Axonogenesis in the brain of zebrafish embryos.

We analyzed the pattern and development of the earliest tracts and followed pathfinding by the growth cones of an identified cluster of neurons in the brain of zebrafish embryos. Neurons were labeled with an antibody which labels many embryonic neurons, a lipophilic axonal tracer dye, and intracellular dye injections. The embryonic brain is extremely simple, and at 28 hr of development, the forebrain and midbrain consist of 8 main axonal tracts which are arranged as a set of longitudinal tracts connected by commissures. Each tract is established by identified clusters of approximately 2-12 neurons found in discrete regions of the brain. Many identified clusters of neurons project axons in a defined direction appropriate for the cluster and have axons with stereotyped trajectories, suggesting that their growth cones follow cell-specific routes. This was confirmed with intracellular dye injections for neurons of the nucleus of the posterior commissure. The growth cones of these neurons arrive at a site in the anterior tegmentum where 4 tracts meet. At this site, they could, in principle, turn in a number of directions but always extend posteriorly into one of the tracts. The pattern of pathfinding by these growth cones suggests the testable hypothesis that the growth cones of identified clusters of neurons establish the simple set of early tracts by selecting cluster-specific pathways at such intersections in order to reach their targets in the brain.