A tissue boundary orchestrates the segregation of inner ear sensory organs

The inner ear contains distinct sensory organs, produced sequentially by segregation from a large sensory-competent domain in the developing otic vesicle. To understand the mechanistic basis of this process, we investigated the changes in prosensory cell patterning, proliferation, and character during the segregation of some of the vestibular organs in the mouse and chicken otic vesicle. We discovered a specialised boundary domain, located at the interface of segregating organs. It is composed of prosensory cells that gradually enlarge, elongate, and are ultimately diverted from a prosensory fate. Strikingly, the boundary cells align their apical borders and constrict basally at the interface of cells expressing or not the Lmx1a transcription factor, an orthologue of Drosophila Apterous. The boundary domain is absent in Lmx1a-deficient mice, which exhibit defects in sensory organ segregation and is disrupted by the inhibition of ROCK-dependent actomyosin contractility. Altogether, our results suggest that actomyosin-dependent tissue boundaries ensure the proper separation of inner ear sensory organs and uncover striking homologies between this process and the compartmentalisation of the Drosophila wing disc by lineage-restricted boundaries.