral in the interstices of the tissue.

Heterogeneous nucleation of calcium phosphates on solid surfaces in aqueous solution

The heterogeneous nucleation of calcium phosphates on solid surfaces of poly(methyl methacrylate) (PMMA), poly-(tetrafluoroethylene-co-hexafluoropropylene) (FEP), silicone rubber, mica, and radiofrequency glow discharge (RFGD)-treated PMMA, FEP, and silicone rubber has been studied in solutions supersaturated with respect to hydroxyapatite. The surface properties of the substrates were characterized by contact angle measurements. For the RFGD-treated surfaces, the Lifshitz-Van der Waals surface tension component changes very little, but the Lewis acid-base surface tension parameters vary greatly depending upon the materials. With scanning electron microscopy, nucleation of calcium phosphates was observed only on the surfaces: mica, RFGD-treated PMMA and FEP, with relatively high values of the Lewis base surface tension parameter. The more hydrophobic surfaces having low Lewis acid-base surface tensions, untreated PMMA and FEP, silicone rubber, and even RFGD-treated silicone rubber showed no nucleation.

Lipid oxidation products have opposite effects on calcifying vascular cell and bone cell differentiation. A possible explanation for the paradox of arterial calcification in osteoporotic patients

Atherosclerotic calcification and osteoporosis often coexist in patients, yielding formation of bone mineral in vascular walls and its simultaneous loss from bone. To assess the potential role of lipoproteins in both processes, we examined the effects of minimally oxidized low-density lipoprotein (MM-LDL) and several other lipid oxidation products on calcifying vascular cells (CVCs) and bone-derived preosteoblasts MC3T3-E1. In CVCs, MM-LDL but not native LDL inhibited proliferation, caused a dose-dependent increase in alkaline phosphatase activity, which is a marker of osteoblastic differentiation, and induced the formation of extensive areas of calcification. Similar to MM-LDL, oxidized 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine (ox-PAPC) and the isoprostane 8-iso prostaglandin E2 but not PAPC or isoprostane 8-iso prostaglandin F2 alpha induced alkaline phosphatase activity and differentiation of CVCs. In contrast, MM-LDL and the above oxidized lipids inhibited differentiation of the MC3T3-E1 bone cells, as evidenced by their stimulatory effect on proliferation and their inhibitory effect on the induction of alkaline phosphatase and calcium uptake. These results suggest that specific oxidized lipids may be the common factors underlying the pathogenesis of both atherosclerotic calcification and osteoporosis.

Modifications induced by atherogenic diet in the capacity of the arterial wall in rats to respond to surgical insult

A study was made of events occurring in the arterial wall of the rat after administration of an atherogenic calcification-inducing diet and of vascular response in a model of combined metabolic aggression (atherogenic, calcification-inducing diet) and surgical aggression (adventitial resection). Female Sprague-Dawley rats were fed an atherogenic, calcification-inducing diet for 24 consecutive days, after which half the rats returned to standard diet (group I, n = 12) and the other half (group II) underwent resection of the adventitia on a segment of common iliac artery on day 25 before returning to standard diet. Normolipemic rats and rats that underwent adventitial resection without the atherogenic, calcification-inducing diet were used as the control groups. The rats were killed at 10 min, 1 and 24 h, 5, 14, 21, 30, 50, 70, 120 and 180 days. Morphologic studies were made with light microscopy and electron microscopy (scanning and transmission), as well as biochemical studies. Monocyte adherence and infiltration of the arterial intima, thickening of the subintimal space, the presence of monocyte-macrophages, calcification in the medial layer, intense adventitial fibrosis, and vacuolization of the endothelial cells of the adventitial microvessels were common findings in the two groups receiving the atherogenic, calcification-inducing diet. However, these groups differed in the intensity of calcification: the deep part of the medial layer did not become calcified when the adventitia was resected. Moreover, adventitial regeneration was delayed in group II with respect to the animals that underwent adventitial resection without atherogenic, calcification-inducing diet. We conclude that this diet induced atherosclerotic lesions in the vessel wall and inhibited adventitial regeneration in the rats that underwent resection.