Electron microscopic observations on myoid cells of the frog's thymus

Myoid cells of the thymus of hibernating frogs were examined by electron microscopy. This cell type is represented by immature, fully‐developed and degenerating cell forms. The immature forms are mainly located at the surface, whereas the developed ones are found in the inside of the thymus. A peculiar type of immature cell containing no thick filaments, but possessing a rich sarcoplasmic reticulum among the thin filaments is described. The developed myoid cells have a myofibrillar apparatus showing regular cross‐striations. The myofibrils are arranged in concentric layers around the nucleus. The proposed role and origin of myoid cells and their probable connection with myasthenia gravis are discussed.