Planar polarity controls the polarity across epithelial sheets using components distinct from those that determine apical-basal differences. These planar polarity components have been well-studied but all act to interpret rather than generate the planar polarity signal.
Many previous workers studied planar polarity using bristle morphology in flies, but the Rockefeller group set out to study the neuromasts of the lateral line organ in zebrafish—a system where the planar polarity is vital for the biology. Hair cells in the neuromasts must be precisely aligned so they can use polarized stereociliary bundles to detect the direction of water movement.
López-Schier found that the neuromasts migrated posteriorly in two waves from two primordia. Hair cells derived from the first migrating primordium differentiated soon after being deposited and were fixed in this anterior-posterior orientation even after a much later ventral migration. But the second primordium received a ventral migration signal when it was still immature and was fixed in a perpendicular orientation. This allows the fish to detect water movements in two distinct axes.
Disruption of the posterior migration cue in mutants and with misexpression altered both migration and polar polarization in equivalent directions. López-Schier thinks that some of the molecules deposited at the front of migrating cells may favor later polarization events.
Migration may be a factor in other polarization events such as those occurring during gastrulation. Zebrafish offer a system where the migration can be tracked without the need for dissection. The fish also regenerate neuromasts after ablation, and López-Schier wants to know if proper polarity can also be recovered. If regenerated hair cells, originating from resident or externally supplied stem cells, “are not polarized properly it would be a major problem,” he says. In the vestibular organ, for example, randomly oriented hair cells might not work properly and “the animal would feel seasick constantly.”