THREE-DIMENSIONAL ULTRASTRUCTURAL ANALYSIS OF EARLY CORTICAL MYELINATION AND NODE OF RANVIER FORMATION

Oligodendrocytes are specialized glia that generate myelin sheaths around axons in the CNS, enabling saltatory conduction and providing essential metabolic support. Although myelin ultrastructure has been extensively described, how individual oligodendrocytes integrate axonal geometry, intrinsic cellular architecture, and intercellular interactions to shape early myelination and node of Ranvier formation remains poorly quantified. We combined high-resolution serial block-face electron microscopy with complete segmentation and 3D reconstruction for quantitative analysis of early-myelinating oligodendrocytes in the developing somatosensory cortex at single-cell and nanoscale resolution. Our analyses revealed that individual oligodendrocytes initiate myelination nearly synchronously across all contacted axons, but the extent of early ensheathment is strongly constrained by intrinsic cell geometry: myelin progression inversely correlates with the distance of axonal targets from the oligodendrocyte soma. We identified a striking cell-specific selectivity for axon caliber, revealing that each oligodendrocyte operates within a restricted axonal preference range that emerges at the onset of myelination and is independent of early wrapping progression. Extending this analysis to axon–glia interfaces at prospective nodes of Ranvier, we uncovered transient intra-axonal cytoplasmic projections arising from neighboring oligodendrocytes during early myelination. These projections arise from presumptive paranodal regions and converge between neighboring oligodendrocytes, suggesting an active and dynamic morphological phase preceding mature nodal architecture. Together, our findings establish a structural framework in which intrinsic oligodendrocyte geometry, axon selection rules, and coordinated intercellular interactions jointly shape early myelination and nodal formation, providing new insight into myelin development and offering mechanistic perspectives relevant to remyelination failure and nodal disruption in demyelinating disease.