Tenocyte contraction induces crimp formation in tendon-like tissue

Abstract

Tendons are composed of longitudinally aligned collagen fibrils arranged in bundles with an undulating pattern, called crimp. The crimp structure is established during embryonic development and plays a vital role in the mechanical behaviour of tendon, acting as a shock-absorber during loading. However, the mechanism of crimp formation is unknown, partly because of the difficulties of studying tendon development in vivo. Here, we used a 3D cell culture system in which embryonic tendon fibroblasts synthesise a tendonlike construct comprised of collagen fibrils arranged in parallel bundles. Investigations using polarised light microscopy, scanning electron microscopy and fluorescence microscopy showed that tendon constructs contained a regular pattern of wavy collagen fibrils. Tensile testing indicated that this superstructure was a form of embryonic crimp producing a characteristic toe region in the stress-strain curves. Furthermore, contraction of tendon fibroblasts was the critical factor in the buckling of collagen fibrils during the formation of the crimp structure. Using these biological data, a finite element model was built that mimics the contraction of the tendon The results show that the contraction of the fibroblasts is a sufficient mechanical impulse to build a planar wavy pattern. Furthermore, the value of crimp wavelength was determined by the mechanical properties of the collagen fibrils and inter-fibrillar matrix. Increasing fibril stiffness combined with constant matrix stiffness led to an increase in crimp wavelength. The data suggest a novel mechanism of crimp formation, and the finite element model indicates the minimum requirements to generate a crimp structure in embryonic tendon.

3‐D ultrastructure and collagen composition of healthy and overloaded human tendon: evidence of tenocyte and matrix buckling

“…The reason for buckling is not fully understood but is most likely the result of local breakage of collagen fibrils with concomitant contraction of the actinomyosin cytoskeleton. Recent studies using a tendon-like construct system showed that cells exert forces on the surrounding extracellular matrix (Herchenhan et al, 2012, Kalson et al, 2013). In the absence of sufficient restraining force in the matrix (as might occur if a small but significant number of fibrils are severed) cell contraction can result in matrix buckling and nucleus shape changes.…”

Evaluation of Global Load Sharing and Shear-Lag Models to Describe Mechanical Behavior in Partially Lacerated Tendons

“…Due to the complexity and variability in these tissues, the literature is inconsistent for material parameters and geometries. However, some meaningful ranges are provided by Herchenhan et al [32], and ranges for fibril volume fraction can be found in studies by Lavagnino et al [34]. Limited information is available on the sensitivity of these models to a variation in fibril and matrix volume fractions V f and V m , fibril average diameter d 0 , fibril Young's and shear moduli E f and G f , matrix Young's and shear moduli E m and G m , and stress transfer length k c .…”

“…Table 1 lists the parameter values used for the current application as determined from the literature and from the sensitivity analysis starting from the reference ranges provided in Ref. [32].…”

“…3.1.1 Geometrical and Material Parameters. Geometrical and material properties for model components were obtained from the literature [30,[32][33][34][35][36]. Wagner and Eitan's model incorporating either Cox's or Nairn's shear-lag parameter, b, requires knowledge of the geometry and composition of the considered tissue; this presents a significant complication to its application.…”

Modelling approaches for evaluating multiscale tendon mechanics

“…Unfortunately, very few multiscale models have been developed and used to study tendon properties across different length scales. In one example, a computational cell composed of hyperelastic fibril bundles, interfibrillar matrix and embedded tenocytes was created, and FE analyses were conducted to investigate the cause of crimp in tendon [144,145]. In this study, shear stresses on the interface between collagen fibrils and interfibrillar matrix-attributed to cell contraction-caused crimp structure, and the ratio of fibrils to interfibrillar matrix determined crimp length [144].…”