Fig. 1.

Inversions are local movements of nascent sibling hair cells. (A) Scheme of a neuromast, depicting an outer ring of mantle cells (red), internal supporting cells (gray) and central hair cells (light blue) with their axis of planar polarity (dark blue dots). Dashed line indicates the midline of the organ. (B) Scheme of hair-cell development. Unipotent progenitors (UHCP) divide into two hair cells. Sibling hair cells undergo positional inversion to place Notch-on/Emx2(−) and Notch-on/Emx2(+) cells on opposite sides of the epithelium. (C) The inversion is an angular movement of at least 90°. (D) Selected frames from a time-lapse movie of cell-pair inversion in a wild-type neuromast expressing cldnb:EGFP and myo6b:GFP. The timings are relative to the mitotic division that generates the hair-cell pair. Scale bar: 5 µm. (E) One exemplary hair-cell dyad during an inversion. The position of each cell during the inversion is depicted relative to the centroid of the pair. Time is color-coded from dark violet to yellow, where 0 is the time immediately after cell division and 400 is the upper limit of the inversion. (F) Position of the hair-cell progenitor at the time of its mitotic division. The color of the dots indicates whether the resulting hair-cell pair inverts (red) or not (blue). The center was defined as the position of all pre-existing hair cells in the neuromast. Side panels show the density of points along the dorsoventral (D-V) and anteroposterior (A-P) axes of the epithelium. It shows 22 inverting and 18 non-inverting hair-cell pairs from different neuromasts in 22 specimens. (G) Boxplots showing the Pearson correlation coefficient for the movement of cells in the neuromast along the A-P axis. Each point represents the cells during a rotation. Box plots show median values (middle bars) and first (Q1) to third (Q3) interquartile ranges (boxes); upper whisker is either 1.5× the interquartile range or the maximum value (whichever is the smallest) and lower whisker is either 1.5× the interquartile range or the minimum value (whichever is the biggest). For each neuromast, the movement was compared between the rotating hair cells (HC); the rotating hair cells and all other cells; and between all other cells. n=9 independent neuromasts from N=9 different larvae.