The organization of collagen in growing tensile tissues

Collagen-based Tissue Engineering as Applied to Heart Valves

Using the method of directed collagen gel shrinkage, we have been fabricating heart valves and mitral valve chordae [1,2,3]. The principle involves mixing solubilized collagen with the appropriate cells. When the collagen-cell mixture is neutralized, soluble collagen reassembles into fibrils and a gel is created. When the gel is mechanically constrained, the collagen fibrils align in the direction of constraint. The generation of tensile force during contraction is crucial for the formation of highly aligned, compacted collagenous constructs. So far, inappropriate mechanical properties have been one of the main limitations of most collagen-based tissue equivalents. In this study, we focused on providing both biomechanical and biochemical stimuli to increase cellular proliferation, matrix synthesis, and hence improve the mechanical properties of the collagen constructs. We explored a number of holder materials and configurations, with an objective to maximize the lateral compaction of our constructs. We designed a bioreactor that can provide controlled static tension to our collagen constructs. We also developed a nutrition-fortified medium that includes trace elements (Zn2+, Cu2+, Fe2+ and Mn2+), various amino acids, and vitamins (A, B complex, and C). Our ultimate goal was to combine biomechanical and biochemical stimuli, and enhance the mechanical strength of our collagen constructs.

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

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

Microstructural morphology in the transition region between scar and intact residual segments of a healing rat medial collateral ligament

This study used a rat model to investigate the microstructural organization of collagen through the transition from scar to intact residual segments of a healing medial collateral ligament (MCL). Twenty-two male retired breeder Sprague-Dawley rats were randomly separated into two groups. Eleven underwent surgical transections of both MCLs and were allowed unrestricted cage activity until euthanized two weeks post surgery. The remaining eleven rats were used as normal controls. All 44 MCLs were harvested including intact femoral and tibial insertions and prepared for scanning electron microscopy (SEM) imaging. At harvest the scar region in the healing ligaments was more translucent than the normal tissue. Ligaments were viewed from femoral to tibial insertions at magnifications of 100X through 20,000X. Tissue away from the scar region in the transected MCLs was indistinguishable from normal tissue in uninjured ligaments. Collagen fibers and fibrils in these tissues were more aligned along the longitudinal axis of the ligament than in the scar tissue. Continuity of collagen fibers and fibrils were consistently observed from the residual portions of the transected ligament through the scar region. Bifurcations/fusions, but no anastomoses, in fibers and fibrils were observed in both normal and scar tissues of ligaments. Qualitatively, bifurcations were encountered more frequently in scar tissue. In the transition region, larger diameter fibers from the residual tissue bifurcated into smaller diameter fibrils in the scar. This connection between larger diameter and smaller diameter fibers and fibrils indicates that bifurcations/fusions are likely to be the dominant way in which force is transmitted from a region with larger fibrils (residual ligament) into and through a region with smaller fibrils (scar).

Remodeling of a collagenous tissue at fixed lengths

Mature tissues can often adapt to changes in their chemical, mechanical, or thermal environment. For example, in response to sustained increases or decreases in mechanical loads, some tissues grow and remodel so as to restore the stress or strain to its homeostatic state. Whereas most previous work addresses gross descriptors of tissue growth, this paper introduces a possible cell-mediated mechanism by which remodeling may occur in a soft connective tissue--that the kinetics of collagen deposition and degradation is similar regardless of the configuration of the body at which it occurs. The proposed theoretical framework applies to three-dimensional settings, but it is illustrated by focusing on the remodeling of a uniaxial collagenous tissue that is maintained at a fixed length for an extended period. It is shown that qualitative features expected of such remodeling (e.g., an increased compliance and increased stress-free length when remodeling occurs at an extended length) are easily realized. Growth and remodeling are complex phenomena, however, and are likely accomplished via multiple complementary mechanisms. There is a need, therefore, to identify other candidate mechanisms and, of course, to collect experimental data suitable for testing and refining the possible theories.

Excursion is important in regulating sarcomere number in the growing rabbit tibialis anterior

1. The mechanical signal(s) that may be important in regulating the number of sarcomeres in series in skeletal muscle fibres or fascicles (sarcomere number) remain(s) speculative, in part because the in vivo mechanical environment of muscle has not yet been defined during sarcomere number perturbations. In the present study, measurements of the in vivo mechanical environment were used to test the hypothesis that increasing muscle excursion results in increased serial sarcomere addition in growing animals. 2. The tibialis anterior (TA) was released from its retinacular restraint at the ankle joint to increase muscle excursion in 4-week-old female New Zealand White rabbits. Twelve weeks post release, muscle excursion and sarcomere number were significantly increased for released TAs compared with control TAs. 3. General cage activity over a 24 h period and in vivo ankle joint kinematics of the experimental leg during hopping on a treadmill were not significantly different between control and release groups, suggesting that altered animal and joint activity patterns were not responsible for the increased serial sarcomere addition associated with TA release. 4. In vivo TA forces during hopping and during a large force-producing foot-flicking motion were significantly decreased in released TAs compared with control TAs. Chronically decreased force production may have been involved in the decreased longitudinal tendon growth observed for the released TA compared with the control TA, which, in turn, may have stimulated the increase in serial sarcomere addition. However, increasing force production of the released TA by partial ablation of the extensor digitorum longus did not inhibit the increase in serial sarcomere addition. 5. The results of this study support the hypothesis that increasing excursion results in increased serial sarcomere addition in growing animals and, when combined with the results of previous studies indicating that decreasing excursion results in decreased serial sarcomere addition, support the working hypothesis that excursion is important in regulating sarcomere number in growing animals.