Morphological correlates of pyramidal cell axonal myelination in mouse and human neocortex.

The axons of neocortical pyramidal neurons are frequently myelinated. Heterogeneity in the topography of axonal myelination in the cerebral cortex has been attributed to a combination of electrophysiological activity, axonal morphology, and neuronal-glial interactions. Previously, we showed that axonal segment length and caliber are critical local determinants of fast-spiking interneuron myelination. However, the factors that determine the myelination of individual axonal segments along neocortical pyramidal neurons remain largely unexplored. Here, we used structured illumination microscopy to examine the extent to which axonal morphology is predictive of the topography of myelination along neocortical pyramidal neurons. We identified critical thresholds for axonal caliber and interbranch distance that are necessary, but not sufficient, for myelination of pyramidal cell axons in mouse primary somatosensory cortex (S1). Specifically, we found that pyramidal neuron axonal segments with a caliber < 0.24 μm or interbranch distance < 18.10 μm are rarely myelinated. Moreover, we further confirmed that these findings in mice are similar for human neocortical pyramidal cell myelination (caliber < 0.25 μm, interbranch distance < 19.00 μm), suggesting that this mechanism is evolutionarily conserved. Taken together, our findings suggest that axonal morphology is a critical correlate of the topography and cell-type specificity of neocortical myelination.