Electronmicroscopic observation on the degeneration of skeletal muscles in Xenopus laevis during metamorphosis and after denervation.

We observed metamorphosis-related structural changes in the tail muscle of Xenopus laevis tadpoles during tail involution and changes in the calf muscles of adult Xenopus laevis. The specimens were treated with hydrochloric acid for extraction of the fibrous components in the connective tissues. Prior to metamorphosis, the muscle fibers appeared straight and flat. At the start of metamorphosis, they appeared crooked with uneven surfaces. Scanning electronmicroscopy revealed a marked increase in the number of macrophages and fibroblast-like cells in the muscle fibers. Transmission electronmicroscopy revealed an enlargement of the sarcoplasmic reticulum, the appearance of myelin structures and the uptake of muscle fragments by macrophages. The myofilaments in the muscle fragments were in different stages of decomposition. After denervation of the nervus ischiadicus, scanning electron-microscopy showed that the diameter of the muscles was reduced; a fine unevenness of the muscle surface could be observed. However, the cellular component in the muscle fibers was not obviously increased. Transmission electronmicroscopy showed that the sarcolemma and basal lamina were finely waved; cross striae were unclear due to a decrease in myofilaments. There are similarities in the structural changes which occur in the tail muscles of Xenopus laevis during tail involution and calf muscles after denervation, but the processes are not identical. In tadpols, nerves exert a secondary influence on structural changes of the muscles in the process of tail involution during metamorphosis.