The mammalian auditory sensory organ, the organ of Corti, consists of sensory hair cells with uniformly oriented stereocilia on the apical surfaces, displaying a distinct planar cell polarity (PCP) parallel to the sensory epithelium 1-3 . It is not clear how this polarity is achieved during differentiation 4-5 . Here we show that the organ of Corti is formed from a thicker and shorter postmitotic primordium through unidirectional extension, characteristic of cellular intercalation known as convergent extension 6 . Mutations in the PCP pathway interfere with this extension, resulting a shortened and widened cochlea, as well as misorientation of stereocilia. Furthermore, parallel to the homologous pathway in Drosophila 7,8 , a mammalian PCP component Dishevelled2 displays PCPdependent polarized subcellular localization across the organ of Corti. Together, these data suggest a conserved molecular mechanism for PCP pathways in invertebrates and vertebrates, and indicate that the mammalian PCP pathway might directly couple cellular intercalations to PCP establishment in the cochlea.Between embryonic day 13 (E13) and E14, precursors for the organ of Corti can be identified as a zone of non-proliferating cells within the cochlear duct using BrdU pulse-labeling (Fig. 1c, brackets) 4,9-10 . Subsequently, a gradient of cell differentiation within the precursor domain initiates near the base of the cochlea and leads to the patterning of one row of inner and three rows of outer hair cells (Fig. 1d , 1e) along the entire length of the cochlea by E18.5 10 , with uniformly oriented stereocilia on the apical surface of hair cells (Fig. 1e, green). From E14.5 to E18.5, the length of the cochlea is increased approximately 2 fold ( Fig. 1a-b). As viewed in cross sections, the developing organ of Corti in the same period is thinned from a 4-5 cell layered primordium (Fig. 1g) to only two layers of cells (Fig. 1h). One layer of hair cells lies above a layer of supporting cell nuclei whose cytoplasmic phalangeal processes reach the apexes of the hair cells, separating them from each other (Fig. 1h). Based on these observations, we have previously proposed that the longer and thinner mature organ is formed from a defined number of postmitotic precursor cells through integrated cellular intercalation movements 4 . BrdU chasing from E14 to E18 further indicated that there was no detectable cell mixing between the primordial organ of Corti and the surrounding cells (Fig. 1d) the hair cells at E14.5 marked by red fluorescent protein (Math1/RFP) were multi-cell layered ( Fig. 1f) within the sensory primordium (Fig. 1g). The thinning of the sensory primordium is accomplished by E18.5 without detectable cell death ( Fig. 1g-h).To further test our hypothesis, we bisected E14.5 cochlear epithelia into apical and basal portions, cultured them in a defined media. Using the cutting wound sites as reference points, we monitored the extension of the organ of Corti from apical-to-basal or basal-to-apical directions for the apical cultures ...
