The study of neurodegenerative disorders such as multiple sclerosis (MS) is currently hampered by our inability to properly visualize key pathogenic players (blood–brain barrier breakdown, immune cells infiltration, myelin degradation, axonal transection). Although the exact cause is unclear, it is generally accepted that lesions result from infiltrating immune cells that target myelin antigens.1 Briefly, the purpose of myelin is to ensure efficient saltatory conduction of action potentials over large distances by insulating axons between the nodes of Ranvier, where the action potentials are regenerated. The myelin thickness varies with the axonal diameter and the -ratio, defined as the axonal diameter divided by the diameter of the axon and its myelin sheath, is around 0.60 to 0.70 across all species.2 In demyelinating pathologies, the myelin thickness is known to decrease by as much as 20% of its nominal value.3 Magnetic resonance imaging (MRI) allows the evaluation of the status of myelin sheaths (through T2-weighted MRI and with diffuse tensor imaging) and the blood–brain barrier (with gadolinium enhancement) in the central nervous system, but only at macroscopic levels. Consequently, early signs of diseases and subtle differences between therapies usually go undetected with MRI. Immunohistochemistry applied to ex vivo samples can provide sufficient spatial resolution and a variety of contrast agents, but morphological measurements may be affected by deformation artifacts due to fixation, dehydration, paraffin embedding, and mechanical cutting. Additionally, this approach is impractical for studies over large volumes of tissue because of the time required for sample preparation. Moreover, a large number of biological samples at different time points are needed when performing ex vivo studies, therefore masking inter-individual variations in the pathology. Hence, in vivo cellular imaging techniques are needed to accelerate the study and drug developments for neurodegeneration.