Otx2, Gbx2, and Fgf8 expression patterns in the chick developing inner ear and their possible roles in otic specification and early innervation.

The chick inner ear is a complex structure containing auditory and vestibular sensory organs innervated by neurons of the acoustic-vestibular ganglion. The molecular signals involved in the specification and initial innervation of the otic epithelium are poorly understood. Here, we present a detailed description of the Otx2, Gbx2, and Fgf8 gene expression patterns in the chick developing inner ear, comparing them with the Bmp4 expression, a putative sensory-organ marker. The Otx2 expression was detected in the ventro-lateral wall of the otic anlage and could play a role in the segregation of the saccule and utricle maculae. The relationship between Gbx2 and Fgf8 expression changed during inner ear development but was always related to the macula sacculi innervation and endolymphatic duct formation. Our results also suggest that the maculae of the saccule and lagena, and the medial portion of the macula utriculi could arise within a broad Fgf8-positive domain previously observed at the otocyst stage. The spatial and temporal relationships between these gene expression domains and the initial innervation of the epithelium by some subpopulations of otic axons suggest that expression domain boundaries could be involved in the specification and early innervation of presumptive sensory patches.