Distinct roles for FGF3 and FGF10 in the transition from proneuromast initiation to proneuromast maturation and deposition in the zebrafish lateral line primordium

The posterior lateral line primordium (pLLp) migrates caudally and periodically deposits neuromasts to form the pLL system in zebrafish. Wnt‐dependent FGF3 and FGF10 expression initiates proneuromast formation at the leading end of the pLLp. FGF signaling initiates morphogenesis of epithelial rosettes and expression of prosensory hair cell factor, atoh1a, in the maturing proneuromasts. Notch–mediated lateral inhibition restricts atoh1a expression to a central cell. This central atoh1a‐expressing cell becomes a stable focal source of FGF10 as proneuromasts mature and prepare for deposition from the trailing end. Previous studies have shown that FGF signaling induces expression of Wnt antagonist Dkk1, which limits or prevents Wnt activity in the FGF domain. Our studies suggest that Dkk1 expression requires both Wnt and FGF activity and defines an intermediate domain where proneuromasts begin to form. Further maturation of proneuromasts and preparation for deposition requires establishment of a third trailing zone with less Wnt activity. Establishment of this third domain defines a specific role for FGF10, which is less effectively played by FGF3. We show how changing the relative size of the leading Wnt domain; the intervening domain with Wnt and FGF activity; and the trailing domain with primarily FGF activity, alters the pattern of proneuromast initiation, maturation and deposition in the migrating pLLp.