Effects of Developmental Hypothyroidism on Cochlear Hair Cells in Mice

Insufficient thyroid hormone during development results in deafness, but the molecular and structural defects responsible for this deafness are largely unknown. Therefore, we investigated the effects of developmental hypothyroidism on cochlear hair cells of juvenile mice. We first established three separate groups of mice: hypothyroid, control, and a hypothyroid plus thyroid hormone replacement (rescue) group. By doing so, we will be able to compare structural differences in the interior of the cochlea. We then harvested the cochlea of mouse pups on postnatal days 16 and 21. After fixing the extracted cochlea and decalcifying we exposed the apical sensory epithelium to image the stereocillia with a scanning electron microscope. We hypothesize that several structural differences between the hypothyroid and non-hypothyroid mice will be observed; including growth retardation and disorganization of hair stereocillia bundles. Disorganization of the stereocillia is associated with sensory deafness in mice and humans. Therefore, by studying these cochlear defects we will gain a better understanding of sensory deafness and the importance of sufficient thyroid hormone levels to cochlear development. Moreover, by gaining a better understanding of the structural differences between normally developing and hypothyroid mice, we will be contributing to the development of treatments aimed at eliminating cochlear defects in humans