Astrocyte associations with nodes of Ranvier: ultrastructural analysis of HRP-filled astrocytes in the mouse optic nerve

Fate of developing astrocytes in the optic nerve of the myelin-deficient rat

There is considerable debate on the development of a glial cell line in the rat optic nerve, which is characterized by the specific expression of the A2B5 and HNK-1 epitopes. This cell line has been assumed to give rise to oligodendrocytes and so-called type 2 astrocytes. However, it is doubtful that the latter cell type really exists in vivo. In the present study, we have addressed this question by investigating the development of astrocytes in the myelin-deficient (md) rat, which is characterized by dysmyelination and loss of oligodendrocytes. Defective oligodendrocytes were observed by the third postnatal day, well before the generation of type 2 astrocytes. Consequently, the number of type 2 astrocytes was reduced in cultures prepared from optic nerves of md rats vs. controls. This finding was not paralleled in vivo; i.e., no dying astrocytes were observed in md sections by conventional electron microscopy. However, immunoreactivity against the HNK-1 epitope was enhanced in md compared to control sections. Ultrastructurally, HNK-1 immunoreactivity was detected predominantly on the axonal surface at astroaxonal contact sites, which were found only at the nodes of Ranvier within controls but extended to the whole axonal surface in md animals. Only a minor portion of the immunoreactivity derived from glial cells, presumably from oligodendrocytes at the paranodal region in controls. Thus, the HNK-1 epitope is not a useful antigen for distinguishing astrocytes in the rat optic nerve. Accordingly, our results do not provide evidence for the existence of specialized type 2 astrocytes in vivo. In vitro, these cells are probably only oligodendrocytes that mimic some astroglial features if grown in serum-containing media.

The Lineages of Neuroglial Cells

The study of neuroglial cell lineages in the CNS identifies the time in development, when astrocytes and oligodendrocytes diverge from a common precursor cell. Recent studies using retroviral tracing show that the lineages for astrocytes and oligodendrocytes begin to diverge as early as embryonic day 13 (E13) in the cerebellum and as early as E15 in the forebrain. A very small percentage of glial precursor cells present in late pre- and postnatal development are pluripotential, but the vast majority of astrocytes and oligodendrocytes in the brain are derived from “committed” precursors. The precursors for these postmitotic astrocytes and oligodendrocytes are immature astrocytes and oligodendrocytes (progenitors) that express molecular properties unique to each of these cell types.

It is critical to distinguish glial lineage (thought of in terms of a glial cell's ancestry in normal development) from glial plasticity (the potential of a glial cell to alter its fate when its normal environment is changed). In tissue culture, a bipotential cell known as the O-2A cell generates oligodendrocytes under one set of culture conditions and retains plasticity to become a subtype of astrocyte under another set. Whether all cells phenotyped as O-2A cells in culture are bipotential or whether only a subset displays this capacity is still unclear.

Transfer of horseradish peroxidase from oligodendrocyte to axon in the myelinating neonatal rat optic nerve: artefact or transcellular exchange?

In this paper we make the surprising observation that intracellular injection of horseradish peroxidase (HRP) into a single myelinating oligodendrocyte also resulted in localised HRP labelling at the nodes of Ranvier of some axons of the unit. It appeared that HRP had been transferred to the nodal axoplasm from the paranodal loops of the HRP-filled oligodendrocyte. Three HRP-filled oligodendrocytes from isolated optic nerves of 14-day-old rats were analysed by serial section electron microscopy, and HRP was observed in the axonal cytoplasm at three of the nodes of Ranvier delineated by one of the cells. At labelled nodes, HRP was of a uniform intensity throughout the nodal axoplasm. Axonal labelling gradually diminished along the paranodal regions and was not evident in the contiguous internodal axoplasm beyond 20 microns from the node. The myelin sheaths, paranodal loops, and axons appeared normal at labelled nodes, and the paranodal loops and astrocyte perinodal processes adjacent to those of the HRP-filled oligodendrocyte unit did not contain HRP. There was no evidence of extracellular HRP or tissue damage in the surrounding neuropil, and axons neighbouring those enwrapped by the HRP-filled oligodendrocyte did not contain HRP. The possibility that axonal labelling was an artefact of either iontophoretic injection or tissue preparation is discussed. This provocative finding is not definite proof of exchange, but the balance of evidence supports the possibility that there was transcellular exchange of HRP at paranodes between the labelled oligodendrocyte and some of the axons in the unit. The rarity of HRP transfer to axons suggests that it may be a transient or labile event. It is not clear whether oligodendrocyte to axon macromolecular exchange has real physiological and/or pathological significance.

Glial cells in transected optic nerves of immature rats. II. An immunohistochemical study

The glia response to Wallerian degeneration was studied in optic nerves 21 days after unilateral enucleation (PED21) of immature rats, 21 days old (P21), using immunohistochemical labelling. Nerves from normal P21 and P42 nerves were also studied for comparison. At PED21, there was a virtual loss of axons apart from a few solitary fibres of unknown origin. The nerve comprised a homogeneous glial scar tissue formed by dense astrocyte processes, oriented parallel to the long axis of the nerve along the tracks of degenerated axons. Astrocytes were almost perfectly co-labelled by antibodies to glial fibrillary acid protein and vimentin in both normal and transected nerves. However, there was a small population of VIM+GFAP- cells in normal P21 and P42 nerves, and we discuss the possibility that they correspond to O-2A progenitor cells described in vitro. Significantly, double immunofluorescence labelling in transected nerves revealed a distinct population of hypertrophic astrocytes which were GFAP+VIM-. These cells represented a novel morphological and antigenic subtype of reactive astrocyte. It was also noted that the number of oligodendrocytes in transected nerves did not appear to be less than in normal nerves, on the basis of double immunofluorescence staining for carbonic anhydrase II, myelin oligodendrocyte glycoprotein, myelin basic protein, glial fibrillary acid protein and ED-1 (for macrophages), although it was not excluded that a small proportion may have been microglia. A further prominent feature of transected nerves was that they contained a substantial amount of myelin debris, notwithstanding that OX-42 and ED1 immunostaining showed that there were abundant microglia and macrophages, sufficient for the rapid and almost complete removal of axonal debris. In conclusion, glial cells in the immature P21 rat optic nerve reacted to Wallerian degeneration in a way equivalent to the adult CNS, i.e. astrocytes underwent pronounced reactive changes and formed a dense glial scar, oligodendrocytes persisted and were not dependent on axons for their continued survival, and there was ineffective phagocytosis of myelin possibly due to incomplete activation of microglia/macrophages.

Glial cells in transected optic nerves of immature rats. I. An analysis of individual cells by intracellular dye-injection

The glial response to Wallerian degeneration was studied in optic nerves following unilateral enucleation in immature rats, aged 21 days old (P21). The three-dimensional morphology of dye-filled glia was determined in intact nerves, at post-enucleation day 21 in normal nerves from untreated P21 rats, by correlating laser scanning confocal microscopy and camera lucida drawings of single cells. In normal and transected nerves, the majority of dye-filled cells comprized astrocytes (54% and 65%, respectively). In normal P21 nerves, the predominant astrocyte form had a complex stellate morphology and had a centrally-located cell body from which branching processes extended randomly. Two other distinct forms were transverse and longitudinal astrocytes, which had a polarized process extension in a plane perpendicular or parallel to the long axis of the nerve, respectively. These forms were recognized in transected nerves also, but astrocytes in transected nerves had a simple morphology on the whole, and extended few, dense processes which branched infrequently. Quantitative analysis of astrocyte morphology confirmed that individual astrocytes underwent considerable remodelling in response to Wallerian degeneration. A prominent reaction was that astrocytes had withdrawn radial processes and extended a greater proportion of processes longitudinally, parallel to the long axis of the nerve and along the course of degenerated axons. A further, notable feature of transected nerves was the development of novel longitudinal forms and of hypertrophic astroglia. These results indicated that all astrocytes became reactive following enucleation and that glial scar formation was not the function of a single astrocyte subtype. Oligodendrocytes in transected nerves had lost their myelin sheaths and appeared as small cells with numerous bifurcating processes which extended radially, but a small number of oligodendrocytes were recognized which apparently supported myelin sheaths (9%, compared to 40% in normal nerves). In addition, there was a significant population of indeterminate cells in transected nerves (26%, compared to 6% in normal nerves) and, although some of these were identified as microglia/macrophages, it was concluded that many were likely to be dedifferentiated oligodendrocytes.

PNS features of rodent optic nerve axons

Peripheral nerves undulate together, giving them a wavelike appearance. This axonal pattern is also found in one region of the central nervous system, the optic nerve. Undulations provide a degree of compliance, as when a nerve is stretched, they are pulled straight. In the peripheral nervous system, undulations are thought to be conditional on the presence of extrafascicular collagen, which is also present between fascicles in the mammalian optic nerve. The pattern of undulations is described in the rat optic nerve and is related to the nerve's fascicular configuration and the regions between the fascicles, the extrafascicular matrix. The stage at which the undulations appear is determined to ascertain whether they are an intrinsic feature of optic axons or whether they emerge in association with the development of other events. The waves could be traced across the width of the nerve. Their pattern was not altered at fascicular boundaries, where axon groups are segregated. The periodicity of the undulations was constant between the eye and the middle of the nerve's length. Here fascicular divisions are present. Close to the chiasm, where the glial organisation changes and fascicles disappear, the periodicity of the undulations lengthened. They disappeared in the chiasm. Although collagen is a major component of the mammalian optic nerve's extrafascicular matrix, it was not present in the rat. Hence, the wavelike trajectory is independent of connective tissue. The waves are not present during early development and hence not an intrinsic feature of these axons. They appear behind the eye at late prenatal stages, emerging in association with glia in the extrafascicular matrix. They develop caudally, mirroring glial maturation. It is probable that the glia are maturing astrocytes that provide a scaffold for this feature of optic axon trajectory.

Axon-glial relationships in the anterior medullary velum of the adult rat

The anterior medullary velum is a thin sheet of CNS tissue which roofs the rostral part of the IVth ventricle and contains fascicles of myelinated fibres which, in part, arise from the nucleus of the IVth cranial nerve. This study used histochemical, immunohistochemical, and intracellular dye-injection techniques to describe cellular interrelationships in the velum in whole-mounts and in sections. Rip antibody-stained whole mounts provided a unique description of both oligodendrocyte units (defined as an oligodendrocyte and the complement of myelinated internodal segments it forms), and consecutive myelin sheaths along the same axon. A broad range of unit morphologies was categorised into four arbitrary groups, according to classical criteria, which comprised small cells supporting the short, thin myelin sheaths of 15-30 small diameter axons (Type I), through intermediate types (II & III), to the largest cells forming the long, thick myelin sheaths of 1-3 large diameter axons. Rip antibody and ferric ion-ferrocyanide staining, together with intracellular dye injection, revealed oligodendrocyte process branching patterns and their mode of engagement of myelin sheaths, nodes of Ranvier, and the spatial disposition of the outer cytoplasmic rims of myelin sheaths. The latter formed a conspicuous spiral ridge on the exterior surface of myelin sheaths which connected with the paranodal loops at each heminode. Large bundles of axons decussated through the velum, the bulk of which were IVth nerve fibres which constituted the IVth nerve rootlet. The PNS/CNS transitional zone of the IVth nerve was located 0.25-0.50 mm along the root, where astrocytic end-feet defined an abrupt margin, convex towards the periphery, where the heminodes of central and peripheral myelin were apposed, and where the basal lamina tubes of the Schwann cell units were discontinued. The basal processes of ependymal cells lining the ventricular wall of the velum, passed between axon bundles before abutting on the basal lamina of the pia. Many of these processes branched and ran along the axonal bundles. A monolayer of microglia occupied a subependymal stratum in which the non-overlapping dendritic territories of each cell formed a regular mosaic throughout the velum without any obvious interaction with either axons or other glial cells. Astrocytes were also uniformly distributed; their fine processes made up a dense lattice amongst axons, often running parallel and within the fibre bundles; stouter ones had terminal end-feet which undercoated the basal lamina of both the glia limitans externa and the blood vessels in the velum.

Biochemical subtypes of oligodendrocyte in the anterior medullary velum of the rat as revealed by the monoclonal antibody Rip

Oligodendrocytes were studied in the anterior medullary velum (AMV) of the rat using the monoclonal antibody Rip, an oligodendrocyte marker of unknown function. Confocal microscopic imaging of double immunofluorescent labelling with antibodies to Rip and carbonic anhydrase II (CAII) revealed two biochemically and morphologically distinct populations of oligodendrocyte which were either Rip+CAII+ or Rip+CAII-. Double immunofluorescent labelling with Rip and myelin basic protein (MBP) or glial fibrillary acidic protein (GFAP) provided direct evidence that Rip-labelled cells were phenotypically oligodendrocytes and confirmed that Rip did not recognise astrocytes. Oligodendrocytes which were Rip+CAII+ supported numerous myelin sheaths for small diameter axons, whilst Rip+CAII- oligodendrocytes supported fewer myelin sheaths for large diameter axons. Morphologically, Rip+CAII+ oligodendrocytes corresponded to types I or II of classical nomenclature, whilst Rip+CAII- oligodendrocytes corresponded to types III and IV. The results demonstrated a biochemical difference between oligodendrocytes which myelinated small and large diameter fibres.

Three-dimensional morphology of astrocytes and oligodendrocytes in the intact mouse optic nerve

The three-dimensional morphology of astrocytes and oligodendrocytes was analysed in the isolated intact mature mouse optic nerve, by correlating laser scanning confocal microscopy and camera lucida drawings of single cells, dye-filled with lysinated rhodamine dextran or horseradish peroxidase, respectively. These techniques enabled the entire process field of single dye-filled cells to be visualized in all planes and resolved the fine details of glial morphology. Morphometric analysis showed that the processes of all astrocytes had branches ending at the pial surface, on blood vessels, and freely in the nerve; branches ending in the nerve were described to end at nodes of Ranvier in the accompanying paper. Astrocytes were classified into a single morphological population in which each cell subserved multiple functions. The results of this study do not support the contention that astrocytes can be subdivided into two morphological and functional subtypes, namely type-1 and type-2, which have process ending either at the glia limitans or at nodes, respectively. Three-dimensional analysis of oligodendrocyte units, defined as the oligodendrocyte, its processes and the axons it ensheaths, showed the provision of single myelin segments for an average of 19 nearby axons (range 12-35) with a mean internodal length of 138 microns (range 50-350 microns). Mouse optic nerve oligodendrocytes were a homogeneous population and were markedly similar to those in the rat optic nerve. The results of our analysis of oligodendrocyte morphology are consistent with the view that the number and internodal length of myelin sheaths supported by a single oligodendrocyte are related to the diameter of the ensheathed axons.