Effect of thyroid hypofunction on the masseter motor innervation pattern in developing rats

Introduction: the thyroid hormones have profound effects on thedevelopment of neuromuscular system. these hormones exert theirinfluence on both muscle fibers and related motoneurons duringdevelopment. the masseter is one of the most important muscles formastication in mammals. we attempted to evaluate the effect of thyroidhormone deficiency on the morphological characteristics of massetericmotoneurons in the period of alteration from sucking to biting and chewing inthe rat.material and methods: to induce hypothyroidism, timed pregnant damsreceived 50 ppm antithyroid drug propylthiouracil (ptu) in their drinkingwater and ptu was administered to the pups during suckling period.horseradish peroxidase (hrp) was injected into the masseter (0.5-5 ulit,40%) of normal and prenatal hypothyroid pups on postnatal days of 1, 5, 13,and 21 (n=24). 24 to 48 hours later, following transcardial perfusion andfixation the brainstem blocks were cut to 50 ijm thick sections. after tmbhistochemical reaction the morphological characteristics of hrp labeledmotoneurons and their hrp labeling intensity was evaluated. student's t-testand two-way analysis of variance (anova) were used for statistical analysis.results: no significant morphological differences were observed at the endof first week of life. on day 15, hypothyroid labeled masseteric motoneuronsconsisted of 70% small and 30% medium neurons versus 40% and 60% innormal pups respectively (p(less than)0.05). at the time of weaning, the number oflarge motoneurons dropped to 30% of normal value (p(less than)0.001) with few,short, and disoriented dendrites.conclusion: the alteration in particular patterns of masseteric motoneuronmorphology and a severe delay in size transition could affect thedevelopment and plasticity of oral motor behavior under congenitalhypothyroidism.