MULTIPLY MYELINATED AXONS IN THE OPTIC-NERVE OF MICE DEFICIENT FOR THE MYELIN-ASSOCIATED GLYCOPROTEIN

We recently reported that some retinal ganglion cell axons in mice def icient for the myelin-associated glycoprotein are concentrically surro unded by more than one myelin sheath. In the present study, we demonst rate that myelin sheaths displaced from the axon reveal a normal ultra structure of compact myelin, with the only exception that multiple mye lination of axons frequently correlates with the presence of unfused r egions of major dense lines. Supernumerary sheaths terminated on other sheaths or on astrocyte cell surfaces in a pattern closely resembling the morphology of a true paranode. The thickness of compact myelin of multiply myelinated axons was significantly increased when compared w ith axons of similar caliber surrounded by a single myelin sheath. Our observations demonstrate that maintenance of compact myelin and paran odal regions is not dependent on direct axonal contact and that the pr esence of more than one concentric myelin sheath around an axon result s in dysregulation of the axon-to-fiber ratio. (C) 1995 Wiley-Liss, In c.