Age changes in neuromuscular junctions of masseter muscle.

There is little information regarding the relationship between aging and ensuing morphological changes in the neuromuscular junction (NMJ) of the masticatory muscle. The masseter of C57BL/6J male mice at three different ages (young adult, 6 months; mature adult, 12 months; and old, 30 months) was studied. Morphological measurements were taken from zinc iodide osmium stained NMJ. Camera lucida drawings were superimposed on a computer monitor via a video camera and traced using a digitizer. The data were treated statistically using multivariate analysis of variance (MANOVA). No difference was found between 6 and 12 months of age, but a significant decrease in morphological parameters in NMJ from 30-month-old animals was found when compared with mature adult animals. Nerve terminal areas, perimeters, longitudinal extent lengths, and fiber diameters were reduced by 24%, 21%, 15.5% and 23%, respectively. Nerve terminal branches and incidence of sprouts were significantly increased in older animals. Age changes in the NMJ morphology are probably associated with altered balance between degeneration and regeneration of nerve terminals. This view is supported by increased terminal sprouting in old mice which is indicative of the plasticity or remodelling of NMJ during aging.