Cell migration in the postembryonic development of the fish lateral line.

We examine at the cellular level the postembryonic development of the posterior lateral line in the zebrafish. We show that the first wave of secondary neuromasts is laid down by a migrating primordium, primII. This primordium originates from a cephalic region much like the primordium that formed the primary line during embryogenesis. PrimII contributes to both the lateral and the dorsal branches of the posterior lateral line. Once they are deposited by the primordium, the differentiating neuromasts induce the specialisation of overlying epidermal cells into a pore-forming annulus, and the entire structure begins to migrate ventrally across the epithelium. Thus the final two-dimensional pattern depends on the combination of two orthogonal processes: anteroposterior waves of neuromast formation and dorsoventral migration of individual neuromasts. Finally, we examine how general these migratory processes can be by describing two fish species with very different adult patterns, Astyanax fasciatus (Mexican blind cavefish) and Oryzias latipes (medaka). We show that their primary patterns are nearly identical to that observed in zebrafish embryos, and that their postembryonic growth relies on the same combination of migratory processes that we documented in the case of the zebrafish.