Morphological changes of myotendinous junction generated by muscle disuse atrophy

Abstract

The skeletal muscle contraction is transmitted to the tendon through the myotendinous junction (MTJ), where the proximal extremity of the tendon forms characteristic finger-like processes, penetrating into the muscle mass. We recently demonstrated that changes at MTJ level occur as an adaptation to exercise-induced tension increase, allowing a better tension resistance. The aim of this study is to analyse MTJ behavior in disuse condition, furtherly investigating the strict morpho-functional correlation. Therefore, 4 hind-limb suspended (HS) and 4 control (CTRL) rats were studied. After sacrifice, MTJs of plantaris muscle were processed for electron microscopy. After 5 days of suspension, skeletal muscle displays signs of atrophy in response to decreased mechanical loading. In fact, close to muscle-tendon interface, irregular misaligned sarcomeres, with absent Z-lines, were observed. Muscle intermyofibrillar component spreads, especially between terminal filaments and tendon finger-like processes. The surface area between muscle and tendon was evaluated with P/B ratio, where “B” is the base and “P” is the interface length of tendon fingerlike processes at MTJ level. After 5 days of suspension, the IL/B ratio decreases from 6.39 in CTRL to 3.92 in HS and the suspension reduces the mean extension of finger-like processes from 1.84 μm to 0.97 μm. In conclusion, along with muscle atrophy features, ultrastructural changes occur at the MTJ organization level, as an adaptation to muscle unloading.