Anatomy and development of the zebrafish posterior lateral line system

The lateral line is a sensory system of fish and aquatic amphibians that is sensitive to the motion of water about the animal. The lateral line system has long been considered an excellent model system for studies of development because of its simplicity and accessibility. Yet it has received little attention in recent years. I used horseradish peroxidase labeling of cells, scanning electron microscopy, autoradiography, and viewing living specimens with Nomarski optics to provide an anatomical description of the lateral line system of larval zebrafish (Brachydanio rerio). This was to characterize the components of the system and to learn how during development the components are assembled into the system. I found that in the larva there were three components to the lateral line system: (1) typical free sensory neuromasts present in a simple linear array on the side of the fish, (2) afferent neurons which receive contacts from hair cells of the neuromasts and project to a single dorsolateral column of neuropil in the hindbrain, and (3) three types of efferent neurons, including one previously undescribed type found in the diencephalon. A primary system develops in a few day period in the embryo and thus could be functional before the animal hatches. Afferent and efferent neurons are both present before the appearance of the first neuromast. The maturation of neuromasts, which carry out the primary sensory transduction, appears to be the last step in the morphological assembly of the system. The development of the primary system is tightly orchestrated. The first sign of development of this system is the appearance of a cellular primordium, identified at the end of the first day after fertilization, which migrates caudally along the side of the fish. The peripheral growth cones of the afferent neurons grow along with the primordium. Neuromasts then appear during the second day in a rostro-caudal wave which bears a strict temporal relationship to the rate of migration of the mid-body line primordium. In later larva, secondary neuromasts are inserted in between the primary ones in a second rostro-caudal wave which follows a much slower time course.