Stereological analysis of the muscle‐tendon junction in the aging mouse

Transmission of contractile tension from skeletal muscle fibers to connective tissue elements is thought to occur at the muscle‐tendon junctions, specialized regions at the extreme ends of the fibers. Previous stereological studies on adult mouse and chicken fibers have shown that, with reference to equal cross‐sectional areas of myofibrils, the muscle‐tendon junctions of faster fibers have significantly more surface membrane devoted to force transmission than do those of slower fibers (Trotter et al.: Anat. Rec. 213:16–25, 1985a; Trotter et al.: Anat. Rec. 213:26–32, 1985b; Trotter and Baca: Anat. Rec. 218:256–266, 1987). In the present study we have analyzed the muscle‐tendon junctions of 30‐month‐old mice, employing techniques for scanning and transmission electron microscopy and for ultrastructural stereology which are identical to those previously used to study the same muscles in 4‐month‐old mice. Whereas the principal structural features of the muscle‐tendon junctions of fibers from adult and aged mice are indistinguishable, stereological analyses of the fiber‐tendon interfaces indicate that, in aging animals, the interfacial ratio (the ratio of the surface area of force‐transmitting membrane to the cross‐sectional area of force‐generating myofibrils) is significantly reduced. While the interfacial ratio of fast‐twitch fibers of the adult plantaris is about 14.5, the corresponding ratio in aged animals is about ten. In the predominantly slow‐twitch fibers of the adult soleus, the interfacial ratio is about ten at the insertion and about 12.7 at the origin, whereas the corresponding ratios in the aged animals are both about ten. These findings are consistent with the notion that the amount of surface area specialized for force transmission is related to the contractile speed of the fiber.

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