Fig. 2.

(A) Histological cross-section of the supraspinatus tendon to bone insertion (rat sample) showing the two regions used to measure orientation of collagen fibers with polarized light (T-INS: tendon end of the insertion, B-INS: bone end of the insertion). (B) Bell-like frequency distribution of collagen fibers orientation for the tendon insertion (T-INS) and the bony insertion (B-INS). Note that the 90° orientation lies along the main axis of the tendon. The distribution at the bone insertion shows a slightly higher heterogeneity. (C) Bright field microscopy of the supraspinatus insertion showing the regions (A–E) used for collagen orientation assessment on both sides of the tidemark (unmineralized fibrocartilage, UFC and mineralized fibrocartilage, MFC). (D) Resulting angular deviation measured with polarized light showing that fibers in the center (B–D) are more organized at the MFC compared to the UFC. (E) Spatial variation of angular deviation (which defines how well the fibers are aligned along a common direction) across the tendon (position 0) to bone (position 1) interface. Smaller values indicate higher alignment. (F) Spatial variation in elastic modulus computed with a continuum mechanics approach assuming the angular dispersion visualized in (C) and an ideal enthesis with no mineral. Decreasing fiber alignment causes a sudden and substantial drop in modulus. (G) Organization of collagen fibers across the anterior cruciate ligament (ACL) to bone insertion. Light micrograph (left) with the different tissues encounter at the enthesis (L: ligament, NFC: non-MFC, B: bone). Polarized light images (middle) of sections stained with Picrosirius red to highlight collagen arrangement and (left) corresponding FTIR spectroscopic maps (fiber orientation is color-coded as follows: blue perpendicular to interface; yellow: mixed orientation; red: parallel to interface). (H) Line profile analyses of spectroscopic maps for five samples (full line is the mean and shaded area shows standard deviation for each sample) revealing a clear transition in fiber organization. Bold grey arrows represent the tendon to bone direction. Figures modified from (Thomopoulos et al., 2003; Qu et al., 2017; Thomopoulos et al., 2006; Genin et al., 2009), with permission.