Abstract: ［Abstract］ ObjectiveTo investigate the influence of increasing occlusal vertical dimension(iOVD) on mRNA of myosin heavy chain and myogenic regulatory factors in deep masseter of rat at different ages. MethodsFortyeight male Wistar rats were divided into two groups: teenage group(1.5 months, n=24) and young adult group(8 months, n=24). Then the two groups were randomly divided into control and experimental group(n=12). The experimental groups were iOVD by placing composite resin on molars. So there were four groups according to the age and experimental intervention, namely teenager control group, teenager experimental group, young adult control group and young adult experimental group. The mRNA of the MHC, MyoD and myogenin of deep masseter were observed on day 7 and day 14 after iOVD. ResultsOn the 7th day of iOVD, the relative level of MHCⅡa mRNA in group TE was higher than that in group TC. Aslo MHCⅡa mRNA levels in group YAC and group YAE were lower than that in group TC and TE（P＜005）. On the 14th day of iOVD, the relative level of MHCⅡa mRNA in group TE and YAE was higher than that in group TC and YAC. MHCⅡa mRNA levels in group YAC and group YAE were lower than that in group TC and TE（P＜005）. The relative level of MHCⅡx mRNA in group TE and group YAE was lower than that in group TC and group YAC on the 7th and the 14th day of iOVD（P＜005）. On the 7th day of iOVD, the relative level of MHCⅡb mRNA in group TE was higher than that in group TC. Aslo MHCⅡb mRNA level in group YAC and group YAE were higher than that in group TC and TE（P＜005）. On the 14th day of iOVD, the relative level of MHCⅡb mRNA in group TE and YAE was higher than that in group TC and YAC. Aslo MHCⅡb mRNA level in group YAC and group YAE were lower than that in group TC and TE（P＜005）. On the 7th and the 14th day of iOVD, the relative level of MyoD mRNA in group TE and group YAE was higher than that in group TC and YAC. The relative level of MyoD mRNA in group TE and YAE was higher than that in group TC and YAC(P＜005). The relative level of myogenin mRNA and myogenin:MyoD mRNA in group TE and group YAE was higher than that in group TC and group YAC, group YAC and group YAE was lower than group TC and group TE on the 7th and the 14th day of iOVD（P＜005）.There was positive correlation between the myogenin/MyoD mRNA ratio and MHC Ⅱa mRNA(r=0.954， P＜0.05), and negative correlation between myogenin/MyoD mRNA ratio and MHC Ⅱb mRNA(r=-0.755， P＜0.05). ConclusionThe deep masseter of teenager rat in experimental group changes to MHC Ⅱa mRNA earlier and more than that of young adult rat. The MyoD and myogenin which belong to myogenic regulatory factors plays an important role in fiber reconstruction.

Key words: masseter muscle, myosins, myogenic regulatory factor