The slow progression of weakness and atrophy of muscle and the prominence of myotonia suggested that the disease observed in thedy2jstrain of the Bar Harbor 129 mice is similar to the human form of myotonic dystrophy. Bright field microscopy and histochemical studies disclosed primary myopathic changes which were at first focal, becoming more diffuse as the disease progressed. The ultrastructural studies revealed two main types of alteration. The most obvious was the dystrophic process in muscle. Attention was focused on the second category of change, observed at every motor end plate, and consisting of a reduction in number and size and a distortion in shape of the terminal axoplasmic vesicles. In addition, at a number of end plates there was retraction of axolemma from the plasma membrane of the sareolemma. At still other end plates, processes of Schwann eytoplasm separated axolemma from plasmalemma. The fine structure of 46 motor end plates was altered while the structure of 12 motor end plates in the control muscles was normal. Considering the preservation of the post-synaptic architecture and the integrity of the axon with its myelin up to its termination, there is reason to believe that the described abnormality is primary at the motor end plate and not secondary to diseased nerve or muscle.
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