Ultrastructural and light-microscopic studies of the developing feline spinal cord white matter. II. Cell death and myelin sheath disintegration in the early postnatal period

Characterization of Superficial Dorsal Horn Neurons from "Tamamaki" Mice and Stability of their GAD67-EGFP Phenotype in Defined-Medium Organotypic Culture

Defined medium organotypic cultures (DMOTC) containing spinal dorsal horn neurons are especially useful in studying the etiology and pharmacology of chronic pain. We made whole-cell recordings from neurons in acutely isolated mouse spinal cord slices or from those maintained in DMOTC for up to 6 weeks. In acute slices, neurons in the substantia gelatinosa exhibited 7 different firing patterns in response to 800-ms depolarizing current commands; delay (irregular), delay (tonic), tonic, regular firing, phasic, initial bursting and single spiking. Initial bursting and regular firing neurons are not found in rat substantia gelatinosa. In acute slices from "Tamamaki" mice that express enhanced green fluorescent protein (EGFP) under the control of the glutamic acid decarboxylase 67 (GAD67) promotor, tonic, phasic and regular firing neurons exhibited the strongest GABAergic (GAD67-EGFP+) phenotype. Delay (tonic) and delay (irregular) neurons almost never expressed GAD67 (GAD67-EGFP-) and are likely glutamatergic. All seven phenotypes were preserved in mouse spinal cord neurons in DMOTC prepared from e12 embryos and the GAD67-EGFP+ phenotype continued to associate with phasic and regular firing neurons. Only 3 out of 51 GAD67-EGFP+ neurons exhibited a delay (tonic) firing pattern. Modifications to the mouse genome thus continue to be expressed when embryonic neurons develop in vitro in DMOTC. However, analysis of the amplitude and interevent interval of spontaneous EPSCs (sEPSCs) indicated substantial re-arrangement of synaptic connections within the cultures. Despite this, the characteristics and age-dependence of asynchronous oscillatory activity, as monitored by multiphoton Ca²⁺ imaging, were similar in acute slices and in DMOTC.

Is There Evidence for Myelin Modeling by Astrocytes in the Normal Adult Brain?

A set of astrocytic process associated with altered myelinated axons is described in the forebrain of normal adult rodents with confocal, electron microscopy, and 3D reconstructions. Each process consists of a protuberance that contains secretory organelles including numerous lysosomes which polarize and open next to disrupted myelinated axons. Because of the distinctive asymmetric organelle distribution and ubiquity throughout the forebrain neuropil, this enlargement is named paraxial process (PAP). The myelin envelope contiguous to the PAP displays focal disruption or disintegration. In routine electron microscopy clusters of large, confluent, lysosomes proved to be an effective landmark for PAP identification. In 3D assemblies lysosomes organize a series of interconnected saccules that open up to the plasmalemma next to the disrupted myelin envelope(s). Activity for acid hydrolases was visualized in lysosomes, and extracellularly at the PAP-myelin interface and/or between the glial and neuronal outer aspects. Organelles in astrocytic processes involved in digesting pyknotic cells and debris resemble those encountered in PAPs supporting a likewise lytic function of the later. Conversely, processes entangling tripartite synapses and glomeruli were devoid of lysosomes. Both oligodendrocytic and microglial processes were not associated with altered myelin envelopes. The possible roles of the PAP in myelin remodeling in the context of the oligodendrocyte-astrocyte interactions and in the astrocyte's secretory pathways are discussed.

Mechanisms of functional loss and recovery in spinal cord damage

Two main classes of morphological change follow trauma to central nerve fibres: (1) axonal disruption leads to total disintegration of the fibre distal (with respect to the cell body) to the lesion; (2) less severe trauma produces focal demyelination with preservation of axonal continuity. Large experimental demyelinating lesions produce complete conduction block. The histologically normal portions of the fibres, proximal and distal to the lesion, retain the ability to transmit impulses. Smaller lesions allow conduction to continue, but at a reduced velocity, and the ability of the fibres to carry long trains of impulses faithfully is impaired. All three defects of conduction contribute to functional loss. After acute transient compression of the spinal cord of the cat, demyelination increases during the first week. Evidence of remyelination appears in the third week. Inappropriately thin myelin is seen surrounding histologically normal axons. By one month, 90% of the fibres in the lesion have acquired new sheaths. Studies on single fibres have shown that the myelin is organized into segments bounded by nodes. The segments are abnormally thin and short. The myelin increases in thickness with time but thin segments are still present at 18 months. Electron microscopy shows that many of the known ultrastructural prerequisites for conduction are present in the new segments. It is not yet known, however, whether the chains of very short internodes which occur on some fibres allow conduction to be restored.

Central neuron-glial and glial-glial interactions following axon injury

Axon injury rapidly activates microglial and astroglial cells close to the axotomized neurons. Following motor axon injury, astrocytes upregulate within hour(s) the gap junction protein connexin-43, and within one day glial fibrillary acidic protein (GFAP). Concomitantly, microglial cells proliferate and migrate towards the axotomized neuron perikarya. Analogous responses occur in central termination territories of peripherally injured sensory ganglion cells. The activated microglia express a number of inflammatory and immune mediators. When neuron degeneration occurs, microglia act as phagocytes. This is uncommon after peripheral nerve injury in the adult mammal, however, and the functional implications of the glial cell responses in this situation are unclear. When central axons are injured, the glial cell responses around the affected neuron perikarya appears to be minimal or absent, unless neuron degeneration occurs. Microglia proliferate, and astrocytes upregulate GFAP along central axons undergoing anterograde, Wallerian, degeneration. Although microglia develop into phagocytes, they eliminate the disintegrating myelin very slowly, presumably because they fail to release molecules which facilitate phagocytosis. During later stages of Wallerian degeneration, oligodendrocytes express clusterin, a glycoprotein implicated in several conditions of cell degeneration. A hypothetical scheme for glial cell activation following axon injury is discussed, implying the injured neurons initially interact with adjacent astrocytes. Subsequently, neighbouring resting microglia are activated. These glial reactions are amplified by paracrine and autocrine mechanisms, in which cytokines appear to be important mediators. The specific functional properties of the activated glial cells will determine their influence on neuronal survival, axon regeneration, and synaptic plasticity. The control of the induction and progression of these responses are therefore likely to be critical for the outcome of, for example, neurotrauma, brain ischemia and chronic neurodegenerative diseases.

Regulation of oligodendrocyte development and CNS myelination by growth factors: prospects for therapy of demyelinating disease

Multiple sclerosis (MS), the most common neurological disorder diagnosed in young adults, is characterized by autoimmune demyelination in the central nervous system (CNS). Promotion of remyelination in the brain and spinal cord is a potential strategy for therapeutic intervention in MS and other demyelinating diseases. Recent studies have shown that the development of oligodendrocytes, the myelin-forming cells of the CNS, is extensively controlled by growth factors. These factors regulate the proliferation, migration, differentiation, survival and regeneration of oligodendroglial cells and the synthesis of myelin, and often interact in a complex manner. Moreover, insulin-like growth factor I (IGF-I) has proven effective for therapy of experimental autoimmune encephalomyelitis (EAE), an animal model of autoimmune demyelination. In this review we summarize recent findings on the regulation of oligodendrocyte development and CNS myelination by growth factors, and discuss these findings in the context of possible clinical application for the therapy of neurological disease in humans.

The mouse dystonia musculorum gene is a neural isoform of bullous pemphigoid antigen 1

Dystonia musculorum (dt) is a hereditary neurodegenerative disease in mice that leads to a sensory ataxia. We describe cloning of a candidate dt gene, dystonin, that is predominantly expressed in the dorsal root ganglia and other sites of neurodegeneration in dt mice. Dystonin encodes an N-terminal actin binding domain and a C-terminal portion comprised of the hemidesmosomal protein, bullous pemphigoid antigen 1 (bpag1). dt and bpag1 are part of the same transcription unit which is partially deleted in a transgenic strain of mice, Tg4, that harbours an insertional mutation at the dt locus, and in mice that carry a spontaneous dt mutation, dtAlb. We also demonstrate abnormal dystonin transcripts in a second dt mutant, dt24J. We conclude that mutations in the dystonin gene are the primary genetic lesion in dt mice.

Expression of the receptor tyrosine kinase c-kit in oligodendrocyte progenitor cells

The growth and differentiation of neural precursor cells in the central nervous system (CNS) are regulated by their response to polypeptide growth factors which interact with specific transmembrane receptor tyrosine kinases (RTKs). We demonstrate that rat oligodendrocyte-type 2 astrocyte (O-2A) glial progenitor cells, precursors of the myelin-forming cells in the CNS, express the transmembrane RTK c-kit, the gene product of the murine dominant white spotting (W) locus and receptor for stem cell factor. Expression of c-kit transcripts and immunoreactive protein is lost when O-2A progenitors differentiate into post-mitotic oligodendrocytes. Analysis of developing rat brain revealed an increase in the expression of c-kit transcripts between postnatal days 10 and 12, a window of time preceding the emergence of oligodendrocytes and the onset of myelination in vivo. Expression of c-kit in vitro and in vivo suggests a role for this receptor and its ligand during oligodendrocyte development.

Oligodendrocyte differentiation and progenitor cell proliferation are independently regulated by cyclic AMP

Oligodendrocytes, the glial cells specialized to synthesize myelin in the central nervous system, differentiate in primary rat brain cell cultures on a schedule similar to that observed in vivo. The schedule of oligodendrocyte differentiation and the rate of oligodendroglial progenitor cell proliferation in vitro are both modulated by 3',5'-cyclic AMP (cAMP). A 24-hour exposure to 1 mM N6,2'O-dibutyryladenosine 3',5'-cyclic monophosphate (dbcAMP) induced a wave of oligodendrocyte differentiation but inhibited proliferation of oligodendroglial progenitors, and reduced by 30-fold the proliferation of progenitors in response to platelet-derived growth factor (PDGF). When cells were grown in the presence of maximally stimulating concentrations of PDGF, the inhibitory effect of cAMP on progenitor cell proliferation was abolished while the stimulatory effect of cAMP on oligodendrocyte differentiation remained, demonstrating that these two cAMP-regulated events are independent.

Cell death in the oligodendrocyte lineage

We have recently found that about 50% of newly formed oligodendrocytes normally die in the developing rat optic nerve. When purified oligodendrocytes or their precursors are cultured in the absence of serum or added signalling molecules, they die rapidly with the characteristics of programmed cell death. This death is prevented either by the addition of medium conditioned by cultures of their normal neighboring cells in the developing optic nerve, or by the addition of platelet-derived growth factor (PDGF) or insulin-like growth factors (IGFs). Increasing PDGF in the developing optic nerve decreases normal oligodendrocyte death by up to 90% and doubles the number of oligodendrocytes, suggesting that this normally occurring glial cell death might result from a competition for limiting amounts of survival signals. These results suggest that competition for limiting amounts of survival factors is not confined to developing neurons, and raise the possibility that a similar mechanism may be responsible for some naturally occurring cell deaths in nonneural tissues.

Cell death and control of cell survival in the oligodendrocyte lineage

Dead cells are observed in many developing animal tissues, but the causes of these normal cell deaths are mostly unknown. We show that about 50% of oligodendrocytes normally die in the developing rat optic nerve, apparently as a result of a competition for limiting amounts of survival signals. Both platelet-derived growth factor and insulin-like growth factors are survival factors for newly formed oligodendrocytes and their precursors in culture. Increasing platelet-derived growth factor in the developing optic nerve decreases normal oligodendrocyte death by up to 90% and doubles the number of oligodendrocytes in 4 days. These results suggest that a requirement for survival signals is more general than previously thought and that some normal cell deaths in nonneural tissues may also reflect competition for survival factors.

Relations between axons and oligodendroglial cells during initial myelination. II. The individual axon

Axo-glial relations in the ventral funiculus of the spinal cord (SC) and in the corpus callosum (CC) of the cat were examined by electron microscopy during initial myelination. In addition to random transverse and longitudinal sections from several stages, two series of sections were studied. As a first step in myelination the axons become ensheathed by one to three uncompacted glial lamellae (E-sheaths). E-sheaths present a length range from less than 5 microns to 149 microns (SC) or to 93 microns (CC). E-sheaths are more frequent along SC-axons than CC-axons, and the mean E-sheath is 3.3-fold longer in the former compared to the latter. In both areas naked axon portions occur between successive E-sheaths, but these gaps are insufficient to allow elongation of all short E-sheaths into long ones. Sheaths composed of mixed compacted (M-sheaths) and uncompacted segments have a length range of 66-212 microns in the SC and 66-171 microns in the CC. In relation to the undifferentiated terminations of E-sheaths or mixed E/M-sheaths, undercoated axolemmal domains are always lacking. Fully compacted sheaths were not found in the series from the SC. In the CC, 141-212 microns long compact sheaths were found, with tight axoglial junctions at their terminations. Axolemmal domains with a 'nodal' undercoating occur in relation to some of these terminations. In both areas, individual developing axons present a chaotic mixture of naked, ensheathed and myelinated portions; bulges with clusters of vesiculotubular profiles are frequent along naked and ensheathed axonal portions, particularly in the SC. The axon diameter is clearly larger in myelinated than in naked portions of the same axon. On the basis of these results, we propose that the early glial sheaths of developing CNS axons actively elongate and undergo extensive remodelling before compaction. The maximal length of uncompacted E-sheaths, and the sheath length at which axoglial junctions and nodes of Ranvier form, are markedly different in the two areas.

Myelin breakdown in the posterior funiculus of the kitten after dorsal rhizotomy. A qualitative and quantitative light and electron microscopic study

Morphological aspects of myelin breakdown in the posterior funiculus during Wallerian degeneration were studied in kittens subjected to lumbosacral dorsal rhizotomies 6-8 days after birth. The first sign of myelin breakdown was characterized by swollen or shrunken nerve fibers. Shortly thereafter there was an increased occurrence of collapsed myelin sheaths and later of rounded myelin bodies. Myelin was clearly seen in microglial cells. Correlative observations on Marchi-stained material indicted the simultaneous and frequent appearance of Marchi-positive bodies (MPB:s) and myelin bodies. Due to the rapidity of the degeneration process in the kitten, the increase in the occurrence of Marchi-positive granules (MPG:s) seemed to start concomitantly with increased occurrence of MPB:s. However, the frequent occurrence of MPG:s outlasted that for MPB:s. The findings indicate that the MPB:s may be the counterpart to myelin bodies and the MPG:s to lipid droplets. Microglial cells may be responsible for the primary uptake of degenerating myelin and the subsequent transformation of myelin bodies to lipid droplets. The much faster breakdown of myelin and elimination of lipid material in the degenerating posterior funiculus of the kitten, as compared to the adult, seemed to be due not only to the lower myelin content in the kitten, but also to a higher density of microglial and a greater efficiency in the myelin breakdown process in the degenerating posterior funiculus of the kitten.

Quantitative electron microscopic observations on the non-neuronal cells and lipid droplets in the posterior funiculus of the kitten after dorsal rhizotomy

Kittens were subjected to lumbosacral dorsal rhizotomies at the age of 6-8 days postnatally. After postoperative survival times of 1-25 days the number of non-neuronal cells and lipid droplets in each cell type in the posterior funiculus at L1 were counted at the ultrastructural level. Intact control animals were analyzed in the same way. The number of astrocytes and oligodendrocytes decreased with increasing postoperative survival time in the degenerating zone. This was also the case in the white matter of control animals with increasing age of sacrifice. However, in the degenerating zone of operated animals the decrease was more extensive for oligodendrocytes starting at 5 days after surgery, and possibly also for astrocytes at 25 days postoperatively. The number of microglial cells in the degenerating zone was markedly increased 2-10 days after surgery compared to the controls. The number of non-pericytic perivascular cells seemed to be somewhat increased from 9 days after surgery, while the number of pericytes remained unchanged during the experimental period. Lipid droplets in the degenerating white matter were mainly located in microglial cells and astrocytes and only to a small extent in non-pericytic perivascular cells. These findings suggest that lipid material produced during anterograde fiber degeneration in the immature white matter is mainly metabolized in glial cells.

Early postnatal development of glial cells in the canine cervical spinal cord

To study qualitative and quantitative changes in the glial cell population of young postnatal dogs, the cervical spinal cords of 20 beagle pups, ranging in age from 1 to 28 days, were prepared for light and electron microscopy. Glial cells in the lateral corticospinal tract were classified and quantified directly on the electron microscope. Quantification was performed by means of a stereological method designed to correct for sampling bias, and glia were classified according to morphological criteria as immature glial cell precursors, light and dark oligodendrocytes, astrocytes, and microglia. Glial cell precursors, which include undifferentiated glioblasts, oligodendroblasts, and astroblasts, predominated in the first few days after birth, constituting 43% of the glial cell population, and then declined to less than 5% by 28 days. Light and dark oligodendrocytes differed morphologically in their electron density and the appearance of their organelles. Light oligodendrocytes increased slightly prior to myelination, and then declined, whereas dark oligodendrocytes continued to increase throughout the 4-week period and became the predominant cell type at 28 days (66%). In contrast to the oligodendroglial population, the sizes of the astroglial and microglial cell populations were relatively stable. This study shows that the population of immature glial cell precursors, abundant at birth in the lateral corticospinal tract, appear to be differentiating primarily into oligodendroglia, because this population exhibits a rapid increase in size, and relatively little change occurs in the astrocyte population. The trends in glial cell development in the dog are similar to those reported for rodents, although there may be some variation in the maturation and activity of oligodendrocytes.

Growth cones, dying axons, and developmental fluctuations in the fiber population of the cat's optic nerve

We have studied the rise and fall in the number of axons in the optic nerve of fetal and neonatal cats in relation to changes in the ultrastructure of fibers, and in particular, to the characteristics and spatiotemporal distribution of growth cones and necrotic axons. Axons of retinal ganglion cells start to grow through the optic nerve on the 19th day of embryonic development (E-19). As early as E-23 there are 8,000 fibers in the nerve close to the eye. Fibers are added to the nerve at a rate of approximately 50,000 per day from E-28 until E-39--the age at which the peak population of 600,000-700,000 axons is reached. Thereafter, the number decreases rapidly: About 400,000 axons are lost between E-39 and E-53. In contrast, from E-56 until the second week after birth the number of axons decreases at a slow rate. Even as late as postnatal day 12 (P-12) the nerve contains an excess of up to 100,000 fibers. The final number of fibers--140,000-165,000--is reached by the sixth week after birth. Growth cones of retinal ganglion cells are present in the optic nerve from E-19 until E-39. At E-19 and E-23 they have comparatively simple shapes but in older fetuses they are larger and their shapes are more elaborate. As early as E-28 many growth cones have lamellipodia that extend outward from the core region as far as 10 microns. These sheetlike processes are insinuated between bundles of axons and commonly contact 10 to 20 neighboring fibers in single transverse sections. At E-28 growth cones make up 2.0% of the fiber population; at E-33 they make up about 1.0%; from E-36 to E-39 they make up only 0.3% of the population. Virtually none are present in the midorbital part of the nerve on or after E-44. At all ages growth cones are more common at the periphery of the nerve than at its center. This central-to-peripheral gradient increases with age: at E-28 the density of growth cones is two times greater at the edge than at the center but by E-39 the density is four to five times greater. Necrotic fibers are observed as early as E-28 in all parts of the nerve. Their axoplasm is dark and mottled and often contains dense vesiculated structures.(ABSTRACT TRUNCATED AT 400 WORDS)

Myelin deficient (shimld) mutant allele: morphological comparison with shiverer (shi) allele on a B6C3 mouse stock

A new B6C3 stock of shimld mutant mice is compared in terms of behavior and CNS morphology with both a B6C3 shi stock and reports on other shimld animals. Defects of B6C3 shimld myelination seen at postnatal day 21 (P-21) are comparable to those in B6C3 shi with respect to % axons myelinated, sheath thickness, errors in the wrapping and targeting of myelin and abnormal oligodendrocyte shape. The two mutations are similarly expressed in cerebellar organotypic cultures. However, the major dense line (MDL) is present in a few shimld myelin sheaths at P-21 and a few sheaths show myelin basic protein by immunocytochemistry, while neither phenomenon is seen in shi at this age in the same CNS regions. Shimld mice survive their disease significantly better than shi. The shimld stock currently under study elsewhere differs from this B6C3 stock in that MDL was reported only in older animals, and behavior and survival were severely compromised.

Dimensions and branching patterns of triceps surae alpha-motor axons and their recurrent axon collaterals in the spinal cord during the postnatal development of the cat

Triceps surae alpha motoneurons in the cat were stained intracellularly with horseradish peroxidase (HRP) at different postnatal ages from birth to the adult stage. The motor axons and axon collaterals were studied with regard to length, diameter and branching pattern. The postnatal increase of internodal length, measured as the distance between two subsequent axon collateral origins, was about 100% which paralleled the total length increase of the main axon in the grey matter. The axon collaterals were unmyelinated at birth and branched exclusively dichotomously until after 3 weeks of age when a substantial fraction of the branching points gave off 3-5 daughter branches. This was interpreted as signs of a fusion between neighboring branching points during the period of myelination of the axon collaterals. The length analysis of the collaterals indicated that the postnatal elimination of collateral branches described previously is preferentially located in the distal parts of the collateral tree.

Postnatal differentiation of rat optic nerve fibers: electron microscopic observations on the development of nodes of Ranvier and axoglial relations

The postnatal differentiation of rat optic nerve fibres was examined by transmission electron microscopy. The results show that many early developing axons contain clusters of vesiculotubular profiles prior to myelination. At places vesicular elements appear to fuse with the axolemma, and, in addition, some axons exhibit deep axolemmal invaginations and axoplasmic lamellated bodies. It is suggested that these features might reflect axolemmal remodelling, possibly involving axoglial signalling and/or functional differentiation of the axolemma. The size distribution of unmyelinated optic nerve axons changes little during development. Ensheathment of larger axons commences 6 days postnatally. The subsequent formation of compact myelin sheaths is accompanied by an increase in axonal diameter. The early sheaths are a few microns long and separated by long bare axon segments. In optic nerves from 10-12-day-old rat pups, a few sheaths consisting of about five layers border primitive asymmetric nodes with a patchy axolemmal undercoating. Extensions from one of the terminating sheaths are often associated with undercoated patches of axolemma. Relatively differentiated nodes of Ranvier first appear 14-16 days after birth. The continued nodal maturation involves establishment of a regular nodal geometry, increasing distinctness of the axolemmal undercoating, and formation of perinodal astrocytic processes embedded in an extracellular node gap substance. The results are compared with available data on the conduction properties of rat optic nerve fibres during development.

Immunohistochemical studies of blood monocytes infiltrating into the neonatal rat brain

Brains of normal rats ranging in age from newborn to adult were observed with immunofluorescence technique using anti- granulomonocyte antiserum. For the first 10 days after birth, many cells with positive fluorescence were found in the white matter, the subependyma , the extra-parenchymal spaces, and the leptomeninx , but very few in the gray matter. They were mononuclear, rich in cytoplasm, and globular or irregular in shape. After about day 10 p.n., the positive cells decreased in number and became slender. However, there was no change in the distribution pattern. After about 3 weeks of age, no positive cells were detected in the brain parenchyma, except for very rare necrobiotic ones. It was suggested that blood monocytes infiltrate into the brain parenchyma of normal neonatal rat, but only for a while in the limited areas (white matter and subependyma ). They have the morphology and distribution of the "ameboid microglia" of neonatal brain. These monocytes disappear from the brain finally by the end of month 1 p.n.

'Shaking pups': a disorder of central myelination in the spaniel dog. III. Quantitative aspects of glia and myelin in the spinal cord and optic nerve

Quantification of glial cells, axonal size and myelin thickness and volume were carried out in selected areas of the three funiculi of the cervical spinal cord and the optic nerve of 'shaking pups' and normal littermates at 4 and 8 weeks of age. There was a marked reduction of oligodendrocytes in the affected pups with many of these cells having distended rough endoplasmic reticulum. Oligodendrocyte death was not noticeable. Astrocyte numbers were similar in both normal and affected pups. Axonal diameters were not reduced in the affected pups and there was no apparent correlation between myelination and axonal size in these animals. Total myelin volume and thickness were greatly reduced in the 'shaking pups.' Impaired stem cell division together with metabolic disturbance of oligodendrocytes are considered to be the main causes of the hypomyelination in this mutant.

Nerve fibres in spinal cord impact injuries. Part 1. Changes in the myelin sheath during the initial 5 weeks

The spinal cords of cats were subjected to an impact injury using a "weight dropping" technique and sequential changes in the sheaths of non-degenerate myelinated fibres studied over a 3-week period. By 1 1/2 h after impact fibres showed retraction of some lateral loops from one paranode. The extent and severity of this change increased over the first week so that partial and full thickness demyelination were seen frequently. Partial demyelination most commonly resulted from the internodal termination of the innermost lamellae at an internodal location often associated with a Schmidt-Lantermann incisure. Remyelination by both Schwann cells and oligodendroglia occurred at the end of the second week. Oligodendroglial myelin showed many features of immaturity, similar to those found during development. It is suggested that the very earliest myelin damage is mechanical but is aggravated by other factor(s) one of which is probably ischaemia. Within the most severely injured areas there is death of oligodendroglia and any surviving axons are remyelinated principally by Schwann cells. In intermediate and minimally damaged areas of white matter oligodendroglial remyelination predominates.

Cultured neonatal rat oligodendrocytes elaborate myelin membrane in the absence of neurons

We have utilized transmission electron microscopy to study oligodendrocyte-enriched cell cultures established from dissociated neonatal rat cerebra by the method of McCarthy and de Vellis [1980]. Cells were examined after 14 and 26 days in vitro. The overall morphology of the cells from cultures at both time periods was similar and consistent with previous reports of light (immature) oligodendrocyte fine structure. The cells contained an eccentrically located nucleus, prominent Golgi regions, numerous free ribosomes, and microtubules. Large numbers of processes with varying diameter were also observed. There was some indication of cytoplasmic maturation from the younger to the older cultures. The most important feature of the 26-day cultures was the large quantity of intercellular membranes which were shown to be continuous with oligodendrocyte processes. These membranes often exhibited the appearance of "loose myelin" and were therefore not normally compacted. Layers of membrane with the morphologic appearance of compact myelin were observed on an occasional oligodendrocyte perikaryon or process. This finding necessitates a reevaluation of the widely held theory that oligodendrocytes are not able to elaborate myelin in the absence of neurons.

Electron-microscopic identification of Gomori-positive rings in normal spinal cord white matter

Longitudinal vibratome slices from glutaraldehyde-fixed adult guinea pig lateral funiculus white matter were incubated in a Gomori medium, stained with ammonium sulfide, osmicated and section-embedded in vestopal. Following light-microscopic localization of ring-shaped deposits of reaction product series of thin sections were cut from these areas. In the electron microscope ring-shaped deposits of reaction product were consistently found to surround lamellated myelinoid bodies within glial cells. By following the series some of these cells could be identified as microglia. Sparse aggregates of reaction product were occasionally seen outside myelinoid bodies in typical astrocytes. Precipitates were not related to myelinoid bodies associated with typical oligodendrocytes. The findings suggest that microglia and possibly astroglia participate in the breakdown of spontaneously formed myelin fragments in normal adult white matter.

Affinity of chick microglia in vitro for ricin 120

The distribution of binding sites for ricin 120 in cell cultures of spinal cord from the chick embryo was examined. A characteristic pattern was observed, which remained similar in cultures fixed by a variety of methods. Light microscopy demonstrated that the most prominent staining was of small rounded or amoeboid cells. Electron microscopy showed that ricin was bound over their entire surfaces. The ultrastructural characteristics of these cells suggest their identification as microglia.

Cholesterol esterifying enzyme in normal and degenerating peripheral nerve

The cholesterol esterifying enzyme which incorporates exogenous free [1-14C]oleate into cholesteryl ester is present in rat sciatic endoneurium. Cholesterol esterification is optimal at pH 4.8. Exogenous ATP, CoA, and oleyl-CoA do not greatly affect its activity. Various detergents and bile salts are inhibitory. Enzyme activity does not change appreciably during storage at 4 degrees C for up to 4 days or at -70 degrees C for up to 1 month. Of the subcellular fractions, the microsomal fraction exhibits the highest specific activity. Over 75% of enzyme activity is recovered, with equal amounts in the microsomal and soluble fractions. During nerve fiber degeneration an increase (more than fivefold) in cholesterol esterifying activity, which peaks 6 days after crush, is observed. Elevated levels of enzyme activity persist for 90 days after crush, by which time nerve regeneration is well established. Thus, it is concluded that an increase in cholesterol esterifying activity in degenerating nerve is primarily responsible for cholesterol esterification during Wallerian degeneration. The maximum increase in cholesterol esterifying activity is associated temporally with axonal degeneration and, particularly, with the formation of myelin ovoids.

Hematogenous cells in the central nervous system of developing human embryos and fetuses

Examination of large blocks of Epon-embedded, 1.0-micrometer sections of human embryos and fetuses reveal the presence of hematogenous cells in various stages of differentiation in neural tissue. In every embryo and fetus of 10 weeks ovulation age and younger, hematogenous cells are found randomly scattered in the cerebrum, cerebellum, and spinal cord. Many of these cells appear to undergo spontaneous degeneration in neural tissue and become rarer in older fetuses. Also identified in the neuropil of normal embryos and fetuses are cells with the typical morphological appearance of macrophage containing numerous inclusions of various kinds, both inside and outside the blood vessels. In addition, scattered in the subpial, perivascular, and perineuronal regions of the neural parenchyma are small cells with fusiform nuclei and a small amount of cytoplasm as well as cells with a moderate amount of elongated cytoplasm containing various inclusions and oblong nuclei. All of these cells have clumped heterochromatin along the nuclear membrane which differs from other neuroectodermal cells of the developing human CNS. Although there is no direct evidence to indicate transformation of macrophages to "microglia" or vice versa, the presence of cells having similar nuclear morphology and chromatin pattern while appearing to be transitional forms of macrophage, varying from undifferentiated to fully developed, suggest a common lineage of these latter types. It is concluded that migration of hematogenous cells into neural tissue is a ubiquitous developmental phenomenon in young human embryos and fetuses.

Chronological study of oligodendroglial alterations and myelination in quaking mice

Chronological morphological investigation was carried out in the spinal cord of quaking mice from day 3 to day 130. Numbers of myelinated fibres were far fewer in quaking mice at day 3 compared to controls. However, when the animals became older, myelination progressed and numbers of myelinated fibres increased although myelin sheaths remained far thinner than the size of axons. Many oligodendroglia during day 5 to 15 in quaking mice revealed prominent dilation and proliferation of smooth walled vesicles and cisterns but after 20 days, such changes were no longer observed. Tortuous bizarre oligodendroglial processes, aberrant myelination and myelin figures were very prominent around day 5--15, but such changes also gradually subsided. Density of glial cells during pre-myelination gliosis was similar in both quaking and control mice. However, glial cell population decreased far slower pace than controls when myelination progressed. Thus, glial cell density remained proportionally higher in quaking than controls although the density declined with age in both.

Quantitative examination of internodal length of remyelinated nerve fibres in the central nervous system

The internodal length of remyelinated internodes was examined by observations on teased CNS nerve fibres following primary demyelination induced by intraspinal injections of lysolecithin into the white matter of cats. A remyelinated internode was identified as a thinly-myelinated internode, where a node of Ranvier was bounded by a thickly-myelinated internode on one side and a less-thickly-myelinated internode on the other; as an internode of less than 100 microns in length or below 2 standard deviations from the normal regression line of internodal length against fibre diameter; as an internode joined to one of similar myelin sheath thickness which fulfilled either or both of the previous two criteria. Using the above criteria, remyelinated internodes were found to be shorter than normal; some were very short with no relationship to axon diameter while others were longer, falling within the predicted range for a given fibre diameter. This study illustrates that examination of teased CNS fibres in pathological situations can yield valuable information. However, sampling and technical difficulties are far greater than in comparable studies on peripheral nerve fibres.

Use of carbon labeling to demonstrate the role of blood monocytes as precursors of the 'ameboid cells' present in the corpus callosum of postnatal rats

Cells with features suggestive of ameboid motion and phagocytic properties are observed in the rat corpus callosum during the first few days of life. These cells, hereafter referred to as 'ameboid cells', have been investigated in several ways. An electron microscopic study of the corpus callosum in 5- to 7-day-old rats indicated that most 'ameboid cells' were typical macrophages, but some displayed features of monocytes, while others appeared to be transitional between the two types. These observations raised the possibility that blood monocytes were the precursors of 'ameboid cells'. This possibility was tested by injecting a suspension of carbon particles into the circulation of rats of various ages to label and trace monocytes. Within 15 minutes after injection, carbon particles were seen between cells in blood smears as well as in the lumen of capillaries, but not between cells and axons in corpus callosum. By a half hour, a few of the circulationg monocytes, and with time, up to half of them, contained carbon particles. Five days after injection, carbon particles were observed in cells of the corpus callosum identified as 'ameboid cells' of the monocytic and macrophagic type. Such carbon-containing cells were seen in many of the animals injected at the age of 0-1 day, in few of those injected at 3-5 days, and in none of the older animals. Since free carbon had not been observed in corpus callosum spaces, it was concluded that 'ameboid cells' did not pick up carbon locally. The alternative was that blood monocytes, after ingesting carbon particles in the circulation, migrated to the corpus callosum and settled as 'ameboid cells'. In the hope of obtaining a direct confirmation of this conclusion, blood cells obtained from carbon-injected Lewis rats were centrifuged in a Percoll gradient to obtain a fraction which contained 70-80% monocytes, less than 2% granulocytes, and 20-30% lymphocytes. Carbon was present in up to half of the monocytes and 1% of the granulocytes, but not in the lymphocytes; and it was calculated that over 99% of the carbon-labeled cells were monocytes. The cell fraction was then introduced into the blood circulation of 2- to 3-day-old syngeneic Lewis rats, and the animals were sacrificed 5 days later. Occasional carbon-labeled cells appeared not only in liver, spleen and connective tissue, but also in the corpus callosum, where they were identified as 'ameboid cells' of the monocytic and macrophagic type. Even though such cells were infrequent, their presence conclusively demonstrated that at least some 'ameboid cells' of the corpus callosum were derived from circulating blood monocytes.

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.

Remodelling of optic nerve myelin sheaths and axons during metamorphosis in Xenopus laevis

Whole mounts and transverse sections of Xenopus optic nerves were examined with the light and electron microscopes before, during, and after metamorphosis. In stage 52--58 tadpoles, almost all myelin sheaths were circular in transverse sections. Early in metamorphosis (stages 60--61) large redundant myelin loops surrounded many large axons in central regions of the nerve. The loops subsequently were broken down into ovoids and lamellar segments that remained mostly within oligodendrocytes. These myelin changes were not observed in the chiasm or next to the eye. They were not associated with significant axonal degeneration and were no longer apparent in optic nerves of young frogs. Xenopus optic nerves also became shorter during metamorphosis. We therefore suggest that myelin sheaths with redundant loops which degenerate and disappear are being remodelled as the nerve decreases in length.

Radioautographic investigation of gliogenesis in the corpus callosum of young rats. II. Origin of microglial cells

Microglial cells are absent from the corpus callosum of newborn rats. In the hope of finding out when and how microglial cells appear with age, 3H-thymidine was given intraperitoneally as single or three shortly spaced injections to 5-day-old rats weighing about 15 g; and these animals were sacrificed at various time intervals from 2 hours to 35 days later. Pieces of corpus callosum were taken near the superior lateral angle of the lateral ventricles; and semithin sections were radioautographed and stained with toluidine blue. The corpus callosum of 5-day-old rats is composed of loosely arranged unmyelinated fibers and scattered cells. Among these cells, microglia are rare; there are a few astrocytes, many immature glial cells, rare pericytes, and 6--7% of phagocytic "ameboid cells" consisting of a few monocytes and many macrophages. In the animals sacrificed two hours after 3H-thymidine administration, label is present only in immature cells and "ameboid cells." As time elapses and the fibers of corpus callosum become myelinated, oligodendrocytes and, later, microglial cells appear. At the age of 12 days, microglial cells are present in substantial number; and by 19 days, the number doubles to reach a plateau. Many of the new microglial cells are labeled, e.g., 78.1% in 12-day-old animals (7 days after 3H-thymidine administration). The labeled microglial cells must have come from the transformation of cells that acquired label early, that is, from the immature cells or the "ameboid cells." The height of the peaks of labeling--59.8% at nine days for immature cells and 77.8% at 12 days for "ameboid cells"--points to the latter as precursors of the highly labeled microglial cells. Furthermore, the "ameboid cells" disappear as microglial cells appear and there are transitional elements between these two cell types. Cell counts suggest that about a third of the "ameboid cells" transform into microglial cells, while the others degenerate and die. Thus, the microglial cells which appear in the corpus callosum during the first three weeks of life result from transformation of the "ameboid cells"--a group of macrophages showing various stages of transition from monocytes. As for the occasional microglial cell appearing after the third week or in the adult, they presumably come directly from monocytes. In either case, monocytes would be the initial precursors.

Age-related changes in occurrence of Marchi-positive granules and Marchi-positive myelinoid bodies in postnatally developing feline white matter

The occurrence of clusters of Marchi-positive granules and of Marchi-positive myelinoid bodies at different postnatal developmental stages was examined lightmicroscopically in Vibratome sections from the cervical lateral funiculus of the cat, after perfusion-fixation with glutaraldehyde. The findings show that clusters of Marchi-positive granules are most common at birth and rapidly decrease in number with development, being largely absent in animals older than 1 month. The pattern of change resembles the postnatal changes in content of esterified cholesterol in the cervical lateral funiculus and is compatible with the view that the clusters of Marchi-positive granules may result from spontaneous myelin sheath disintegration occurring early postnatally. The incidence of Marchi-positive myelinoid bodies increases 7 times during the first 4 months after birth to a peak and declines about 40% during late maturation. The size spectrum of the Marchi-positve bodies shifts markedly towards larger sizes with development and presents a close to mature picture from 120 days on. Comparisons between the Marchi-positive myelinoid bodies and the myelin sheaths in the same region, with respect to postnatal change in occurrence and size spectrum, suggest that the Marchi-positive bodies are related to myelin sheaths of large fibres or fibres destined to become large.

The spinal cord-ventral root junction in the trembler mouse

The Trembler mouse suffers from an hereditary demyelinating neuropathy. Schwann cell myelination of peripheral nerve fibres in the Trembler mouse is abnormal. Myelination of central nerve fibres in the deeper layers of white matter of the spinal cord is normal. At the junction between the peripheral nervous system and the central nervous system in the ventral roots in the Trembler mouse central-type nerve fibres are abnormally thinly myelinated. It is suggested that the normal process of myelination of central nerve fibres in this region is affected by abnormalities of the Schwann cell in the peripheral nervous system.

Ultrastructure of the neuroglial fatty metamorphosis (Virchow) in the perinatal period

The ultrastructure of neuroglial fatty metamorphosis (GFM) has been investigated in the telencephalic white matter of 12 premature and mature infants (gestational age 22-40 weeks; survival 0-96 days). GFM was found in all cases apart from a 22-week-old fetus, and involves predominantly astrocytic cells (68.8%), then glioblasts (43.5%), but only 7.4% of oligodendrocytes. GFM, therefore, seems to be independent of the myelination process and indicates the vulnerability of the immature neuroglial population in the metabolic and circulatory disorders of the perinatal period. Since GFM is found in almost all children dying within the early postnatal period, this subtle alteration reflects a special form of minimal brain damage. The relationship between GFM, astrocytic hypertrophy and periventricular leucomalacia and their role in the telencephalic leucoencephalopathy are discussed.

Pericytes and perivascular microglial cells in the basal forebrain of the neonatal rabbit

Three types of pericytes outline the vascular bed in Golgi preparations of the newborn rabbit brain. Elongate cells (Type I) are restricted to capillaries, elements resembling smooth muscle cells (Type II) surround vessels of intermediate size, and large flat forms (Type III) cover the surface of arterioles and venules. Electron microscopy shows all types to be located within a well defined perivascular basement membrane. It also reveals the presence of filaments in the cytoplasm of some pericytes resembling the myofilaments of smooth muscle cells. It suggests the possibility that some pericytes are capable of contraction and may participate in regulating blood flow in small vessels. Microglia cells bear no resemblance to pericytes in terms of their shape, distribution or staining characteristics. Microglia cells are located outside the vascular basement membrane (external basal lamina) in the brain parenchyma, and they vary in form according to their location and the character of the surrounding extracellular space. This study does not support the hypothesis that microglia cells arise from pericytes but indicates that they originate either by in situ division or from hematogenous elements that enter the brain by crossing the vessel wall.

Early degenerative nerve alterations in feline resorbing deciduous incisors as observed by electron microscopy

Electron microscopy was used to assess degenerative nerve alterations in resorbing deciduous incisors of the cat. Intradental nerve structural changes are described when minimal signs of root resorption are present. Intradental nerves in resorbing deciduous incisors are compared with those of fully developed deciduous incisors.