Mechanisms establishing tonotopic organization in the vertebrate cochlea

Abstract

The auditory organ is able to discriminate the sound frequencies. Sound frequency discrimination starts at the basilar papilla in birds and the organ of Corti in mammals. These auditory organs tonotopically organized with a basal end (proximal region) that detects high frequencies and an apical end (distal region) that senses low frequencies. Sonic Hedgehog (Shh) secreted from the notochord and floor plate is crucial for cochlear duct development in species. Early otocyst is required high level of Shh signaling secreted in ventral midline source at apex but not at base in chicken and mouse. In this study, gain-of-function experiment involving of Shh-soaked beads in chicken embryo in ovo and in a continuously activated Shh signaling pathway mouse model, demonstrated the specification of the positional identity of the cochlear duct in these animal species. However, region specified regulator genes are not conserved between the both species. In chicken, expression of these altered genes, led to morphological and physiological characteristic alterations in hair cells. These features included tonotopic properties, such as the total number of stereocilia per hair cell and gene expression of an inward rectifier potassium channel (IRK), Kcnj2, which is an inherent characteristics of apical hair cells in the basilar papilla. In addition, this study suggests that Shh from the ventral midline, is required for the tonotopic organization during the developing cochlear duct in avian and mammal species.