Background: The formation of the posterior lateral line of teleosts depends on the migration of a primordium that originates near the otic vesicle and moves to the tip of the tail. Groups of cells at the trailing edge of the primordium slow down at regular intervals and eventually settle to differentiate as sense organs. The migration of the primordium is driven by the chemokine SDF1 and by its receptor CXCR4, encoded respectively by the genes sdf1a and cxcr4b. cxcr4b is expressed in the migrating cells and is down-regulated in the trailing cells of the primordium. sdf1a is expressed along the path of migration. There is no evidence for a gradient of sdf1a expression, however, and the origin of the directionality of migration is not known.

The lateral line of fish and amphibians is a sensory system that comprises a number of individual sense organs, the neuromasts, arranged in a defined pattern on the surface of the body. A conspicuous part of the system is a line of organs that extends along each flank (and which gave the system its name). At the end of zebrafish embryogenesis, this line comprises 7-8 neuromasts regularly spaced between the ear and the tip of the tail. The neuromasts are deposited by a migrating primordium that originates from the otic region. Here, we follow the development of this pattern and show that heterogeneities within the migrating primordium prefigure neuromast formation.

The posterior lateral line (PLL) of zebrafish comprises seven to eight sense organs at the end of embryogenesis, arranged in a single antero-posterior line that extends along the horizontal myoseptum from the ear to the tip of the tail. At the end of larval life, four antero-posterior lines extend on the trunk and tail, comprising together around 60 sense organs. The embryonic pattern is largely conserved among teleosts, although adult patterns are very diverse. Here we describe the transition from embryonic to juvenile pattern in the zebrafish, to provide a framework for understanding how the diversity of adult patterns comes about. We show that the four lines that extend over the adult body originate from latent precursors laid down by migrating primordia that arise during embryogenesis. We conclude that, in zebrafish, the entire development of the PLL system up to adulthood can be traced back to events that took place during the first 2 days of life. We also show that the transition from embryonic to adult pattern involves few distinct operations, suggesting that the diversity of patterns among adult teleosts may be due to differential control of these few operations acting upon common embryonic precursors.