Some observations on the fine structure of the myotendinous junction in myotomal muscle of the tadpole tail.

Myotendinous junctions in the myotomal tail muscles of the tadpole of Rana rugosa were examined by electron microscopy. At the site of the myotendinous junction, the sarcolemma is covered on its sarcoplasmic aspect by the connecting filament layer and the attachment layer, and on the extracellular aspect by the intermediary later and the external lamina, with associated collagen fibrils. The intermediary layer consists of filamentous structures which closely resemble "microfibrils" (Hanak and Böck, 1971), "spine-like or thread-like profiles" (Korneliussen, 1973) and "intermediary layer" (Nakao, 1975a, b) in the myotendinous junctions of other vertebrate skeletal muscles. Particularly interesting is the fact that all the coverings and linings of the sarcolemma, including the external lamina, are completely absent in the terminal segment of the finger-like sarcolemmal invagination characteristic of the myotendinous junction. Furthermore, special types of coupling between a sac of sarcoplasmic reticulum and a part of the sarcolemmal invagination are frequently observed. These couplings always occur along the region of the sarcolemma where the external lamina is absent. The couplings show features similar to those to the triad, such as "SR feet", "scalloped SR membranes" and "granular content of the SR sac", suggesting that they are analogous and functionally similar to the triad and other equivalent structures.