Volume electron microscopy reveals oligodendrocyte axon selection patterns and spatial assembly of myelin and nodes

Oligodendrocytes myelinate multiple axons, requiring single cells to coordinate target selection, sheath growth, and node formation across complex cortical geometry. We combined serial block-face electron microscopy, complete segmentation, and three-dimensional reconstruction to analyze early-myelinating oligodendrocytes in the somatosensory cortex of a postnatal day 14 rat at single-cell and nanoscale resolution. Individual oligodendrocytes initiated myelination across all contacted axons, while early ensheathment decreased with increasing process distance from the soma. Each oligodendrocyte also contacted axons within a restricted caliber range, independently of wrapping progression. At prospective nodes of Ranvier, transient cytoplasmic projections extended into axonal domains from adjacent oligodendrocytes and converged near presumptive paranodes. These observations support a structural model in which oligodendrocyte geometry, cell-specific axon selection, and intercellular interactions jointly organize early cortical myelination and nodal assembly, providing testable hypotheses for developmental myelination and repair after demyelination.