A confirmation of the oligodendroglial origin of myelin in the adult rat

Glutamate delta-1 receptor regulates oligodendrocyte progenitor cell differentiation and myelination in normal and demyelinating conditions

In this study, we investigated the role of glutamate delta 1 receptor (GluD1) in oligodendrocyte progenitor cell (OPC)-mediated myelination during basal (development) and pathophysiological (cuprizone-induced demyelination) conditions. Initially, we sought to determine the expression pattern of GluD1 in OPCs and found a significant colocalization of GluD1 puncta with neuron-glial antigen 2 (NG2, OPC marker) in the motor cortex and dorsal striatum. Importantly, we found that the ablation of GluD1 led to an increase in the number of myelin-associated glycoprotein (MAG+) cells in the corpus callosum and motor cortex at P40 without affecting the number of NG2+ OPCs, suggesting that GluD1 loss selectively facilitates OPC differentiation rather than proliferation. Further, deletion of GluD1 enhanced myelination in the corpus callosum and motor cortex, as indicated by increased myelin basic protein (MBP) staining at P40, suggesting that GluD1 may play an essential role in the developmental regulation of myelination during the critical window period. In contrast, in cuprizone-induced demyelination, we observed reduced MBP staining in the corpus callosum of GluD1 KO mice. Furthermore, cuprizone-fed GluD1 KO mice showed more robust motor deficits. Collectively, our results demonstrate that GluD1 plays a critical role in OPC regulation and myelination in normal and demyelinating conditions.

The history of myelin

Andreas Vesalius is attributed the discovery of white matter in the 16th century but van Leeuwenhoek is arguably the first to have observed myelinated fibers in 1717. A globular myelin theory followed, claiming all elements of the nervous system except for Fontana's primitive cylinder with outer sheath in 1781. Remak's axon revolution in 1836 relegated myelin to the unknown. Ehrenberg described nerve tubes with double borders in 1833, and Schwann with nuclei in 1839, but the medullary sheath acquired its name of myelin, coined by Virchow, only in 1854. Thanks to Schultze's osmium specific staining in 1865, myelin designates the structure known today. The origin of myelin though was baffling. Only after Ranvier discovered a periodic segmentation, which came to us as nodes of Ranvier, did he venture suggesting in 1872 that the nerve internode was a fatty cell secreting myelin in cytoplasm. Ranvier's hypothesis was met with high skepticism, because nobody could see the cytoplasm, and the term Schwann cell very slowly emerged into the vocabulary with von Lenhossék in 1895. When Cajal finally admitted the concept of Schwann cell internode in 1912, he still firmly believed myelin was secreted by the axon. Del Río-Hortega re-discovered oligodendrocytes in 1919 (after Robertson in 1899) and named them oligodendroglia in 1921, thereby antagonizing Cajal for discovering a second cell type in his invisible third element. Penfield had to come to del Río-Hortega's rescue in 1924 for oligodendrocytes to be accepted. They jointly hypothesized myelin could be made by oligodendrocytes, considered the central equivalent of Schwann cells. Meanwhile myelin birefringence properties observed by Klebs in 1865 then Schmidt in 1924 confirmed its high fatty content, ascertained by biochemistry by Thudichum in 1884. The 20th century saw X-ray diffraction developed by Schmitt, who discovered in 1935 the crystal-like organization of this most peculiar structure, and devised the g-ratio concept in 1937. A revolution happened around the same time: saltatory conduction, the very reason for myelin existence, discovered by Tasaki in 1939 and confirmed by Huxley and Stämpfli in 1949. After the second world war, widely available electron microscopes allowed Geren to finally discover the origin of myelin in 1954, exactly a century after Virchow coined 'myelin' in 1854. Geren had the genial insight that the Schwann cell wraps around the axon and generates a spiral of compacted membrane-myelin. The central origin of myelin took a little longer due to the special configuration of oligodendrocyte distanced from the axon, but in 1962 the Bunges established the definitive proof that oligodendrocyte secretes myelin. The era of myelin biology had begun. In 1973 Norton devised a method to purify myelin which launched the modern molecular era.

A critical cell-intrinsic role for serum response factor in glial specification in the CNS

Astrocytes and oligodendrocytes play crucial roles in nearly every facet of nervous system development and function, including neuronal migration, synaptogenesis, synaptic plasticity, and myelination. Previous studies have widely characterized the signaling pathways important for astrocyte differentiation and unveiled a number of transcription factors that guide oligodendrocyte differentiation in the CNS. However, the identities of the transcription factors critical for astrocyte specification in the brain remain unknown. Here we show that deletion of the stimulus-dependent transcription factor, serum response factor (SRF), in neural precursor cells (NPCs) (Srf-Nestin-cKO) results in nearly 60% loss in astrocytes and 50% loss in oligodendrocyte precursors at birth. Cultured SRF-deficient NPCs exhibited normal growth rate and capacity to self-renew. However, SRF-deficient NPCs generated fewer astrocytes and oligodendrocytes in response to several lineage-specific differentiation factors. These deficits in glial differentiation were rescued by ectopic expression of wild-type SRF in SRF-deficient NPCs. Interestingly, ectopic expression of a constitutively active SRF (SRF-VP16) in NPCs augmented astrocyte differentiation in the presence of pro-astrocytic factors. However, SRF-VP16 expression in NPCs had an inhibitory effect on oligodendrocyte differentiation. In contrast, mice carrying conditional deletion of SRF in developing forebrain neurons (Srf-NEX-cKO) did not exhibit any deficits in astrocytes in the brain. Together, our observations suggest that SRF plays a critical cell-autonomous role in NPCs to regulate astrocyte and oligodendrocyte specification in vivo and in vitro.

Small Molecule Induction of Human Umbilical Stem Cells into MBP-positive Oligodendrocytes in a Defined Three-Dimensional Environment

Stem cells from umbilical cord would be a favorable alternative to embryonic stem cells for therapeutic applications. In this study, human multipotent progenitor cells (MLPCs) from umbilical cord were differentiated into oligodendrocytes by exposure to a range of microenvironmental chemical and physical cues. Chemical cues were represented by a novel defined differentiation medium containing the neurotransmitter norepinephrine (NE). In traditional 2 dimensional (2D) conditions, the MLPCs differentiated into oligodendrocyte precursors, but did not progress further. However, in a 3 dimensional (3D) environment, the MLPCs differentiated into committed oligodendrocytes that expressed MBP. The apparent method of interaction of NE in stimulating the differentiation process was identified to occur through the adenergic pathway while all prior differentiation methods have used other routes. This novel method of obtaining functional human oligodendrocytes from MLPCs would eliminate many of the difficulties associated with their differentiation from embryonic stem cells.

Hypothesis of an energetic function for myelin

Nervous system is a great oxygen consumer, but the site of oxygen absorption has remained elusive. Four proteomic studies have shown that the respiratory complexes I to V may be expressed in isolated myelin. Myelin is an outgrowth of glial cells, surrounding many axons in multiple spires both in peripheral and central nervous system. Recent quantitative analyses strongly support the daring hypothesis that myelin is functional in aerobic ATP production, to supply the neuron with chemical energy. A vision of myelin sheath as a structure devoted to the oxygen absorbance for glucose combustion in nervous system thank to its enormous surface, would be also supported by an impressive series of characteristics and properties of myelin that do not presently find an explanation, all of which are herein examined.

[Fine structure of neuronal and glial processes in neuropathology, a personal historical note]

Neurons and glia are characterized by their well formed processes and by cell-to-cell relationships. Neurons show cylindrical processes, which form synaptic junctions. On the other hand, the peripheral parts of the glial cells are sheet-like in nature. Thus, the oligodendroglial cells form shovel-shaped myelin sheets around axons. The astrocytes also form delicate sheet-like processes, which separate the central nervous system from the mesodermal tissue and surround neuronal soma, dendrites and synapses. Fine structural studies in neuropathological material provide many interesting new findings on neuronal and glial processes. This communication highlights my exciting experience studying neuropathology for over 50 years.

Fine structure of neuronal and glial processes in neuropathology

The cells of the nervous system are characterized by their well-formed cell processes and by cell-to-cell relationships that they form. The neuron reveals essentially cylindrical processes, which form synaptic junctions. On the other hand, the peripheral parts of the glial cells are mainly sheet-like in nature. Thus, the oligodendroglial cell elaborates many sheet-like processes, each of which forms a segment of the myelin sheath. Unique cell junction, transverse bands are present at the interface of oligodendroglial processes and the axon. Finally, the astrocytes also form elaborate sheet-like processes, which separate most of the CNS from the mesodermal tissue as well as surrounding certain neuronal surfaces, including synapses. Punctate adhesions, gap junctions and other adhesive devices are present between astrocytic processes. Defects or anomalies in the neuronal and glial cell processes characterize numerous pathological conditions.

The role of electron microscopy in neuropathology

The electron microscope has been an essential instrument for elucidating the morphology of cells and tissue. Fine structural investigation of nervous tissue has disclosed numerous new findings, which were once invisible. In this communication I present my personal perspective on the role of electron microscopy in neuropathology through the presentation of selected electron microscopic projects engaged in our institution. These include brain edema, structural analysis of the myelin in the central nervous system, endodermal cysts in the central nervous system, aberrant synaptic development, toxoplasmosis in AIDS, and Hirano bodies.

Age-related changes in oligodendrocytes in monkey cerebral cortex

Compared with those in young monkeys (5-12 years of age), oligodendrocytes in area 46 of frontal cortex and primary visual cortex of monkeys over 25 years of age develop bulbous swellings along their processes. Such swellings are filled with characteristic inclusions that resemble age pigment, and other accumulations of these inclusions occur within the cell bodies of the oligodendrocytes in old monkeys. In addition, whereas the oligodendrocytes in young monkeys most commonly occur singly, in old monkeys it is common to find oligodendrocytes in groups or rows. These aggregates are often situated close to capillaries, and in some instances it is found that the cell bodies of the oligodendrocytes abut the basal lamina surrounding the capillary, so that the normally intervening astrocytic glial limiting membrane is absent. In these groups and rows, the perikarya of the oligodendrocytes are squashed close together, and it is common to find tight junctions formed between them. The cortices of the old monkeys also show extensive degeneration of myelin, and it is supposed that this is linked to the changes in the oligodendrocytes. It is hypothesized that the alterations in the oligodendrocyte-myelin system lead to changes in the rates of conduction along fibers whose myelin sheaths are affected, and this may be one of the causes of the behavioral changes associated with aging in primates.

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.

Confocal imaging of glial cells in the intact rat optic nerve

Astrocytes and oligodendrocytes in the isolated intact mature rat optic nerve have been computer imaged in three dimensions by laser scanning confocal microscopy of single cells, dye-filled with lysinated rhodamine dextran (LRD). Our results illustrate the first application of these techniques to an intact CNS white matter tract and provide comparative data for previous studies on neonatal rat optic nerve (Butt and Ransom: Glia 2:470-475, 1989; Butt and Ransom: J Comp Neurol 338:141-158, 1993). The combined use of intracellular injection of LRD and confocal imaging significantly improves the resolution of glial cell structure, particularly that of mature astrocytes, for a number of reasons. 1) Single mature dye-filled glia can be imaged, because LRD does not pass through gap junctions. 2) The entire process field of astrocytes can be visualized in a single two-dimensional image. 3) Cell images can be rotated through 360 degrees in all planes to provide a new perspective of glial cell structure in the intact tissue. 4) Reconstruction of optical sections, within a narrow focal plane, provides a high definition and resolution of the finer details of glial morphology. Using these techniques, three astrocyte subclasses were distinguished on morphological criteria. It is the conclusion of this study that the majority of these forms represent a single population of fibrous astrocytes which are well-suited to perform the multiple functions attributed to astrocytes in the CNS. The morphology of mature myelin-forming oligodendrocytes was also described.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution and morphological characteristics of oligodendrocytes in the rat hippocampus in situ and in vitro: an immunocytochemical study with the monoclonal Rip antibody

Oligodendrocytes in the rat hippocampus in situ and in organotypic slice cultures were studied by light and electron microscopic immunocytochemistry using the monoclonal Rip antibody. Our results confirm that this antibody exclusively stains oligodendrocytes, while astrocytes and neurons are not labelled. In the light microscope, immunopositive cells had the appearance of myelinating oligodendrocytes with their characteristic tubular processes. In the electron microscope, stained cells showed intimate contacts with myelin sheaths but not with the basal laminae of endothelial cells. Rip-positive oligodendrocytes were unevenly distributed in the adult rat hippocampal formation. In general, they were abundant in layers known to contain many afferent and efferent fibres. In the hippocampus proper, there was a particularly strong immunolabelling of stratum radiatum of field CA2. In the fascia dentata, the hilar region displayed a high cell density, especially in the vicinity of the granule cell layer. A similar distribution of immunopositive cells was found in young animals (15-18 days old); however, the density of labelled cells was lower, particularly in the hilus. Immunolabelled cells in slice cultures of hippocampus displayed the characteristics of myelinating oligodendrocytes. Moreover, they showed an organotypic distribution, although afferent and efferent fibre projections normally myelinated by these cells were absent under these conditions.

Morphology of astrocytes and oligodendrocytes during development in the intact rat optic nerve

The detailed three-dimensional morphology of macroglial cells was determined throughout postnatal development in the intact rat optic nerve, a central nervous system white matter tract. Over 750 cells were analyzed by intracellular injection of horseradish peroxidase or Lucifer Yellow to provide a new perspective of glial differentiation in situ. Retrograde analysis of changes in glial morphology allowed us to identify developmental timetables for three morphological subclasses of astrocytes and oligodendrocytes, and to estimate their time of emergence from undifferentiated glial progenitors. Glial progenitors were recognised throughout postnatal development and persisted in 35-day-old nerves, where we suggest they represent adult progenitor cells. Astrocytes were present at birth, but the majority of these cells developed over the first week as three morphological classes emerged having either transverse, random, or longitudinal process orientation. Several lines of evidence led us to believe that the majority of astrocytes in the rat optic nerve were morphological variations of a single cell type. Young oligodendrocytes were first observed 2 days after birth, indicating that they diverged from progenitors at or near this time. During early development these cells extended a large number of fine processes, which then bifurcated and extended along axons. Later, as myelination proceeded, oligodendrocytes exhibited fewer processes which grew symmetrically and uniformly along the axons, resulting in a highly stereotypic mature oligodendrocyte form. Our analysis of oligodendrocyte growth suggests that these cells did not myelinate axons in a random manner and that axons may influence the myelinating processes of nearby oligodendrocytes.

Oligodendrocytes and myelin formation along the optic tract of the developing hamster: an immunohistochemical study using the Rip antibody

The monoclonal antibody Rip recognizes an antigen specific to oligodendrocytes and their processes (Friedman et al: Glia 2:380, 1989). We have used this antibody to document the appearance of oligodendrocytes and the sequence of ensheathment of axons along the optic tract (OT) and within its major target areas in neonatal (P3-P21) and adult hamsters. Myelination of axons in the visual pathway follows an overall proximo-distal gradient. On P3, immunopositive, pre-ensheathing oligodendrocytes are detected in the OT ventral to the lateral geniculate body (LGB) whereas myelin segments are present around OT axons by P5. The first pre-ensheathing oligodendrocytes are detected medially in the LGB on P7 and myelinated axons in the overlying OT by P11. In the superior colliculus, pre-ensheathing oligodendrocytes are present in the optic fiber layer (SO) on P7, but not in the superficial gray layer (SGS) until P11. Myelination of axons within SO proceeds along a marked rostro-caudal gradient. On P14, axons in rostral SO are heavily myelinated; thereafter, ensheathment continues caudally within the SO and the SGS. The progressive invasion of oligodendrocytes along the proximo-distal axis of the optic pathway, and the corresponding myelination of OT axons, are discussed in the context of a possible inhibitory role of oligodendrocytes in regulating the regenerative propensity of retinotectal axons.

Ultrastructural identification of HRP-injected oligodendrocytes in the intact rat optic nerve

Glial cells in the rat optic nerve were visualized by intracellular injections of horseradish peroxidase (HRP). A novel class of cell was encountered that was presumed to be an oligodendrocyte on the basis of arguments related to its light microscopic appearance after intracellular staining (Butt and Ransom, Glia 1989;2:470-475). These cells had 10-20 parallel processes 200-300 microns long that were oriented exclusively along the long axis of the optic nerve; the parallel processes were connected to the cell body by thin branches 15-30 microns long. To determine if these HRP-filled cells were oligodendrocytes, they were examined ultrastructurally; all cells examined in this way were unequivocally found to be myelin-producing oligodendrocytes. The oligodendrocytes contained intracellular organelles that were characteristic of this cell type, including abundant Golgi profiles and microtubules. In addition, HRP was found to fill the inner and outer tongue processes of myelin sheaths and the paranodal loops at nodes of Ranvier, proving that the entire cytoplasmic border surrounding the myelin sheath rapidly communicates by intracellular diffusion with the cell body. This electron microscopic study demonstrates that oligodendrocytes in the rat optic nerve can be positively identified by their distinctive light microscopic appearance after intracellular dye injection, and provides light microscopic criteria for establishing the number, distribution, and dimensions of the myelin segments provided by individual oligodendrocytes.

Primary dissociated cell culture of fetal rat central nervous tissue. II. Immunocytochemical and ultrastructural studies of myelinogenesis

It is generally considered that the dissociated cell culture is a type of monolayer culture, and when this method is used with the central nervous system (CNS) cells, synapses may be formed but myelin is either not formed at all or formed in very small quantities. So we tried to establish a dissociated cell culture of the CNS which contains many myelinated axons. Cerebra from 18-day-old fetal rat were dissociated into single cells and plated onto poly-D-lysine-precoated coverslips at high cell density. With the progress of cell maturation, neuronal and non-neuronal cell processes intermingled and piled up on each other, forming 3-dimensional structures. We were able to observe not only many mature neurons and remarkable synapses but also many myelinated axons. Moreover direct connections of oligodendrocyte (ODC) somas to myelin sheaths were demonstrated either immunocytochemically or ultrastructurally. The formation of the 3-dimensional structure is considered to provide a favorable microenvironment for the intimate interaction of neurons and ODCs leading to the formation of many myelinated axons. So this culture system may provide a useful model for the investigation of the details in myelinogenesis under physiological conditions and demyelination or remyelination under pathological conditions such as multiple sclerosis and allied diseases.

Topology of proteolipid protein in the myelin membrane of central nervous system. A study using antipeptide antibodies

Peptides according to amino-acid sequences of the N- and C-terminus of lipophilin (proteolipid protein, PLP) (Gly1-Phe15 = 1; Thr261-Phe276 = 6) and of the other four hydrophilic domains (Glu37-Leu60 = 2; Arg97-Leu112 = 3; Gly119-Gly127 = 3A; Trp144-Tyr156 = 3B; Lys191-Ala203 = 4; Asn222-Phe232 = 5) have been synthesized by the solid-phase Fmoc method, linked covalently to keyhole limpet hemocyanin (KLH) and used as antigens. Monospecific antibodies against these antigens were isolated by affinity chromatography. Each antibody recognized its epitope in isolated partially delipidated PLP with the ELISA technique, western blot, thin sections of paraffin embedded rat brains and in the plasma membrane of appropriately fixed/permeabilized rat oligodendrocytes in culture. After fixation with formaldehyde antipeptide 3A antibody stained intact non-permeabilized cells. Therefore the epitope 3A must be located on the extracellular surface of the membrane. This is in full support of our previous biochemical results on the orientation of lipophilin in the myelin membrane.

Visualization of oligodendrocytes and astrocytes in the intact rat optic nerve by intracellular injection of lucifer yellow and horseradish peroxidase

The morphology of glial cells in the intact rat optic nerve, a central nervous system (CNS) white matter tract, was analysed by filling over 500 macroglial cells intracellularly with horseradish peroxidase (HRP) or Lucifer yellow (LY). Two main cell types were distinguished: fibrous astrocytes and cells presumed to be oligodendrocytes. Intracellularly stained astrocytes were highly complex, with 50-60 long branching processes which passed radially from the cell body and terminated in end-feet at the pial surface or on blood vessels; some processes ended freely in the nerve parenchyma. Astrocytes filled with LY were usually dye-coupled to other astrocytes after the first week of life. Filled oligodendrocytes had a unique appearance that unmistakably distinguished them from astrocytes and were occassionally dye-coupled to nearby oligodendrocytes. These cells had 20-30 longitudinally oriented processes 150-200 microns long, which passed exclusively along the long axis of the nerve parallel to axons; the longitudinal processes were connected to the cell body by thin branches 15-30 microns long. The longitudinal processes probably represent the tongue processes of the internodal myelin sheaths, and thus each oligodendrocyte appears to myelinate 20-30 axons with sheaths that are 150-200 microns in length.

In situ demonstration of mature oligodendrocytes and their processes: an immunocytochemical study with a new monoclonal antibody, rip

This paper introduces "Rip" a monoclonal antibody that produces relatively complete staining of oligodendrocytes and their processes in the adult central nervous system (CNS). The distribution of Rip immunoreactivity coincides with that of myelinated axons in both the spinal cord and the cerebellum. In addition, double-immunolabeling experiments demonstrate that Rip stains processes containing myelin basic protein but does not stain processes that express glial fibrillary acidic protein. These results indicate that Rip selectively stains oligodendrocytes but not astrocytes. Moreover, individual Rip-stained oligodendrologial somata and their cytoplasmic processes were observable at both the light microscopic and electron microscopic level when the staining of myelin was reduced. This was accomplished by omitting detergents from antibody incubation steps. Rip-stained oligodendrocytes have multiple processes of varying thickness, some of which end in close proximity to myelin sheaths. These immunostained profiles, reminiscent of those observed in oligodendrocytes stained by Golgi methods, are unique to Rip and indicate that its immunoreactivity is distinct from that of existing serological markers for oligodendrocytes.

Insulin/insulin-like growth factor I and other epigenetic modulators of myelin basic protein expression in isolated oligodendrocyte progenitor cells

The expression of myelin basic protein by the oligodendrocyte is an integral event in the maturation of central nervous system function. Although much is known concerning the various myelin basic protein species, their temporal expression, and processing of RNA transcripts, little is known about the epigenetic factors responsible for the regulation of myelin basic protein (MBP) expression. In this study, we present evidence that insulin/insulin-like growth factor-I can increase the levels of MBP protein in isolated oligodendrocyte progenitor cells cultured in a serumless, chemically defined medium (ODM). Insulin was found to increase MBP protein in a dose-responsive manner, reaching a maximal level at 72 hr of exposure. Both insulin-like growth factor-I (IGF-I) and insulin were demonstrated to have no effect on MBP RNA levels. These data indicate that insulin/IGF-I increased MBP protein levels at a level distal to transcription The dose response of insulin action suggests that it may have a MBP regulatory function, distinct from IGF-I. When added individually, the other supplements of ODM, transferrin (500 ng/ml), and basic fibroblast growth factor (5 ng/ml) had no effect on MBP expression. However, when all three components were combined, a synergistic effect resulting in increased MBP protein and total RNA levels was found. The phorbol ester 12-O-tetradecanoyl phorbol acetate was found to reduce intracellular MBP RNA levels. The cAMP analogue/dibutyryl cAMP had contrasting effects on MBP RNA levels; no effect occurred in cultures grown in fetal calf serum, but a reduction in RNA levels was found in cultures grown in ODM. These data suggest that only a select range of extrinsic factors may be involved in MBP regulation, and depending on the environmental milieu, epigenetic agents may modulate gene activity differently.

Isolation and cultivation of mature oligodendroglial cells

CNS axons are ensheathed by myelin which is produced and maintained by oligodendrocytes. A disorder of this assembly results in functional disturbances, e.g., paralysis in multiple sclerosis. Methods are now available to isolate and cultivate oligodendrocytes in vitro. Thus, basic oligodendroglial properties can be now investigated: signals for oligodendroglial gene expression and their role in myelinogenesis and the interaction between oligodendrocytes and other neural cells by, e.g., the release of informational substances.

Specificity in central myelination: evidence for local regulation of myelin thickness

The ventral funiculi of normal and X-irradiated 13-day-old rats were studied by electron microscopy. In both tissues, oligodendrocytes form myelin sheaths around multiple axons, with a single oligodendrocyte associated with several axons of different sizes. Despite their origin from the same glial cell, the myelin sheaths are thicker for larger axons. Polyribosomes and rough endoplasmic reticulum are observed in distal oligodendrocyte processes, in proximity to the forming myelin sheaths. These results indicate that myelin sheath thickness is matched to axon size via local mechanisms, and suggest a role of polyribosomes and/or rough endoplasmic reticulum in myelin formation.

Expression of glial fibrillary acidic protein in immature oligodendroglia

In the human fetal spinal cord at 15 to 16 weeks, glial fibrillary acidic protein (GFAP) was demonstrated within the cytoplasm and processes of cells having the cytological, ultrastructural, and immunocytochemical features of oligodendrocytes--including processes that extend into and contribute to the formation of myelin sheaths. By 17 to 18 weeks, however, GFAP immunoreactivity was no longer evident within such cells. Thus GFAP is expressed by myelin-forming oligodendroglia early in their development.

Do the Purkinje cells have a special type of oligodendrocyte as satellites?

Two types of oligodendrocytes considered to be a constant feature in the cerebellar cortex of the rat are described. One cell type (I) exhibits rounded or elliptical nuclei, whereas the other type (II) presents more irregular nuclear and cellular contours and wider perinuclear cisternae. The latter cell type shows a more electron-dense cytoplasm with more heavily clumped heterochromatin, contrasting strongly with the euchromatin; also long and parallel cisternae of rough endoplasmic reticulum are more frequent. The percentages of both types of oligodendrocytes in relation to the total population of common glial cell types were calculated in the cortical layers and at several levels in these layers. The distribution of oligodendrocytes in the associated white matter was also carried out for purposes of comparison. The results provide evidence the the Purkinje cells may have a special kind of oligodendrocyte (Type II) as satellites.

Immunocytochemical localization of glycerol-3-phosphate dehydrogenase in rat oligodendrocytes

In this study, two indirect immunoperoxidase staining procedures were used to investigate the cellular localization of rat brain glycerol-3-phospate dehydrogenase (EC 1.1.1.8;GPDH). At the light and electron microscopic level, we found that the use of monospecific rabbit antibodies to GPDH consistently resulted in the specific staining of only one glial cell population. GPDH-positive cells in perineuronal, interfascicular and perivascular positions were identified as oligodendrocytes by classical morphological criteria. The specificity of GPDH antigen-antibody reaction was determined by qualitative and quantitative immunochemical methods and by imunocytochemical controls for immunologic and methodologic sources of nonspecific reaction product. The illustrative data from this study serve to qualitatively define GPDH as a biochemical marker for oligodendrocytes in rat central nervous tissue. In view of the fact that the synthesis of rat brain GPDH is specifically regulated by glucocorticoids, the positive results obtained in this study further warrant the interpretation that rat oligodendrocytes are target cells for glucocorticoids.

Ultrastructural study of myelinating cells and sub-pial astrocytes in developing rat spinal cord

The anterior funiculus of the spinal cervical cord of post-natal rats was examined ultrastructurally. The myelinating cells found one day after brith contained a large amount of evenly distributed ribosomes up to the outer tongue of mesaxons, representing the cytoplasmic density. These cells were separated by astrocytic processes from the pial basement membrane, even when they were located on the pial surface. Astrocytes contained glial fibrils from one day onwards and often attached their processes to the pial basement membrane. Although the cytoplasmic processes of astrocytes occasionally wrapped axons, they were never shown to form the initial layer of myelin sheaths. However, the tenuous processes of the sub-pial astrocytes were occasionally rolled in myelin lamellae, as if a part of the myelin sheaths was constructed by astrocytic processes. The interpretation for this finding is discussed in relation to function and potency of the astrocytes, and variations and anomalies of nervous ontogeny.

Immune response to isolated oligodendrocytes

Oligodendrocytes were isolated from bovine white matter and were injected with complete Freund's adjuvant (CFA) into experimental animals. Indirect immunofluorescence studies using fluoresceinated goat anti-rabbit or anti-guinea pig immunoglobulin (GARIg; GAGPIg) showed that rabbit and guinea pig anti-oligodendrocyte (RAO, GPAO) sera reacted specifically with the surface of isolated oligodendrocytes in suspension, as well as with oligodendroglia in bovine and human brain sections, and in mouse cerebellum cultures. This activity of RAO was blocked by non-fluoresceinated GARIg and by GPAO, and absorbed by oligodendrocyte preparation (OP) or whole white matter, but not by purified myelin, neuroblastoma or non-brain tissue. Low levels of anti-basic protein antibodies were found in many RAO (but not GPAO) sera by radioimmunoassay, and a few showed significant anti-galactocerebroside antibody by agglutination and radioimmunoprecipation techniques. Guinea pigs sensitized with isolated oligodendrocytes in CFA showed cell-mediated immunity (CMI) to OP as manifested by delayed type skin test and induced in vitro lymphocyte transformation. CMI to purified myelin basic protein was not detected. The demonstration of humoral and CMI to the cell responsible for the production of CNS myelin may be related to some aspects of the immunopathogenesis of demyelinating disorders.

Immunocytochemical method to identify basic protein in myelin-forming oligodendrocytes of newborn rat C.N.S

An immunocytochemical method for detecting myelin basic protein in oligodendrocytes and myelin of newborn rat C.N.S. is described. C.N.S. tissue is perfused and fixes in HgCl2--formaldehyde and 20 micron Vibratome sections are treated with antibodies to myelin basic protein using the peroxidase--antiperoxidase method. Oligodendrocytes in the newborn rat are intensely stained by antiserum to basic protein and multiple stained processes extend from the perikaryon to myelin sheaths. With this procedure it is possible to demonstrate the geometric relationships between a single oligodendrocyte and multiple myelin sheaths. Stained oligodendrocytes and myelin are present in newborn cervical spinal cord, medulla oblongata, pons and midbrain. By 25 days of age, staining in oligodendrocytes is less intense than in newborn rats and differences in amount of staining can be detected in areas that are myelinating at different rates. With anticerebroside serum, cerebroside, of newborn and developing rat C.N.S. tissue is localized only in myelin. In the developing P.N.S., myelin basic protein is localized in Schwann cell cytoplasm and myelin sheaths of the trigeminal ganglion. Cerebroside is found only in myelin.

Mechanism of demyelination in JHM virus encephalomyelitis. Electron microscopic studies

Weanling mice were given intraperitoneal inoculations of the neurotropic, JHM strain of mouse hepatitis virus, the virulence of which had been altered by repeated mouse passages. Five to seven days later many animals developed hind leg paralysis. The pathology consisted of an acute encephalomyelitis with patchy demyelinating lesions in the brain stem and spinal cord. Virus particles, consistent with the appearance of corona viruses, were found in the cytoplasm of cells that were identified as oligondendrocytes by demonstrating connections of their plasma membranes with myelin lamellae. Following the degeneration of oligodendrocytes the myelin sheaths disintegrated or were stripped off intact axons by cytoplasmic tongues of polymorpho- and mononuclear leucocytes that intruded between myelin lamellae. The findings indicate that JHM virus has an affinity for oligodendrocytes in weanling mice and that demyelination occurs subsequently to the degeneration of the infected oligodendrocytes.