The topographic projection from the eye to the tectum (amphibians and fish)/superior colliculus (birds and mammals) is a paradigm model system for studying mechanisms of neural wiring development. It has previously been proposed that retinal ganglion cell axons use distinct guidance strategies in fish vs. mammals, with direct guidance to the tectal target zone in the former and overshoot followed by biased branching toward the target zone in the latter. Here we visualized individual retinal ganglion cell axons as they grew over the tectum in zebrafish for periods of 10-21 hours and analyzed these results using an array of quantitative measures. We found that, although axons were generally guided directly toward their targets, this occurred without growth cone turning. Instead, axons branched dynamically and profusely throughout pathfinding, and successive branches oriented growth cone extension toward a target zone in a stepwise manner. These data suggest that the guidance strategies used between fish and mammals may be less distinct than previously thought.

The zebrafish retinotectal projection provides an attractive model system for studying many aspects of topographic map formation and maintenance. Visual connections initially start to form between 3 and 5 days postfertilization, and remain plastic throughout the life of the fish. Zebrafish are easily manipulated surgically, genetically, and chemically, and a variety of molecular tools exist to enable visualization and control of various aspects of map development. Here, we review zebrafish retinotectal map formation, focusing particularly on the detailed structure and dynamics of the connections, the molecules that are important in map creation, and how activity regulates the maintenance of the map.

BackgroundNormal brain function depends on the development of appropriate patterns of neural connections. A critical role in guiding axons to their targets during neural development is played by neuronal growth cones. These have a complex and rapidly changing morphology; however, a quantitative understanding of this morphology, its dynamics and how these are related to growth cone movement, is lacking.ResultsHere we use eigenshape analysis (principal components analysis in shape space) to uncover the set of five to six basic shape modes that capture the most variance in growth cone form. By analysing how the projections of growth cones onto these principal modes evolve in time, we found that growth cone shape oscillates with a mean period of 30 min. The variability of oscillation periods and strengths between different growth cones was correlated with their forward movement, such that growth cones with strong, fast shape oscillations tended to extend faster. A simple computational model of growth cone shape dynamics based on dynamic microtubule instability was able to reproduce quantitatively both the mean and variance of oscillation periods seen experimentally, suggesting that the principal driver of growth cone shape oscillations may be intrinsic periodicity in cytoskeletal rearrangements.ConclusionsIntrinsically driven shape oscillations are an important component of growth cone shape dynamics. More generally, eigenshape analysis has the potential to provide new quantitative information about differences in growth cone behaviour in different conditions.

The relative importance of neural activity versus activity-independent cues in shaping the initial wiring of the brain is still largely an open question. While activity is clearly critical for circuit rearrangements after initial connections have been made, whether it also plays a role in initial axon pathfinding remains to be determined. Here, we investigated this question using the guidance of zebrafish retinal ganglion cell axons to their targets in the tectum as a model. Recent results have implicated biased branching as a key feature of pathfinding in the zebrafish tectum. Using tetrodotoxin to silence neural activity globally, we found a decrease in the area covered by axon branches during pathfinding. After reaching the target, there were dynamic differences in axon length, area and the number of branches between conditions. However, other aspects of pathfinding were unaffected by silencing, including the ratio of branches directed toward the target, length, and number of branches, as well as turning angle, velocity, and number of growth cones per axon. These results challenge the hypothesis that neural connections develop in sequential stages of molecularly guided pathfinding and activity-based refinement. Despite a maintenance of overall guidance, axon pathfinding dynamics can nevertheless be altered by activity loss.

Abstract We investigated how the two rounds of whole-genome duplication that occurred at the base of the vertebrate lineage have impacted ancient microsyntenic associations involving developmental regulators (known as genomic regulatory blocks, GRBs). We showed that the majority of GRBs identified in the last common ancestor of chordates have been maintained as a single copy in humans. We found evidence that dismantling of the duplicated GRB copies occurred early in vertebrate evolution often through the differential retention of the regulatory gene but loss of the bystander gene’s exonic sequences. Despite the large evolutionary scale, the presence of duplicated highly conserved noncoding regions provided unambiguous proof for this scenario for multiple ancient GRBs. Remarkably, the dismantling of ancient GRB duplicates has contributed to the creation of large gene deserts associated with regulatory genes in vertebrates, providing a potentially widespread mechanism for the origin of these enigmatic genomic traits.