Morphological changes of neuromuscular junctions in the dystrophic (dy) mouse: a scanning and transmission electron microscopic study.

Neuromuscular junctions in the extensor digitorum longus muscle of control and dystrophic (dy) mice at stages ranging from 30 to 120 postnatal days were examined by scanning and transmission electron microscopy (SEM and TEM). In control mice, the neuromuscular junctions consisted of thin ramifying nerve endings and of complex labyrinthine gutters with a great number of slit-like junctional folds. The subneural apparatuses of dy mice on the 30th and the 60th postnatal days were composed of undeveloped gutters or a number of cup-like depressions containing a few pit-like or slit-like junctional folds. In dystrophic neuromuscular junctions on the 120th postnatal day, subneural apparatuses were classified into two types: one had a cluster of continuous cup-like depressions with or without slit-like junctional folds, and the other consisted of discrete depressions containing slit-like junctional folds. Moreover, a small number of dystrophic muscle fibers were endowed with two subneural apparatuses which were 20 to 30 microns away from each other. Small round pits (0.5-1.0 microns in diameter) in some cup-like depressions were occasionally protruded by short processes of terminal axons. The present findings suggest that the age-dependent postsynaptic structural changes of dystrophic dy mice induce a complex morphological reorganization of presynaptic nerve endings. This might help the nerve endings compensate for the functional deterioration of dystrophic muscle fibers.