Ultrastructure of the muscle spindle in dystrophia myotonica. I. The intrafusal muscle fibres.

The abnormal intrafusal muscle fibres of four spindles located in extensor indicis biopsies from two patients with dystrophia myotonica have been examined by electron microscopy. In all cases, the fibres had undergone severe longitudinal fragmentation in their equatorial, proximal-polar and mid-polar zones. It was not possible to differentiate the fibres into nuclear bag and nuclear chain types. Many of the fragments were arranged in clusters, each cluster being bounded by a single basement membrane. Numerous partially divided fibres were seen in mid-polar sections, and a morphological picture of the fragmentation process has been presented. The ultrastructural appearance of individual fibre fragments ranged from normality through to severe atrophy. Intrafusal satellite cells showed an increase in numbers, presumably in response to the pathological processes affecting the muscle fibres. The nature of this response is discussed.