Multicellular rosettes organize neuropil formation

Neuropils are compartments in the nervous system containing dense networks of neurites and synapses which function as information processing centers. Neuropil formation requires structural and functional organization at and across different scales, achieving single-axon precision for circuits that carry out the core functions while simultaneously accommodating variability among individuals1-4. How these organizational features emerge over development is poorly understood. The nerve ring is the primary neuropil in C. elegans, and its structure is thoroughly mapped 5,6. We show that prior to axon outgrowth, nerve ring neurons form a ring of multicellular rosettes with surrounding cells to organize the stratified nerve ring structure 7,8. Axon bundles which correspond to future nerve ring strata grow from rosette centers, travel along the ring on "bridge" cells that are simultaneously engaged in adjacent rosettes, and assemble into a topographic scaffold of the nerve ring. SAX-3/Robo is required for proper rosette formation and outgrowth from the center. Furthermore, axon contact sites that form early in development are more conserved than the later ones, indicating a temporal component in neuropil structural variability. Our results reveal an unexpected and critical role of collective cell behaviors prior to innervation to pattern a complex neuropil and orchestrate its formation across scales.