Structural and developmental principles of neuropil assembly in C. elegans

Neuropil is a fundamental form of tissue organization within brains1. In neuropils, densely packed neurons synaptically interconnect into precise circuit architecture(2,3), yet the structural and developmental principles governing this nanoscale precision remain largely unknown(4,5). Here, we use diffusion condensation, an iterative data coarse-graining algorithm6, to identify nested circuit structures within the C. elegans neuropil (called the nerve ring). We show that the nerve ring neuropil is largely organized into four strata composed of related behavioral circuits. The stratified architecture of the neuropil is a geometrical representation of the functional segregation of sensory information and motor outputs, with specific sensory organs and muscle quadrants mapping onto particular neuropil strata. We identify groups of neurons with unique morphologies that integrate information across strata and that create neural structures that cage the strata within the nerve ring. We use high resolution light-sheet microscopy(7,8), coupled with lineage-tracing and cell-tracking algorithms(9,10), to resolve the developmental sequence and reveal principles of cell position, migration and outgrowth that guide stratified neuropil organization. Our results uncover conserved structural design principles underlying nerve ring neuropil architecture and function, and a pioneer-neuron-based, temporal progression of outgrowth that guides the hierarchical development of the layered neuropil. Our findings provide a systematic blueprint for using structural and developmental approaches to understand neuropil organization within brains.