Scanning electron microscopical study of skeletal muscle fiber ends in normal and dystrophic mice.

Muscle fiber ends at myotendinous junctions were examined by scanning electron microscopy after removal of tendon connective tissue components by HCl hydrolysis in the extensor digitorum longus muscle of 30-, 60- and 120-day-old normal and dystrophic (dy) mice. A remarkable morphological difference between normal and dystrophic mice was observed. In normal 30-day-old mice, muscle fiber ends had already assumed a complicated three-dimensional morphology with many thin cytoplasmic processes and lateral longitudinal clefts. On the other hand, in dystrophic animals at the 60th day, muscle fiber ends were characterized by a simple conical form with a rather smooth surface in which existed a number of pit-like sarcolemmal invaginations, and short and narrow longitudinal slits, possibly an indication of developmental immaturity. Thereafter, the slits increased in number and in length, although the linear elongation of the slits seems to be caused by the fusion of adjacent pits to one another and to existing slits. From these findings, the fiber ends in the adult (120-day-old) dystrophic mouse are suggested to retain a preceding state of their unique structural differentiation.