Juvenile and adult rat neuromuscular junctions: density, distribution, and morphology.

Anatomical and physiological differences in neuromuscular junctions (NMJs) between juvenile and adult muscle may partially explain the variability in clinical results following chemodenervation with botulinum toxin or nerve repair in children and adults. We evaluated NMJ density, distribution, and morphometry in biceps brachii and gastrocnemius muscles from juvenile and adult rats. Motor endplates were stained with Karnovsky-Roots methods, and NMJ density (number/gram muscle tissue) was determined. The NMJ morphometry was quantitated with alpha-bungarotoxin labeling using confocal microscopy. Juvenile rats had a greater NMJ density in both muscles compared with adult rats. Juveniles and adult rats had a similar NMJ distribution in both muscles. In juvenile muscles NMJs occupied approximately 50% of the surface area and 70% of the length, width, circumference, and gutter depth compared with adult muscles. Our study demonstrates that although NMJs are smaller, juvenile muscles have a higher NMJ density than do adult muscles with similar distributions. If these age-dependent differences in NMJ density are obtained in humans, they may account, at least in part, for the better recovery that occurs in children than adults after nerve repair and also suggest that the dosage of botulinum toxin (units per kg) for chemodenervation may need to be increased in juveniles.