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Background and Aim of the Study: Mitral valve chordae tendineae are an essential component for correct functioning of the human heart. The microstructural make-up of the chordae is responsible for their tensile properties, and is seen gradually to change with age. However, little is known of the maturation of chordae tendineae and their microstructure., Methods: To examine such maturation, structural changes in chordae tendineae were studied in rats at 1, 3, 7, 15 and 30 days of postnatal life, and in adult rats. Differences in the chordae microstructure of each age group were observed using light microscopy. The collagen fibril crimp period was determined using polarized light microscopy., Results: At day 1 after birth the chordae had yet to develop, and the lateral sides of the mitral valve leaflets were completely attached to the papillary muscles. Chordae developed through the formation of gaps in the leaflet tissue. From day 7 on, numerous chordae were seen. As the chordae matured, crimped collagen fibrils were formed and began to align in a longitudinally packed core with increasing density. The collagen fibril crimp period increased significantly with the age of the animal., Conclusion: Rat chordae tendineae have yet to develop at postnatal day 1. Morphological development and microstructural maturation of the chordae are not completed until adulthood (>30 days). A further understanding of the development of mitral valve chordae tendineae will provide insight for the use of tissue-engineered chordae in surgical repair.

Over the past five years, transcatheter valves have stimulated the attention of physicians, engineers, and investors. Transcatheter valve design and implantation techniques depart from the time-proven features of surgical valves, and this has an important impact on the safety and efficacy of prosthetic valve therapy. Herein is reviewed the performance of transcatheter valve procedures in comparison to surgical valves, together with a summary of the specific design features of several emerging transcatheter valves. How the current and future generation transcatheter valves are likely to impact on patient treatment is also explored.

Background and Aim of the Study: The chordae tendineae, which form an important component of the mitral valve apparatus, experience continuous cyclic loading and are thus well-adapted for effectively storing and dissipating energy. An understanding of their microstructure would be expected to shed light on the mechanism of their remarkable durability., Methods: In these studies, porcine mitral valve chordae from freshly slaughtered pigs were used. Histological samples of Picrosirius Red-stained and Movat's pentachrome-stained chordae were examined with optical microscopy and laser scanning confocal microscopy. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to study the ultrastructure at high magnification., Results: Both, optical microscopy and SEM revealed that the waviness of collagen fibers was uniform across the thickness of the chordae, with no straight fibers in the core. Wavy fibers and fiber bundles were found to be in skewed-register, rather than transverse. Collagen fiber bundles were found to undulate in a three-dimensional path, rather than the planar waveform, as reported previously. TEM showed that different types of chordae had different fibril configurations. Marginal chordae had smaller diameters but a higher fibril density than did basal and strut chordae., Conclusion: The configuration of collagen fibrils in the mitral valve chordae is more complex than initially thought, and different chordae have morphologies that are likely specific to their mechanical role in the mitral apparatus. These findings provide insight into possible improvements for chordal repair surgery, and form a structural basis for accurate computational modeling.

Background and Aim of the Study: The aortic valve cusp is commonly described as a three-layered structure containing circumferentially aligned fiber bundles. Little is known, however, regarding fiber bundle sizes, branching patterns, or how they are connected. This is because previous morphological studies relied primarily on histological sectioning and staining techniques, which tend to affect all of the collagen, regardless of structure or orientation., Methods: To address this problem, a novel system was developed for the visualization and analysis of the intermediate-scale 'mesostructures' of aortic valve cusps. Mesostructures are defined as the branching fiber bundle and membrane structures that make up the valve. This system uses elliptically polarized light to provide contrast between collagen mesostructures without the need for embedding, staining, or other contrast-enhancing techniques. Using this system, high-resolution images of 42 whole porcine aortic valve cusps were acquired in an unloaded (i.e. resting) condition and during application of controlled manipulation. Image-processing algorithms were developed to quantify fiber bundle morphological features and produce detailed maps of the fiber bundle patterns., Results: Fiber bundle sizes and patterns were found to be significantly different for each of the three cusps. The non-coronary cusp had a significantly smaller bundle diameter (0.9 +/- 0.07 mm) than the left and right coronary cusps (1.1 +/- 0.08 mm). The left and non-coronary cusps appeared to be mirror images of each other, whereas the right coronary cusp was self-symmetric. When applying controlled loads to the cusp specimens, thin, overlapping, collagenous membranes were often found which connected the fiber bundles. Interesting pinnate fiber branching patterns were also found., Conclusion: These morphological results were strikingly different than the currently accepted three-layer description, and may provide valuable insight into aortic valve structure-function relationships.

Background and Aim of the Study: Organ cultures maintain cells within their native microstructural environment, and thus offer greater potential for studying tissue disease and remodeling than do monolayer cell cultures or pathological examinations of diseased tissue. To validate an in-vitro heart valve organ culture model, cell viability was examined within valve tissues over sustained culture periods., Methods: Following culture of blocks of valve tissue for 1 to 49 days, cross-sections were cut with a vibratome, stained with a LIVE/DEAD kit, and imaged with confocal microscopy to quantify the number of live and dead cells present., Results: In numerous organ cultures, valvular interstitial cells were found to be viable beyond 30 days. Live cells were abundant in the central region of the valve, but more sparse in the deepest central regions. Dead cells were found mainly on the surface of both fresh tissues and tissues after prolonged culture, with few dead cells occurring centrally., Conclusion: This is the first reported mapping of cell viability within heart valve organ cultures, and results suggest that extended organ culture of valve leaflets is indeed possible. The derived viability staining methods have wide applicability for organ cultures of other tissues as well as tissue-engineered matrices.

Background and Aims of the Study: Testing whole explanted porcine bioprosthetic valves provides information on changes the valves undergo over time in vivo, and may help elucidate a possible mechanism for bioprosthetic valve failure. The study aim was to measure the static pressure/strain response of whole explanted porcine bioprostheses., Methods: Three bioprosthetic valves were explanted prophylactically prior to structural failure. Loading curves (pressure versus strain curves) were generated for these valves using a whole-valve optical strain mapping system. Maximum strain, stiffness, and the occurrence of 'negative compliance' (inverse relationship between cusp strain and applied pressure) were obtained from these curves., Results: All three cusps had very low extensibility (3-10% in the radial direction; 2-4% in the circumferential direction). The non-coronary (NC) cusp was less extensible than the right (RC) or left coronary (LC) cusps in two of the valves. In one valve, the smallest strains were in the RC cusp, where large amounts of calcification were noted. Negative strains were present primarily in one valve., Conclusion: The extensibilities of the three explanted valves were lower than those seen in fresh valves, due to either glutaraldehyde fixation and/or extension of the fibrosal corrugations. Furthermore, the uniqueness of the mechanical properties of the NC cusp previously seen in fresh porcine valves remained through glutaraldehyde fixation and stenting. It is concluded that cusp anisotropy and heterogeneity, as well as the occurrence of negative strains seen in fresh porcine valves, are preserved during the fabrication of porcine bioprostheses.

Background and Aim of the Study: Rupture of chordae tendineae is the main cause of mitral valve insufficiency, and often requires corrective surgery. The precise mechanisms of chordal rupture, however, are unknown., Methods: Failure mechanics were measured in porcine mitral valve chordae (37 anterior marginal, 40 anterior basal, 35 posterior marginal, and 38 posterior basal). Full-length chordae were weighed, measured, and stretched to failure in an Instron tensile testing machine. The ruptured ends were characterized under a dissecting microscope., Results: Marginal chordae had 68% thinner cross-sectional areas and failed at 68% less load and 28% less strain than basal chordae. Chordae from the posterior leaflet were 35% thinner and failed at 43% less load and 22% less strain than anterior leaflet chordae. Failure strength was lowest for posterior marginal chordae. Chordae most frequently tore just below the leaflet insertion, in what was often their narrowest section., Conclusion: Overall, the marginal chordae and posterior leaflet chordae were thinner and required less strain and load to fail than basal chordae and anterior leaflet chordae, respectively. These results support previous reports of decreased extensibility in marginal chordae. The high incidence of ruptures in the posterior marginal chordae of diseased mitral valves may be due to an inherent weakness in these chordae.

Background and Aim of Study: Previous studies in our laboratory have revealed the presence of compressive strains at the base of the non-coronary (NC) cusp of porcine aortic valves. Since these strains have been shown to damage bioprosthetic valve tissue, the mechanism by which they are induced in valve tissues should be investigated., Methods: Whole porcine aortic valves were pressurized and the induced strain patterns measured on aortic valve cusps and roots. Measurements were made before and after trimming of excess ventricular muscle and mitral valve tissue., Results: Negative (compressive) radial strains were found only in the NC cusp. Trimming of surrounding muscle increased the magnitude of both the tensile circumferential strains and compressive radial strains in the NC cusp. The left and right coronary cusps (LC, RC respectively) had similar positive radial and circumferential strains, and trimming had inconsistent effects on the strain patterns. Trimming of the aortic root increased root strains adjacent to the NC cusp, thus increasing positive circumferential strains and negative radial strains in the NC cusp. Analysis of images obtained during loading indicated that a wrinkle formed at the base of the NC cusp in 66% of cases in which negative strains were observed., Conclusion: Negative radial strains were induced in the NC cusp as it stretched circumferentially during static pressurization. In many valves, negative strains resulted from radial folding or wrinkling of the valve cusp. Since the RC and LC cusps had circumferential and radial strains that were not similar to those of the NC cusp, it is likely that the NC cusp has a different internal fibrous organization.