Myelin Ultrastructure Terminology in Disease and Recovery Processes

Ultrastructural evaluation of myelin coat helps to understand the possible background of pathological changes leading to deterioration or complete loss of nerve functions. A number of terms were previously introduced to describe the fine structural changes in myelin under various conditions. We believe that using a common terminology will be helpful to interpret the structure/function relationship in neurological disorders empowering the diagnosis and possible therapeutical approaches. In this paper, we present examples of ultrastructural changes in myelin during myelination, demyelination, re-myelination and dysmyelination processes and we reviewed the terminology previously used. We tried to include all studies reporting ultrastructural findings with no limitation to the experimental conditions, the species used and the disorders. The terminology used to describe the structural findings included compacted myelin, partially compacted myelin, noncompacted myelin, redundancy (hypermyelination, tomacula, myelinosome), splitting, complete circular splitting, myelin degradation, concentric lamellar bodies (myelin figures), loss of myelin lamellae, polyaxonal Schwann cells and necrotic cell debris. Ultrastructural data described in this paper aimed to provide a guide for future studies. We concluded that the evaluation of ultrastructural changes in any neurological disorder is greatly valuable for a better understanding of pathological and physiological changes occured. We also believe that supporting the ultrastructural findings with quantitative methods in the future will be of great value.