In vitro tendon tissue development from human fibroblasts demonstrates collagen fibril diameter growth associated with a rise in mechanical strength

Background: Collagen‐rich tendons and ligaments are important for joint stability and force transmission, but the capacity to form new tendon is poorly understood. In the present study, we investigated mechanical strength, fibril size, and structure during development of tendon‐like tissue from adult human tenocytes (termed tendon constructs) in vitro over 5 weeks in 3D tissue culture. Results: The constructs displayed large elongated tendon cells aligned along the tendon axis together with collagen fibrils that increased in diameter by 50% from day 14 to 35, which approaches that observed in adult human tendon in vivo. The increase in diameter was accompanied by a 5‐fold increase in mechanical strength (0.9±0.1 MPa to 4.9±0.6 MPa) and Young's modulus (5.8±0.9 MPa to 32.3±4.2 MPa), while the maximal strain at failure (16%) remained constant throughout the 5‐week culture period. Conclusions: The present study demonstrates that 3D tendon constructs can be formed by isolated human tendon fibroblasts, and when these constructs are subjected to static self‐generated tension, the fibrils will grow in size and strength approaching that of adult human tendon in vivo. Developmental Dynamics 242:2–8, 2013. © 2012 Wiley Periodicals, Inc.     

Investigating Tendon Fascicle Structure–Function Relationships in a Transgenic-Age Mouse Model Using Multiple Regression Models

Proper replacement or repair of damaged tendons or ligaments requires functionally engineered tissue that mimics their native mechanical properties. While tendon structure-function relationships are generally assumed, there exists little quantitative evidence of the roles of distinct tendon components in tendon function. Previous work has used linear correlations to assess the independent, univariate effects of one structural or one biochemical variable on mechanics. The current study's objective was to simultaneously and rigorously evaluate the relative contributions of seven different structural and compositional variables in predicting tissue mechanical properties through the use of multiple regression statistical models. Structural, biochemical, and mechanical analysis were all performed on tail tendon fascicles from different groups of transgenic mice, which provide a reproducible, noninvasive, in vivo model of changes in tendon structure and composition. Interestingly, glycosaminoglycan (GAG) content was observed to be the strongest predictor of mechanical properties. GAG content was also well correlated with collagen content and mean collagen fibril diameter. Collagen fibril area fraction was a significant predictor only of material properties. Therefore, in a large multivariate model, GAG content was the largest predictor of mechanical properties, perhaps both through direct influence and indirectly through its correlation with collagen content and fibril structure.     

Mechanical,Compositional, and Structural Properties of the Post-natal Mouse Achilles Tendon

During post-natal development, tendons undergo a well orchestrated process whereby extensive structural and compositional changes occur in synchrony to produce a normal tissue. Conversely, during the repair response to injury, structural and compositional changes occur, but in this case, a mechanically inferior tendon is produced. As a result, the process of development has been postulated as a potential paradigm through which improved adult tissue healing may occur. In this study we measured the mechanical, compositional, and structural properties in the post-natal mouse Achilles tendon at 4, 7, 10, 14, 21, and 28 days old. Throughout post-natal development, the mechanical properties, collagen content, fibril diameter mean, and fibril diameter standard deviation increased. Biglycan expression decreased and decorin expression and fiber organization were unchanged. This study provides a new mouse model that can be used to quantitatively examine mechanical development, as well as compositional and structural changes and biological mechanisms, during post-natal tendon development. This model is advantageous due to the large number of genetically modified mice and commercially available assays that are not available in other animal models. A mouse model therefore allows future mechanistic studies to build on this work.     

Ultrastructural Determinants of Murine Achilles Tendon Strength During Healing

The mechanisms by which tendon strength is established during growth and development and restored following injury are not completely understood and are likely to be complex, multifactorial processes. Several studies examining the relationship between mechanical behavior and ultrastructural characteristics of tendons and ligaments during growth and maturation suggest that collagen fibril diameter is strongly correlated with tendon strength. Because of the similarities between development and repair processes of musculoskeletal tissues, increases in tendon strength during healing may be related to increases in fibril ultrastructural parameters such as fibril size, numerical density, and area fraction. In this study, we compared murine Achilles tendons at various time points after tenotomy with sham-operated controls in tensile tests to failure and examined tendons using electron microscopy to assess collagen fibril ultrastructure. We found that in the 6-week period following Achilles tenotomy, fibril mean diameter remained significantly smaller than sham-side diameter by a factor of 2–3. Despite the persistently small fibril size, increasing numerical density resulted in a gradual increase in fibril area fraction. Biomechanical strength did not reach that of intact tendons until some time between 5 and 7 weeks, approximately the same time period when fibril area fraction began to approach sham values. These data suggest that parameters other than collagen fibril size are most responsible for increased tendon strength during healing.     

Crimp morphology in relaxed and stretched rat Achilles tendon

Fibrous extracellular matrix of tendon is considered to be an inextensible anatomical structure consisting of type I collagen fibrils arranged in parallel bundles. Under polarized light microscopy the collagen fibre bundles appear crimped with alternating dark and light transverse bands. This study describes the ultrastructure of the collagen fibrils in crimps of both relaxed and in vivo stretched rat Achilles tendon. Under polarized light microscopy crimps of relaxed Achilles tendons appear as isosceles or scalene triangles of different size. Tendon crimps observed via SEM and TEM show the single collagen fibrils that suddenly change their direction containing knots. The fibrils appear partially squeezed in the knots, bent on the same plane like bayonets, or twisted and bent. Moreover some of them lose their D-period, revealing their microfibrillar component. These particular aspects of collagen fibrils inside each tendon crimp have been termed 'fibrillar crimps' and may fulfil the same functional role. When tendon is physiologically stretched in vivo the tendon crimps decrease in number (46.7%) (P<0.01) and appear more flattened with an increase in the crimp top angle (165 degrees in stretched tendons vs. 148 degrees in relaxed tendons, P<0.005). Under SEM and TEM, the 'fibrillar crimps' are still present, never losing their structural identity in straightened collagen fibril bundles of stretched tendons even where tendon crimps are not detectable. These data suggest that the 'fibrillar crimp' may be the true structural component of the tendon crimp acting as a shock absorber during physiological stretching of Achilles tendon.     

Ultrastructural determinants of murine achilles tendon strength during healing

The mechanisms by which tendon strength is established during growth and development and restored following injury are not completely understood and are likely to be complex, multifactorial processes. Several studies examining the relationship between mechanical behavior and ultrastructural characteristics of tendons and ligaments during growth and maturation suggest that collagen fibril diameter is strongly correlated with tendon strength. Because of the similarities between development and repair processes of musculoskeletal tissues, increases in tendon strength during healing may be related to increases in fibril ultrastructural parameters such as fibril size, numerical density, and area fraction. In this study, we compared murine Achilles tendons at various time points after tenotomy with sham-operated controls in tensile tests to failure and examined tendons using electron microscopy to assess collagen fibril ultrastructure. We found that in the 6-week period following Achilles tenotomy, fibril mean diameter remained significantly smaller than sham-side diameter by a factor of 2-3. Despite the persistently small fibril size, increasing numerical density resulted in a gradual increase in fibril area fraction. Biomechanical strength did not reach that of intact tendons until some time between 5 and 7 weeks, approximately the same time period when fibril area fraction began to approach sham values. These data suggest that parameters other than collagen fibril size are most responsible for increased tendon strength during healing.     

Effect of ageing on ultrastructure of slow and fast skeletal muscle tendon in rabbit Achilles tendons

This reports presents the changing morphological characteristics of collagen and fibroblasts in the soleus and gastrocnemius muscle tendon of female Japanese white rabbits with ageing. The fibroblasts decreased in number per 37 μm² with ageing in each group, and their morphology became longer and more slender through ageing. The mean fibril area and diameter of the collagen fibrils of soleus muscle tendon (SMT) and lateral gastrocnemius muscle tendon (GMT) in 8- to 10-month old rabbits were significantly higher than those of 3-wk-old rabbits during growth (P < 0.01). The mean area and diameter of collagen fibrils of SMT and GMT decreased during senescence: the values for 4- to 5-yr-old rabbits were lower than those for 8- to 10-month-old rabbits, but the difference was not significant. Statistically significant differences in fibril area and diameter between the SMT and GMT were not found during ageing. The number of thick fibrils increased during growth, but decreased in senescence. There were more thin fibrils (30–60 nm) in the 3-wk-old rabbits than in the 8- to 10-month old and 4 to 5-yr-old groups, and the large-diameter collagen (300–360 nm) was more abundant in the 8- to 10-month-old group than in the 3-wk-old and 4- to 5-yr-old groups. Differences in fibril size between slow and fast muscle tendons were not observed during ageing.     

Tendon and ligament fibrillar crimps give rise to left-handed helices of collagen fibrils in both planar and helical crimps

Collagen fibres in tendons and ligaments run straight but in some regions they show crimps which disappear or appear more flattened during the initial elongation of tissues. Each crimp is formed of collagen fibrils showing knots or fibrillar crimps at the crimp top angle. The present study analyzes by polarized light microscopy, scanning electron microscopy, transmission electron microscopy the 3D morphology of fibrillar crimp in tendons and ligaments of rat demonstrating that each fibril in the fibrillar region always twists leftwards changing the plane of running and sharply bends modifying the course on a new plane. The morphology of fibrillar crimp in stretched tendons fulfills the mechanical role of the fibrillar crimp acting as a particular knot/biological hinge in absorbing tension forces during fibril strengthening and recoiling collagen fibres when stretching is removed. The left‐handed path of fibrils in the fibrillar crimp region gives rise to left‐handed fibril helices observed both in isolated fibrils and sections of different tendons and ligaments (flexor digitorum profundus muscle tendon, Achilles tendon, tail tendon, patellar ligament and medial collateral ligament of the knee). The left‐handed path of fibrils represents a new final suprafibrillar level of the alternating handedness which was previously described only from the molecular to the microfibrillar level. When the width of the twisting angle in the fibrillar crimp is nearly 180° the fibrils appear as left‐handed flattened helices forming crimped collagen fibres previously described as planar crimps. When fibrils twist with different subsequent rotational angles (< 180°) they always assume a left‐helical course but, running in many different nonplanar planes, they form wider helical crimped fibres.     

Engineering of extensor tendon complex by an ex vivo approach

Engineering of extensor tendon complex remains an unexplored area in tendon engineering research. In addition, less is known about the mechanism of mechanical loading in human tendon development and maturation. In the current study, an ex vivo approach was developed to investigate these issues. Human fetal extensor tenocytes were isolated, expanded and seeded on polyglycolic acid (PGA) fibers that formed a scaffold with a shape mimicking human extensor tendon complex. After in vitro culture for 6 weeks, 7 cell-scaffold constructs were further in vitro cultured with dynamic mechanical loading for another 6 weeks in a bioreactor. The other 14 constructs were in vivo implanted subcutaneously to nude mice for another 14 weeks. Seven of them were implanted without loading, whereas the other 7 were sutured to mouse fascia and animal movement provided a natural dynamic loading in vivo. The results demonstrated that human fetal cells could form an extensor tendon complex structure in vitro and become further matured in vivo by mechanical stimulation. In contrast to in vitro loaded and in vivo non-loaded tendons, in vivo loaded tendons exhibited bigger tissue volume, better aligned collagen fibers, more mature collagen fibril structure with D-band periodicity, and stronger mechanical properties. These findings indicate that an extensor tendon complex like structure is possible to generate by an ex vivo approach and in vivo mechanical loading might be an optimal niche for engineering functional extensor tendon.     

Age-related changes of tendon fibril micro-morphology and gene expression

Aging is hypothesized to be associated with changes in tendon matrix composition which may lead to alteration of tendon material properties and hence propensity to injury. Altered gene expression may offer insights into disease pathophysiology and thus open new perspectives toward designing pathophysiology-driven therapeutics. Therefore, the current study aimed at identifying naturally occurring differences in tendon micro-morphology and gene expression of newborn, young and old horses. Age-related differences in the distribution pattern of tendon fibril thickness and in the expression of the tendon relevant genes collagen type 1 (Col1), Col3, Col5, tenascin-C, decorin, tenomodulin, versican, scleraxis and cartilage oligomeric matrix protein were investigated. A qualitative and quantitative gene expression and collagen fibril diameter analysis was performed for the most frequently injured equine tendon, the superficial digital flexor tendon, in comparison with the deep digital flexor tendon. Most analyzed genes (Col1, Col3, Col5, tenascin-C, tenomodulin, scleraxis) were expressed at a higher level in foals (age ≤ 6 months) than in horses of 2.75 years (age at which flexor tendons become mature in structure) and older, decorin expression increased with age. Decorin was previously reported to inhibit the lateral fusion of collagen fibrils, causing a thinner fibril diameter with increased decorin concentration. The results of this study suggested that reduction of tendon fibril diameters commonly seen in equine tendons with increasing age might be a natural age-related phenomenon leading to greater fibril surface areas with increased fibrillar interaction and reduced sliding at the fascicular/fibrillar interface and hence a stiffer interfascicular/interfibrillar matrix. This may be a potential reason for the higher propensity to tendinopathies with increasing age.     

Demonstrating collagen tendon fibril segments involvement in intrinsic tendon repair

Severed tendons can undergo regenerative healing, intrinsic tendon repair. Fibrillogenesis of chick tendon involves “collagen fibril segments” (CFS), which are the building blocks of collagen fibers that make up tendon fascicles. The CFS are 10.5 micron in length, composed of tropocollagen monomers arranged in parallel arrays. Rather than incorporating single tropocollagen molecules into growing collagen fibers, incorporating large CFS units is the mechanism for generating collagen fibers. Is intrinsic tendon repair through the reestablishment of tendon embryogenesis? Gentamicin treated 10-day-old chick embryo tendons released CFS were fluorescently tagged with Rhodamine (Rh). Organ cultured severed 14-day-old embryo tendon explants received Rh tagged CFS. At day 4 auto fluorescent tagged CFS were identified at the severed tendon ends by fluorescent microscopy. Accumulation of fluorescent tagged CFS was exclusively localized to the severed ends of tendon explants. Parallels between collagen fiber growth during embryonic fibrillogenesis and tendon repair reveal CFS incorporation is responsible for collagen fibers growth. CFS incorporation into frayed collagen fibers from severed tendons is the proposed mechanism for intrinsic tendon repair, which is an example of regenerative repair.     

Collagen fibril morphology and mechanical properties of the Achilles tendon in two inbred mouse strains

The relationship between collagen fibril morphology and the functional behavior of tendon tissue has been investigated in numerous experimental studies. Several of these studies suggest that larger fibril radius is a primary determinant of higher tendon stiffness and strength; others have shown that factors apart from fibril radius (such as fibril–fibril interactions) may be critical to improved tendon strength. In the present study, we investigate these factors in two inbred mouse strains that are widely used in skeletal structure–function research: C57BL/6J (B6) and C3H/HeJ (C3H). The aim was to establish a quantitative baseline that will allow one to assess how regulation of tendon extracellular matrix architecture affects tensile mechanical properties. We specifically focused on collagen fibril structure and glycosaminoglycan (GAG) content – the two primary constituents of tendon by dry weight – and their potential functional interactions. For this purpose, Achilles tendons from both groups were tested to failure in tension. Tendon collagen morphology was analyzed from transmission electron microscopy images of tendon sections perpendicular to the longitudinal axis. Our results showed that the two inbred strains are macroscopically similar, but C3H mice have a higher elastic modulus (P < 0.05). Structurally, C3H mice showed a larger collagen fibril radius compared to B6 mice (96 ± 7 nm and 80 ± 10 nm respectively). Tendons from C3H mice also showed smaller specific fibril surface (0.015 ± 0.001 nm nm⁻² vs. 0.017 ± 0.003 nm nm⁻² in the B6 tendons, P < 0.05), and accordingly a lower concentration of GAGs (0.60 ± 0.07 μg mg⁻¹ vs. 0.83 ± 0.11 μg mg⁻¹, P < 0.05). As in other studies of tendon structure and function, larger collagen fibril radius appears to be associated with stiffer tendon, but this functional difference could also be attributed to reduced potential surface area exchange between fibrils and the surrounding proteoglycan-rich matrix, in which the hydrophilic GAG side chains may promote inter-fibril sliding. This study provides an architectural and functional baseline for a comparative murine model that can be used to investigate the genetic regulation of tendon biomechanics.     

An Age-Related Study of Morphology and Cross-Link Composition of Collagen Fibrils in the Digital Flexor Tendons of Young Thoroughbred Horses

The superficial digital flexor tendon is the most commonly injured tendon in the racing Thoroughbred. Despite the clinical significance of this structure, only limited data exist regarding normal age-related morphology of the tensile units, the collagen fibrils. The age at which these collagen fibrils become mature in composition and structure may be of importance. Consequently, the association of age and collagen fibril crosslink composition, diameter distribution and crimp morphology in the superficial and deep digital flexor tendons of Thoroughbreds up to and including three years of age has been studied. Replacement of immature crosslinks, peaking of the collagen fibril mass-average diameter and collagen fibril index, and stabilization of collagen crimp morphology changes supported the hypothesis that both digital flexor tendons become mature in structure by two years of age.     

bFGF复合膜对同种异体肌腱移植的作用的实验研究

目的　用bFGF复合膜 ,包裹在腱移植处 ,探讨腱内部再生是否快于腱周结缔组织增生 ,从而达到防止或减轻腱粘连的目的。方法　用 - 80℃冰箱冷存 10天的同种异体跟腱缝接 30只兔左肢跟腱缺损 2 .0cm处 ,分两组 :A组在腱移植处包裹bFGF复合非降解膜 (药膜 )为实验组 ,B组在腱移植处包裹无药的非解降解膜 (无药膜 )为对照组 ,术后不同时间 ,切取腱移植部位 ,常规制成光、电镜标本 ,再行镜下观察 ,图像分析仪测定和腱周粘连定量测定。结果　包药膜腱移植段内部的成纤维细胞和胶原纤维明显较无药膜的多 (<0 0 1) ,包无药膜腱周结缔组织的成纤维细胞和胶原纤维明显较包膜的多 (<0 0 1) ,包药膜腱移植段内部的成纤维细胞和胶原纤维明显较腱周结缔组织的多 (<0 1) ,无药膜的明显较腱周的少 (<0 0 1)。结论　bFGF复合膜有增强腱内部再生和减轻或防止腱粘连的作用     

Concerted and adaptive alignment of decorin dermatan sulfate filaments in the graded organization of collagen fibrils in the equine superficial digital flexor tendon

The equine superficial digital flexor tendon (SDFT) has a graded distribution of collagen fibril diameters, with predominantly small-diameter fibrils in the region of the myotendinous junction (MTJ), a gradual increase in large-diameter fibrils toward the osteotendinous junction (OTJ), and a mixture of small- and large-diameter fibrils in the middle metacarpal (MM) region. In this study, we investigated the ultrastructure of the SDFT, to correlate the spatial relationship of the collagen fibrils with the graded distribution. The surface-to-surface distances of pairs of fibrils were found to be almost constant over the entire tendon. However, the center-to-center distances varied according to fibril diameter. Decorin is the predominant proteoglycan in normal mature tendons, and has one dermatan sulfate (DS) or chondroitin sulfate (CS) filament as a side chain which is associated with the surfaces of the collagen fibrils via its core protein. We identified a coordinated arrangement of decorin DS filaments in the equine SDFT. The sizes of the decorin DS filaments detected by Cupromeronic blue staining showed a unique regional variation; they were shortest in the MM region and longer in the MTJ and OTJ regions, and a considerable number of filaments were arranged obliquely to adjacent collagen fibrils in the MTJ region. This regional variation of the filaments may be an adaptation to lubricate the interfibrillar space in response to local mechanical requirements. The results of this study suggest that the MTJ region, which receives the muscular contractile force first, acts as a buffer for mechanical forces in the equine SDFT.     

Glutaraldehyde Cross-Linking of Tendon—Mechanical Effects at the Level of the Tendon Fascicle and Fibril

Conclusive insight into the microscopic principles that govern the strength of tendon and related connective tissues is lacking and the importance of collagen cross-linking has not been firmly established. The combined application of whole-tissue mechanical testing and atomic force spectroscopy allowed for a detailed characterization of the effect of cross-linking in rat-tail tendon. The cross-link inducing agent glutaraldehyde augmented the tensile strength of tendon fascicles. Stress at failure increased from ～8 MPa to ～39 MPa. The mechanical effects of glutaraldehyde at the tendon fibril level were examined by atomic force microscopy. Peak forces increased from ～1379 to ～2622 pN while an extended Hertz fit of force-indentation data showed a ～24 fold increase in Young's modulus on indentation. The effect of glutaraldehyde cross-linking on the tensile properties of a single collagen fibril was investigated by a novel methodology based on atomic force spectroscopy. The Young's modulus of a secluded fibril increased from ～407 MPa to ～1.1 GPa with glutaraldehyde treatment. Collectively, the findings indicate that cross-linking at the level of the collagen fibril is of key importance for the mechanical strength of tendon tissue. However, when comparing the effects at the level of the tendon fascicle and fibril, respectively, further questions are prompted regarding the pathways of force through the tendon microstructure as fibril strength seems to surpass that of the tendon fascicle.     

Analysis of mineral deposition in turkey tendons and self-assembled collagen fibers using mechanical techniques

During limb movement and locomotion, animals store elastic energy in the tendons of the feet, legs, and other limbs. In the turkey, much of the force generated by the gastrocnemius muscle during locomotion is stored as elastic energy through deformation of the tendon. During growth and development, the leg tendons in some avians, including turkeys, mineralize and result in an increase in tensile strength and modulus. The purpose of our study was to evaluate the effects of mineralization on elastic energy storage and transmission in turkey tendons. Elastic and viscous stress-strain curves and elastic energy storage behavior were used to compare the behavior of mineralized turkey gastrocnemius tendons and mineralized self-assembled type I collagen fibers. Based on analysis of these two systems, we concluded that a simple mineralized fibrillar collagenous substrate can mimic the behavior of a more complex fibrillar collagenous substrate such as mineralized turkey tendon; however, the exact mechanism of mineralization may be different between the two substrates. Changes in mechanical properties of turkey tendon were consistent with a model in which mineralization appears to increase the effective collagen fibril length by efficiently transferring stress between neighboring collagen fibrils. Mineralization in self-assembled collagen fibers increased elastic energy storage less efficiently as compared with turkey tendon suggesting that the noncollagenous components of mineralizing tissue may act to promote collagen fibril to collagen fibril interactions.     

Repair of Achilles tendon defect with autologous ASCs engineered tendon in a rabbit model

Adipose derived stem cells (ASCs) are an important cell source for tissue regeneration and have been demonstrated the potential of tenogenic differentiation in vitro. This study explored the feasibility of using ASCs for engineered tendon repair in vivo in a rabbit Achilles tendon model. Total 30 rabbits were involved in this study. A composite tendon scaffold composed of an inner part of polyglycolic acid (PGA) unwoven fibers and an outer part of a net knitted with PGA/PLA (polylactic acid) fibers was used to provide mechanical strength. Autologous ASCs were harvested from nuchal subcutaneous adipose tissues and in vitro expanded. The expanded ASCs were harvested and resuspended in culture medium and evenly seeded onto the scaffold in the experimental group, whereas cell-free scaffolds served as the control group. The constructs of both groups were cultured inside a bioreactor under dynamic stretch for 5 weeks. In each of 30 rabbits, a 2 cm defect was created on right side of Achilles tendon followed by the transplantation of a 3 cm cell-seeded scaffold in the experimental group of 15 rabbits, or by the transplantation of a 3 cm cell-free scaffold in the control group of 15 rabbits. Animals were sacrificed at 12, 21 and 45 weeks post-surgery for gross view, histology, and mechanical analysis. The results showed that short term in vitro culture enabled ASCs to produce matrix on the PGA fibers and the constructs showed tensile strength around 50 MPa in both groups (p > 0.05). With the increase of implantation time, cell-seeded constructs gradually form neo-tendon and became more mature at 45 weeks with histological structure similar to that of native tendon and with the presence of bipolar pattern and D-periodic structure of formed collagen fibrils. Additionally, both collagen fibril diameters and tensile strength increased continuously with significant difference among different time points (p < 0.05). In contrast, cell-free constructs failed to form good quality tendon tissue with fibril structure observable only at 45 weeks. There were significant differences in both collagen fibril diameter and tensile strength between two groups at all examined time points (p < 0.05). The results of this study support that ASCs are likely to be a potential cell source for in vivo tendon engineering and regeneration.     

Analysis of Mineral Deposition in Turkey Tendons and Self-Assembled Collagen Fibers Using Mechanical Techniques

During limb movement and locomotion, animals store elastic energy in the tendons of the feet, legs, and other limbs. In the turkey, much of the force generated by the gastrocnemius muscle during locomotion is stored as elastic energy through deformation of the tendon. During growth and development, the leg tendons in some avians, including turkeys, mineralize and result in an increase in tensile strength and modulus. The purpose of our study was to evaluate the effects of mineralization on elastic energy storage and transmission in turkey tendons.

Elastic and viscous stress-strain curves and elastic energy storage behavior were used to compare the behavior of mineralized turkey gastrocnemius tendons and mineralized self-assembled type I collagen fibers. Based on analysis of these two systems, we concluded that a simple mineralized fibrillar collagenous substrate can mimic the behavior of a more complex fibrillar collagenous substrate such as mineralized turkey tendon; however, the exact mechanism of mineralization may be different between the two substrates. Changes in mechanical properties of turkey tendon were consistent with a model in which mineralization appears to increase the effective collagen fibril length by efficiently transferring stress between neighboring collagen fibrils. Mineralization in self-assembled collagen fibers increased elastic energy storage less efficiently as compared with turkey tendon suggesting that the noncollagenous components of mineralizing tissue may act to promote collagen fibril to collagen fibril interactions.     

Biomechanical properties and dry weight content of the developing superficial digital flexor tendon in rabbit

Composition and structural organization of tendon changes during aging and these alterations affect the mechanical behaviors of this structure. Therefore, this experiment was designed to study the biomechanical properties together with changes in dry weight content of normal superficial digital flexor tendon of rabbits from pre-natal stage to 112 days post-natally. Forty-two White New Zealand rabbits were assigned to seven different age groups (from 5–7 days before birth to 112 days after birth), each consisting of six animals. The right superficial digital flexor tendons were used for biomechanical studies and the left ones for percentage dry weight investigation. Ultimate tensile strength, stiffness, maximum energy, and percentage dry weight values significantly increased in each higher age group compared to those of the younger group and the yield strain and maximum strain decreased comparatively as a function of age. This improvement in the mechanical behavior of tendons during aging could be correlated with increase in collagen content, alteration in the collagen fibril differentiation and distribution from small-sized unimodal fibrils to trimodally distributed collagen fibrils, improvement in quantity and quality of the cross-linking, fibril continuity, type of collagen, development and maturation of crimp pattern, tissue alignment and organization. Therefore, characterization of mechanical behavior and tissue dry weight, as an index of collagen content, from fetal stage to skeletally mature animals is essential in better understanding the tissue structural development and hierarchical organization coincidental with the material properties of this organ.