Regeneration of Hair Cells and Afferents in Adult Vestibular Organs

Studying regeneration in the vestibular system after ototoxic damage offers unique advantages in that the system can be examined from genetic determinants to reacquisition of appropriate function. Our recent work has focused on the morphological and physiological development of spatial tuning and dynamic properties of regenerating vestibular afferents. Adult pigeons were used with a single intralabyrinthine dose of 2 mg streptomycin producing complete hair cell loss in all vestibular organs in 72 hours. Birds were then examined at discrete intervals ranging through poststreptomycin day (PSD) to 9 months. Hair cell type, location, and total distribution in the semicircular canals and otolith organs were quantified using a scanning electron microscope (SEM) or stereology, or both. Afferent innervation patterns of the cristae and maculae were reconstructed and quantified using three-dimensional visual imaging software (Neurolucida). At PSD 4, a null time point defining regeneration initiation, no hair cells were present and afferent fibers had mostly degenerated to beneath the stroma. Between 1 and 4 weeks PSD, only type II hair cells developed, with a striola becoming well-defined. Only bouton-type innervation patterns were observed. At 6 weeks PSD, type I hair cells began to form but were only innervated by dimorph afferents. These dimorphs lay near the striola, with simple calyces containing only one hair cell. At 3 months PSD, type I cells were more numerous with true calyx afferents present. The striola consisted of a distinct type II band of four to six cells where reversed morphological polarizations occurred. No type I cells were present in the type II band. Outside the band, type I cells predominated, yet dimorph and bouton afferents far outnumbered calyx units. At 6 months PSD, hair cell number, type, and location resembled those of a normal adult pigeon. Calyx, dimorph, and bouton afferent innervation patterns were regionally distributed; however, differences in afferent size, terminal number, innervation area, and branching patterns are currently being quantified and compared for different time points of regeneration.