Selective myelin defects in the anterior medullary velum of the taiep mutant rat

The taiep rat is a myelin mutant in which initial hypomyelination is followed by progressive demyelination of the CNS. An in vitro study suggests that accumulation of microtubules within oligodendrocytes is the cause of the taiep myelin defects (Song et al., 1999). In this article, we analyze microtubule accumulation in relation to taiep myelin defects in vivo in the anterior medullary velum (AMV), a CNS tissue that enables entire oligodendrocyte units to be resolved. Immunohistochemical analysis demonstrated notably high levels of beta-tubulin and the microtubule associated protein tau in the somata and processes of taiep oligodendrocytes. This was correlated with markedly reduced expression of the myelin proteins, proteolipid protein (PLP), myelin basic protein (MBP), 2',3 -cyclic nucleotide 3'-phosphodiesterase, and both large (L) and small (S) isoforms of myelin-associated glycoprotein (MAG). Moreover, PLP and L-MAG, which are dependent on the microtubule system for intracellular transport, accumulated in the perinuclear cytoplasm of the taiep oligodendrocyte. The myelin deficit was most marked in the area of the AMV populated by the small somata oligodendrocytes that have fine long processes that support numerous myelin sheaths of small diameter axons. Type III/IV oligodendrocytes, which have large somata and short processes that support a small number of myelin sheaths of large diameter axons, were also affected to a certain degree in compact myelin sheath formation. These results support the hypothesis that myelin loss and oligodendrocyte disruption in the taiep mutant result from a defect in the microtubule system that transports myelin components from the somata to the myelin sheath.

Oligodendrocytes and the control of myelination in vivo: new insights from the rat anterior medullary velum

The rat anterior medullary velum (AMV) is representative of the brain and spinal cord, overall, and provides an almost two-dimensional preparation for investigating axon-glial interactions in vivo. Here, we review some of our findings on axon-oligodendrocyte unit relations in our adult, development, and injury paradigms: (1) adult oligodendrocytes are phenotypically heterogeneous, conforming to Del Rio Hortega's types I-IV, whereby differences in oligodendrocyte morphology, metabolism, myelin sheath radial and longitudinal dimensions, and biochemistry correlate with the diameters of axons in the unit; (2) oligodendrocytes derive from a common premyelinating oligodendrocyte phenotype, and divergence of types I-IV is related to the age they emerge and the presumptive diameter of axons in the unit; (3) during myelination, axon-oligodendrocyte units progress through a sequence of maturation phases, related to axon contact, ensheathment, establishment of internodal myelin sheaths, and finally the radial growth and compaction of the myelin sheath; (4) we provide direct in vivo evidence that platelet-derived growth factor-AA (PDGF-AA), fibroblast growth factor (FGF-2), and insulin-like growth factor-I (IGF-I) differentially regulate these events, by injecting the growth factors into the cerebrospinal fluid of neonatal rat pups; (5) in lesioned adult AMV, transected central nervous system (CNS) axons regenerate through the putatively inhibitory environment of the glial scar, but remyelination by oligodendrocytes is incomplete, indicating that axon-oligodendrocyte interactions are defective; and (6) in the adult AMV, cells expressing the NG2 chondroitin sulphate have a presumptive adult oligodendrocyte progenitor antigenic phenotype, but are highly complex cells and send processes to contact axolemma at nodes of Ranvier, suggesting they subserve a specific perinodal function. Thus, axons and oligodendrocyte lineage cells form interdependent functional units, but oligodendrocyte numbers, differentiation, phenotype divergence, and myelinogenesis are governed by axons in the units, mediated by growth factors and contact-dependent signals.

The NG2 chondroitin sulfate proteoglycan: role in malignant progression of human brain tumours

The expression and function of NG2, a transmembrane chondroitin sulfate proteoglycan was studied in human gliomas of various histological types in culture using immunocytochemistry and flow cytometry. NG2 was differentially expressed in the neoplasms, with higher expression in high compared to low-grade gliomas. In acutely isolated cells from human biopsies, NG2 +ve and NG2 -ve populations were morphologically distinct from each other, and NG2 +ve cells were more proliferative than NG2 -ve cells. The mitogens platelet derived growth factor (PDGF-AA) and basic fibroblast growth factor (bFGF) added in combination to serum-free medium (SFM) upregulated NG2 expression on glioblastoma multiforme cells in culture but had little effect on NG2 expression on the anaplastic astrocytoma cells. Furthermore, NG2 was colocalised with the platelet derived growth factor alpha receptor (PDGFalphaR) and antibody blockade of the PDGF-alphaR ablated NG2 expression on the glioblastoma multiforme cells, suggesting that increased NG2 expression in the presence of PDGF-AA is mediated via the PDGF-alphaR. Assays of migration and invasion indicate that NG2 +ve glioma cells migrated more efficiently on collagen IV and that NG2 -ve cells were more invasive than their NG2 +ve counterparts. The results indicate that NG2 may be, respectively, positively and negatively related to the proliferative and invasive capacity of glioma cells. Thus, expression of the NG2 proteoglycan may have major implications for malignant progression in glial neoplasms and may prove a useful target for future therapeutic regimens.

In vivo actions of fibroblast growth factor-2 and insulin-like growth factor-I on oligodendrocyte development and myelination in the central nervous system

The in vivo effects of fibroblast growth factor-2 (FGF-2) and insulin-like growth factor-I (IGF-I) on oligodendrocytes and CNS myelination were determined in the postnatal rat anterior medullary velum (AMV) following injection of both cytokines into the cerebrospinal fluid. Either FGF-2, IGF-I, or saline were administered via the lateral ventricle, twice daily commencing at postnatal day (P) 6. At P9, AMV were immunohistochemically labeled with the Rip antibody, to enable analysis of the numbers of myelin sheaths and of promyelinating and myelinating oligodendrocytes; promyelinating oligodendrocytes are a recognisable immature phenotype which express myelin-related proteins prior to forming myelin sheaths. In parallel experiments, AMV were treated for Western blot analysis to determine relative changes in expression of the myelin proteins 2', 3'-cyclic nucleotide 3'-phosphohydrolase (CNP) and myelin oligodendrocyte glycoprotein (MOG), which, respectively, characterise early and late stages of myelin maturation. In FGF-2-treated AMV, the number of promyelinating oligodendrocytes increased by 87% compared to saline-injected controls. The numbers of myelinating oligodendrocytes and myelin sheaths were not decreased, but conspicuous unmyelinated gaps within fibre tracts were indications of retarded myelination following FGF-2 treatment. Western blot analysis demonstrated decreased expression of CNP and a near-total loss of MOG, confirming that FGF-2 decreased myelin maturation. In contrast, IGF-I had no effect on the number of promyelinating oligodendrocytes, but increased the numbers of myelinating oligodendrocytes and myelin sheaths by 100% and 93%, respectively. Western blot analysis showed that the amount of CNP was increased following IGF-I treatment, correlating with the greater number of oligodendrocytes, but that MOG expression was lower than in controls, suggesting that the increased number of myelin sheaths in IGF-I was not matched by increased myelin maturation. The results provide in vivo evidence that FGF-2 and IGF-I control the numbers of oligodendrocytes in the brain and, respectively, retard and promote myelination.

Adenosine 5' triphosphate evoked mobilization of intracellular calcium in central nervous system white matter of adult mouse optic nerve

Although it has been established that immature glial cells express functional purinergic receptors, the responsiveness of mature glial cells in vivo had not been elucidated. This question was addressed using fura-2 ratiometric measurements of [Ca2+]i in the adult mouse optic nerve, a central nervous system (CNS) white matter tract, taking advantage of the facts that (i), the optic nerve contains glial cells but not neurons and (ii), that fura-2 loads primarily astrocytes in isolated intact optic nerves. We show that adenosine 5' triphosphate (ATP) evoked an increase in [Ca2+]i in a concentration-dependent manner with a half-maximal effect at 3 microm ATP, and with a rank order of agonist potency of ATP > ADP > alpha,beta-methyline-ATP > UDP > adenosine. The results indicate mainly P2Y and P2X components, consistent with the in vitro astroglial purinergic receptor profile. The in vivo response of mature glia to ATP may be important in their response to CNS damage.

Cells expressing the NG2 antigen contact nodes of Ranvier in adult CNS white matter

The NG2 antibody, which recognises an integral membrane chondroitin sulphate, labels a significant population of cells in adult CNS white matter tracts of the rat optic nerve and anterior medullary velum (AMV). Adult NG2+ cells are highly complex with multiple branching processes and we show by EM immunocytochemistry that they extend perinodal processes, which contact nodes of Ranvier. NG2+ cells do not react to conventional immunohistochemical markers for adult glia and so we reservedly term them NG2P cells. In vitro, NG2 labels oligodendrocyte-type-2 astrocyte (O-2A) progenitors that can give rise to oligodendrocytes or type-2 astrocytes, depending on the culture medium. Thus, it is possible that NG2P cells may be derived from the same stem cells as oligodendrocytes. Interestingly, NG2+ cells identified previously in adult CNS displayed phenotypic characteristics of O-2Aadult progenitors and it is possible that, like them, NG2P cells might retain the capacity of generating oligodendrocytes in the adult CNS. This may be an important role of NG2P cells in demyelinating diseases such as multiple sclerosis. It is significant therefore that the perinodal processes of NG2P cells contact the only sites of exposed axolemma in myelinated axons, so that NG2P cells are ideally situated to detect and respond to changes in axonal function during demyelination. A further implication of our finding is that NG2P cells may perform functions at nodes of Ranvier previously attributed to perinodal astrocytes, including the clustering and maintenance of sodium channels in the axon membrane at nodes, during development and following demyelination.

Axon-glial relations during regeneration of axons in the adult rat anterior medullary velum

The anterior medullary velum (AMV) of adult Wistar rats was lesioned in the midsagittal plane, transecting all decussating axons including those of the central projection of the IVth nerve. At selected times up to 200 days after transection, the degenerative and regenerative responses of axons and glia were analyzed using transmission and scanning electron microscopy and immunohistochemistry. In particular, both the capacity of oligodendrocytes to remyelinate regenerated fibers and the stability of the CNS/PNS junctional zone of the IVth nerve rootlet were documented. Transected central AMV axons exhibited four patterns of fiber regeneration in which fibers grew: rostrocaudally in the reactive paralesion neuropil (Group 1); randomly within the AMV (Group 2); into the ipsilateral IVth nerve rootlet, after turning at the lesion edge and growing recurrently through the old degenerated contralateral central trochlear nerve trajectory (Group 3); and ectopically through paralesion tears in the ependyma onto the surface of the IVth ventricle (Group 4). Group 1-3 axons regenerated unperturbed through degenerating central myelin, reactive astrocytes, oligodendrocytes, microglia, and large accumulations of hematogenous macrophages. Only Group 3 axons survived long term in significant numbers, and all became myelinated by oligodendrocytes, ultimately establishing thin sheaths with relatively normal nodal gaps and intersegmental myelin sheath lengths. Schwann cells at the CNS/PNS junction of the IVth nerve rootlet did not invade the CNS, but astrocyte processes grew across the junction into the PNS portion of the IVth nerve. The basal lamina of the junctional glia limitans remained stable throughout the experimental period.

Differential expression of the L- and S-isoforms of myelin associated glycoprotein (MAG) in oligodendrocyte unit phenotypes in the adult rat anterior medullary velum

We have previously demonstrated differences in the expression of carbonic anhydrase II (CAII) in oligodendrocyte units myelinating small and large diameter fibres in the anterior medullary velum (AMV) of the adult rat (each unit comprises the cell body, processes and myelin sheaths). Others have indicated that myelin composition may also vary with respect to myelin basic protein (MBP) and proteolipid protein (PLP), and the small (S)- and large (L)-isoforms of myelin associated glycoprotein (MAG). In this study, we have determined the expression of myelin proteins in oligodendrocyte unit phenotypes I-IV, which myelinate fibres ranging in diameter from 0.3-12 microns diameter in the AMV, by using double immunolabelling for Rip, which labels entire units, and MBP, PLP, myelin oligodendrocyte glycoprotein (MOG), L-MAG and S-MAG. We show differences in the expression of L- and S-MAG in units which myelinate different diameter fibres: (1) type I/II units myelinating small diameter fibres had a L-MAG+/S-MAG-/CAII+ phenotype; (2) type II/III units myelinating different diameter fibres had a L-MAG+/S-MAG+/CAII+ phenotype; (3) type III/IV units myelinated large diameter fibres had a L-MAG+/S-MAG+/CAII- phenotype. All units, irrespective of fibre diameter, expressed Rip, MBP, PLP and MOG. The results indicate that type I-IV units may be variants of a single oligodendrocyte population and that phenotypic differences are determined by the diameter of fibres within the unit. The possible significance of metabolic and biochemical differences between oligodendrocytes myelinating small and large diameter axons are discussed with reference to the pathology of demyelination.

Axon-myelin sheath relations of oligodendrocyte unit phenotypes in the adult rat anterior medullary velum

Axon-oligodendrocyte relations of Rip-immunolabelled and dye-injected oligodendrocyte units are characterised in the adult rat anterior medullary velum (AMV). Each oligodendrocyte unit comprised the oligodendrocyte cell body, processes and the internodal myelin segments they support. Oligodendrocyte units corresponded to classically described type I/II or type III/IV unit phenotypes which respectively myelinated discrete populations of small and large diameter axons, delineated by a myelinated fire diameter of 2-4 microns (diameter of the axon plus its myelin sheath). Within units, mean fibre diameter was directly related to mean internodal length and inversely related to the number of myelin sheaths in the unit. The relationship between fibre diameter and internodal length was retained in units which myelinated axons of different diameters, indicating that axon diameter was an important determinant of the longitudinal dimensions of myelin sheaths. We also show that type III/IV units maintained a far greater volume of myelin than type I/II units. It was concluded that type I/II and III/IV oligodendrocytes represent two functionally and morphologically distinct phenotypes whose distribution densities were determined by the diameter and spatial dispersion of axons.

Optic neuritis in chronic relapsing experimental allergic encephalomyelitis in Biozzi ABH mice: demyelination and fast axonal transport changes in disease

The encephalitogenicity of optic nerve tissue was demonstrated in Biozzi ABH (H-2(dq1)) mice. Acute experimental allergic encephalomyelitis (EAE) occurred in 11/14 animals and 4/5 exhibited relapse. The involvement of the optic nerve in spinal cord homogenate induced chronic relapsing EAE (CREAE) was demonstrated by mononuclear cell infiltration and myelin degradation in the optic nerve prior to and during clinical disease. During the relapse phase gross pathological assessment revealed swollen and translucent plaques on the optic nerves. Advanced lesions showed widespread demyelination, astrocytic gliosis and fibrotic changes of the blood vessels. Physiologically, the fast axonal transport of proteins from the retina to the optic nerve and superior colliculus was significantly decreased during relapse. The association of inflammation and demyelination with physiological deficit in the optic nerve highlights the usefulness of this model in the study of multiple sclerosis in which acute monosymptomatic unilateral optic neuritis is a common manifestation. Furthermore, the novel induction of CREAE with optic nerve homogenate suggests that optic neuritis is a common significant role in the pathophysiology and progression of neurological disease in CREAE which may be relevant to studies of optic neuritis in multiple sclerosis.

Expression of GABA transporter mRNAs in the developing and adult rat optic nerve

There is electrophysiological evidence of a functional role for gamma-aminobutyric acid (GABA) and GABA transporters (GATs) in the neonatal rat optic nerve, and that they are down-regulated during development. The results of the present study demonstrate directly by reverse transcription-polymerase chain reaction (RT-PCR) that the mRNAs encoding for GAT-1, -2 and -3 are expressed in the optic nerves of both neonatal (5 day old) and adult rats. The results support a role for GABA in the developing rat optic nerve, a typical white matter tract which contains axons and glia, but neither neuronal cell bodies nor synapses. Significantly, the persistence of GAT mRNAs suggests an enduring function for both GABA and glial uptake mechanisms in the adult optic nerve.

Platelet-derived growth factor delays oligodendrocyte differentiation and axonal myelination in vivo in the anterior medullary velum of the developing rat

The AA dimeric form of platelet-derived growth factor (PDGF-AA) is implicated in the differentiation of cells of the oligodendrocyte lineage, which express PDGF receptors of the alpha subunit type (PDGF-alphaR). In the present study, we show that a single injection of PDGF-AA into the cerebrospinal fluid of neonatal rats delays oligodendrocyte differentiation and interrupts the progress of myelination in the anterior medullary velum (AMV), a white matter tract roofing the IVth ventricle of the brain. PDGF-AA or saline was injected intrathecally in postnatal day (P) 7 rats, and the AMV was subsequently removed and immunolabelled with the oligodendrocyte-specific antibody Rip, at P9, P12, and P21, corresponding to postinjection days (PID) 2, 5, and 14. At P9 (PID2), myelination was retarded in PDGF-AA-treated rats as opposed to saline-treated controls but progressed rapidly after P12 (PID5). Quantification supported the qualitative observations that PDGF-AA mediated an acute decrease in the number of Rip+ oligodendrocytes at P9-12, which largely recovered by P21, suggesting that PDGF-AA may have delayed recruitment of myelinating oligodendrocytes. However, the definitive number of Rip+ oligodendrocytes in the AMV was not increased, suggesting that its action as a promoter of early oligodendrocyte survival may not ultimately affect the definitive number of myelinating oliogdendrocytes in vivo. We discuss the possibilities that excess PDGF-AA may have acted on early oligodendrocytes (precursors or preoligodendrocytes) to either (1) delay their differentiation by maintaining them in the cell cycle or (2) accelerate their differentiation, which may result in premature cell death in the absence of synchronised survival signals. This study supports a role for PDGF-AA in the timing of oligodendrocyte differentiation in vivo, as has been shown in vitro.

The relationship between developing oligodendrocyte units and maturing axons during myelinogenesis in the anterior medullary velum of neonatal rats

Myelinogenesis was investigated in whole-mounted anterior medullary vela from rats aged postnatal day (P) 10-12, using double immunofluorescence labelling with Rip and anti-neurofilament 200 (NF200) antibodies, to identify oligodendrocytes and axons, respectively. A number of discrete phases of maturation of oligodendrocyte units were recognised. (1) Promyelinating oligodendrocytes co-expressed Rip and Myelin basic Protein and formed axonal associations, prior to ensheathment. (2) Transitional oligodendrocytes contained both ensheathing and non-ensheating processes. (3) Myelinating oligodendrocytes were established after a period of remodelling (in which non-ensheathing processes were lost), appearing as oligodendrocyte unit morphological phenotypes with a definitive number of incipient myelin sheaths. (4) Maturation of myelinating oligodendrocytes was defined as the establishment of internodal sheath lengths and the redistrubution of myelin basic protein from the cell somata and radial processes into the myelin sheaths only. Myelination was probably related to the maturational state of the axons, since it was initiated when the latter had attained a critical diameter of between approximately 0.2 and 0.4 micron, coincident with the expression of NF200. Oligodendrocyte differentiation and myelination of the AMV were asynchronous and multifocal, and at P10: (1) axons which were destined to be of the largest calibre in the adult AMV were already myelinated by early developing oligodendrocytes, whilst those which were destined to be the smallest calibre in the adult were unmyelinated, but ultimately became ensheathed by late developing oligoendrocytes; (2) axons were sequentially ensheathed by early developing myelinating oligodendrocytes and late developing promyelinating oligodendrocytes; (3) all axons were small calibre; (4) oligodendrocyte units exhibited polymorphism. Thus, the development of oligodendrocyte morphological phenotypes was not related solely to either the physical dimension of axon calibre at the time of ensheathment, nor oligodendrocyte birth dates.

PDGF-alpha receptor and myelin basic protein mRNAs are not coexpressed by oligodendrocytes in vivo: a double in situ hybridization study in the anterior medullary velum of the neonatal rat

Platelet-derived growth factor (PDGF) is a growth-regulatory dimer with A and B subunits. PDGF-AA, acting via PDGF receptors of the alpha-unit subtype (PDGF-alphaR), is implicated in the differentiation of oligodendrocyte precursors and in the survival of newly formed oligodendrocytes, which gradually lose expression of PDGF-alphaR. However, it is unclear whether terminally differentiated oligodendrocytes express PDGF-alphaR in vivo. To address this question, and to help clarify the role of PDGF-AA in late oligodendrocyte differentiation, we have used double in situ hybridization with digoxigenin- and fluorescein-labeled riboprobes to relate PDGF-alphaR mRNA and myelin basic protein (MBP) mRNA expression in the isolated intact anterior medullary velum (AMV) of rats ages Postnatal Day (P) 10-12 and P30-32. In parallel experiments, AMV were immunolabeled with the oligodendrocyte-specific monoclonal antibody Rip to provide information on oligodendrocyte development and the extent of myelination. At P10, the AMV contained tracts in which axons ranged from unmyelinated to fully myelinated, whereas myelination was complete in P30-32 AMV. The first oligodendrocytes to express MBP mRNA or Rip were promyelinating oligodendrocytes, which had a "star-burst" morphology and had not yet begun to form myelin sheaths. As myelination proceeded, MBP mRNA became dispersed throughout oligodendrocyte units, comprising cell somata, processes, and internodal myelin sheaths. By P30-32, MBP mRNA had been redistributed to the myelin sheaths only, reflecting a change in the site of protein synthesis in mature myelinated axon tracts. At no stage of oligodendrocyte differentiation did we observe cellular coexpression of mRNA for PDGFalphaR and MBP. Our results indicated that oligodendrocytes lost the expression of PDGFalphaR prior to gaining that of myelin gene products, and preclude an action of PDGF-AA on Rip+/MBP+ star-burst promyelinating oligodendrocytes. The spatial and temporal expression of PDGF-alphaR mRNA in the AMV was inversely related to the pattern of maturation of both myelin and oligodendrocytes, and is consistent with PDGF-alphaR being expressed by pro-oligodendrocytes. A notable finding was the high level of expression of PDGF-alphaR mRNA in the AMV of juvenile rats, localized to cell bodies within the myelinated axon tracts, strongly suggesting that oligodendrocyte precursors persisted in the mature velum.

Morphology of oligodendrocytes during demyelination in optic nerves of mice infected with Semliki Forest virus

Multiple sclerosis (MS) is a demyelinating disease which affects oligodendrocytes, the myelinating cells of the CNS. Demyelination is known to occur in the optic nerves of Balb/c mice infected with the avirulent A7(74) strain of Semliki Forest virus (SFV), and many of the changes are similar to those of patients with MS. The aim of the present study was to determine how demyelination proceeds in individual oligodendrocytes in SFV infection, to help in understanding the pathology of demyelination and remyelination in MS. The whole-cell morphology of individual oligodendrocyte units (defined as the oligodendrocyte, its processes and the internodal myelin segments of the axons it ensheaths) was characterized using intracellular dye injection in isolated intact optic nerves. In untreated control mice, oligodendrocytes had a relatively uniform morphology and each cell on average provided 20 or so nearby axons with single myelin sheaths with internodal lengths of approximately equal to 150 microns. In SFV infected mice, during the peak of demyelination at post-inoculation days 14-21, 55% of oligodendrocytes displayed a range of morphological abnormalities, which most likely represented sequential changes in oligodendrocytes during demyelination. Thus, at the earliest stage of demyelination oligodendrocytes developed swellings or vacuolations along their internodal myelin sheaths, which became gradually attenuated and were completely lost in extreme cases. The results show that whole oligodendrocyte units were affected during SFV-induced demyelination and this is the basis of the focal nature of lesions in this viral model of MS. Individual oligodendrocyte units which had lost their full complement of myelin sheaths had the appearance of immature oligodendrocytes, suggesting they had undergone dedifferentiation. We concluded that these cells may not be destroyed during demyelination and it is possible they are capable of remyelination which is a feature of SFV infection in mice and MS in humans.

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.

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.

Relationship between myelin sheath diameter and internodal length in axons of the anterior medullary velum of the adult rat

Relations between myelin sheath diameters and internodal lengths were measured in whole mounts of osmium stained intact anterior medullary velum (AMV) from glutaraldehyde perfused adult rats. The AMV is a sheet of CNS tissue which roofs the IVth ventricle and contains fascicles of myelinated fibres which arise mainly from the nucleus of the IVth cranial nerve. These fibers displayed a broad range of myelin sheath external diameters and internodal lengths, from < 1-12 microns and 50-750 microns, respectively. Myelin sheath external diameter was a measurement of the axonal diameter plus the thickness of its myelin sheath, while internodal length was measured as the distance between consecutive nodes. There was a broadly linear relationship between myelin sheath diameters and internodal lengths, with the smaller diameter sheaths tending to have shorter internodes than the larger. However, the correlation was weak and for any given diameter myelin sheaths displayed considerable variation in their internodal lengths. The smallest diameter myelin sheaths, < 4 microns, consistently had shorter internodes than predicted by a linear regression and, in an analysis of consecutive internodes in single fibres, the slope was flattened in fibres with a diameter > 4 microns. Our results indicated that small and large calibre fibres may have different myelin sheath diameter-internodal length interrelations.

Effect of inflammatory agents on electrical resistance across the blood-brain barrier in pial microvessels of anaesthetized rats

The effect of histamine, bradykinin and serotonin on blood-brain barrier permeability was investigated using in situ measurement of transendothelial electrical resistance in pial microvessels of anaesthetized rats. Mean resistance of vessels superfused with artificial cerebrospinal fluid was 1800 omega cm2, indicating a tight barrier with extremely low ion permeability. In paired experiments from continuous measurements in single vessels, addition of 10(-3) M serotonin to the solution bathing the brain had no marked effect on resistance; whereas both histamine and bradykinin, applied at a concentration of 10(-4) M, caused a rapid and reversible decrease in resistance. Mean resistance was 408 and 505 omega cm2 in 10(-4) M histamine and bradykinin, respectively, and approximately 50% of vessels had a resistance less than 250 omega cm2, compared to 12% in controls, indicating a leaky blood-brain barrier that is not capable of normal brain ion homeostasis. Histamine and bradykinin had similar dose-response relations, and a maximal effect was observed between 20 and 50 microM. Thus, histamine and bradykinin act at the abluminal (brain-facing) membranes of the cerebral endothelium to mediate blood-brain barrier opening. These results support a role for histamine and bradykinin in brain oedema formation.

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.

The astrocyte response to gamma-aminobutyric acid attenuates with age in the rat optic nerve

There is increasing evidence that glial cells respond to the inhibitory neurotransmitter gamma-aminobutyric acid (GABA), and astrocytes have been shown to possess GABAA receptors both in vivo and in vitro. A recent study by Sakatani et al. (Proc. R. Soc. Lond. B247, 155 (1992)) demonstrated the transient expression of functional GABAA receptors in the developing rat optic nerve, but axonal and glial components of the response were not distinguished. To help address this problem, we have determined the electrophysiological response to GABA in astrocytes of the isolated intact optic nerves from neonatal rats, identified morphologically following intracellular injection of horseradish peroxidase. Astrocytes responded to GABA by a GABAA receptor-mediated depolarization which attenuated gradually during post-natal development; astrocytes in 21-day-old nerves were not observed to respond to GABA. The results indicate the transient presence of functional GABAA receptors in developing rat optic nerve astrocytes in situ, and we speculate upon a role for GABA in glial signalling and the organization of axonglial interrelations during development.

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

Astrocytes are implicated in the function of nodes of Ranvier because their perinodal processes form contacts with the axonal membrane at nodes. We have filled astrocytes iontophoretically with horseradish peroxidase in the intact mouse optic nerve to resolve the precise relationship between perinodal processes and astrocyte three dimensional structure. We confirm that nodal contacts were formed either by single processes which almost completely enveloped nodes, or by delicate, finger-like projections from larger processes which made discrete nodal contacts. A single perinodal process can form multiple contacts with a node and nodes were contacted by processes from more than one astrocyte. Perinodal processes emanated from larger processes, which terminated as end-feet on blood vessels and at the pia, as well as collateral branches which subsequently ended at nodes; these latter may specifically subserve nodes. Perinodal contacts were also formed directly by the soma and cytoplasmic expansions of the cell body. Both primary processes and collateral branches formed multiple associations with nodes which often appeared in clusters. Thus, all astrocytes formed multiple contacts with nodes, blood vessels and the subpial glia limitans. We conclude that perinodal processes are not formed by a specialized astrocyte in the mouse optic nerve.