Cochlear synaptogenesis in the hypothyroid rat

The effects of congenital hypothyroidism on cochlear synaptogenesis were studied using transmission electron microscopy in 30 propylthiouracil-treated rat pups 1-35 days of age. No difference with controls was observed at the level of the inner hair cells which are known to mature essentially during the prenatal period. On the contrary, hypothyroidism resulted in numerous abnormalities in synaptogenesis at the level of the outer hair cells: abnormal persistence of numerous afferent dendrites and presynaptic specializations, incomplete development of efferent terminals and absence of formation of postsynaptic cisterns. It can be concluded that hypothyroidism results in severe retardation in the postnatal development of the innervation of outer hair cells.