The effects of mandibular hypofunction on the development of the mandibular disc in the rabbit

The effect of a reduced functional dentition on the development of the mandibular disc in young rabbits was studied by measuring cell proliferation within the disc following tooth extraction. Maxillary and mandibular incisor teeth were extracted from 18 animals at 5 weeks of age. At 12 weeks the rabbits received 0.25 mg/kg vincristine sulphate. Groups of three animals were killed 1.5, 3, 6, 12, 24 and 48 h after the injection of vincristine and the mitotic rate determined across the anterior, intermediate and posterior bands of the disc. Eighteen age- and sex-matched control rabbits with intact dentitions were treated in parallel. In the absence of incisor teeth, reflex gnawing and incising failed to develop, resulting in altered jaw movements and muscle force requirements. The mitotic rate in the anterior band was reduced significantly (p = 0.0117); rates for the intermediate and posterior bands were not significantly affected. There was an associated reduction in alveolar bone mass and deformation of the developing craniomandibular complex. As the lateral pterygoid inserts into the anterior band of the mandibular disc, it is proposed that altered activity within this muscle, combined with a modified loading of the joints, both secondary to incisor removal, resulted in a reduced mitotic rate in the anterior band of the developing mandibular disc.