An Electron Microscopic Studay on the Development of Cross-striated Muscles in Ephyrae of Aurelia aurita

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The development of circular cross-striated muscles in ephyrae of Aurelia aurita was investigated electron microscopically. Ephyrae cultured in the laboratory were fixed at the following three stages: 1) Before release by the strobilae, 2) 1 day after release, and 3) 7 days after release. Young ephyrae in the first stage usually did not show complete pulsation. Ephyrae in the second stage sank to the bottom of the petri-dish. When they were fully developed, they moved quickly with active pulsation. Muscle cells in the circular muscle were small in number at the first stage, but when fully developed, they increased to 200-300 in number. Myofibrils at the first stage, where two types of myofilaments are arranged in a smooth-type muscle, partially contained incomplete A-bands. Newly constructed A-bands included many short microtubules of about 20 nm in diameter running parallel to each other. Two types of myofilaments ran in all directions at the first stage, They were then gradually rearranged into an ordered lattice (thick ones) or a hexagonal pattern (thin ones). The myofibril bore short sarcomeres, few sarcoplasmic reticular systems and dotted Z-membranes which showed an arrangement of J-granules.
1983-09-25