Postnatal Expressions of Myosins in Masseter Muscle of Microphthalmic Mouse

Objectives: The properties of mouse masseter muscle are remarkably changed during the postnatal development, in particular before and after a transition in feeding behavior from suckling to chewing at 3 ~ 4 weeks of age. Since in microphthalmic mice (mi/mi) with a mutation in the MITF gene teeth do not erupt, a transition in feeding behavior does not seem to occur. The purpose of the present study is to determine what extent a transition in feeding behavior contributes the changes in the properties of masseter muscle during the postnatal development using mi/mi mice. Methods: We analyzed the mRNA expressions of myosin heavy chain (MyHC) IIa, IIb, IIx and I, as indicators of muscle properties, in the masseter and gastrocnemius muscles obtained from 1, 2, 3, 4 and 8 week-old mi/mi and control mice using the competitive, reverse transcriptase-polymerase chain reaction method. After all mice were weaned around 3 weeks of age, mi/mi and control mice were fed a powdered and pellet diet, respectively. Results: In the gastrocnemius muscle, no significant difference in expression levels of all MyHC mRNAs was observed between mi/mi and control mice during the whole postnatal period studied. In the masseter muscle, except that the mRNA expression levels of MyHC IIx in the mi/mi mice were significantly less than those in the control mice at 3 and 4 weeks of age (p<0.05), no significant difference was found: Conclusions: Since the result of gastrocnemius suggests that the mutation in the MITF gene does not alter the transcription of MyHCs, the significant changes in MyHC IIx between mi/mi and control mice seem to be ascribed to a transition in feeding behavior, but the contribution of a transition in feeding behavior to postnatal changes in masseter muscle does not seem remarkably.