Figure 1. Schematic of bidirectional hair cell (HC) establishment in the zebrafish neuromast.

(A) The lateral line system in zebrafish showing the top view of an anterior-posterior (A-P) and dorsal-ventral (D-V) neuromast, based on their hair bundle orientation. Side view of two hair bundles, each comprises a kinocilium (black) and a stereociliary staircase (pink or blue), arranged in opposite orientation atop of an Emx2-positive (pink) and Emx2-negative (blue) HC. (B) An approximate developmental time-line of HC formation in zebrafish neuromast. A HC precursor (gray) divides to form two daughter HCs, which roll to exchange positions with each other 50% of the time before differentiating into mature HCs with opposite hair bundle orientation. The entire process takes approximately 8 hr from detectable Gfp signal driven by the myosin6b promoter to HCs with polarized hair bundles based on our live-imaging analyses. (C) Emx2, which is important for establishing the bidirectional HC pattern, can be expressed early in the HC precursors, which could affect the HC rearrangement (scenario 1) or later during hair bundle establishment after HCs are formed (scenario 2). A combination of both scenarios is also possible (not shown). Black dots represent the position of the kinocilium.