The presence of adipocytes at the myotendinous junction (MTJ) has not previously been reported in mammals. However, during studies of the MTJ, our group has noticed the presence of adipocytes as a general and surprising phenomenon.

The main aim of this study was to describe and quantify the presence of adipocytes in relation to the MTJ in healthy human subjects. In addition, we wanted to investigate whether adipocytes are also found at the MTJ in mice, which is a commonly used species in muscle and tendon research. Samples from the semitendinosus MTJ from 10 healthy human subjects and tibialis anterior and soleus from 6 mice were prepared for electron microscopy or for immunohistochemical labeling against perilipin (PLIN1, to identify adipocytes), collagen XXII (representing the myotendinous junction) and nuclei (DAPI).

In all humans and mice, numerous adipocytes were present at the MTJ. Electron microscopy of human samples showed that adipocytes were located near the characteristic foldings of the muscle membrane at the MTJ.

The large number of adipocytes at the MTJ in humans and mice suggests that they have an important function in the interplay between skeletal muscle and tendon in this region.