Use of phase contrast microscopy to determine the height of the organ of Corti in whole-mount preparations.

A technique has been developed to measure the height of the organ of Corti (OC) in the whole-mount preparations of the cochlear duct. The technique corrects for variations in the microscope system, such as the magnification of the objective lens and the mechanical properties of the fine-focus knob, as well as the refractive index of the embedding medium and the angle of specimens with respect to the optical axis of the microscope. At 11 percentage locations from apex to base, the height of the OC in ten chinchilla cochleas was measured at three positions: (1) the lateral edge of the inner hair cell (IHC); (2) the medial edge of the first row outer hair cell (OHC1); and (3) the lateral edge of the third row outer hair cell (OHC3). These measurements were compared to measurements made on radial sections from five other cochleas, with very good agreement at IHC and OHC3, and fairly good agreement at OHC1. The height at OHC3 varied almost linearly with percentage distance along the OC, ranging from 96 microns (apical end) to 51 microns (basal end). The height at the OHC1 varied from 77 to 49 microns, but did not vary linearly. The height of the IHC was relatively constant, from 50 to 60 microns, except at the basal end, where it decreased to 42 microns.

Interaural correlations in normal and traumatized cochleas: length and sensory cell loss.

Sizable intraspecies variations have been found in both the length of the organ of Corti (OC) and the amount of damage resulting from exposure to a particular ototraumatic agent. These variations have made it difficult to address certain research questions such as the susceptibility of the previously injured ear to further damage. If intra-animal correlation is high, the variability problem could be circumvented by using the two ears from a given animal for different aspects of the same study. Therefore, correlation coefficients were calculated for OC length and for percentage of missing inner (IHCs) and outer hair cells (OHCs) in a large sample of chinchillas which included controls and animals which had been exposed to noise or treated with ionizing radiation. The correlation coefficients were +0.96 for OC length, +0.93 for IHC loss, and +0.97 for OHC loss.

Damage to the cochlea following interrupted exposure to low frequency noise.

This study determines how the magnitude and pattern of cochlear damage is altered when exposure to noise is interrupted by regularly spaced rest periods. Groups of chinchillas were exposed for six hours per session to an octave band of noise with a center frequency of 0.5 kHz. The rest interval between successive exposures varied from 18 to 162 hours. The total energy in these exposures was equal to that in a nine-day continuous exposure at 95 dB sound pressure level. The ears of all animals were prepared for histological study so that the extent of cochlear damage could be determined. The pattern of damage produced by interrupted exposure to low frequency noise was the same as that found with continuous exposure, while the magnitude of damage was usually reduced. The amount of protection provided by a particular rest period was found to be different for the low and high frequency regions of the cochlea. The significance of these findings with regard to the mechanisms of noise damage is discussed.