| The effects of occlusion on the development and outcome of diseases in the stomatognathic system have always been focused in related studies. Numerous experimental and clinical studies have shown an association between obnormal changes of biting condition and masticatory system disorders. The abnormal changes of occlusion could lead to certain organization structure’s functional disorders of stomatognathic system, such as masticatory muscles and temporomandibular joint （TMJ）. It even leads to pathologic lesion when the abnormal state exceeds the adaptability capacity. Changes in occlusal vertical dimension （OVD） have been claimed to cause obnormal biting condition. Appropriate and stable occlusal vertical dimension is the indispensable premise in stabilizing the structure and optimizing the utility of masticatory muscle. Masseter muscle is the most important masticatory muscle which acts strongly in elevating the mandible. It’s speciality in locus of dissection and function makes it the most related muscle to OVD. The stability of OVD plays an important role in maintaining the normal organizational structure and function of masseter muscle.Related studies on the effect of changes of OVD have been carried out both in home and abroad. However, emphasis has been focused on how to value and restore the normal OVD rationally, and the influence of components within the masticatory system with the rise of OVD. However, researches on how decreased OVD influence the organizational structure and function of masticatory system remain to be confined to clinical observations. Related fundamental researches have been rarely reported. Based on establishing rat animal model with decreased OVD, the present study, by techniques such as histopathology, enzymohistochemistry and biochemistry, aims to investigate the changes of rat’s masseter muscles in histopathology, enzymohistochemistry and energy metabolism after the decrease of OVD.Objective:Firstly, establish rats’animal model with the decreased OVD. Then, the changes of masseter muscle in histopathology, enzymohistochemistry and energy metabolism will be analyzed by relevant techniques. The present study will help to clarify the adaptive changes of masticatory structures and functions after the decrease of OVD, providing experimentally theoretical support for the clinical manifestations and treatments.Methods:1. The crown height of the upper molars and the lower incisors of adult rats was decreased artificially and mechanically. The animals were divided into experimental groups and sham-operated groups. Each group was further divided into four subgroups according to different sacrifice time-ponts.2. The samples of masseter muscle were prepared by cryotomy and ultramicrotomy, then the microstructure and ultrastructure were observed and analyzed under microscope and TEM.3. The enzyme histochemical staining （the NADH-TR staining） was carried out on the sections of deep masseter muscle.The mean cross-sectional area （mCSA） of myofibers and fiber type proportion were analyzed by microscope image analysis system.4. The decreased activity of Ca2+- ATPase、Na+- K+ATPase in masseter muscle were measured via chemocolorimetric analysis. Commercial kits were used in the tests according to protocols provided by the manufacturers.Results:1. The animal model appears to be rapid, controllable, stable and repeatable. The weights, habits and behaviors of experimental animals haven’t been affected.2. The changes of microstructure and ultrastructure were induced by decreased OVD. Structural damage of mitochondria is regarded as typical feature. In comparison to the sham-operated group, the 10-day experimental group showed vacuolar changes of mitochondria, cristae loss and reduced matrix density in the masseter muscles.Even myelin figure were observed in 30-day experimental group. However, damage was only confined in isolated mitochondria, and the organizational structure recovered gradually with time.3. The mean cross-sectional area of singleⅡa fiber （mCSA-Ⅱa） in 60-day experimental group was lower than that in 3d experimental group and its age-matched controls.The mean cross-sectional area of singleⅡb fiber （mCSA-Ⅱb） in 60-day experimental group was higher than that in 3-day experimental group and its age-matched controls.The proportion ofⅡb fiber in 60-day experimental group was higher than that in 3-day experimental group and its age-matched controls.4. The decreased activity of Ca2+- ATPase and Na+- K+ATPase in the masseter macles was observed at the early stage of decreased OVD,especially in 30-day experimental group. With the duration of state, the activity of Ca2+- ATPase and Na+- K+ATPase recovered gradually.Conclusion:The regional damages in the ultrastructure, the blocked energy metabolism of masseter muscle can be induced by dOVD in rats.However,with the duration of dOVD state, muscle fibers of masseter muscles undergo the adaptive remodelling in propotion of muscle types, and the regional damages in the ultrastructure , and the blocked energy metabolism will disappear. |
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