To get further experimental data for the elaboration of a sound-impairment-model, guinea pigs were sounded with pure tone of a frequency of 2700 Hz and 130 dB in a closed system for 1 and 2 h. The cochleae were investigated using the technique of Spoendlin and Brun (1974) after different periods of recovery. This technique made a complete estimation of the damage of the hair cells possible.
Loss of hair cells due to the sound could be observed mostly in the second and in the lower half of the third cochlear turn. After 2 h sounding the number of the inner (IHZ) and of the first row of the outer hair cells (OHZ) in the part of maximal damage was strongly diminished already on the 5th day of recovery, however, in the 2nd and 3rd row the loss of hair cells was less pronounced.
After l h sounding only the outer hair cells — especially the 1st row — were found to be lost on the 5th day of recovery, but in most of the inner hair cells the beginning of degeneration was indicated by distorsions of the cells, damaged cilia, pyknotic nuclei, and dense cytoplasm with vacuoles. On the 10th day of recovery a great number of the inner hair cells were damaged, however, a reduction in the number of the outer hair cells did not show a further increase.
The pillar cells also were impaired at distinct loci situated in the part of maximal damage as well as at the basal and apical border to unchanged parts of the Corti organ.
Variations in the sounding times and the recovery periods revealed differences in the degeneration behaviour of the inner and the outer hair cells, and a differentiation in the sensitivity of the outer and inner hair cells to sound-impairment caused by mechanical and metabolic changes. The data are discussed with respect to impairment mechanisms of the Corti organ after pure tone sounding.