ALT Schematics of structural changes in the cochlea induced by expressional alternations of Neurod1 and/or Atoh1 as a combination of cues for guiding neuronal axons to their target sensory organs. In the control cochlea, spiral ganglion (SG) neurons are radially connected (RF, radial fibers, red) with a single ending to IHC (10-20 neurons/ 1IHC). The loss of Atoh1 results in a flat epithelium without any sensory cells and innervation, reduction of SG neurons, and retraction of radial fibers. Absence of Neurod1 correlates with reduced number of SG neurons, a loss of apex-base and vestibular fiber segregation, multiple endings of a single neuron to HCs, and the disorganized epithelium in the apex of Neurod1CKO. In Atoh1/Neurod1CKO neuronal fibers ramify inside the flat epithelium that replaces the organ of Corti. Magenta color depicts neurons and fibers in the cochlea labeled by the dye application to the vestibular end organs.