title: Human intraretinal myelination: axon diameters and axon/myelin thickness ratios.
abstract: OBJECTIVE
  Human intraretinal myelination of ganglion cell axons occurs in about 1% of the population. We examined myelin thickness and axon diameter in human retinal specimens containing myelinated retinal ganglion cell axons.
  
  
  METHODS
  Two eyes containing myelinated patches were prepared for electron microscopy. Two areas were examined in one retina and five in the second retina. Measurements were compared to normal retinal and optic nerve samples and the rabbit retina, which normally contains myelinated axons. Measurements were made using a graphics tablet.
  
  
  RESULTS
  Mean axon diameter of myelinated axons at all locations were significantly larger than unmyelinated axons (P ≤ 0.01). Myelinated axons within the patches were significantly larger than axons within the optic nerve (P < 0.01). The relationship between axon diameter/fiber diameter (the G-ratio) seen in the retinal sites differed from that in the nerve. G-ratios were higher and myelin thickness was positively correlated to axon diameter (P < 0.01) in the retina but negatively correlated to axon diameter in the nerve (P < 0.001).
  
  
  CONCLUSIONS
  Intraretinally myelinated axons are larger than non-myelinated axons from the same population and suggests that glial cells can induce diameter changes in retinal axons that are not normally myelinated. This effect is more dramatic on intraretinal axons compared with the normal transition zone as axons enter the optic nerve and these changes are abnormal. Whether intraretinal myelin alters axonal conduction velocity or blocks axonal conduction remains to be clarified and these issues may have different clinical outcomes.
authors.affiliation: Department of Clinical Ophthalmology; School of Medical Sciences and Bosch Institute, Discipline of Anatomy and Histology, University of Sydney, NSW 2006, Australia, .
authors.forename: Thomas
authors.initials: T
authors.lastname: FitzGibbon
authors.forename: Zoran
authors.initials: Z
authors.lastname: Nestorovski
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D000818:Animals; D001369:Axons / Q000648:ultrastructure*; D002452:Cell Count; D005260:Female; D006801:Humans; D008854:Microscopy, Electron; D009186:Myelin Sheath / Q000648:ultrastructure*; D009900:Optic Nerve / Q000648:ultrastructure*; D011817:Rabbits; D012165:Retinal Ganglion Cells / Q000648:ultrastructure*