The expression of retinoid receptors has been studied in detail in the developing mouse inner ear [Raz and Kelley, 1999; Romand et al., 2002; Romand et al., 1998]. Expression of all three Rar genes was observed in the developing otocyst as early as E10.5 [Romand et al., 2002], and persisted until prenatal stages [Romand et al., 1998]. The expression patterns were largely non-overlapping, Rara being predominantly expressed in the developing sensory epithelium, Rarb in inner ear mesenchymal tissues, and Rarg in the differentiating otic capsule. Interestingly, expression of the three Rar genes was further detected in the adult inner ear. Rara and Rarg, in particular, are expressed in the organ of Corti (the cochlear auditory epithelium) [Romand et al., 2002]. RARα and RARγ were shown to be necessary for embryonic inner ear development [Romand et al., 2002]. Whether the retinoid receptors may play a role postnatally in the auditory system remains unclear. However, Rara-null mutants exhibit a hearing deficiency related in part to middle ear functional deficits (R. Romand, personal communication).
