ANALYSIS OF SMALL HAIR BUNDLES IN THE UTRICLES OF MATURE GUINEA-PIGS

Hypothesis: This study aimed to determine whether hair cells with imma ture hair bundles exist in the normal utricular maculae of mature guin ea pigs. Background: A low rate of hair cell ''turnover'' occurs in th e vestibular organs of normal adult birds. Small, immature-appearing h air cells have been identified in the utricles of juvenile guinea pigs , but their existence in the vestibular system of mature mammals has n ot been confirmed. Methods: Nine utricles from 14- to 16-week-old guin ea pigs were processed for scanning electron microscopy. A systematic search for small hair bundles was made. These bundles were classified as either ''newborn-like'' or ''intermediate'' depending on specific c haracteristics of the apical cell surface, the stereocilia, and the ki nocilium. Results: A mean of 7 newborn-like and 41 intermediate hair b undles per utricle were identified. The average hair cell count for th e guinea pig utricular macula was 7,200; thus, these small hair bundle s comprise 0.7% of the total. Conclusions: The small hair bundles are interpreted as developing vestibular hair cells, produced to replace h air cells lost to normal processes. This is thought to represent a bio logic phenomenon that is in some ways similar, but in other ways disti nct, from hair cell regeneration after trauma.