Cyp1B1 expression patterns in the developing chick inner ear

Background: Retinoic acid (RA) plays an important role in organogenesis as a paracrine signal through transcriptional regulation of an increasing number of known downstream target genes, regulating cell proliferation and differentiation. During the development of the inner ear, RA directly governs the morphogenesis and specification processes mainly by means of RA‐synthesizing retinaldehyde dehydrogenase (RALDH) enzymes. Interestingly, CYP1B1, a cytochrome P450 enzyme, is able to mediate the oxidative metabolisms also leading to RA generation, its expression patterns being associated with many known sites of RA activity. Results: This study describes for the first time the presence of CYP1B1 in the developing chick inner ear as a RALDH‐independent RA‐signaling mechanism. In our in situ hybridization analysis, Cyp1B1 expression was first observed in a domain located in the ventromedial wall of the otic anlagen, being included within the rostralmost aspect of an Fgf10‐positive pan‐sensory domain. As development proceeds, all identified Fgf10‐positive areas were Cyp1B1 stained, with all sensory patches being Cyp1B1 positive at stage HH34, except the macula neglecta. Conclusions: Cyp1B1 expression suggested a possible contribution of CYP1B1 action in the specification of the lateral‐to‐medial and dorsal‐to‐ventral axes of the developing chick inner ear