HEARING-LOSS AND COCHLEAR ABNORMALITIES IN THE CONGENITAL HYPOTHYROID(HYT HYT) MOUSE/

The congenital hypothyroid (hyt/hyt) mouse has been described as havin g a homozygous recessive mutation of a single locus on chromosome 12 w hich results in significant endocrine hypofunction and retarded growth . Although a distinct correlation between inherited hypothyroidism and hearing loss in humans exists, there has been no previous evaluation of the auditory system in these mutant mice. We determined hearing thr esholds by auditory-evoked brainstem response testing and noted a 40-4 5 dB elevation in the hyt/hyt mouse compared to littermate heterozygot e (hyt/+) animals and normal progenitor controls BALB/cByJ (+/+). Conv entional light microscopy was used to examine the general anatomy of t he cochlea in these animals, and the surface structure of the organ of Corti was further evaluated with scanning electron microscopy. Hetero zygote and normal control mice had no significant abnormalities of the cochlea, however the hyt/hyt mice displayed consistent morphologic ab normalities of the stereocilia on both inner and outer hair cell syste ms. The surrounding and supporting cells were identified in the cochle as of the hypothyroid mouse and control animals and showed no signific ant histologic abnormalities. The auditory, histologic, and ultrastruc tural characterization of this model provides a foundation for evaluat ing the effects of true inherited hypothyroidism on auditory pathway d evelopment.