Interplay between medial nuclear stalling and lateral cellular flow underlies cochlear duct spiral morphogenesis

A notable example of spiral architecture in organs is the mammalian cochlear duct, where the duct morphology is critical for hearing function. Molecular genetics has revealed the necessary signaling molecules for the formation of spirals in organs, but it remains unclear how cellular dynamics generate bending and coiling of the cochlear duct during development. Here we show two modes of multicellular dynamics underlying the morphogenetic process by combining deep tissue live-cell imaging, Förster resonance energy transfer (FRET)-based quantitation, and mathematical modeling. First, surgical separation of the cochlear duct revealed that bending forces reside primarily in the medial side of the duct. In the medial pseudostratified epithelium, we found that nuclei stall at the luminal side during interkinetic nuclear migration, which would cause the extension of the luminal side, thereby bending the duct. Second, long-term organ-scale FRET imaging of extracellular signal-regulated kinase (ERK) activity showed that helical ERK activation waves propagate from the duct tip concomitant with the reverse multicellular flow in the lateral side of the duct, resulting in advection-based duct elongation. We propose an interplay of distinct multicellular behaviors underpinning spiral morphogenesis in the developing cochlear duct.