Ultrastructural studies of young and old mouse neuromuscular junctions.

The ultrastructure of the neuromuscular junction of young and old male CBF-1 mice was analysed both qualitatively and quantitatively. The age-related findings were similar in both the phasic extensor digitorum longus muscle and the tonic soleus muscle but more pronounced in the latter. Presynaptic terminals of old mice compared to young showed decreases in nerve terminal area, mitochondria and synaptic vesicles, but increases in smooth endoplasmic reticulum, coated vesicles, cisternae, microtubules and probably neurofilaments. On the postsynaptic side there were increases in complexity of junctional folds and subsarcolemmal vesicles, and the appearance of lipofuscin deposits. Occasional denervated postsynaptic regions were encountered in old neuromuscular junctions, but the predominant characteristics of aging changes were not those of denervation. Rather, a unique and uniform process involving most of the population of nerve terminals, possibly of physiologically adaptive significance, appears to occur with age in both phasic and tonic limb muscles.