Emx2 Lineage Tracing Reveals Antecedent Patterns of Planar Polarity in the Mouse Inner Ear

The planar polarized organization of vestibular hair cells in the utricle and saccule is unique because these inner ear sensory organs contain two groups of hair cells with oppositely oriented stereociliary bundles that meet at a Line of Polarity Reversal (LPR). This organization allows the utricle or the saccule to detect motions directed in opposite directions, and is coordinated with patterns of neural innervation. EMX2 is a transcription factor that is only expressed by hair cells located on one side of the utricle or saccule where it reverses the orientation of their bundles and thereby establishes the position of the LPR. We generated Emx2-CreERt2 transgenic mice for genetic lineage tracing and demonstrate robust Emx2 expression at embryonic day 11.5 (E11.5), before hair cell specification, and when the nascent utricle and saccule have not yet segregated from a common prosensory domain. All hair cells derived from Emx2-CreERt2 lineage tracing at E11.5 are restricted to one side of the LPR in the mature utricle or saccule indicating that an antecedent LPR may be established by EMX2 at that stage. Consistent with this, Emx2-CreERt2 lineage tracing at E11.5 in Dreher mutant mice, where the utricle and saccule fail to segregate, labels a continuous field of cells distributed along one side of a fused utricular-saccular-cochlear organ. Altogether these observations reveal that the origin of the LPR is established in the developing prosensory domain, and that the presence or absence of Emx2 expression defines progenitor cells with distinct lineages that include hair cells with oppositely oriented stereociliary bundles.