Pathfinding by identified growth cones in the embryonic zebrafish brain.

Abstract

The embryonic zebrafish (Brachydanio rerio) brain was studied to explore its potential as a system to investigate mechanisms of growth cone guidance in the early vertebrate brain. The early brain tracts were identified and their development was traced with (1) antibodies that label neurons soon after axonogenesis, (2) fluorescent dyes that retrogradely or orthogradely label neurons, and (3) electron microscopy. The embryonic zebrafish brain was found to contain a few longitudinal tracts connected by commissures, which together form an early axon scaffold. Small clusters of neurons were identified that either establish the axon scaffold or later extend growth cones along specific parts of the axon scaffold. Neurons within an identifiable cluster were found to extend growth cones along well defined stereotyped pathways suggesting that their growth cones follow cell specific trajectories by responding to specific cues in their environment. Neurons in the nucleus of the posterior commissure (nucPC), one such identifiable cluster, project growth cones ventrally along the posterior commissure (PC) to an intersection in the anterior tegmentum where the commissure crosses two longitudinal tracts, the TPOC (tract of the postoptic commissure) and the MLF (medial longitudinal fasciculus). Once at the intersection nucPC growth cones turn posteriorly onto the TPOC in the dorsal tegmentum and follow it to the hindbrain. Elimination of the TPOC caused nucPC growth cones to make more errors and extend along aberrant paths in the anterior tegmentum and at the midbrain/hindbrain boundary. This suggests that fasciculation with axons in the TPOC helps to guide the growth cones along their normal pathway in these two regions. However, many nucPC growth cones select an appropriate pathway in the absence of the TPOC. This suggests that other cues associated with the anterior tegmentum and the midbrain/hindbrain boundary, independent of the TPOC, also help to guide the nucPC growth cones. These studies show that the embryonic zebrafish is an attractive system to investigate mechanisms of growth cone guidance in the vertebrate brain. They suggest that error-free navigation in the CNS may normally be brought about by the simultaneous operation of multiple guidance mechanisms.