Effect of third molar absence on development of masticatory muscles in EL mice

We investigated the effect of the lack of the third molar on synaptogenesis at the neuromuscular junction of masticatory muscles in EL/ Sea (Sea) mice, in which tooth bud of all third molars disappears in the process of development. We analyzed the mRNA expression levels of the nicotinic acetylcholine receptor (nAChR) α, ε, and γ subunits at 3, 6, and 12 weeks of age in the masseter and temporalis muscles of Sea mice, and in the control strain EL/Kwl (Kwl) mice, using real-time PCR. In the Sea, no third molars erupted over the oral mucous membrane during the experimental period. In the Kwl, at 3 weeks of age, no third molars had erupted over the oral mucous membrane, but they partially had emerged over the maxilla and mandible and, after 3 weeks of age, completely erupted over the oral mucous membrane. In the masseter, the expression levels of the α, ε, and γ subunit mRNAs in the Sea mice were 92% (P< 0.05), 100% (P< 0.05), and 91% (not significant) higher than in the Kwl mice at 3 weeks of age, but steadily decreased until 12 weeks of age. The expression levels in the Kwl mice were stable throughout the experimental period. In the temporalis and gastrocnemius, there were no significant differences in the expression levels of any of the subunits studied between the Sea and Kwl mice. This result suggests that the difference in the expression of nAChR subunits in the masseter muscle between the Sea and Kwl mice is associated with the eruption of the third molars, and that the absence of the third molars has different effects between the masseter and temporalis.