Second-order projection from the posterior lateral line in the early zebrafish brain

In this paper we examine the second-order projection from the posterior lateral-line system in late embryonic/early larval zebrafish. At four days after fertilization the synaptic field of the sensory neurons can be accurately targeted, allowing a very reproducible labeling of second-order neurons. We show that second-order projections are highly stereotyped, that they vary according to rhombomeric identity, and that they are almost completely lateralized. We also show that the projections extend not only to the contralateral hindbrain and torus semicircularis but to many other brain centers as well, including gaze- and posture-controlling nuclei in the midbrain, and presumptive thalamic nuclei.We propose that the extensive connectivity observed in early brain development reveals a basic scaffold common to most vertebrates, from which different subsets are later reinforced in various vertebrate groups. The large repertoire of projection targets provides a promising system to study the genetic encoding of this differential projection capacity.The sensory input measured by vertebrate mechanosensory hair cells is hair deflection, yet this input can convey information about a number of different stimuli, such as sound waves, angular acceleration of the head, body movement, or posture. The process by which sensory transduction translates into perception depends on the structure of the sense organ as well as on the distribution of sensory information to specific brain centers through second- (and third-) order projections. Here we examine the second-order projection of a particular set of sensory organs, the mechanosensory organs of the lateral-line system, in the zebrafish embryo.In amniotic vertebrates, hair cells are restricted to the inner ear where they mediate audition and vestibular proprioceptive functions. In fish and amphibians, mechanosensory hair cells are also present in another sensory system, the lateral line. The lateral-line system is closely related to t