Effects of restriction of fetal jaw movement on prenatal development of the temporalis muscle.

Jaw movement affects masticatory muscles during the postnatal period. Prenatal jaw movement has also been implicated in the development of the temporomandibular joint; however, its effect on prenatal development of the masticatory muscles has not been extensively analysed. In the present study, we examined the effects of the restriction of fetal jaw movement on the temporalis muscle, a major masticatory muscle, in mice by suturing the maxilla and mandible (sutured group) using an exo utero development system. We compared the morphology of the temporalis muscle between sutured, sham-operated and normal in utero groups. At embryonic day (E) 18.5, the volume of muscle fibres, but not that of connective tissue, in the temporalis muscle was decreased in the sutured group. The E18.5 temporalis muscle in the sutured group appeared morphologically similar to that of the E17.5 in utero group, except for frequent muscle fibre irregularities. By transmission electron microscopy, in the sutured group, the myofibrils were immature and scattered, the nuclei appeared comparatively immature, the mitochondria were expanded in volume with fewer cristae, and cytoplasmic inclusion bodies were frequently observed. Expression of Myf-6, a late myogenic transcription factor, by real-time RT-PCR was not significantly different between the sutured and sham-operated groups. These findings demonstrated approximately 1-day delay in the morphological development of the temporalis muscle in the sutured group, and some abnormalities were observed, although Myf-6 level was not affected in the sutured group. The present study revealed that the prenatal jaw movement influences the development of the temporalis muscle.

Fetal jaw movement affects development of articular disk in the temporomandibular joint.

Previous studies suggest that jaw movement is an important factor in the development of cartilage in the temporomandibular joint during the prenatal and postnatal periods. In the present study, the effects of fetal jaw movement on the articular disk were studied in mice by restraining the opening movement of the mouth using the mouse exo utero development system. At embryonic day 18.5, the articular disk was reduced in size in the embryos whose maxilla and mandible were sutured (sutured group) and there were changes in the cellular morphology of the mesenchymal cells in the disk. The volume of the articular disk, the total number of cells and the number of 5-bromo-2'-deoxyuridine-positive cells in the articular disk were significantly lower in the sutured group than in the non-sutured control group. Our data revealed that fetal jaw movement affects the development of the articular disk in the temporomandibular joint.