Myelin – the multilayer membrane that envelops axons – is a facilitator of rapid nerve conduction. Oligodendrocytes form CNS myelin; the prevailing hypothesis being that they do it by extending a process that circumnavigates the axon. It is pertinent to ask how myelin is built because oligodendrocyte plasma membrane and myelin are compositionally different. To this end, we examined oligodendrocyte cultures and embryonic avian optic nerves by electron microscopy, immuno-electron microscopy and three-dimensional electron tomography. The results support three novel concepts. Myelin membranes are synthesized as tubules and packaged into “myelinophore organelles” in the oligodendrocyte perikaryon. Myelin membranes are matured in and transported by myelinophore organelles within an oligodendrocyte process. The myelin sheath is generated by myelin membrane fusion inside an oligodendrocyte process. These findings abrogate the dogma of myelin resulting from a wrapping motion of an oligodendrocyte process and open up new avenues in the quest for understanding myelination in health and disease.
It has been thought that a growing myelin sheath winds around the axon by an advancing inner tongue underneath the previously deposited membrane in the centre of the myelin segment”. There is the transport of membrane from a secretory pathway leading to the tounges.
In this study it is demonstrated that: (1) on reaching an axon, the oligodendrocyte process splits in two (bifurcates) with each of its arms embracing it with a slight overlap and probable fusion at the opposite end; and (2) myelin membranes are preformed and packaged as tubules into specific organelles – myelinophore organelles – within the Oligodendrocyte, transported and assembled by a building block fusion process inside an oligodendrocyte. This is an interesting new concept and I will return to this once a review is written makes some diagrams as it will make it easier to explain