Intermediate voltage electron microscopy of transverse tubules at myotendinous junctions.

The 3-dimensional distribution of transverse (T) tubules at myotendinous junctions (MTJs) was studied by intermediate voltage (400 kV) electron microscopy of thick sections of rat vastus intermedius and chicken pectoralis muscles stained with lanthanum nitrate. Transversely oriented T tubules were seen to run at the level of A-I junctions (the rat vastus intermedius) and Z bands (the chicken pectoralis), but were absent from such levels adjacent to the end of MTJ processes. These tubules opened to the lateral wall of sarcolemmal infoldings of MTJs and to the lateral cell surface. Longitudinally running T tubules were seen to connect with the transverse T tubules and to open at the bottom of junctional folds. The lack of T tubules at the final sarcomeric regions seems to indicate that the terminal sarcomeric half in close proximity to MTJs may be activated from the sarcoplasmic reticulum which forms couplings with the MTJ sarcolemma and/or longitudinal tubules in the MTJ processes.