Inductive processes leading to inner ear formation during Xenopus development.

This study examines the spatial and temporal attributes of inner ear induction in Xenopus embryos. These results are compared to recent experiments concerning lens induction to assess whether head sensory structures share common ontogenetic features. Ectoderm from different regions and stages was transplanted to the presumptive ear region of hosts of either early (neural plate) or late (neural tube) stages. Explants of the presumptive ear ectoderm were also taken from embryos at these stages to establish the time of otic ectoderm specification. We find that ectodermal competence for otic vesicle formation extends through neural plate stages, far longer than for lens formation. Otic vesicle specification occurs substantially earlier, at neural plate stages, than lens specification. Competent ectoderm forms otic vesicles in a high fraction of cases when exposed to the ear-inducing environment of either neural plate stages or neural tube stages, a result which contrasts with lens induction where the neural tube stage embryo provides a much weaker inducing environment than earlier stages. Otic vesicles induced in neural tube stage hosts are primarily in contact with presumptive hindbrain, suggesting that this neural tissue may be sufficient for otic vesicle induction. These studies reveal overall similarities between lens and inner ear induction, but sufficient differences to propose that some facets of determination of these sensory tissues may occur by independent mechanisms and not via a common developmental state.