Naturally-occurring degeneration in chick muscle development: ultrastructure of the M. complexus.

Ultrastructural study of the M. complexus muscle of the chick embryo has demonstrated two populations of fibres: the more common is the normal myotube or muscle fibre, which was observed in various stages of development; the other shows myofibrillar contractions which, at their greatest degree, produce the appearance of an amorphous mass of myofilaments. The contracted fibre had rounded and swollen mitochondria, and vacuoles (autophagic) containing glycogen, and it exhibited a cleavage of the cell which isolated the nuclear region from the main body of the fibre. When the fibres were less contracted they resembled degenerative mammalian fast-twitch-oxidative-glycolytic fibres after immobilization. The more normal fibre population was identical with that of the pectoralis muscle, which was used as a control. These results suggest that the contracted fibres are degenerating, which agrees with conclusions by earlier investigators using light miscroscopy.