Ultrastructure and differentiation of ascidian muscle. II. Differentiation of the caudal muscle cells in the larva of Diplosoma macdonaldi.

The larval muscle cells of Diplosoma macdonaldi contain subcortical and medullary myofibrils which are invested by fenestrated sheets of the sarcoplasmic reticulum. Cisternae of the sarcoplasmic reticulum are coupled with tubular invaginations of the sarcolemma. To appreciate better such uncommon features of cellular organization, six embryonic stages were selected for an ultrastructural study of myogenesis. The proliferative, synthetic, and elaborative phases of myogenesis were represented by embryos ranging from neurulae to prehatching larvae. The contractile apparatus originates during the synthetic phase of myogenesis, when thick and thin myofilaments appear in the cortical sarcoplasm at the epidermal and notochordal poles of the cell. The myofilaments promptly aggregate into unstriated fascicles, and the fascicles unite in series to establish the rudimentary myofibrils. All major sarcomeric bands, except the Z-lines, are evident along the myofibrils. Cisternae of the sarcoplasmic reticulum form peripheral couplings with the overlying sarcolemma, and they also form interior couplings with sarcolemmal invaginations from the ends of the cell. The interior couplings localize over the I-bands of the myofibrils. In the elaborative phase of myogenesis, mitochondria invade the cortical sarcoplasm, and the contractile apparatus passively shifts to the subcortex and medulla of the cell. Relocation of the myofibrils coincides with the disappearance of all peripheral couplings. Cisternae of the sarcoplasmic reticulum anastomose around the myofibrils, creating the fenestrated sheets that extend between sarcomeres. As Z-lines begin to bisect the I-bands, the perifibrillar cisternae become confluent with the cisternae in the precocious interior couplings.