(A-C) In situ hybridization (ISH) was used to analyze the cochlear expression pattern of Inhba and Fst at the onset of (A, E13.5) and during hair cell differentiation (B, E14.5 and C, E15.5) in adjacent serial cochlear sections. Sox2 transcript marks the pro-sensory/sensory domains, Atoh1 transcript marks nascent hair cells. Brackets mark the pro-sensory/sensory domains with the cochlear duct. Abbreviations: m, medial; l, lateral. Scale bar, 100 µm. (D) Schematics of longitudinal (apical–basal) and radial (medial-lateral) expression gradients of Inhba (green) and Fst (red) within the auditory sensory epithelium.