Muscle fibers from dysgenic mouse in vivo lack a surface component of peripheral couplings.

We have studied the structure of developing normal and dysgenic (mdg/mdg) mouse muscle fibers in vivo, with special attention to the components of the junctions between the sarcoplasmic reticulum and either the surface membrane or the transverse tubules. Triads and dyads are rare in dysgenic muscle fibers, but have apparently normal disposition of feet and calsequestrin. Peripheral couplings in normal developing muscle fibers have junctional tetrads in their surface membrane in association with the junctional feet. Muscle fibers in dysgenic mice lack junctional tetrads. This provides indirect evidence for the identification of the components of junctional tetrads with dihydropyridine receptors, which are known to be absent in dysgenic muscle fibers.