Ultrastructure of the caudal muscle cells in the larva of a polyclinid ascidian

Abstract

Two paraxial bands of somatic striated muscle occur in the tail of the larva of the compound ascidian Aplidium ?constellatum. The mononucleate muscle cells of each band align in longitudinal rows between the epidermis and the notochord. The cross-striated myofibrils, originating and terminating at intermediate junctions on the transverse cellular boundaries, are indiscrete. They follow a spiral course through the subcortical and medullary sarcoplasm, bypassing the nucleus and the other organelles and inclusions in the center of the cell. Cisternae of the sarcoplasmic reticulum envelop the myofibrils, forming compact fenestrated sheets that are continuous between the sarcomeres and locally undifferentiated with respect to the myofibrillar striations. Cisternae of the perifibrillar sarcoplasmic reticulum near each sarcomeric Z-line establish dyadic interior couplings with a network of tubular invaginations of the sarcolemma. The sarcolemmal tubules can originate from any surface, including the transverse cellular boundaries. Near the half I-bands of the terminal sarcomeres at the intermediate junctions, the perifibrillar cisternae frequently leave the fenestrated sheets and extend to the overlying sarcolemma, becoming the sub-sarcolemmal cisternae of dyadic peripheral couplings.