Biochemical characterization of individual normal, floppy and rheumatic human mitral valves

A Novel Rheumatic Mitral Valve Disease Model with Ex Vivo Hemodynamic and Biomechanical Validation

PURPOSE

Rheumatic heart disease is a major cause of mitral valve (MV) dysfunction, particularly in disadvantaged areas and developing countries. There lacks a critical understanding of the disease biomechanics, and as such, the purpose of this study was to generate the first ex vivo porcine model of rheumatic MV disease by simulating the human pathophysiology and hemodynamics.

METHODS

Healthy porcine valves were altered with heat treatment, commissural suturing, and cyanoacrylate tissue coating, all of which approximate the pathology of leaflet stiffening and thickening as well as commissural fusion. Hemodynamic data, echocardiography, and high-speed videography were collected in a paired manner for control and model valves (n = 4) in an ex vivo left heart simulator. Valve leaflets were characterized in an Instron tensile testing machine to understand the mechanical changes of the model (n = 18).

RESULTS

The model showed significant differences indicative of rheumatic disease: increased regurgitant fractions (p < 0.001), reduced effective orifice areas (p < 0.001), augmented transmitral mean gradients (p < 0.001), and increased leaflet stiffness (p = 0.025).

CONCLUSION

This work represents the creation of the first ex vivo model of rheumatic MV disease, bearing close similarity to the human pathophysiology and hemodynamics, and it will be used to extensively study both established and new treatment techniques, benefitting the millions of affected victims.

Mechanics and Microstructure of the Atrioventricular Heart Valve Chordae Tendineae: A Review

The atrioventricular heart valves (AHVs) are responsible for directing unidirectional blood flow through the heart by properly opening and closing the valve leaflets, which are supported in their function by the chordae tendineae and the papillary muscles. Specifically, the chordae tendineae are critical to distributing forces during systolic closure from the leaflets to the papillary muscles, preventing leaflet prolapse and consequent regurgitation. Current therapies for chordae failure have issues of disease recurrence or suboptimal treatment outcomes. To improve those therapies, researchers have sought to better understand the mechanics and microstructure of the chordae tendineae of the AHVs. The intricate structures of the chordae tendineae have become of increasing interest in recent literature, and there are several key findings that have not been comprehensively summarized in one review. Therefore, in this review paper, we will provide a summary of the current state of biomechanical and microstructural characterizations of the chordae tendineae, and also discuss perspectives for future studies that will aid in a better understanding of the tissue mechanics-microstructure linking of the AHVs' chordae tendineae, and thereby improve the therapeutics for heart valve diseases caused by chordae failures.

Effect of glutaraldehyde based cross-linking on the viscoelasticity of mitral valve basal chordae tendineae

BACKGROUND

Mitral valve failure can require repair or replacement. Replacement bioprosthetic valves are treated with glutaraldehyde prior to implantation. The aim of this study was to determine the changes in mechanical properties following glutaraldehyde fixation of mitral valve chordae.

METHODS

To investigate the impact of glutaraldehyde on mitral valve chordae, 24 basal chordae were dissected from four porcine hearts. Anterior and posterior basal (including strut) chordae were used. All 24 chordae were subjected to a sinusoidally varying load (mean level 2N, dynamic amplitude 2N) over a frequency range of 0.5-10 Hz before and after glutaraldehyde treatment.

RESULTS

The storage and loss modulus of all chordal types decreased following glutaraldehyde fixation. The storage modulus ranged from: 108 to 119 MPa before fixation and 67.3-87.4 MPa following fixation for basal chordae; 52.3-58.4 MPa before fixation and 47.9-53.5 MPa following fixation for strut chordae. Similarly, the loss modulus ranged from: 5.47 to 6.25 MPa before fixation and 3.63-4.94 MPa following fixation for basal chordae; 2.60-2.97 MPa before fixation and 2.31-2.93 MPa following fixation for strut chordae.

CONCLUSION

The viscoelastic properties of mitral valve chordae are affected by glutaraldehyde fixation; in particular, the reduction in storage moduli decreased with an increase in chordal diameter.

Development and Characterization of a Porcine Mitral Valve Scaffold for Tissue Engineering

Decellularized scaffolds represent a promising alternative for mitral valve (MV) replacement. This work developed and characterized a protocol for the decellularization of whole MVs. Porcine MVs were decellularized with 0.5% (w/v) SDS and 0.5% (w/v) SD and sterilized with 0.1% (v/v) PAA. Decellularized samples were seeded with human foreskin fibroblasts and human adipose-derived stem cells to investigate cellular repopulation and infiltration, and with human colony-forming endothelial cells to investigate collagen IV formation. Histology revealed an acellular scaffold with a generally conserved histoarchitecture, but collagen IV loss. Following decellularization, no significant changes were observed in the hydroxyproline content, but there was a significant reduction in the glycosaminoglycan content. SEM/TEM analysis confirmed cellular removal and loss of some extracellular matrix components. Collagen and elastin were generally preserved. The endothelial cells produced newly formed collagen IV on the non-cytotoxic scaffold. The protocol produced acellular scaffolds with generally preserved histoarchitecture, biochemistry, and biomechanics.

Myxomatous Degeneration of the Canine Mitral Valve: From Gross Changes to Molecular Events

Myxomatous mitral valve disease (MMVD) is the single most common acquired heart disease of the dog, but is also of emerging importance in human medicine, with some features of the disease shared between both species. There has been increased understanding of this disease in recent years, with most research aiming to elucidate the cellular and molecular events of disease pathogenesis. For gross and histological changes, much of our understanding is based on historical studies and there has been no comprehensive reappraisal of the pathology of MMVD. This paper reviews the gross, histological, ultrastructural, cellular and molecular changes in canine MMVD.

Differential cell-matrix responses in hypoxia-stimulated aortic versus mitral valves

Tissue oxygenation often plays a significant role in disease and is an essential design consideration for tissue engineering. Here, oxygen diffusion profiles of porcine aortic and mitral valve leaflets were determined using an oxygen diffusion chamber in conjunction with computational models. Results from these studies revealed the differences between aortic and mitral valve leaflet diffusion profiles and suggested that diffusion alone was insufficient for normal oxygen delivery in mitral valves. During fibrotic valve disease, leaflet thickening due to abnormal extracellular matrix is likely to reduce regional oxygen availability. To assess the impact of low oxygen levels on valve behaviour, whole leaflet organ cultures were created to induce leaflet hypoxia. These studies revealed a loss of layer stratification and elevated levels of hypoxia inducible factor 1-alpha in both aortic and mitral valve hypoxic groups. Mitral valves also exhibited altered expression of angiogenic factors in response to low oxygen environments when compared with normoxic groups. Hypoxia affected aortic and mitral valves differently, and mitral valves appeared to show a stenotic, rheumatic phenotype accompanied by significant cell death. These results indicate that hypoxia could be a factor in mid to late valve disease progression, especially with the reduction in chondromodulin-1 expression shown by hypoxic mitral valves.

Extracellular matrix remodeling in wound healing of critical size defects in the mitral valve leaflet

The details of valvular leaflet healing following valvuloplasty and leaflet perforation from endocarditis are poorly understood. In this study, the synthesis and turnover of valvular extracellular matrix due to healing of a critical sized wound was investigated. Twenty-nine sheep were randomized to either CTRL (n = 11) or HOLE (n = 18), in which a 2.8-4.8 mm diameter hole was punched in the posterior mitral leaflet. After 12 weeks, posterior leaflets were harvested and histologically stained to localize extracellular matrix components. Immunohistochemistry was also performed to assess matrix components and markers of matrix turnover. A semi-quantitative grading scale was used to quantify differences between HOLE and CTRL. After 12 weeks, the hole diameter was reduced by 71.3 ± 1.4 % (p < 0.001). Areas of remodeling surrounding the hole contained more activated cells, greater expression of proteoglycans, and markers of matrix turnover (prolyl 4-hydroxylase, metalloproteases, and lysyl oxidase, each p ≤ 0.025), along with fibrin accumulation. Two distinct remodeling regions were evident surrounding the hole, one directly bordering the hole rich in versican and hyaluronan and a second adjacent region with abundant collagen and elastic fiber turnover. The remodeling also caused reduced delineation between valve layers (p = 0.002), more diffuse staining of matrix components and markers of matrix turnover (p < 0.001), and disruption of the collagenous fibrosa. In conclusion, acute valve injury elicited distinct, heterogeneous alterations in valvular matrix composition and structure, resulting in partial wound closure. Because these changes could also affect leaflet mechanics and valve function, it will be important to determine their impact on healing wounds.

Characterization of biomechanical properties of aged human and ovine mitral valve chordae tendineae

The mitral valve (MV) is a highly complex cardiac valve consisting of an annulus, anterior and posterior leaflets, chordae tendineae (chords) and two papillary muscles. The chordae tendineae mechanics play a pivotal role in proper MV function: the chords help maintain proper leaflet coaptation and rupture of the chordae tendineae due to disease or aging can lead to mitral valve insufficiency. Therefore, the aim of this study was to characterize the mechanical properties of aged human and ovine mitral chordae tendineae. The human and ovine chordal specimens were categorized by insertion location (i.e., marginal, basal and strut) and leaflet type (i.e., anterior and posterior). The results show that human and ovine chords of differing types vary largely in size but do not have significantly different elastic and failure properties. The excess fibrous tissue layers surrounding the central core of human chords added thickness to the chords but did not contribute to the overall strength of the chords. In general, the thinner marginal chords were stiffer than the thicker basal and strut chords, and the anterior chords were stiffer and weaker than the posterior chords. The human chords of all types were significantly stiffer than the corresponding ovine chords and exhibited much lower failure strains. These findings can be explained by the diminished crimp pattern of collagen fibers of the human mitral chords observed histologically. Moreover, the mechanical testing data was modeled with the nonlinear hyperelastic Ogden strain energy function to facilitate accurate computational modeling of the human MV.

A triphasic constrained mixture model of engineered tissue formation under in vitro dynamic mechanical conditioning

While it has become axiomatic that mechanical signals promote in vitro engineered tissue formation, the underlying mechanisms remain largely unknown. Moreover, efforts to date to determine parameters for optimal extracellular matrix (ECM) development have been largely empirical. In the present work, we propose a two-pronged approach involving novel theoretical developments coupled with key experimental data to develop better mechanistic understanding of growth and development of dense connective tissue under mechanical stimuli. To describe cellular proliferation and ECM synthesis that occur at rates of days to weeks, we employ mixture theory to model the construct constituents as a nutrient-cell-ECM triphasic system, their transport, and their biochemical reactions. Dynamic conditioning protocols with frequencies around 1 Hz are described with multi-scale methods to couple the dissimilar time scales. Enhancement of nutrient transport due to pore fluid advection is upscaled into the growth model, and the spatially dependent ECM distribution describes the evolving poroelastic characteristics of the scaffold-engineered tissue construct. Simulation results compared favorably to the existing experimental data, and most importantly, distinguish between static and dynamic conditioning regimes. The theoretical framework for mechanically conditioned tissue engineering (TE) permits not only the formulation of novel and better-informed mechanistic hypothesis describing the phenomena underlying TE growth and development, but also the exploration/optimization of conditioning protocols in a rational manner.

A meso-scale layer-specific structural constitutive model of the mitral heart valve leaflets

Fundamental to developing a deeper understanding of pathophysiological remodeling in mitral valve (MV) disease is the development of an accurate tissue-level constitutive model. In the present work, we developed a novel meso-scale (i.e. at the level of the fiber, 10-100 μm in length scale) structural constitutive model (MSSCM) for MV leaflet tissues. Due to its four-layer structure, we focused on the contributions from the distinct collagen and elastin fiber networks within each tissue layer. Requisite collagen and elastin fibrous structural information for each layer were quantified using second harmonic generation microscopy and conventional histology. A comprehensive mechanical dataset was also used to guide model formulation and parameter estimation. Furthermore, novel to tissue-level structural constitutive modeling approaches, we allowed the collagen fiber recruitment function to vary with orientation. Results indicated that the MSSCM predicted a surprisingly consistent mean effective collagen fiber modulus of 162.72 MPa, and demonstrated excellent predictive capability for extra-physiological loading regimes. There were also anterior-posterior leaflet-specific differences, such as tighter collagen and elastin fiber orientation distributions (ODF) in the anterior leaflet, and a thicker and stiffer atrialis in the posterior leaflet. While a degree of angular variance was observed, the tight valvular tissue ODF also left little room for any physically meaningful angular variance in fiber mechanical responses. Finally, a novel fibril-level (0.1-1 μm) validation approach was used to compare the predicted collagen fiber/fibril mechanical behavior with extant MV small angle X-ray scattering data. Results demonstrated excellent agreement, indicating that the MSSCM fully captures the tissue-level function. Future utilization of the MSSCM in computational models of the MV will aid in producing highly accurate simulations in non-physiological loading states that can occur in repair situations, as well as guide the form of simplified models for real-time simulation tools.

A novel role of sympathetic activity in regulating mitral valve prolapse

BACKGROUND

Increased sympathetic activity, commonly reported in mitral valve prolapse, indicates that the sympathetic nervous system might play an important role in regulating mitral interstitial cells. Hence, the aim of this study is to determine the level and pattern of adrenergic receptors expressed in human mitral valve leaflets and to investigate the effect of norepinephrine on physiologic behaviors of mitral interstitial cells.

METHODS AND RESULTS

Immunohistochemistry displayed significantly increased expressions of β1, β2, and α1 adrenergic receptors in mitral valve prolapse. Norepinephrine was found to activate the phenotype of interstitial cells with increased α-SMA expression (2.26 fold). In synthesis, norepinephrine downregulated levels of mRNA for type I to type III collagen in ratio, but increased the elastin gene transcription and glycosaminoglycan levels in valve interstitial cells greatly. In view of the extracellular matrix remodel, sympathetic effects presented catabolic metabolism displaying significantly increased expressions of total, secretory and active MMP-2 protein (matrix metalloproteinase-2), as well as MMP-9 protein. Diminished MMP inhibitor expression, TIMP2, also could reflect this effect in the norepinephrine medium.

CONCLUSIONS

A novel role for the sympathetic effect in influencing physiologic behaviors in mitral interstitial cells was identified. It is indicated that sympathetic activity could promote myxomatous degeneration in mitral valve prolapse, propagating the disease severity, which might identify potential therapeutic targets.

Clinical and pathological features of degenerative mitral valve disease: billowing mitral leaflet versus fibroelastic deficiency

PURPOSE

Degenerative mitral valve disease is distinguished with billowing mitral leaflet (BML) or fibroelastic deficiency (FED). The purpose of this study is to evaluate the clinical characteristics and the pathohistological differences between BML and FED.

METHODS

A total of 73 patients who diagnosed as degenerative mitral valve disease pathologically after mitral valve surgery for severe mitral regurgitation were enrolled. On the basis of echocardiographic features and gross appearances, they were classified as BML (9 cases) and FED (64 cases).

RESULTS

In the BML group, multiple segments of the leaflet showed billowing with elongated chordae. Therefore excessive valve tissue needed to be removed by multiple resection and suture. The FED patients had focal myxomatous changes with ruptured chordae, a single resection and suture was frequently employed. In pathological examination, the valve thickness of the BML was nearly twice as thick as the FED, and the mucopolysaccharide accumulation of the Spongiosa in the BML was over 50%, while 30% in the FED.

CONCLUSION

BML presents the characteristic valve thickening due to its abnormal production of mucopolysaccharide. Since excessive tissue was voluminous in the BML, high-grade plasty techniques, such as combination of multiple resection and chordal reconstruction were required.

Structural analysis of chordae tendineae in degenerative disease of the mitral valve

BACKGROUND

Degenerative disease of the mitral valve (DDMV) is always accompanied by lengthening and/or rupture of chordae tendineae. However, the mechanisms and the mode of chordal rupture remain controversial, and the pathologic anatomy of the apparently healthy chordae has mostly been overlooked. We analyze the structural aspects of both ruptured and intact chordae tendineae in DDMV.

METHODS AND RESULTS

Structural and ultrastructural microscopic analyses indicate that both the extracellular matrix and the interstitial cells are severely affected. Degenerative chordae show alterations in the synthesis and deposition of collagen and elastin, disorganization of collagen bundles and rupture of collagen fibres, accumulation of proteoglycans and of cellular and vesicular remnants, and cell transformation into a myofibroblast phenotype. Structural disruption makes the spongiosa and the dense collagenous core separate and break. Degeneration of the chordae is segmental, affecting both chordae that are clearly abnormal, and chordae that appear healthy on visual inspection.

CONCLUSIONS

Changes in both matrix synthesis and degradation disturb the ordered collagen arrangement and modify the structural and physical properties of the chordae. Progressive structural disruption of the diseased chordae is the cause of chordal rupture. Mitral surgery corrects the damage, but the underlying causes of DDMV are not corrected. Thus, progression of the disease and affectation of additional chordae may be at the basis of the late complications and the recurrent mitral regurgitation which occurs several years after surgery. Our results indicate that a more aggressive approach to surgery may be needed.

The mechanobiology of mitral valve function, degeneration, and repair

In degenerative valve disease, the highly organized mitral valve leaflet matrix stratification is progressively destroyed and replaced with proteoglycan rich, mechanically inadequate tissue. This is driven by the actions of originally quiescent valve interstitial cells that become active contractile and migratory myofibroblasts. While treatment for myxomatous mitral valve disease in humans ranges from repair to total replacement, therapies in dogs focus on treating the consequences of the resulting mitral regurgitation. The fundamental gap in our understanding is how the resident valve cells respond to altered mechanical signals to drive tissue remodeling. Despite the pathological similarities and high clinical occurrence, surprisingly little mechanistic insight has been gleaned from the dog. This review presents what is known about mitral valve mechanobiology from clinical, in vivo, and in vitro data. There are a number of experimental strategies already available to pursue this significant opportunity, but success requires the collaboration between veterinary clinicians, scientists, and engineers.

Biomechanical assessment of surgical repair of the mitral valve

Repair of the mitral valve is defined (loosely) as a procedure that alters the valve structure, without replacement, enabling the natural valve itself to continue to perform under the physical conditions to which it is exposed. As the mitral valve is driven by flow and pressure, it should be feasible to analyse and assess its function, failure and repair as a mechanical system. This article reviews the current state of mechanical evaluation of surgical repairs of the failed mitral valve of the heart. This review describes the anatomy and physiology of the mitral valve, followed by the failure of the mitral valve from a mechanical point of view. The surgical methods used to repair failed valves are introduced, while the use of engineering analysis to aid understanding of mitral valve repair is also reviewed. Finally, a section on recommendations for development and future uses of engineering techniques to surgical repair are presented.

Design and validation of a novel splashing bioreactor system for use in mitral valve organ culture

Previous research in our lab suggested that heart valve tissues cultured without mechanical stimulation do not retain their in vivo microstructure, i.e., cell density decreased within the deep tissue layers and increased at the periphery. In this study, a splashing rotating bioreactor was designed to apply mechanical stimulation to a mitral valve leaflet segment. Porcine valve segments (n = 9-10 per group) were cultured in the bioreactor for 2 weeks (dynamic culture), negative controls were cultured without mechanical stimulation (static culture), and baseline controls were fresh uncultured samples. Overall changes in cellularity and extracellular matrix (ECM) structure were assessed by H&E and Movat pentachrome stains. Tissues were also immunostained for multiple ECM components and turnover mediators. After 2 weeks of culture, proliferating cells were distributed throughout the tissue in segments cultured in the bioreactor, in contrast to segments cultured without mechanical stimulation. Most ECM components, especially collagen types I and III, better maintained normal expression patterns and magnitudes (as found in baseline controls) over 2 weeks of dynamic organ culture compared to static culture. Lack of mechanical stimulation changed several aspects of the tissue microstructure, including the cell distribution and ECM locations. In conclusion, mechanical stimulation by the bioreactor maintained tissue integrity, which will enable future in vitro investigation of mitral valve remodeling.

The effects of mitral regurgitation alone are sufficient for leaflet remodeling

BACKGROUND

Although chronic mitral regurgitation results in adverse left ventricular remodeling, its effect on the mitral valve leaflets per se is unknown. In a chronic ovine model, we tested whether isolated mitral regurgitation alone was sufficient to remodel the anterior mitral leaflet.

METHODS AND RESULTS

Twenty-nine sheep were randomized to either control (CTRL, n=11) or experimental (HOLE, n=18) groups. In HOLE, a 2.8- to 4.8-mm diameter hole was punched in the middle scallop of the posterior mitral leaflet to create "pure" mitral regurgitation. At 12 weeks, the anterior mitral leaflet was analyzed immunohistochemically to assess markers of collagen and elastin synthesis as well as matrix metalloproteinases and proteoglycans. A semiquantitative grading scale for characteristics such as intensity and delineation of stain between layers was used to quantify differences between HOLE and CTRL specimens across the heterogeneous leaflet structure. At 12 weeks, mitral regurgitation grade was greater in HOLE versus CTRL (3.0+/-0.8 versus 0.4+/-0.4, P<0.001). In HOLE anterior mitral leaflet, saffron-staining collagen (Movat) decreased, consistent with an increase in matrix metalloproteases throughout the leaflet. Type III collagen expression was increased in the midleaflet and free edge and expression of prolyl-4-hydroxylase (indicating collagen synthesis) was increased in the spongiosa layer. The proteoglycan decorin, also involved in collagen fibrillogenesis, was increased compared with CTRL (all P</=0.05).

CONCLUSIONS

In HOLE anterior mitral leaflet, the increased expression of proteins related to collagen synthesis and matrix degradation suggests active matrix turnover. These are the first observations showing that regurgitation alone can stimulate mitral leaflet remodeling. Such leaflet remodeling needs to be considered in reparative surgical techniques.

Overexpression of transforming growth factor-beta 1 in the valvular fibrosis of chronic rheumatic heart disease

For the purpose of determining the pathogenic role of transforming growth factor-beta1 (TGF-beta 1) in the mechanism of chronic rheumatic heart disease, we evaluated the expression of TGF-beta 1, proliferation of myofibroblasts, and changes in extracellular matrix components including collagen and proteoglycan in 30 rheumatic mitral valves and in 15 control valves. High TGF-beta 1 expression was identified in 21 cases (70%) of rheumatic mitral valves, whereas only 3 cases (20%) of the control group showed high TGF-beta 1 expression (p<0.001). Additionally, increased proliferation of myofibroblasts was observed in the rheumatic valves. High TGF-beta1 expression positively correlated with the proliferation of myofibroblasts (p=0.004), valvular fibrosis (p<0.001), inflammatory cell infiltration (p=0.004), neovascularization (p=0.007), and calcification (p<0.001) in the valvular leaflets. The ratio of proteoglycan to collagen deposition inversely correlated with TGF-beta 1 expression in mitral valves (p=0.040). In conclusion, an ongoing inflammatory process, the expression of TGF-beta 1, and proliferation of myofibroblasts within the valves have a potential role in the valvular fibrosis, calcification, and changes in the extracellular matrix that lead to the scarring sequelae of rheumatic heart disease.

Mitral valvular interstitial cells demonstrate regional, adhesional, and synthetic heterogeneity

BACKGROUND/AIMS

Because various regions of the mitral valve contain distinctive extracellular matrix enabling the tissues to withstand diverse mechanical environments, we investigated phenotype and matrix production of porcine valvular interstitial cells (VICs) from different regions.

METHODS

VICswere isolated from the chordae (MCh), the center of the anterior leaflet (AlCtr), and the posterior leaflet free edge (PlFree), then assayed for metabolic, growth, and adhesion rates; collagen and glycosaminoglycan (GAG) production, and phenotype using biochemical assays, flow cytometry, and immunocytochemistry.

RESULTS

The AlCtr VICs exhibited the fastest metabolism but slowest growth. PlFree cells grew the fastest, but demonstrated the least smooth muscle alpha-actin, vimentin, and internal complexity. AlCtr VICs secreted less collagen into the culture medium but more 4-sulfated GAGs than other cells. Adhesion-based separation resulted in altered secretion of sulfated GAGs by MCh and AlCtr cells but not by the PlFree cells.

CONCLUSIONS

VICs isolated from various regions of the mitral valve demonstrate phenotypic differences in culture, corresponding to the ability of the mitral valve to accommodate the physical stresses or altered hemodynamics that occur with injury or disease. Further understanding of VIC and valve mechanobiology could lead to novel medical or tissue engineering approaches to treat valve diseases.

The Role of Collagen Cross-Links in Biomechanical Behavior of Human Aortic Heart Valve Leaflets—Relevance for Tissue Engineering

A major challenge in tissue engineering of functional heart valves is to determine and mimic the dominant tissue structures that regulate heart valve function and in vivo survival. In native heart valves, the anisotropic matrix architecture assures sustained and adequate functioning under high-pressure conditions. Collagen, being the main load-bearing matrix component, contributes significantly to the biomechanical strength of the tissue. This study investigates the relationship between collagen content, collagen cross-links, and biomechanical behavior in human aortic heart valve leaflets and in tissue-engineered constructs. In the main loading direction (circumferential) of native valve leaflets, a significant positive linear correlation between modulus of elasticity and collagen cross-link concentration was found, whereas no correlation between modulus of elasticity and collagen content was found. Similar findings were observed in tissue-engineered constructs, where cross-link concentration was higher for dynamically strained constructs then for statically cultured controls. These findings suggest a dominant role for collagen cross-links over collagen content with respect to biomechanical tissue behavior in human heart valve leaflets. They further suggest that dynamic tissue straining in tissue engineering protocols can enhance cross-link concentration and biomechanical function.

Planar biaxial creep and stress relaxation of the mitral valve anterior leaflet

A fundamental assumption in mitral valve (MV) therapies is that a repaired or replaced valve should mimic the functionality of the native valve as closely as possible. Thus, improvements in valvular treatments are dependent on the establishment of a complete understanding of the function and mechanical properties of the native normal MV. In a recent study [Grashow et al. ABME 34(2), 2006] we demonstrated that the planar biaxial stress-strain relationship of the MV anterior leaflet (MVAL) exhibited minimal hysteresis and a stress-strain response independent of strain rate, suggesting that MVAL could be modeled as a "quasi-elastic" material. The objective of our current study was to expand these results to provide a more complete picture of the time-dependent mechanical properties of the MVAL. To accomplish this, biaxial stress-relaxation and creep studies were performed on porcine MVAL specimens. Our primary finding was that while the MVAL leaflet exhibited significant stress relaxation, it exhibited negligible creep over the 3-h test. These results furthered our assertion that the MVAL functionally behaves not as a linear or non-linear viscoelastic material, but as an anisotropic quasi-elastic material. These results appear to be unique in the soft tissue literature; suggesting that valvular tissues are unequalled in their ability to withstand significant loading without time-dependent material effects. Moreover, insight into these specialized characteristics can help guide and inform efforts directed toward surgical repair and engineered valvular tissue replacements.

The Relation Between Collagen Fibril Kinematics and Mechanical Properties in the Mitral Valve Anterior Leaflet

We have recently demonstrated that the mitral valve anterior leaflet (MVAL) exhibited minimal hysteresis, no strain rate sensitivity, stress relaxation but not creep (Grashow et al., 2006, Ann Biomed Eng., 34(2), pp. 315–325;Grashow et al., 2006, Ann Biomed. Eng., 34(10), pp. 1509–1518). However, the underlying structural basis for this unique quasi-elastic mechanical behavior is presently unknown. As collagen is the major structural component of the MVAL, we investigated the relation between collagen fibril kinematics (rotation and stretch) and tissue-level mechanical properties in the MVAL under biaxial loading using small angle X-ray scattering. A novel device was developed and utilized to perform simultaneous measurements of tissue level forces and strain under a planar biaxial loading state. Collagen fibril D-period strain (εD) and the fibrillar angular distribution were measured under equibiaxial tension, creep, and stress relaxation to a peak tension of 90N∕m. Results indicated that, under equibiaxial tension, collagen fibril straining did not initiate until the end of the nonlinear region of the tissue-level stress-strain curve. At higher tissue tension levels, εD increased linearly with increasing tension. Changes in the angular distribution of the collagen fibrils mainly occurred in the tissue toe region. Using εD, the tangent modulus of collagen fibrils was estimated to be 95.5±25.5MPa, which was ∼27 times higher than the tissue tensile tangent modulus of 3.58±1.83MPa. In creep tests performed at 90N∕m equibiaxial tension for 60min, both tissue strain and εD remained constant with no observable changes over the test length. In contrast, in stress relaxation tests performed for 90minεD was found to rapidly decrease in the first 10min followed by a slower decay rate for the remainder of the test. Using a single exponential model, the time constant for the reduction in collagen fibril strain was 8.3min, which was smaller than the tissue-level stress relaxation time constants of 22.0 and 16.9min in the circumferential and radial directions, respectively. Moreover, there was no change in the fibril angular distribution under both creep and stress relaxation over the test period. Our results suggest that (1) the MVAL collagen fibrils do not exhibit intrinsic viscoelastic behavior, (2) tissue relaxation results from the removal of stress from the fibrils, possibly by a slipping mechanism modulated by noncollagenous components (e.g. proteoglycans), and (3) the lack of creep but the occurrence of stress relaxation suggests a “load-locking” behavior under maintained loading conditions. These unique mechanical characteristics are likely necessary for normal valvular function.

Biaixal stress-stretch behavior of the mitral valve anterior leaflet at physiologic strain rates

Characterization of the mechanical properties of the native mitral valve leaflets at physiological strain rates is a critical step in improving our understanding of MV function and providing experimental data for dynamic constitutive models. We explored, for the first time, the effects of strain rate (from quasi-static to physiologic) on the biaxial mechanical properties of the native mitral valve anterior leaflet (MVAL). A novel high-speed biaxial testing device was developed, capable of achieving in vitro strain rates reported for the MVAL (Sacks et al., Ann. Biomed. Eng. 30(10):1280-1290, 2002). Porcine MVAL specimens were loaded to physiological load levels with cycle periods of 15, 1, 0.5, 0.1, and 0.05 s. The resulting loading stress-strain responses were found to be remarkably independent of strain rate. The hysteresis, defined as the fraction of the membrane strain energy between the loading and unloading curves tension-areal stretch curves, was low (approximately 12%) and did not vary with strain rate. The results of the present work indicated that MVAL tissues exhibit complete strain rate insensitivity at and below physiological strain rates under physiological loading conditions. These novel results suggest that experimental tests utilizing quasi-static strain rates are appropriate for constitutive model development for mitral valve tissues. The mechanisms underlying this quasi-elastic behavior are as yet unknown, but are likely an important functional aspect of native mitral valve tissues and clearly warrant further study.

Mitral valve stiffening in end-stage heart failure: evidence of an organic contribution to functional mitral regurgitation

OBJECTIVE

Mitral regurgitation is a complication for many patients with congestive heart failure. Although this regurgitation is considered purely functional, we hypothesize that the alterations in cardiac geometry and function induce dysfunctional remodeling of the mitral valve, which can be demonstrated by alterations in the material behavior of the leaflets and chordae.

METHODS

Mitral leaflets and chordae from 23 valves from transplant recipient hearts (11 with dilated and 12 with ischemic cardiomyopathy) and from 21 normal valves (from autopsy) were mechanically tested.

RESULTS

Radially oriented anterior mitral leaflet strips from failing hearts were 61% stiffer and 23% less viscous on average than those from autopsy control hearts. The mean stiffness of circumferentially oriented anterior leaflet strips was 50% higher than that of control hearts. Leaflet extensibility was reduced 35% overall. Likewise, the failing heart chordae were an average of 16% stiffer (all P < or = .05).

CONCLUSIONS

Mitral valves in congestive heart failure have significantly altered mechanics that suggest that the tissue is permanently distended and fibrotic and might be unable to stretch sufficiently to cover the valve orifice. These material changes in the valve tissues accompany the biochemical alterations in extracellular matrix composition that we have previously reported. Our finding of leaflet and chordal remodeling suggests that mitral regurgitation in patients experiencing heart failure might not be purely functional and that these mitral valves should not be considered normal. Moreover, there are implications for strategies of mitral valve surgery or percutaneous approaches in this patient population.

The relationship of normal and abnormal microstructural proliferation to the mitral valve closure sound

BACKGROUND

Many diseases that affect the mitral valve are accompanied by the proliferation or degradation of tissue microstructure. The early acoustic detection of these changes may lead to the better management of mitral valve disease. In this study, we examine the nonstationary acoustic effects of perturbing material parameters that characterize mitral valve tissue in terms of its microstructural components. Specifically, we examine the influence of the volume fraction, stiffness and splay of collagen fibers as well as the stiffness of the nonlinear matrix in which they are embedded.

METHODS AND RESULTS

To model the transient vibrations of the mitral valve apparatus bathed in a blood medium, we have constructed a dynamic nonlinear fluid-coupled finite element model of the valve leaflets and chordae tendinae. The material behavior for the leaflets is based on an experimentally derived structural constitutive equation. The gross movement and small-scale acoustic vibrations of the valvular structures result from the application of physiologic pressure loads. Material changes that preserved the anisotropy of the valve leaflets were found to preserve valvular function. By contrast, material changes that altered the anisotropy of the valve were found to profoundly alter valvular function. These changes were manifest in the acoustic signatures of the valve closure sounds. Abnormally, stiffened valves closed more slowly and were accompanied by lower peak frequencies.

CONCLUSION

The relationship between stiffness and frequency, though never documented in a native mitral valve, has been an axiom of heart sounds research. We find that the relationship is more subtle and that increases in stiffness may lead to either increases or decreases in peak frequency depending on their relationship to valvular function.

Molecular and functional characterization of ovine cardiac valve-derived interstitial cells in primary isolates and cultures

At present the involvement of cardiac valve interstitial cells (VICs) in growth, repair, and tissue engineering is understudied. Therefore, this study aims at characterizing ovine VICs in order to provide a solid base for tissue engineering of heart valves. Ovine ICs of the four heart valves were isolated by the explant outgrowth method and expanded in vitro up to passage 5. Vimentin and collagen I gene expression from freshly isolated or cultured ICs was measured by reverse transcriptase-polymerase chain reaction. Immunocytochemical stainings of vimentin, alpha-smooth muscle actin (ASMA), smooth muscle myosin, and procollagen I were performed on aortic VICs. In addition, migration and extracellular matrix deposition were studied in vitro in aortic VICs. ICs show stable vimentin and collagen I expression in culture. Expression is approximately doubled in cultured ICs compared with fresh isolates. More than 95% of ICs in each passage stain for vimentin and procollagen I. Freshly isolated ICs are ASMA and myosin negative, but ICs in culture partially stain for these contractile markers. ICs have stable matrix production for up to five passages, associated with stable migration of the cells. We conclude that ovine valve interstitial cells undergo phenotypic modulation to activated myofibroblasts under culture conditions but retain stable matrix production.

Apparently normal mitral valves in patients with heart failure demonstrate biochemical and structural derangements

OBJECTIVES

This study assessed apparently normal mitral valves from patients with congestive heart failure (CHF) using biochemical and echocardiographic measures of extracellular matrix (ECM) and anatomy.

BACKGROUND

Mitral regurgitation (MR) is frequently found in patients with CHF. This MR is considered purely functional, yet animal studies suggest that altered left ventricular (LV) function leads to increased cellularity and fibrosis of the mitral valve. Therefore, we hypothesized that patients with CHF might have partly organic MR, via dysfunctional valvular remodeling.

METHODS

Mitral valves from transplant recipient hearts of patients with CHF (23 dilated, 14 ischemic) were analyzed for deoxyribonucleic acid (DNA), collagen, glycosaminoglycan (GAG), and water concentrations and compared with autopsy controls. Cardiac dimensions and functional parameters (measured from recent echocardiograms) were compared with biochemical parameters using a repeated measures generalized linear model.

RESULTS

The mitral valves in CHF had up to 78% more DNA (p <0.03), 59% more GAGs (p <0.02), and 15% more collagen (p <0.007), but 7% less water (p <0.05) than normal. The absence of anterior leaflet redundancy was associated with these deranged biochemical measures (p <0.03). Associations were found between leaflet thickness and DNA concentration (+, p=0.003), annular diameter and chordal collagen (+, p=0.03), and water concentration and both left atrial diameter (-, p=0.008) and LV collagen concentration (-, p=0.04).

CONCLUSIONS

Mitral valves in CHF are biochemically different from normal, with ECM changes that are influenced by the altered cardiac dimensions. This remodeling suggests that MR in patients with CHF may not be purely functional, and that these valves are not "normal."

Nonhomogeneous Deformation in the Anterior Leaflet of the Mitral Valve

In the mitral valve, regional variations in structure and material properties combine to affect the biomechanics of the entire valve. Previous biaxial testing has shown that mitral valve leaflet tissue is highly extensible, and exhibits nonlinear, anisotropic material properties. In this study, experimental measurements of mitral valve leaflet deformation under quasi-static pressure loading were performed on isolated porcine hearts. Biplane video images of markers placed on the anterior leaflet surface were used to reconstruct the 3D position of the markers at several pressure levels over the physiological range. A least-squares finite-element method was used to fit parametric models to the markers and to calculate the deformation over the surface. The results showed that the leaflet deformations were anisotropic, exhibiting a large nonhomogeneous radial stretch and a small circumferential stretch. This information can be used to better understand how the valve deforms under physiological loading, and to help design treatments for valve problems, such as mitral regurgitation.

The Structure and Mechanical Properties of the Mitral Valve Leaflet-Strut Chordae Transition Zone

Biaxial testing, histological measurements and theoretical continuum mechanics modeling were employed to investigate the structure and mechanical properties of the mitral valve leaflet-strut chordae transition zone (LCT). The results showed that geometry changes and collagen fiber angle distribution contribute to variations in mechanical properties in the LCT zone. A simple three-coefficient exponential constitutive law was able to simulate the variation in stress-stretch behavior in the LCT zone by spatially varying a single coefficient and incorporating collagen fiber angle and degree of alignment. This quantitative information can greatly improve the predictions from biomechanical valve models by incorporating regional variations of structure and properties in the mitral leaflet-chordae tendineae system. These data provide the foundation for a computational model for studying stress distributions before and following chordal rupture, which may indicate the underlying reasons for the development of valve insufficiency in patients.

Esophageal dysmotility and acid sensitivity in patients with mitral valve prolapse and chest pain

Mitral valve prolapse (MVP) patients often experience non-cardiac chest pain. The aims of this study were to determine, in patients with non-cardiac chest pain: (i) whether esophageal dysmotility is more common in patients with MVP than in patients without MVP; and (ii) if acid sensitivity is an important cause of the chest pain in MVP patients. Esophageal manometry and acid perfusion testing were performed in 277 consecutive patients with non-cardiac chest pain. Patients with MVP (13 female, one male; mean age 49 years) were more likely (P = 0.01) to have esophageal dysmotility, while acid perfusion was less likely (P < 0.05) to provoke their chest pain, than in patients without MVP. The most common esophageal motor abnormalities detected in patients with and without MVP were diffuse esophageal spasm (prevalence, 57%) and non-specific motor disorder (prevalence, 9%), respectively. This study, the first large prospective series examining possible esophageal sensorimotor correlates of chest pain in MVP patients, demonstrates that in the absence of a cardiac cause for chest pain, a specific esophageal motility disorder should be excluded, rather than assuming the chest pain is likely to be due to acid sensitivity.

Glycosaminoglycan profiles of myxomatous mitral leaflets and chordae parallel the severity of mechanical alterations

OBJECTIVES

This biochemical study compared the extracellular matrix of normal mitral valves and myxomatous mitral valves with either unileaflet prolapse (ULP) or bileaflet prolapse (BLP).

BACKGROUND

Myxomatous mitral valves are weaker and more extensible than normal valves, and myxomatous chordae are more mechanically compromised than leaflets. Despite histological evidence that glycosaminoglycans (GAGs) accumulate in myxomatous valves, previous biochemical analyses have not adequately examined the different GAG classes.

METHODS

Leaflets and chordae from myxomatous valves (n = 41 ULP, 31 BLP) and normal valves (n = 27) were dried, dissolved, and assayed for deoxyribonucleic acid, collagen, and total GAGs. Specific GAG classes were analyzed with selective enzyme digestions and fluorophore-assisted carbohydrate electrophoresis.

RESULTS

Biochemical changes were more pronounced in chordae than in leaflets. Myxomatous leaflets and chordae had 3% to 9% more water content and 30% to 150% higher GAG concentrations than normal. Collagen concentration was slightly elevated in the myxomatous valves. Chordae from ULP had 62% more GAGs than those from BLP, primarily from elevated levels of hyaluronan and chondroitin-6-sulfate.

CONCLUSIONS

The GAG classes elevated in the myxomatous chordae are associated with matrix microstructure and elastic fiber deficiencies and may influence the hydration-related "floppy" nature of these tissues. These abnormalities may be related to the reported mechanical weakness of myxomatous chordae. The biochemical differences between ULP and BLP confirm previous mechanical and echocardiographic distinctions.

The challenge of defining normality for human mitral and aortic valves: geometrical and compositional analysis

Advances in digital imaging technology and in tools for obtaining detailed quantitation of morphological features have facilitated a new approach to pathological assessment of many tissues, including heart valves. In the present study, we quantitatively examined the tissue geometry and composition of structurally normal mitral and aortic valves removed at autopsy or surgery from patients aged 15-84 years. Through univariate analyses of quantitative variables, we have determined which features change distinctively with age. The anterior mitral valve leaflet (AMV) underwent a statistically significant decrease in area of the valve proper and an increase in the number of superficial tissue accumulations called onlays as the patients aged. For all geometric variables measured in the aortic valve, increases were seen with age, leading to a thicker valve, with enlargement of the valve proper and onlays, and with changes in the number of onlays. The mitral valve proper, composed largely of collagen in younger individuals, showed significant increases in glycosaminoglycans and elastin and a relative decrease in collagen with age. The compositional characteristics of the aortic valve proper were similar to those of the mitral valve, with a dramatic relative increase in elastin and a decrease in collagen with age. Valve onlays, when present, were similar in composition to the valve proper for both valves. Our findings regarding normal valve tissue composition, when taken in the context of geometrical features, and together with evidence of age-related changes in the relative amounts of specific constituents, provide a basis on which to analyze human heart valves affected by various known or putative diseases.

N-Linked glycans of proteins from mitral valves of normal pigs and pigs affected by endocardiosis

Endocardiosis, a degenerative and dystrophic process affecting cardiac valves and described in many mammalian species, is characterized by the accumulation of glycosaminoglycans, in particular hyaluronic acid, in the extracellular matrix. The glycoprotein patterns of pig mitral valves in normal animals and animals affected by endocardiosis were investigated. A different N-linked glycosylation pattern of glycoproteins was detected in affected valves compared with normal ones. In either normal or pathological species, the detected N-linked glycans were of the complex type. However, in samples from affected valves, sialic acid showed a prevalence of the alpha2,6 linkage to the galactosyl residue, whereas in normal samples the most frequent linkage was of the alpha2,3 type. In normal valves, the majority of complex oligosaccharides presented two outer branches with different degrees of fucosylation and sialylation, whereas in pathological samples we noted an increased number of glycans having up to four outer branches.

Species differences in expression of angiotensin II receptors and angiotensin-converting enzyme in human, canine and rat mitral valve leaflets

In normal valvular collagen turnover in the rat, angiotensin (Ang) II and angiotensin-converting enzyme (ACE) seem to be involved. In common human and canine valvular diseases, changes in valvular collagen play a pathogenetic role and the valvular renin-angiotensin system is therefore of particular interest in these species. Healthy mitral valve leaflets and adjacent left ventricular myocardium were taken from five rats and five dogs immediately after euthanasia, and from five humans at autopsy. The valvular and myocardial Ang II receptors and ACE were detected and measured by quantitative autoradiography. In rat valves, high levels of Ang II receptors and ACE were found. In human and canine valves, insignificant levels were found. Significant myocardial levels of Ang II receptors and ACE were found only in the rat. The study demonstrated major species differences regarding the level of valvular and myocardial Ang II receptors and ACE in man, dog and rat. The lack of valvular Ang II receptors and ACE in man and dog, suggest that the renin-angiotensin system plays a minor, if any, role in the physiological valvular collagen formation in these two species. The findings in humans, however, need to be confirmed using fresh material.

Immunolocalization of elastin, collagen type I and type III, fibronectin, and vitronectin in extracellular matrix components of normal and myxomatous mitral heart valve chordae tendineae

The identification, distribution, and localization of matrix proteins and the proteins associated with normal and degenerated elastic fibers and collagen fibrils of myxomatous chordae tendineae were studied with immunoelectron microscopy. Ultrathin sections of L R White-embedded tissue were processed by indirect immunogold cytochemistry using primary antibodies against human alpha elastin, collagen types I and III, fibronectin, and vitronectin. In normal chordae tendineae, alpha elastin antibody heavily labeled the elastic fibers in spongiosa and fibrosa, but microfibrils around them were not labeled. Antibodies to collagen type I, collagen type III, and fibronectin all labeled the collagen fibers and microfibrils in the spongiosa. Fibronectin antibody labeling was higher than collagen type III, whereas labeling by anticollagen type I was lower. Intense labeling by vitronectin was observed on the microfibrils in the spongiosa and on electron-dense material around elastic fibers in the spongiosa and fibrosa. In myxomatous chordae tendineae, alpha elastin antibody heavily labeled degenerated elastic fibers, previously unidentified reticulated structures, and other moderately electron-dense material, both in the spongiosa and in the fibrosa, but not the electron-dense fibrous material around them. Antibodies to collagen types I, III, and fibronectin heavy labeled electron-dense aggregates of fibrous material. Vitronectin labeling was observed on electron-dense longitudinally running microfibrils and on the electron-dense microfibrils around degenerated elastic fibers.

Ultrastructure abnormalities in proteoglycans, collagen fibrils, and elastic fibers in normal and myxomatous mitral valve chordae tendineae

Normal and myxomatous chordae tendineae were studied using light and electron microscopy, to assess the alterations in the appearance and mutual arrangement of proteoglycans, collagen fibrils, and elastic fibers. Specific staining with ruthenium red and cuprolinic blue in a critical electrolyte concentration mode were used to localize proteoglycans. Fresh tissues were fixed in glutaraldehyde containing the cationic dyes and embedded into Spurr resin. Semithin sections of LR White (London Resin Co., Basingstoke, U.K.)-embedded tissue were used for histochemistry. In normal chordae tendineae, the fibrosa comprised close-packed collagen fibrils intermixed with elastic fibers. These were surrounded by a thin layer of elastic fibers and collagen fibrils, both of which were closely associated with proteoglycans. In myxomatous chordae tendineae, alterations were observed in the connective tissue. Proteoglycans were more abundant and were distributed throughout the tissue. The outermost layer was transformed into an undifferentiated electron-dense mass surrounding the central fibrosa, which contained degraded elastic fibers and collagen fibrils. Collagen fibrils had faint banding or lacked a banding pattern altogether. Spaces between collagen fibrils were occupied by abnormal proteoglycans or proteoglycan aggregates. Elastic fibers showed varying degrees of degeneration and were occasionally replaced by electron-lucent spaces containing microfibrils. Accumulation of abnormal proteoglycan was also observed around degenerated elastic fibres and collagen fibrils.

Altered collagen concentration in mitral valve leaflets: biochemical and finite element analysis

BACKGROUND

Ischemic mitral regurgitation or ventricular wall motion abnormalities will alter the stress distribution in the mitral valve. We hypothesize that in response, the regional collagen concentration will be altered and will significantly impact the stress distribution in the mitral valve.

METHODS

Two sheep served as normal (sham) controls. Two other sheep had coronary ligation resulting in abnormal ventricular wall motion. Four sheep underwent ligation to infarct the posteromedial papillary muscle, resulting in ischemic regurgitation. After 4 or 8 weeks, the mitral valves were excised, and the anterior leaflet sections were subjected to an assay for collagen concentration. Next, in a finite element model, to simulate changes in collagen concentration, the tissue stiffness was increased by 20%, and then decreased by 20%. In another model, the thickness of the tissue was increased by 20%, and then combined with decreased tissue stiffness. Physiologic loading pressures were applied, and leaflet stress, chordal stress, and coaptation results were analyzed.

RESULTS

The average collagen concentration in the normal sheep leaflets was 59.2% (dry weight), 50.6% in the ischemic controls, and 45.8% in the papillary muscle infarct group. Collagen concentration was greatest at the midline and decreased toward the commissures. Increased tissue stiffness resulted in increased leaflet and chordal stresses, as well as reduced coaptation. Decreased stiffness resulted in the opposite. Increased tissue thickness reduced leaflet and chordal stresses, but also reduced coaptation. The combination of increased tissue thickness and decreased stiffness demonstrated the greatest reduction in leaflet and chordal stress, while maintaining normal leaflet coaptation.

CONCLUSIONS

The observed changes may demonstrate an early effort to compensate for increased leaflet stress. Microstructural alterations may demonstrate an early effort to compensate for altered physiologic loading to reduce stress and maintain coaptation. It is crucial in repairing or partially replacing thickened tissue that normal geometry and physiology be restored.

A constitutive law for mitral valve tissue

Biaxial mechanical testing and theoretical continuum mechanics analysis are employed to formulate a constitutive law for cardiac mitral valve anterior and posterior leaflets. A strain energy description is formulated based on the fibrous architecture of the tissue, accurately describing the large deformation, highly nonlinear transversely isotropic material behavior. The results show that a simple three-coefficient exponential constitutive law provides an accurate prediction of stress-stretch behavior over a wide range of deformations. Regional heterogenity may be accommodated by spatially varying a single coefficient and incorporating collagen fiber angle. The application of this quantitative information to mechanical models and bioprosthetic development could provide substantial improvement in the evaluation and treatment of valvular disease, surgery, and replacement.

Abnormalities in elastic fibers and other connective-tissue components of floppy mitral valve

Histologic, immunohistochemical, and ultrastructural studies were performed on 12 floppy mitral valves, 4 mitral valves showing focal myxomatous changes without prolapse, and 3 normal mitral valves. All floppy mitral valves were thickened by deposits of proteoglycans and also showed diverse structural abnormalities in collagen and elastic fibers. From these observations we conclude that (1) the structure of all major components of connective tissue in floppy mitral valves is abnormal; (2) alterations in collagen and accumulations of proteoglycans are nonspecific changes that may be caused by the abnormal mechanical forces to which floppy mitral valves are subjected because of their excessively large surface area; (3) the presence of excessive amounts of proteoglycans may interfere with the normal assembly of collagen and elastic fibers; (4) abnormalities of elastic fibers resemble those in other conditions characterized by structural dilatation or tissue expansion; and (5) alterations in elastin could result from defective formation, increased degradation, or both.

Interstitial cells from the atrial and ventricular sides of the bovine mitral valve respond differently to denuding endocardial injury

The mitral valve has atrial and ventricular sides, each lined by endocardial cells. The valve stroma contains alpha smooth muscle actin positive interstitial cells, collagen, glycosaminoglycans, and elastic tissue. To eliminate the effect of endocardium on wound repair in bovine mitral valve organ culture, the endocardium was removed from both sides of the valve. At 6 days, organ cultures of these preparations revealed surface cells on the ventricular side but not in the atrial side. Ventricular surface cells were negative for Factor VIII-related antigen, and positive for alpha smooth muscle actin. Immunoperoxidase staining for proliferating cell nuclear antigen/cyclin, a marker for cell proliferation, revealed a positive labeling index of (mean +/- standard deviation) 0.08 +/- 0.16% for interstitial cells from the atrial side and 0.14 +/- 0.19% for ventricular side interstitial cells in uncultured preparations (not significant), and 0.44 +/- 0.69% for atrial side interstitial cells and 2.25 +/- 1.64% for ventricular side interstitial cells in the cultured preparations (significant, P < 0.0006). The results suggest that in organ culture, interstitial cells from the ventricular side of the mitral valve respond to a denuding endocardial injury by proliferating and migrating onto the adjacent surface whereas interstitial cells from the atrial side do not. This difference in the response to injury of interstitial cells from the atrial and ventricular sides of the valve may reflect differences in phenotype or may be due to effects of extracellular matrix on interstitial cell behavior. The latter is possible because of differences in the extracellular matrix of the atrial and ventricular sides of the valve.

Collagen types I and III, collagen content, GAGs and mechanical strength of human atherosclerotic plaque caps: span-wise variations

Measurements of total collagen, of the ratio of collagen types III/(I+III) and of sulphated glycosaminoglycans (GAGs) were compared with mechanical strength for individual ulcerated and non-ulcerated human aortic plaque caps and with intima adjacent to the plaques. The distributions of the collagen type ratio were similar for both ulcerated and non-ulcerated plaque caps but different from that of the adjacent intima. The proportions of different collagen types were not related to fracture stress and are thus unlikely to affect the potential to ulcerate. The distributions of the sulphated GAGs showed lower amounts for the plaque caps compared with the nearby intima, with the centres of ulcerated plaque caps having the lowest values. Total collagen had higher values in the peripheries of plaque caps compared with the nearby intima, but was distinctly lower in the centres of ulcerated plaque caps. Plaque caps appeared to require a higher collagen content than adjacent intima to support a given level of mechanical strength, suggesting that while collagen production had occurred in the plaque caps it was not as efficiently organized to resist fracture as a similar amount of collagen in the adjacent intima. Ulcerated plaque caps are notable for much larger transverse (centre vs. periphery) gradients of connective tissue constituents than for non-ulcerated plaque caps. The development of these transverse gradients may be a critical aspect in determining the propensity of a plaque to ulcerate.

Myxoid heart disease: an assessment of extravalvular cardiac pathology in severe mitral valve prolapse

Because of the microscopic features of the affected leaflets in mitral valve prolapse (MVP), myxoid degeneration of the valve is a common pathologic designation applied to this condition. We undertook this study as a means of gaining an insight into the occurrence and prevalence of extravalvular cardiac alterations in hearts with severe MVP. Tissues of 24 hearts with severe myxomatous transformation of the mitral valve as the sole cardiac abnormality were examined. Eighteen of the 24 subjects with severe MVP died suddenly. Only two of these had pathologic evidence of severe mitral insufficiency. Twenty-four normal hearts served as controls. The two groups of hearts came from victims of homicide, suicide, accident, or natural death. Sections of the mitral valve, working myocardium, conduction system, and cardiac nerves and ganglia were studied by routine and special connective tissue and proteoglycan stains. Similar to the findings in severely affected mitral valves, prominent deposits of proteoglycans in neural and conduction tissue readily distinguished hearts with myxomatous valve changes from the control hearts. We conclude that the commonly recognized local derangement of valvular tissue in MVP is but one specific reflection of a more general myxomatous alteration in cardiac connective tissue.

Genetic evidence that mutations in the COL1A1, COL1A2, COL3A1, or COL5A2 collagen genes are not responsible for mitral valve prolapse

DNA markers were used to assess the segregation of genes encoding the collagen types that predominate in the mitral valve (types I, III, and V) in two family pedigrees that are phenotypically different but showed dominantly inherited mitral valve prolapse. The inheritance of these markers was compared with the segregation of the phenotype for mitral valve prolapse in both families. In one family it was shown that the COL1A1, COL1A2, COL3A1, and COL5A2 genes segregated independently of the phenotype; in the other family the results for COL1A1, COL1A2, and COL5A2 were similar but analysis at the COL3A1 locus was not possible. These data indicate that in these families mitral valve prolapse does not arise from a defect in one of these collagen genes.