A morphometric analysis of the muscle-tendon junction.

Transmission of the contractile force produced by skeletal muscle fibers from myofilaments to tendon fibers occurs at the muscle-tendon junction. This interface between muscle and tendon is characterized by an amplification of the membrane area across which force is transmitted, with the result that stress at the muscle-tendon interface is less than it would be if the muscle fiber ended without surface folding. The amount of stress reduction is proportional to the degree of surface amplification. Because an understanding of the mechanical properties of the muscle-tendon junction requires a quantitative appreciation of the amplification of interfacial area, and hence the reduction of stress, produced by membrane elaboration at the muscle-tendon junction, we have developed a reliable morphometric approach for quantifying this surface amplification. The approach reported here makes use of point-counting techniques applied to thin sections of murine muscle-tendon junctions, together with a statistical analysis of the data. The results indicate that the load on the cell membrane at the muscle-tendon junction is reduced approximately 1 order of magnitude by membrane amplification, compared to the load calculated to occur if the muscle fiber ended as a right cylinder. In addition, significant differences in the degree of membrane amplification have been detected among the four muscle-tendon junctions analyzed in this study. These results and methods should prove useful in future analyses of normal and abnormal muscle-tendon junctions.