Abstract
The developmental pattern of the neuromuscular junction (NMJ) in masseter muscles of the Japanese field vole, Microtus montebelli, was investigated using acetylcholinesterase (AChE) staining and electron microscopy. At birth, intense AChE activity limited to the site of the NMJ where many axon terminals with the cholinergic nature were converging was observed, indicating that cholinergic neuronal activity in the vole masseter muscle begins at this stage. The major morphogenesis of the NMJ such as : AChE staining reaction, concentration of myonuclei at the subneural site, elimination of the axon terminals, formation of the myeline sheath in the intramuscular axons, and the appearance of numerous junctional folds in the postsynaptic membrane was accelerated after postnatal day five, and amended dramatically at day ten with the maturation time of these NMJ components at around day fifteen. From the combination of the present and previous studies, it is clear that both AChE reaction and neuronal structures alter considerably at a time when structural and functional improvements give rise dramatically to muscle fibers. This must be considered in relation to the critical role of the neuronal influence on the differentiation and maturation of the vole masseter muscles that are required for the accomplishment of its own characteristic herbivorous food habits.
