Satellite cells of striated muscle after compression injury so slight as not to cause degeneration of the muscle fibres

Summary Electron microscopy of musculus bulbi rectus superior in the rat reveales increase in the number of. muscle satellite cells during the first 10–12 hours following compression injury so slight as not to cause degeneration of the muscle fibre. These satellite cells are situated close to the muscle nuclei and, in contrast to those in the intact rectus superior, they have a pale nucleus with a conspicuous nucleolus, sparse, pale cytoplasm containing a few small mitochondria, a mainly agranular endoplasmic reticulum and numbers of pinocytotic vesicles along their cell membrane. Later on, between 24 and 48 hours, the cytoplasm increases greatly in amount and extendes in long processess over a considerable length of the muscle fibre. An increase in the number of mitochondria and free ribosomes, the appearance of polysomes and great numbers of Golgi complexes and a decrease in the number of pinocytotic vesicles is noted. On the 4th day, some of the satellite cells have still further extended their cytoplasm beneath the muscle basement membrane, while others are apposed to the muscle fibre only with a minor part of their membrane. On the 9th day, only satellite cells comparable in number and structure with those in intact muscles are found.The possibility is discussed that the increase and subsequent decrease in the number of these satellite cells as well as the changes in their morphology at different periods after the injury reflect morphogenesis of satellite cells originating from the nuclei of the muscle fibre by mitotic division even after a trauma, too slight to cause any visible muscle injury. The observations are interpreted as giving new support to the hypothesis that muscle satellite cells may be of importance in posttraumatic muscle regeneration.