Myogenic Events in Compound Ascidian Larvae

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Abstract
SYNOPSIS. Comparative studies on the compound ascidians Distaplia occidentalis and Diplosonta macdonaldi indicate that larval myogenesis is divisible into three periods. In the prolifnative phase, the presumptive muscle cells divide by mitosis, assume specific relationships to the notochordal and epidermal rudiments of the embryo, and assemble the synthetic machinery to support the ensuing period of myogenesis. In the synthetic phase, the differentiating muscle cells increase in volume, establish the contractile apparatus, and augment the cisternae of agranular sarcoplasmic reticulum. The sarcoplasmic reticulum and sarcolemma form local adhesions that foreshadow structural couplings. In the elaborative phase, the caudal muscle cells refine the contractile apparatus, envelop it with perifibrillar cisternae of sarcoplasmic reticulum, and anchor it to the sarcolemma at the transverse cellular boundaries. Dyadic couplings link the perifibrillar sarcoplasmic reticulum to the sarcolemma. Subsarcolemmal specializations appear at future synaptic sites shortly before hatching.

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