The recessive figure eight mutation causes thyroid defects, neovascular retinaopathy, sperm defects, and hair cell loss in the crista ampullaris and organ of Corti. Homozygotes display head tilting and circling behaviors and an inability to orient in water to swim. By one month of age ABR thresholds are elevated by approximately 40 dB across all frequencies, and by six months of age homozygotes have no detectable ABR, even at 100 dB. Histology of the inner ears at one month of age found no cochlear abnormalities, but by three months of age there is complete degeneration of the organ of Corti. Scanning electron microscopy of the organ of Corti at 5 weeks of age revealed the absence of some outer hair cell bundles, greatest near the apex and base, but normal inner hair cells and normal orientation and patterning of hair cells. Vestibular hair cell loss was also found in cross sections of the crista ampullaris. Scattered depigmented spots of the retina are found at 6 weeks of age, which develop into large depigmented areas by 16 weeks of age and newly formed vessels are found in the retina at both time points. Choroidal neovascularization with leakage was found by fluorescein angiography. Despite these defects, electroretinaograms showed normal amplitudes at 16 weeks of age. Homozygotes also have thyroid defects, with only a few normal follicles and many highly proliferating epithelial cells found at 15 weeks of age. Serum T4 levels are lower than normal, and homozygotes have a reduced body size, with females weighing an average of 5.9g less and males 3g less than controls between 4 and 6 weeks of age. Homozygous males have reduced fertility, but do reproduce. The germinal epithelium of the testes is disorganized, there are fewer maturing sperm cells, reduced total concentration of spermatozoa, and reduced sperm motility, with nearly half of the mature spermatozoa having their tail bent at the mid-piece. Nevertheless, all meiotic stages are present in the seminiferous tubules.
