Molecular structure of the axolemma of developing axons following altered gliogenesis in rat optic nerve

Morphological consequences of myelination in the human retina

Intraretinal myelination of ganglion cell axons occurs in about 1% of humans and when observed ophthalmoscopically, appears as a white or opaque patch within the fiber layer. Previous studies of myelinated retinal tissue have largely been conducted at the light microscopic level. Three retinae with intraretinal myelination and one normal retina were obtained post-mortem and prepared for electron microscopy. The present study showed that myelinated patches in the human retina contained a mixture of unmyelinated and myelinated axons. Within this population of myelinated axons were structures which were abnormal and there were obvious signs of axonal and myelin sheath degeneration within the myelinated patches. Outside these myelin patches the retina appeared normal without signs of degeneration indicating that post-mortem degeneration prior to fixation could not account for all of the degenerative changes observed. The lack of significant numbers of macrophages and lymphocytes indicated that there was no concomitant inflammatory process within the myelin patches. The myelination present within these eyes appeared to be due to the anomalous location of oligodendrocytes. Both unmyelinated and myelinated axons had larger diameter than axons measured within normal areas of the retina or those within the optic nerve.

Macromolecular structure of axon membrane and action potential conduction in myelin deficient and myelin deficient heterozygote rat optic nerves

The macromolecular structure of the axon membrane in optic nerves from 25-day-old male littermate control and myelin deficient (md) rats and 16-month-old md heterozygotic rats was examined with quantitative freeze-fracture electron microscopy. The axon membrane of control optic nerves displayed an asymmetrical partitioning of intramembranous particles (IMPs); P-fracture faces of myelinated internodal axon membrane were more particulate than those of pre-myelinated axons (approximately 1600 v 1100 microns-2, respectively), while relatively few IMPs (approximately 150 microns-2) were present on external faces (E-faces) of internodal or pre-myelinated axon membrane. Amyelinated axons of md optic nerves also exhibited an asymmetrical partitioning of IMPs; protoplasmic membrane face (P-face) IMP densities, taken as a group, exhibited a wide range (approximately 600-2300 microns-2) and, in most regions, E-faces displayed a relatively low IMP density (approximately 175 microns-2). Axons of greater than 0.4 microns diameter exhibited significantly greater mean P-face IMP density than axons less than 0.4 microns diameter. Aggregations of E-face IMPs (approximately 350 microns-2) were occasionally observed along amyelinated axon membrane from md optic nerves. Optic nerves from md heterozygote rats exhibit myelin mosaicism, permitting examination of myelinated and amyelinated axon membrane along the same tract. The axon membrane exhibits different ultrastructure in these two domains. Myelinated internodal axon membrane from md heterozygote optic nerves exhibits similar P- and E-face IMP densities to those of control internodal axolemma (approximately 1800 and 140 microns-2, respectively). Amyelinated axons in the heterozygote exhibit a membrane structure similar to amyelinated axons in md optic nerve. P-face IMP density of large diameter (greater than 0.4 microns) amyelinated axons from md heterozygote optic nerves is significantly greater than that of small calibre (less than 0.4 microns) axons. In most regions, amyelinated axon membrane exhibits a relatively low E-face IMP density (approximately 200 microns-2); however, focal aggregations (approximately 400 microns-2) of E-face particles are present. Electrophysiological recordings demonstrate that amyelinated axons in md optic nerves support the conduction of action potentials. Compound action potentials in md optic nerves exhibit a monophasic configuration, even at 20-days postnatal, similar to that of pre-myelinated optic nerve of 7-day-old normal rats. Moreover, conduction velocities in the amyelinated 20-day-old md optic nerve are similar to those displayed by pre-myelinated axons from 7-day-old optic nerves.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunohistochemical localization of GAP-43 in the developing hamster retinofugal pathway

Metabolic labeling studies have shown that the developing hamster retinotectal pathway is marked by a high level of synthesis and axonal transport of the neuron-specific phosphoprotein GAP-43, which then decline sharply with synaptic maturation. To understand better the relationship of GAP-43 to specific developmental events, we used a monospecific antibody to examine the location of this protein in the optic tract and retinal target areas at various stages. In late embryonic and in neonatal hamsters, dense GAP-43 immunostaining was seen along the entire extent of the optic tract axons, including fascicles coursing over and through the lateral geniculate body (LGB) and within the upper layers of the superior colliculus (SC). The retinal origin of many of these fascicles was confirmed by their rapid disappearance after removal of the contralateral eye. During the first postnatal week, immunostaining in the fiber fascicles showed a marked decline, though the protein was still present throughout the neuropil of the LGB and SC. In the second postnatal week, the neuropil staining also diminished, and by 12 days after birth, both structures showed only light immunoreactivity. The high levels of GAP-43 in embryonic and neonatal optic tract axons coincide temporally with axon elongation, initial target contact, and collateral formation by the retinofugal fibers, whereas subsequent concentration of the protein in the neuropil suggests its involvement in the elaboration of terminal arbors and synaptogenesis.

Unmyelinated and myelinated axon membrane from rat corpus callosum: differences in macromolecular structure

The macromolecular structure of unmyelinated and myelinated internodal axon membrane was examined with freeze-fracture electron microscopy. Unmyelinated axons exhibited a gradient of axonal diameters, generally ranging from 0.1 to 0.5 micron, with some unmyelinated axons of up to 0.7 micron diameter. Myelinated fibers also displayed a range of axonal diameters, with axons generally 0.3-1.0 micron. The overlap in diameters, between unmyelinated and myelinated fibers, permitted a comparison of membrane structure in myelinated and unmyelinated axons of the same diameter. Small (less than 0.5 micron) diameter unmyelinated axons exhibited a moderate density (approximately 700/micron2) of P-face intramembranous particles (IMPs), while large (greater than or equal to 0.5 micron) caliber unmyelinated axons displayed a significantly greater P-face IMP density (approximately 1100/micron2). Internodal membrane of both small (less than 0.5 micron) and large (greater than or equal to 0.5 micron) diameter myelinated fibers exhibited densities of P-face particles (approximately 1400/micron2) that were similar to each other, but significantly different from unmyelinated fibers. These results demonstrate that there are differences in membrane structure between unmyelinated and myelinated axons of similar diameter. These findings also demonstrate that membrane structure of unmyelinated axons is not invariant for all unmyelinated fibers within a given CNS tract but, on the contrary, is related to diameter.

Regional membrane heterogeneity in premyelinated CNS axons: factors influencing the binding of sterol-specific probes

Binding of the sterol-specific probe filipin to developing optic nerve axonal membrane is spatially heterogeneous prior to association of glial cells with the axons. Experiments were performed using different sterol binding probes (filipin, tomatin, and saponin), at different temperatures (4 degrees C, 23 degrees C, and 37 degrees C), after incubation in different ionic conditions (10 mM Ca2+, 10 mM EGTA, and 20 mM Mg2+), to examine factors that may be responsible for this membrane heterogeneity in rat optic nerve. The patchy pattern of filipin binding is apparent with each sterol-specific probe, even prior to glial ensheathment, and is retained when membrane fluidity is increased at higher temperatures. Increased Ca2+ concentration increased membrane stability, and increased Mg2+ reduced the patchiness of filipin binding. After tannic acid staining, regions of the cytoskeleton are seen associated with the membrane via filaments extending from microtubules to the membrane, preferentially in regions where filipin interaction with the membrane is inhibited. The non-uniform interaction of filipin with the axolemma suggests an underlying heterogeneity in the sterol composition and stability of the membrane. Heterogeneity of premyelinated axonal membrane may provide an important formative influence in the differentiation of axons to their mature morphology and function.

Effects of delayed myelination by oligodendrocytes and Schwann cells on the macromolecular structure of axonal membrane in rat spinal cord

The macromolecular structure of axonal membrane from dorsal funiculi of control and irradiated spinal cord of 45-day-old rats was examined with freeze-fracture electron microscopy. In control spinal cords, virtually all myelination is mediated by oligodendrocytes, and the internodal axonal membrane of these fibres displays highly asymmetrical partitioning of intramembranous particles (IMPs). The internodal P-face particle density is approximately 2350IMPs per micron 2, whereas the E-face IMP density is approximately 150 per micron 2. In control dorsal spinal roots, myelination is mediated by Schwann cells, and the ultrastructure of the internodal axolemma of the myelinated fibres is similar to that displayed by myelinated fibres of dorsal funiculi. On the internodal P-face of Schwann cell-myelinated fibres the IMP density is approximately 2350 per micron 2, whereas on the E-face the density is approximately 175 per micron 2. Irradiation of the lumbosacral spinal cord at 3 days of age results in a glial cell-deficient region within the spinal cord such that myelination in irradiated dorsal funiculi is delayed and subsequent myelination is mediated by both oligodendrocytes and Schwann cells. By 45 days of age, dorsal funiculi of irradiated spinal cords are well populated with fibres myelinated by oligodendrocytes and Schwann cells. However, fibres myelinated by oligodendrocytes display very thin myelin sheaths whereas Schwann cell-myelinated fibres exhibit myelin sheaths with normal thicknesses. Internodal membrane of fibres myelinated by Schwann cells and oligodendrocytes exhibit similar macromolecular structure, with approximately 2400 IMPs per micron 2 on P-faces and approximately 150 IMPs per micron 2 on E-faces. Occasional large (greater than 1.5 micron diameter) axons without glial-Schwann cell ensheathment are observed. These axons display a high density of P-face particles (approximately 2000 per micron 2) and a moderate density (approximately 350 per micron 2) of E-face IMPs on their fracture faces. These results demonstrate that CNS fibers exhibit similar axonal membrane ultrastructure irrespective of whether they are myelinated by Schwann cells or oligodendrocytes, or whether myelination is delayed. Moreover, when myelination does not occur, the axolemmal E-face IMP density, which may be related to the density of voltage-sensitive sodium channels, is not reduced.