Ontogenesis of the axolemma and axoglial relationships in myelinated fibers: electrophysiological and freeze-fracture correlates of membrane plasticity

Morphometric analysis of the developing optic nerve of the F1 heterotic mouse and its parental strains

During the period of active myelination, the CNS of the F1 hybrid mouse, derived from DBA/2J (D2) and C57BL/6J (B6) parental strains, displays levels of myelin-specific markers which are greater than in either parent. This so-called hypermyelination has been attributed to hybrid vigor or heterosis. Morphometric comparison of the optic nerves of F1 and parental strains revealed that the F1 contains larger myelinated axons and fewer premyelinated axons. These observations suggest that, compared with its parental strains, the F1 hybrid shows an early onset and accelerated rate of myelination.

Low density of sodium channels supports action potential conduction in axons of neonatal rat optic nerve

The density of sodium channels in premyelinated axons was estimated from measurements of the binding of [3H]saxitoxin to neonatal rat optic nerve. The maximum saturable binding capacity of the nerve was 16.2 +/- 1.2 fmol/mg of wet weight, with an equilibrium dissociation constant of 0.88 +/- 0.18 nM (mean +/- SEM). These values correspond to a high-affinity saxitoxin-binding site density of approximately 2/microns 2 within premyelinated axon membrane. Action potential propagation in neonatal rat optic nerve is completely blocked by 5 nM saxitoxin, indicating that action potential electrogenesis is mediated by channels that correspond to high-affinity saxitoxin-binding sites. These results demonstrate that action potential conduction is supported by a low density of sodium channels in this system. Since the internodal axon membrane of myelinated fibers may contain a low density of sodium channels, it is possible that restoration of conduction in some demyelinated fibers may not require additional sodium channel incorporation into the demyelinated axon membrane.

Node of Ranvier formation along fibres regenerating through silicone tube implants: a freeze-fracture and thin-section electron microscopic study

Thin-section and freeze-fracture electron microscopy have been used to examine the morphogenesis of the node of Ranvier in peripheral nerves regenerating through silicone tubes. A major question posed by this study is whether node formation in fibres regenerating across a gap recapitulates that occurring in normal development. Node formation occurs concurrently with myelination and follows a similar spatial gradient of progression from a proximal to distal direction along the regenerated nerve. Presumptive nodal sites appear prior to myelin formation and are identified as a prominent subaxolemmal density in thin sections and axonal particle patches in freeze-fracture. Following the appearance of presumptive nodes in regenerating fibres, dimeric particles are inserted into the axolemma adjacent to the node. These particles are in close apposition to the overlying Schwann cell terminal processes and with maturity adopt the same circumferential orientation seen in adult nodes. The nodal axolemma of regenerating fibres shows a characteristic increase in the prominence of its subaxolemmal densification and number of heterogeneously sized particles. Mature regenerated nodes demonstrate a complete annulus of nodal particles indistinguishable from control nodes. The results of the present study show that the nodal architecture of regenerating fibres is a faithful reconstruction of normal mature nodes, thus indicating that the morphological correlates associated with saltatory conduction at the node are present in regenerated nodes.

Optical recording of action potential propagation in demyelinated frog nerve

Conduction in focally demyelinated frog nerves has been measured optically using potential-sensitive dyes. Absorption changes were recorded with an array of photodiodes positioned in the image plane of a microscope. Both the amplitude and conduction velocity of the optical signals decreased in the demyelinated region. Conduction was improved after exposure to the potassium channel blocking agent 4-aminopyridine.

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.

Minisegments of newborn rat optic nerves in vitro: gliogenesis and myelination

The question of whether the development of CNS glial cells requires the presence of axons or not can be studied with in vitro systems. In order to compare the differentiation of glial cells during development in vitro with that in situ, we have selected the optic nerve, which is anatomically as well as histotypically a well defined structure. For the in vitro investigations, small explants, called minisegments, of newborn rat optic nerves were cultivated taking four major conditions into account: the regular size of the minisegments should guarantee a permanent exchange of the culture medium in order to avoid cell death, neither mechanical nor enzymatic dissociation of the tissue were applied, the minisegments were explanted into flasks without substrate for cell adhesion and the minisegments were under constant gyratory agitation. The following in situ results were obtained: optic nerves of newborn rats are morphologically characterized by the presence of naked axons, astrocytes, glial precursors, and the absence of both differentiated oligodendrocytes and myelin. At postnatal day 5 myelin sheaths are still absent. Two weeks after birth, differentiated oligodendrocytes and microglial cells are present and numerous axons are surrounded by compact myelin. The in vitro experiments show the following main results, which were obtained after 14 h, 2 d, 5 d and 14 d in culture: during time in culture, the shape of minisegment of newborn rat optic nerves undergoes drastic changes, which indicate high cellular dynamics. After 14 h in vitro, axonal profiles, cells with pyknotic nuclei as well as clusters of astrocytes and glial precursors are present. After 2 days in culture the axonal profiles disappeared and the number of degenerating cells decreased drastically. Many large cells, probably phagocytes containing inclusions and more cells are differentiated. At the stage of 5 d in vitro 4 major types of cells can be distinguished: differentiated oligodendrocytes, which form compact and loose myelin, astrocytes, large and small glioblasts and phagocytes. Immunoprecipitates for myelin basic protein and/or myelin associated glycoprotein were found in oligodendrocytes, in their processes and associated to the myelin. Processes of some astrocytes showed immunoreactive products of glial fibrillary acidic protein. After two weeks in culture, the minisegments were mostly composed of astrocytes, whereas oligodendrocytes became rare and phagocytes disappeared. It can be concluded that CNS glial cells can attain their structural and immunocytochemical characteristics in the total absence of neuronal cell bodies and axons.(ABSTRACT TRUNCATED AT 400 WORDS)

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

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.

Axonal transport of the voltage-dependent Na+ channel protein identified by its tetrodotoxin binding site in rat sciatic nerves

Na+ channels levels were measured in different segments of rat vagus and sciatic nerves by in vitro binding using a tritiated ethylene-diamine tetrodotoxin derivative ([3H]en-TTX). Binding sites were found to accumulate on both sides of a ligature tied on the sciatic nerve indicating an anterograde and retrograde axoplasmic transport of Na+ channels. Accumulation of Na+ channels at the ligature was time-dependent and appeared to occur through fast axoplasmic transport mechanisms. This accumulation on both sides of a ligature was also visualized by autoradiographic studies in longitudinal sections of sciatic nerves using [3H]en-TTX.

Membrane ultrastructure of developing axons in glial cell deficient rat spinal cord

In order to investigate axolemmal development in a glial cell deficient environment, normal and irradiated dorsal funiculus in rat lumbosacral spinal cord was examined by freeze-fracture electron microscopy. At 3 days of age, normal fibres are all unmyelinated and of small (less than 0.5 micron) diameter. The unmyelinated axons have a moderate density (approximately 850 microns-2) of intramembranous particles (IMPs) on P-fracture faces and a low IMP density (approximately 300 microns-2) on E-faces. IMPs are homogeneously distributed along both fracture faces. By 19 days of age, the normal dorsal funiculus is well populated with myelinated axons and glial cells, as well as a sizable population of unmyelinated fibres. Nearly all of the myelinated fibres have a large (greater than 1.0 micron) diameter; whereas, most unmyelinated axons are of small (less than 0.5 micron) calibre. The axolemma of unmyelinated axons is relatively undifferentiated, with an asymmetrical distribution of IMPs (P-face: approximately 1100 microns-2; E-face: approximately 450 microns-2). Myelinated fibres show nodal and paranodal regions with P-face and E-face ultrastructure similar to previous descriptions. Internodal axolemma appears relatively homogeneous, with P-faces being highly particulate (approximately 2100 microns-2) and a low IMP density (approximately 200 microns-2) on E-faces. Following irradiation of the lumbosacral spinal cord at 3 days of age, there is a severe reduction in the number of glial cells and myelinated fibres in this region when the tissue is examined at 19 days of age. Despite the deficiency of glial cells in this tissue, axonal and axolemmal development continue. Numerous large (greater than 1.0 micron) diameter axons are present in this irradiated tissue. Large diameter axons show a high (approximately 2000 microns-2) density of IMPs on P-faces; E-face IMP density remains at approximately 440 micron-2. Small calibre axons also have an asymmetrical distribution of particles (P-face: approximately 1100 microns-2; E-face: 280 microns-2). The axolemmal E-faces of some glial cell deprived fibres exhibit regions with greater than normal (approximately 750 microns-2) density of IMPs. These results demonstrate that some aspects of axonal and axolemmal development continue in a glial cell deficient environment, and it is suggested that axolemmal ultrastructure is, at least in part, independent of glial cell association.

Myelinated non-axonal neuronal elements in the feline olfactory bulb lack sites with a nodal structural differentiation

Myelinated dendrites in the external plexiform layer (EPL) of the feline olfactory bulb and myelinated axons in the lateral olfactory tract (LOT), were examined by transmission electron microscopy. The results show that the non-axonal myelin sheaths are extremely thin and short and that the number of myelin lamellae does not increase with increasing dendritic diameter. In myelinated LOT axons the sheaths tend to be thicker and the myelin lamellar number increases with axon diameter. Domains with node-like structural characteristics are not encountered along myelinated dendrites, neither between successive myelin sheaths nor where single sheaths terminate. The partly myelinated neuronal perikarya, which occur in the EPL, also lack node-like domains. In contrast, typical nodes are easily found in myelinated LOT axons. In the periglomerular region dendrites and neuronal perikarya are surrounded by non-compacted glial sheets. It is concluded that myelination and node formation are relatively independent events and that morphogenetic glial-neuronal interactions may give different results in different parts of the same neuron.