Functional neural circuits require accurate wiring by axons. Axonal targeting is mediated by a unique repertoire of guidance receptors differentially expressed on the axons of given neuronal populations. These receptors recognize spatially distributed guidance cues across an axon’s trajectory to guide directional axonal growth and ultimately the formation of appropriate synaptic connections. This complex cell to cell signaling is dynamic, spatially-regulated, and differs between neuronal populations and their targets. In a nervous system vulnerable to injury, regeneration of damaged motor axons back to their original muscle targets is essential for recovery of coordinated movement. Growing evidence demonstrates that axon regeneration is not simply a recapitulation of development, but that it requires unknown injury-dependent cues. Yet, what are the guidance receptors and cues that mediate topographically correct axon targeting and what is their spatial distribution? How does the cell signaling underlying circuit formation differ between developing and regenerating tissues? Understanding the cellular and molecular mechanisms that mediate regenerative axon guidance represents an important and unaddressed goal that is relevant to recovery from injury or disease. To fill the existing knowledge gaps, I developed a unique model using the motor innervation of the larval zebrafish pectoral fin, which is homologous to tetrapod forelimbs. Importantly, this model allows for the in vivo observation of individual axons and their synaptic targets from development through regeneration. Monitoring distinct sub-populations of motor neurons reveals that they innervate topographically defined domains in the fin musculature. To find their targets, motor neuron axons must first navigate to the fin, sort between the abductor and adductor muscles, and grow to their appropriate muscle domain where they will form functional synapses. I have previously demonstrated that after laser-mediated axon transection, individual regenerating motor axons precisely return to their originally-specified muscle and target domains in the fin via unknown mechanisms. The long-term goal of my research program is to determine the molecular and cellular signaling that enables neuronal populations to re-establish such precise targeting. Given that each of the stepwise choice points along an axon’s route requires distinct guidance signaling mechanisms, a candidate-based approach to test the role of individual guidance cues is not efficient and will not provide a comprehensive understanding of the complex landscape through which an axon is guided. Thus, a critical foundation to support my research is the generation of a spatially-resolved single cell transcriptome of the motor neuron sub-populations as well as the cells of the fin that they target. The Center for Quantitative Biology (CQB) COBRE will advance my research career goals by providing me with the necessary sc...