Molecular structure of the axolemma of developing axons following altered gliogenesis in rat optic nerve |
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| Authors: | J A Black S G Waxman |
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| Affiliation: | 1. Department of Neurology, Stanford University School of Medicine, Palo Alto, California 94304 USA;2. Veterans Administration Medical Center, Palo Alto, California 94304 USA;1. F.M. Kirby Neurobiology Center, Boston Children’s Hospital, and Department of Neurology, Harvard Medical School, 300 Longwood Avenue, Boston, MA 02115, USA;2. Departments of Neurology, Psychiatry and Human Genetics, Semel Institute for Neuroscience and Human Behavior, David Geffen School of Medicine, UCLA, Los Angeles, CA 90095-1761, USA;3. Department of Molecular and Cellular Biology, Center for Brain Science, Harvard University, 52 Oxford Street, Cambridge, MA 02138, USA;1. F.M. Kirby Neurobiology Center, Boston Children’s Hospital, and Department of Neurology and Ophthalmology, Harvard Medical School, Boston, MA, USA;2. Department of Pediatrics, Brain Tumor Center, Division of Experimental Hematology and Cancer Biology, Cincinnati Children’s Hospital Medical Center, Cincinnati, OH 45229, USA;1. Department of Ophthalmology, Stanford University School of Medicine, Palo Alto, CA, USA;2. Department of Neurosurgery Stanford University School of Medicine, Palo Alto, CA, USA;3. Department of Pathology, Stanford University School of Medicine, Palo Alto, CA, USA;4. Department of Neurology, Stanford University School of Medicine, Palo Alto, CA, USA |
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| Abstract: | Axonal and axolemmal development of fibers from rat optic nerves in which gliogenesis was severely delayed by systemic injection of 5-azacytidine (5-AZ) was examined by freeze-fracture electron microscopy. In neonatal (0-2 days) rat optic nerves, all fibers lack myelin, whereas in the adult, virtually all axons are myelinated. The axolemma of neonatal premyelinated fibers is relatively undifferentiated. The P-fracture face (P-face) displays a moderate (approximately 550/micron 2) density of intramembranous particles (IMPs), whereas the E-fracture face (E-face) has few IMPs (approximately 125/micron 2) present. By 14 days of age, approximately 25% of the axons within control optic nerves are ensheathed or myelinated, with the remaining axons premyelinated. The ensheathed and myelinated fibers display increased axonal diameter compared to premyelinated axons, and these larger caliber fibers exhibit marked axonal membrane differentiation. Notably, the P-face IMP density of ensheathed and myelinated fibers is substantially increased compared to premyelinated axolemma, and, at nodes of Ranvier, the density of E-face particles is moderately high (approximately 1300/micron 2), in comparison to internodal or premyelinated E-face axolemma. In optic nerves from 14-day-old 5-AZ-treated rats, few oligodendrocytes are present, and the percentage of myelinated fibers is markedly reduced. Despite delayed gliogenesis, some unensheathed axons within 5-AZ-treated optic nerves display an increased axonal diameter compared to premyelinated fibers. Most of these large caliber fibers also exhibit a substantial increase in P-face IMP density. Small (less than 0.4 micron) diameter unensheathed axons within treated optic nerves maintain a P-face IMP density similar to that of control premyelinated fibers. Regions of increased E-face particle density were not observed. The results demonstrate that some aspects of axolemma differentiation continue despite delayed gliogenesis and the absence of glial ensheathment, and suggest that axolemmal ultrastructure is, at least in part, independent of glial cell association. |
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