Mechanical adaptation on the microstructural components of biological tissues

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Tendons and ligaments have such hierarchical structures as collagen fascicles (the diameter of about 100μm), fibers (the diameter of about 1μm), fibrils (the diameter of 50-500nm), and molecules. The fascicles consist of fibers and fibrils embedded in ground substances such as proteoglycans. Thin sheaths called endotenon surround the fascicles. The basic knowledge of their structure and mechanical properties is essential to the biomechanics of tendons and ligaments.In this study, the fascicles obtained from mouse tail tendons were stirred in a saline solution, and ground substances were removed from the fascicles. These specimens were stretched to failure and their microstructure was observed with a scanning electron microscope (SEM) and atomic force microscope (AFM). The fibrils separated from the fascicles were stretched to failure. We studied the effects of the ground substances on the mechanical properties of the fascicles and the mechanical relationships between the fibrils and fascicles.The stirring of fascicles in a saline solution removed ground substances from the fascicles and decreased their tensile strength and tangent modulus. It is clear that ground substances act as a cementlike substance between collagen fibrils and are a basic component to determine the tensile properties of fascicles. The tensile strength of fibrils (86.3±8.2 MPa (Mean±S.E.)) was 3.3 times larger than that of fascicles. Therefore, fibrils did not fail but ground substances failed at the failure of fascicles.