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.
The force generated within skeletal muscle fibers of vertebrates is transmitted to the tendon at the muscle-tendon junction. Ultrastructural analysis of the murine muscle-tendon junction following a variety of experimental manipulations has produced evidence that the muscle-tendon junction can be described in terms of four principal domains (Trotter and Eberhard, 1983), two of which are discussed in the present report. Each domain is defined by the shape and orientation of its principal components, and by its position with respect to the plasma membrane. The internal lamina is composed of actin filaments, with a center to center spacing of approximately 12 nm, oriented mainly parallel to the principal vector of contractile force, and to the plasma membrane. These filaments are cross-linked into a structural unit, perhaps by the electron-dense structures which are associated with them. The internal lamina is morphologically connected to the external lamina (lamina densa) by a population of fine filaments oriented approximately perpendicular to the principal vector of contractile force. These filaments which constitute the connecting domain, are approximately 2-8 nm in diameter and are at least 50 nm long. They pass through three separate regions: the sarcoplasm between the internal lamina and the plasma membrane; the plasma membrane proper; and the extracellular space between the plasma membrane and the lamina densa. This third region is often referred to as the lamina lucida. These filaments are composed of at least three separate components in series, linked together by noncovalent interactions. The existence of these discrete structural domains implies that each has a different molecular composition and different mechanical properties.
The muscle-tendon junction of murine skeletal muscles has been analyzed by a variety of extraction techniques, by myosin subfragment-1 binding experiments, and by ultrastructural immunocytochemistry . The results indicate that the muscletendon junction is composed of four distinct domains: an intracellular domain, the internal lamina; a domain connecting the internal lamina with the lamina densa of the external lamina, the connecting domain; the lamina densa; and a domain which attaches the lamina densa to the collagen fibers, the matrix. Each of these domains is distinct with respect to position, three-dimensional organization, and molecular composition, and is therefore considered to have a unique role in the transmission of contractile force.Key words: myotendinous junction, laminin, type IV collagen, heparan sulfate proteoglycan, alpha actininThe muscle-tendon junction is the specialized region at the ends of skeletal muscle fibers where force is transmitted from intracellular myofilaments, across the plasma membrane, to extracellular collagen fibers. The most obvious structural feature of the muscle-tendon junction is the folding of the interface between muscle and tendon, such that the two phases are multiply invaginated into one another. This structural arrangement is characteristic of skeletal muscle fibers in numerous species, including arthropods, where tendons are replaced by tendon cells full of microtubules. It is missing in cardiac muscle, in which the fascia adherens has the plasma membrane of adjacent cells parallel to one another and perpendicular to the direction of force. As a consequence of this structural arrangement of the muscle-tendon junction, the interface between muscle fiber and tendon is increased in area, and the greatest part of the interface is nearly parallel to the direction of force. The increased area produces a decrease in stress, while the parallelism of the interface causes the junction to be loaded in shear much more than in tension. These elementary mechanical consideraAddress reprint requests to
At the muscle-tendon junction of skeletal muscle fibers the structural interface between muscle cell and connective tissue is amplified by tapering, by indentation, and by surface folding. The precise form taken by the surface folds has been unknown due to a lack of studies on the three-dimensional geometry of the muscle-tendon junction. Analysis of this region by scanning electron microscopy, using conventional preparative techniques, is uninformative because the muscle surface is covered by connective tissue. Removal of the connective tissue from individual murine muscle fibers by incubation of fixed fibers in hot HCl, followed in some instances by treatment with collagenase, permits SEM analysis of the uncovered fiber ends. The muscle fiber end is characterized by surface specializations in the form of anastamotic cylindrical folds. Transmission electron micrographs of cross sections and of serial longitudinal sections of muscle fiber ends confirm that the SEM observations are correct.
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