Summary
Myotendinous junctions in the parietal muscle of the Atlantic hagfish (Myxine glutinosa, L.) and in the diaphragm of rat were studied with the electron microscope.
The ends of muscle fibers posses deep tube-like invaginations and also finger-like processes. Terminal I-filaments of the myofibrils attach to the plasma membrane of the invaginations. Collagen fibrils of the tendon attach to the external aspect of the lamina densa. The lamina densa and also collagen fibrils continue into the invaginations.
Specializations are present in the zone between the plasma membrane and the lamina densa of the invaginations, being most conspicuous in Myxine. These consist of 60 Å wide spine-like or thread-like profiles 150 Å apart, connecting with both the outer leaflet of the plasma membrane and the lamina densa. Spatially, they form parallel circular ridges oriented transversely in stacks along the invaginations. Similar spine-like profiles are also discernible in the invaginations of rat muscle fibers. In rat, however, their spatial arrangement (ridges or fibrils) remains unknown.
These specializations may serve to attach the lamina densa to the plasma membrane, and may be important for the transmission of force from muscle fibers to tendon.
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The author is indebted to Dr. Finn Walvig, Biological Station, University of Oslo, Drøbak, for supply of hagfishes, and to Mrs. Jorunn Line Vaaland for technical assistance.
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Korneliussen, H. Ultrastructure of myotendinous junctions in Myxine and rat specializations between the plasma membrane and the lamina densa. Z. Anat. Entwickl. Gesch. 142, 91–101 (1973). https://doi.org/10.1007/BF00519878
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DOI: https://doi.org/10.1007/BF00519878