Ultrastructural abnormalities and perifascicular atrophy in childhood dermatomyositis with special reference to transverse tubular system-sarcoplasmic reticulum junctions.

Comparison of ultrastructural abnormalities between perifascicular and centrofascicular myofibers was made in the same muscle fascicles from six patients with childhood dermatomyositis. Consistent abnormalities are seen at the junctional sites between the transverse tubular system (TS) and sarcoplasmic reticulum (SR) as visualized by lanthanum staining. Early conical dilations at the TS-SR junctions have progressed to become connected with large cylindrical or spheroid channels representing SR. These TS-SR anastomoses are far more extensive in the perifascicular than in the centrofascicular myofibers. Honeycomb TS proliferations suggestive of a repair process occasionally occur at the sites of TS-SR anastomoses. These alterations precede myofibrillar derangements and they may be related to the primary pathogenetic process involved in dermatomyositis. It is postulated that leakage of degraded structural protein molecules through TS-SR anastomoses may lead to the perifascicular atrophy seen on light microscopy.