Ultrastructural evidence of continued reorganization at the aging (11–26 months) rat soleus neuromuscular junction

Ultrastructural remodeling, with evidence of focal deafferentation and reinnervation, occurs within normal young adult rat soleus neuromuscular junctions (Cardasis and Padykula, 1981). This may be related to normal variations in function. Recognition of this plasticity provides a basis for analysis of aging changes in junctional ultrastructure. Thirty soleus junctions were studied between 11 and 26 months of life. In these junctions, compared to younger ones (3-5 months) synaptic sites with the conventional ultrastructure become increasingly sparse. There is an increase in extent and frequency of exposed junctional folds, of intervention of Schwann cell cytoplasm between axon and junctional folds, and of numbers of lysosomes in all cytoplasmic profiles. Often primary clefts are shallow or missing, and secondary folds are widened and contain collagen. Features limited largely to these older junctions include highly pleomorphic myonuclei, deeply invaginated by myofibrils, and an increase in cellular profiles between basal lamina and sarcolemma. The identity of these profiles is unknown. At other locations within many of the same endplates, small intact terminals are associated with larger expanses of junctional folds, and several small terminals occur within the same primary cleft. Such terminals frequently contain dense-cored vesicles. These observations suggest continuation of some terminal axonal regeneration. Thus, the ultrastructure of these aging neuromuscular junctions reveals the same degenerative and regenerative events suggested by the ultrastructure of younger junctions, but suggests a shift in the balance between them.