Age-related remodelling of the myotendinous junction in the mouse soleus muscle

The age-related loss of muscle mass and function predominantly affect muscles of the lower limbs and have largely been associated with decline in muscle fibre size and number, although the exact mechanisms underlying these losses are poorly understood. In addition, consistent reports that the loss of muscle strength exceeds that which can be explained by declines in muscle mass has widened the search for causes of sarcopenia to include supporting tissues such as the extracellular matrix and tendons. Although the changes to both muscle and tendon with age are well characterised, little work has focused on the interface between these two tissues, the myotendinous junction (MTJ). Given the crucial role for this structure in force transfer between muscle and tendon, we asked whether the myotendinous junction underwent structural changes with age in lower limb muscle. We used whole muscle to assess gross muscle and tendon morphology, and immunohistochemistry to determine fibre and MTJ profile number in young (6 months), middle aged (18 months) and elderly (24 months) C57BL/6 female mice. MTJ length was quantified using serial cross sections of the soleus muscle. We found an apparent 3.5-fold increase in MTJ profiles per cross section with no increase in fibre number in old mice, and found this to be a result of a doubling in length of the MTJ region with age. This coincided with an increase in proximal tendon length (31%), as well as an increase in collagen deposition between 6 and 24-months of age consistent with an expansion of the fibre termination area. These findings uncover a previously undescribed effect of ageing on the MTJ and open up new lines of investigation into the role of this structure in the age-related loss of muscle function.