Anatomical changes in the inner ear of the bullfrog across metamorphic development

Abstract

Metamorphic development in the bullfrog, Rana catesbeiana, is characterized by widespread changes in peripheral transduction pathways and in the auditory brainstem, in preparation for the transition from a fully aquatic to a semi-terrestrial existence. The time course of development of the inner ear organs has not been as extensively examined. A combination of immunohistochemical, cresyl violet and trichrome staining to were used to delineate the development of the saccule, an otolith organ sensitive to particle motion and to seismic stimuli, across metamorphosis. From early embryonic to metamorphic climax stages and extending to the froglet period, the saccule increases linearly in area, correlated with the growth in body size. Myosin VI label indicates that hair cell density in the central region of the saccule remains relatively stable in tadpoles, but then decreases between froglet and subadult stages. From these results, it is hypothesized that hair cell proliferation occurs more extensively in tadpoles than in froglets.