Assessment criteria for rotated stereociliary bundles in the guinea pig cochlea.

Variant stereociliary bundles of cochlear outer hair cells (OHCs) in guinea pigs were discovered fortuitously in approximately 50% of the animals examined by several investigators (1). We also found this phenomenon frequently in our previous observations of guinea pigs, even in human newborns (2). Accordingly, the anomaly has been described in a subset of animals obtained from animal suppliers, including golden hamsters from Japan (3) and guinea pigs from the U.K. and China (1,3-5). Planar cell polarity (PCP) refers to the polarization of cells within the plane of a cell sheet. A distinctive epithelial PCP in vertebrates is the uniform orientation of stereociliary bundles of the sensory hair cells in the mammalian cochlea (6,7). Planar cell polarity is a fundamental aspect in the development of the inner ear. Not only is the uniform orientation of stereociliary bundles on hair cells one of the clearest examples of planar polarity in a mammalian system, the cochlea is also an ideal model system to study this (8,9). Yoshida and Liberman (10) histologically examined the cochleae from 100 albino guinea pigs and observed 24 of these to have rotated stereociliary bundles, with males and females almost equally represented in the affected population. This congenital anomaly was strictly limited to first-row OHCs, of which 15 to 50% were found to be rotated up to a possible 180 degrees from the normal orientation. Compound action potentials (CAPs) were measured on mice that displayed abnormal hair bundles to analyze whether this structural alteration had a functional consequence. Indeed, this structural change resulted in CAP thresholds increased by 5 to 10 dB on average when compared with thresholds of control animals. The level of CAP threshold increase was correlated with a quantitation of the degree of stereociliary rotation to demonstrate that first-row OHCs play a greater role in amplifying signals within the cochlea than do the second- and third-row OHCs (10). Another study found that OHCs from av6J mutant (6J/6J) mice appear rotated from normal position in the basal turn. Additionally, in av2J mice, OHCs are severely deformed and show abnormal organization of the stereocilia (11), whereas some of the cells have rotated stereocilia bundles, with the tip of the V pointing at an angle of up to 90 degrees away from the normal orientation. Normal cochleae were apparent in the first row; however, 50% OHCs were rotated bundles, but they were considered normal by this investigator. It was concluded by this author that the irregular arrangement and natural loss of stereociliary bundles of the OHCs are not considered to be pathologic (12). In this current study, assessment of cochlear OHC rotation was not based on criteria from previous literature. Rather, our study involved the assessment of hearing acuity in 100 guinea pigs that were then independently examined for cochlear morphology and histology. We measured hearing acuity by both auditory brainstem response (ABR) and distortion product otoacoustic emission (DPOAE) testing. It is well established that DPOAE analysis can discern alterations of OHC functionality that are not detected by ABR analysis (9). The cochlea of each animal was examined by electron microscopy for stereociliary bundle misorientation that was mapped to specific cochlear areas. The histologic results of cochlear damage were compared with the DPOAE analysis to see if both methods concomitantly pinpointed similar areas of stereociliary bundle misorientation. The goal is to use DPOAE analysis to discern subtle changes in stereociliary bundle misorientation.