Collagen fibrils in human tendons, is everything already said? A qualitative ultrastructural assessment

Improving structural knowledge of human tendons is fundamental for a better understanding of pathological and regenerative processes occurring in these key structures. Recent advances in tendon biology indicate cellular heterogeneity between tendons of different muscles. In contrast, very little is known about potential differences in extracellular matrix in tendons across anatomy. In the present work, we aimed to assess collagen fibril organisation ultrastructurally in human supraspinatus, semitendinosus, and quadriceps tendons, as well as in the anterior cruciate and patellar ligaments. Samples were obtained with appropriate permissions from seven different donors belonging to the Anatomy Gift Programme of the Royal College of Surgeons in Ireland in Dublin, that had been previously embalmed for routine anatomical examination. Samples from the tendon mid-body were harvested and processed for Transmission Electron Microscopy (TEM). We examined a total of 34 tendons (n=2 samples of each). Collagen fibrils were well recognisable in 32 tendons. O verall, marked heterogeneity between samples from the same tendon type of different donors, or between the various tendons of the same donor, was observed in terms of collagen fibril shape (rounded vs more irregular-polygonal), size distribution, and width. The greatest similarity in fibril organisation was observed between patellar ligament and quadriceps tendon. The anterior cruciate ligament showed smaller, less rounded fibrils, which were more consistent in size. Supraspinatus also showed bundles perpendicular to the longitudinal tendon axis. Consistency in fibril organisation between both samples of the same individual tendon was observed in 26 cases. This study demonstrates that ultrastructural analysis of collagen fibril is feasible in tissue from anatomical donors despite the embalming process not being a standard procedure of fixation for TEM examination. This first description also offers a basis for a more detailed and quantitative assessment of the collagen fibrils in human tendons. References 1) Franchi, Marco, Trirè, Alessandra, Quaranta, Marilisa, Orsini, Ester, Ottani, Victoria. Collagen Structure of Tendon Relates to Function. The Scientific World Journal, 2007, 7, 132725. 2) Christopher K. Revell, Oliver E. Jensen, Tom Shearer, Yinhui Lu, David F. Holmes, Karl E. Kadler. Collagen fibril assembly: New approaches to unanswered questions. Matrix Biology Plus, 2021, 12, 100079. 3) Baldwin M, Buckley CD, Guilak F, Hulley P, Cribbs AP, Snelling S. A roadmap for delivering a human musculoskeletal cell atlas. Nat Rev Rheumatol. 2023 19, 738-752.