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Apicosome: newly identified cell-type-specific organelle in mouse cochlear and vestibular hair cells

Cochlear and vestibular hair cells in the inner ear are highly specialized sensory receptors for sound waves and acceleration of body movements; these cells can perform their specialized functions because of their distinctive morphology and some unique organelles that they harbor. Here, we report a serendipitous identification in the mouse of a hair-cell-specific organelle, which we name “apicosome.” The apicosome was recognized by anti-FLRT1 antibodies but contains no FLRT1, and the organelle presents several distinctive characteristics: (1) the apicosome typically appears as a single entity (∼500 nm in diameter), but occasionally as two entities, in hair cells; (2) it first appears in the subapical region at the neural side at embryonic day (E) 17–18 in cochlear hair cells, subsequently descends to the perinuclear region during the first postnatal week, and completely disappears around postnatal day (P) 10; (3) in vestibular hair cells, it can be detected in the subapical region of neonatal (P3) cells and persists in adult hair cells although it becomes smaller and more distant from the subapical region; (4) the timing of apicosome translocation and disappearance during development is correlated in kinocilium maintenance; (5) the organelle is potentially associated with microtubules; and (6) the appearance of the apicosome is irregular in supernumerary hair cells and this is likely linked to anomalous lateral inhibition. Thus, our study identifies a previously undescribed organelle in sensory hair cells and lays the foundation for further characterization of this specialized structure potentially linked to hair-cell development and morphogenesis.