The Modular Architecture of the Myelinated Axon

  For vertebrates to evolve and exist, an energy efficient mechanism of fast nerve conduction had to be developed. Two cell types, the neuron and the OLG in the CNS cooperate to achieve this task. What has evolved is a highly sophisticated structure – a modular architecture – where the myelin sheath only covers axonal segments. Here the neuronal signal jumps (saltatory conduction) from module to module (node of Ranvier). Aided by molecular biology and immunohistochemistry scientists were able to identify specific protein complexes and their cell of origin located in defined areas that are critical for proper nerve conduction. However, how this organization is achieved is unknown. To corroborate that the axonal and OLG plasma membranes are in apposition, we captured images at a time when the OLG process has already embraced the axon, but there are only a few planar MyMs and no myelin sheath. We noticed that in a restricted segment, there was a larger gap between them than in the rest of the contour. Significantly, confined within this space, we detected microvesicles budding off from each of these membranes. By definition these are ectosomes. Ectosomes are known to establish cell-to-cell communication; this fact taken together with their spatial and temporal appearance is highly suggestive that they carry the information to coordinate and orchestrate the modular structure of the myelinated axon. This finding per se is a landmark discovery to celebrate for its innovation and impact!