Tubular aggregates in type II muscle fibers: Ultrastructural and histochemical correlation

Abstract

Aggregates rich in DPNH and TPNH tetrazolium reductase activity appeared only in type II muscle fibers of some humans with mild or no clinical neuromuscular disease. They were composed of long tubules containing one or more inner tubules and some saccular dilations. Some abnormal tubules and sacs contained amorphous material like that in normal lateral sacs of the sacroplasmic reticulum (SR). The only relationship of the tubules with normal structures was a contiguity with, as if they were arising from, the lateral sacs of SR. The tubular aggregates were thought to be massive proliferations of the longitudinal component and/or lateral sacs of SR. The possibility was raised that they were formed in response to exogenous or endogenous “toxins”.