Targeted cell interconversions reveal inner hair cell control of organ of Corti cytoarchitecture

The mammalian organ of Corti is a precisely intercalated mosaic of two types of mechanosensory hair cells (HCs) and six types of supporting cells (SCs) arranged in 11 parallel rows. Differentially specialized SCs surround inner HCs (IHCs) and outer HCs (OHCs). To elucidate the developmental roles of IHCs and OHCs in the formation, differentiation, and assembly of the various SCs, we genetically switched HC identities at several developmental stages and also generated mice lacking IHCs. We find that IHCs promote or induce the (i) differentiation of one SC type (outer pillar) at the expense of another (Deiters'), so that each completely and exclusively populates separate rows; (ii) packing density, but not identity, of inner pillar cells; and (iii) embryonic formation and (iv) cytoplasmic attraction of adjacent inner phalangeal cells, which envelop IHCs. Hence, developing IHCs dictate major aspects of SC identity and distribution to assemble the complex organ of Corti.