Functionally Distinct Tendons From Elastin Haploinsufficient Mice Exhibit Mild Stiffening and Tendon-Specific Structural Alteration

Abstract

Elastic fibers are present in low quantities in tendon, where they are located both within fascicles near tenocytes and more broadly in the interfascicular matrix (IFM). While elastic fibers have long been known to be significant in the mechanics of elastin-rich tissue (i.e., vasculature, skin, lungs), recent studies have suggested a mechanical role for elastic fibers in tendons that is dependent on specific tendon function. However, the exact contribution of elastin to properties of different types of tendons (e.g., positional, energy-storing) remains unknown. Therefore, this study purposed to evaluate the role of elastin in the mechanical properties and collagen alignment of functionally distinct supraspinatus tendons (SSTs) and Achilles tendons (ATs) from elastin haploinsufficient (HET) and wild type (WT) mice. Despite the significant decrease in elastin in HET tendons, a slight increase in linear stiffness of both tendons was the only significant mechanical effect of elastin haploinsufficiency. Additionally, there were significant changes in collagen nanostructure and subtle alteration to collagen alignment in the AT but not the SST. Hence, elastin may play only a minor role in tendon mechanical properties. Alternatively, larger changes to tendon mechanics may have been mitigated by developmental compensation of HET tendons and/or the role of elastic fibers may be less prominent in smaller mouse tendons compared to the larger bovine and human tendons evaluated in previous studies. Further research will be necessary to fully elucidate the influence of various elastic fiber components on structure-function relationships in functionally distinct tendons.

The “other” 15–40%: The Role of Non‐Collagenous Extracellular Matrix Proteins and Minor Collagens in Tendon

“…145,146 Surprisingly, elastin-deficient tendons showed no alterations in biomechanical properties, with the exception of a slight increase in linear stiffness. 145 FBN1 and FBN2 were found in bovine flexor tendon and in canine flexor digitorum profundus tendon where FBN2 was localized in the PCM of the central tenocytes, while FBN1 localized to the PCM of the peripheral tenocyte layers. 143,147 The PCM around the tenocytes is rich in FBN2, which co-localized with versican, where it forms a sturdy ECM that allows for the proteolytic and mechanical isolation of intact tenocyte arrays.…”

“…Elastin haploinsufficiency in mice resulted in larger collagen fibrils in the Achilles tendon but not in the supraspinatus tendon . Surprisingly, elastin‐deficient tendons showed no alterations in biomechanical properties, with the exception of a slight increase in linear stiffness . FBN1 and FBN2 were found in bovine flexor tendon and in canine flexor digitorum profundus tendon where FBN2 was localized in the PCM of the central tenocytes, while FBN1 localized to the PCM of the peripheral tenocyte layers .…”

“…Elastic fibers are also found in the interfascicular matrix (IFM), specifically in energy storing tendons and here, elastic fibers become disorganized during aging . Elastin haploinsufficiency in mice resulted in larger collagen fibrils in the Achilles tendon but not in the supraspinatus tendon . Surprisingly, elastin‐deficient tendons showed no alterations in biomechanical properties, with the exception of a slight increase in linear stiffness .…”

Elastic fibers in orthopedics: Form and function in tendons and ligaments, clinical implications, and future directions

“…16,[23][24][25][26] 3 | CONTENT AND DISTRIBUTION Excluding the elastin-rich spinal ligaments such as the nuchal ligament and ligamentum flavum, elastin content in tendon and ligament has been reported to range from as low as 0.25% to as high as 10% of the tissue's dry weight, with various reports of intermediate values. [27][28][29] Much of the variability in elastin content can be attributed to tissue specialization in order to accomplish specific functional requirements.…”

Elastase treatment of tendon specifically impacts the mechanical properties of the interfascicular matrix

“…Previous studies focussed on elucidating the contribution of elastin to tendon mechanics have identified alterations in failure stress and strain [15], stiffness [16] and shear response [17] as a result of elastin depletion, utilising either heterozygous knockout models or enzymatic digestion of elastin. However, to the authors' knowledge, no studies have investigated the effect of elastin depletion in energy storing tendons, or directly determined the effect of elastin depletion on IFM mechanics.…”