Development of the gerbil inner ear observed in the hemicochlea

A frequency-dependent change in hearing sensitivity occurs during maturatio n in the basal gerbil cochlea. This change takes place during the first wee k after the onset of hearing. It has been argued that the mass of a given c ochlear segment decreases during development and thus increases the best fr equency. Changes in mass during cochlear maturation have been estimated pre viously by measuring the changes in cochlear dimensions. Fixed, dehydrated, embedded, or sputter-coated tissues were used in such work. However, dehyd ration of the tissue, a part of most histological techniques, results in se vere distortion of some aspects of cochlear morphology. The present experim ents, using a novel preparation, the hemicochlea, show that hydrated struct ures, such as the tectorial membrane and the basilar membrane hyaline matri x, are up to 100% larger than estimated previous studies. Therefore, the he micochlea was used to study the development of cochlear morphology in the g erbil between the day of birth and postnatal day 19. We used no protocols t hat would have resulted in severe distortion of cochlear elements. Conseque ntly, a detailed study of cochlear morphology yields several measures that differ from previously published data. Our experiments confirm growth patte rns of the cochlea that include a period of remarkably rapid change between postnatal day 6 and 8. The accelerated growth starts in the middle of the cochlea and progresses toward the base and the apex. In particular, the inc rease in height of Deiters' cells dominated the change, "pushing" the tecto rial membrane toward scala vestibuli. This resulted in a shape change of th e tectorial membrane and the organ of Corti. The tectorial membrane was pro perly extended above the outer hair cells by postnatal day 12. This time co incides with the onset of hearing. The basilar membrane hyaline matrix incr eased in thickness, whereas the multilayered tympanic cover layer cells dec reased to a single band of cells by postnatal day 19. Before and after the period of rapid growth, the observed gross morphological changes are rather small. It is unlikely that dimensional changes of cochlear structures betw een postnatal daps 12 and 19 contribute significantly in the remapping of t he frequency-place code in the base of the cochlea. Instead, structural cha nges affecting the stiffness of the cochlear partition might be responsible for the shift in best frequency.