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

introduction: the thyroid hormones have profound effects on the development of neuromuscular system. these hormones exert their influence on both muscle fibers and related motoneurons during development. the masseter is one of the most important muscles for mastication in mammals. we attempted to evaluate the effect of thyroid hormone deficiency on the morphological characteristics of masseteric motoneurons in the period of alteration from sucking to biting and chewing in the rat. materials and methods: to induce hypothyroidism, timed pregnant dams received 50 ppm antithyroid drug propylthiouracil (ptu) in their drinking water and ptu was administered to the pups during suckling period. horseradish peroxidase (hrp) was injected into the masseter (0.5-5 μlit, 40%) of normal and prenatal hypothyroid pups on postnatal days of 1, 7, 15, and 23 (n=24). after 24-48 hours, the 50 μm thick brainstem sections containing trigeminal motor nucleus were processed for tmb histochemical procedure and morphological characteristics of hrp labeled motoneurons and their hrp labeling intensity was evaluated. student's t-test and two-way analysis of variance (anova) were used for statistical analysis. results: no significant morphological differences were observed at the end of first week of life. on day 15, hypothyroid labeled masseteric motoneurons consisted of 70% small and 30% medium neurons versus 40% and 60% in normal pups respectively (p<0.05). at the time of weaning, the number of large motoneurons dropped to 30% of normal value (p<0.001) with few, short, and disoriented dendrites. conclusion: the alteration in particular patterns of masseteric motoneuron morphology and a severe delay in size transition could affect the development and plasticity of oral motor behavior under congenital hypothyroidism.