Cell migration plays an essential role in many morphogenetic processes, and its deregulation has many dramatic consequences. Yet how migration is controlled during normal development is still a largely unresolved question. We examined this process in the case of the posterior lateral line (PLL), a mechanosensory system present in fish and amphibians. In zebrafish, the embryonic PLL comprises seven to eight sense organs (neuromasts) aligned from head to tail along the flank of the animal and is formed by a primordium that originates from a cephalic placode. This primordium migrates along a stereotyped pathway toward the tip of the tail and deposits in its wake discrete groups of cells, each of which will become a neuromast. We show that a trail of SDF1-like chemokine is present along the pathway of the primordium and that a CXCR4-like chemokine receptor is expressed by the migrating cells. The inactivation of either the ligand or its receptor blocks migration, whereas in mutants in which the normal SDF1 trail is absent, the primordium path is redirected to the next, more ventral sdf1 expression domain. In all cases, the sensory axons remain associated to the primordium, indicating that the extension of the neurites to form the PLL nerve depends on the movement of the primordium. We conclude that both the formation and the innervation of this system depend on the SDF1-CXCR4 system, which has also been implicated in several migration events in humans, including metastasis formation and lymphocyte homing.

The lateral line of the zebrafish has many of the advantages that made the sensory organs of Drosophila a very productive model system: 1) it comprises a set of discrete sense organs (neuromasts) arranged in a defined, species-specific pattern, such that each organ can be individually recognized; 2) the neuromasts are superficial and easy to visualize, and the innervating neurons are easy to label; 3) the sensory projection is simple yet reproducibly organized. Here we describe some of the tools that can be used to investigate the development of this system, and we illustrate their usefulness with specific exemples. We conclude that the lateral line is uniquely suited among vertebrate sensory systems for a molecular, cellular and genetic analysis of pattern formation and of neural development.

The lateral line system of teleosts has recently become a model system to study patterning and morphogenesis. However, its embryonic origins are still not well understood. In zebrafish, the posterior lateral line (PLL) system is formed in two waves, one that generates the embryonic line of seven to eight neuromasts and 20 afferent neurons and a second one that generates three additional lines during larval development. The embryonic line originates from a postotic placode that produces both a migrating sensory primordium and afferent neurons. Nothing is known about the origin and innervation of the larval lines. Here we show that a "secondary" placode can be detected at 24 h postfertilization (hpf), shortly after the primary placode has given rise to the embryonic primordium and ganglion. The secondary placode generates two additional sensory primordia, primD and primII, as well as afferent neurons. The primary and secondary placodes require retinoic acid signaling at the same stage of late gastrulation, suggesting that they share a common origin. Neither primary nor secondary neurons show intrinsic specificity for neuromasts derived from their own placode, but the sequence of neuromast deposition ensures that neuromasts are primarily innervated by neurons derived from the cognate placode. The delayed formation of secondary afferent neurons accounts for the capability of the fish to form a new PLL ganglion after ablation of the embryonic ganglion at 24 hpf.