The fine structure of myo-tendon junction in some mammalian skeletal muscles

Abstract

The fine structures of the myo-tendon junctions in the gastrocnemius muscles from some mammals (dogs, cats, guinea-pigs, rats, mice and human beings) were studied with the electron microscope. Observations confirmed that each myo-tendon junction has basically identical structures as reported previously by other investigators. The end of the muscle fiber showed many processes and invaginations of the sarcolemma. Such a complicated end of the muscle fiber was definitely enclosed by a continuous sarcolemma. Each myofibril always ended at the level of the Z-disc and seemed to be embedded in the dense area just inside the sarcolemma. The subsarcolemmal dense area contained longitudinally arranged fine filaments, with which the actin myofilaments of the ending sarcomere appeared to connect. The fine filaments in the dense area showed about the same diameter as that of the actin myofilament. The nature of the dense area were discussed with special reference to the Z-disc. The collagen fibrils which left the surface of the end of the muscle fiber ran for some distance and joined the main bundles of typical tendon fibrils. Their average diameter was considerably smaller than that of the typical tendon. It was emphasized that there were the connective tissues connecting between the end of the muscle fiber and the typical tendon tissue. Several fibroblasts were usually seen in close association with the end of the muscle fiber. They sent many flattened or cylindrical projections into the clefts or the invaginations of the sarcolemma. Most finger-like processes of the muscle fiber end were circularly surrounded by the fibroblastic projections. The possible mechanism of the addition of new sarcomeres at the end of the muscle fiber were discussed. © 1965, International Society of Histology and Cytology. All rights reserved.