In vertebrates, placodes are transient epithelial thickenings (Figure 1) within the nonneural ectoderm that give rise to sensory neurons of the cranial ganglia (Figure 2, arrowheads) as well as the sensory structures of the nose and ear.  The neurogenic placodes include the trigeminal placode that forms neurons of the trigeminal ganglia; the epibranchial (EB) placodes that generate the sensory neurons of the facial, glossopharyngeal, and vagal ganglia; and lateral line placodes that give rise to the lateral line ganglia and mechanosensory neuromasts in aquatic vertebrates.  EB neurons innervate internal organs to transmit information such as heart rate, blood pressure, and visceral distension from the periphery to the CNS.

In our lab, we are interested in defining the cellular mechanisms of EB placode formation (see placode formation movie) and uncovering roles of various signaling pathways during early placode development (reverse genetic approach). In addition to a candidate gene approach, we are conducting a mutagenesis screen to find genes necessary for EB placode and ganglia development (forward genetic approach). We have already isolated a number of mutants defective in EB placode and ganglia development and efforts are underway to find mutated genes.

Figure 2. This confocal projection was obtained from a 4-day old neurod:gfp transgenic zebrafish. Neurod is a proneural transcription factor expressed in differentiating neurons of the CNS and peripheral nervous system (olfactory neurons, epiphysis, cranial ganglia, retina, etc.). The gfp expression also allows visualization of the central and peripheral neuronal projections. We use this zebrafish strain to conduct mutagenesis screens and characterize various mutants defective in cranial ganglia (arrowheads) development.

II. Development of lateral line system

Lateral line system in aquatic vertebrates consists of mechanosensory organs called neuromasts (NM) (Figure 3, arrowheads) and lateral line nerves that innervate them. Each NM is a volcano-shaped structure with mechanosensory hair cells projecting microtubule-containing kinocilia and actin-based stereocilia through a central pore (Movie 1). Hair cells are surrounded by basally-located support cells. Since lateral line sensory cells are on the surface of the body, they are readily accessible for visualization and manipulation. The lateral line system is important for various behaviors, such as feeding, schooling, obstacle avoidance, and prey detection.

Figure 3. ET20 transgenic  zebrafish embryo at 2 days post-fertilization.  GFP reporter is expressed in a subset of support cells in each NM (arrowheads).

The zebrafish lateral line is emerging as an excellent system to understand basic developmental events such as collective cell migration (see lateral line primordium migration movie) , proliferation and differentiation as well as mechanosensory hair cell development and regeneration in a relatively simple vertebrate system.

Movie 1. This confocal movie was obtain from 4-day old ET20 transgenic embryo that also carried an alpha-tubulin:dTomato transgene (a gift from T. Schilling lab). This transgene allows visualization of mechanosensory hair cells (center, red).