Gliogenesis in organotypic tissue culture of the spinal cord of the embryonic mouse. I. Immunocytochemical and ultrastructural studies

Astrocytes display ultrastructural alterations and heterogeneity in the hippocampus of aged APP-PS1 mice and human post-mortem brain samples

The past decade has witnessed increasing evidence for a crucial role played by glial cells, notably astrocytes, in Alzheimer's disease (AD). To provide novel insights into the roles of astrocytes in the pathophysiology of AD, we performed a quantitative ultrastructural characterization of their intracellular contents and parenchymal interactions in an aged mouse model of AD pathology, as aging is considered the main risk factor for developing AD. We compared 20-month-old APP-PS1 and age-matched C57BL/6J male mice, among the ventral hippocampus CA1 strata lacunosum-moleculare and radiatum, two hippocampal layers severely affected by AD pathology. Astrocytes in both layers interacted more with synaptic elements and displayed more ultrastructural markers of increased phagolysosomal activity in APP-PS1 versus C57BL6/J mice. In addition, we investigated the ultrastructural heterogeneity of astrocytes, describing in the two examined layers a dark astrocytic state that we characterized in terms of distribution, interactions with AD hallmarks, and intracellular contents. This electron-dense astrocytic state, termed dark astrocytes, was observed throughout the hippocampal parenchyma, closely associated with the vasculature, and possessed several ultrastructural markers of cellular stress. A case study exploring the hippocampal head of an aged human post-mortem brain sample also revealed the presence of a similar electron-dense, dark astrocytic state. Overall, our study provides the first ultrastructural quantitative analysis of astrocytes among the hippocampus in aged AD pathology, as well as a thorough characterization of a dark astrocytic state conserved from mouse to human.

TGF-β1 promotes cerebral cortex radial glia-astrocyte differentiation in vivo

The major neural stem cell population in the developing cerebral cortex is composed of the radial glial cells, which generate glial cells and neurons. The mechanisms that modulate the maintenance of the radial glia (RG) stem cell phenotype, or its differentiation, are not yet completely understood. We previously demonstrated that the transforming growth factor-β1 (TGF-β1) promotes RG differentiation into astrocytes in vitro (Glia 2007; 55:1023-33) through activation of multiple canonical and non-canonical signaling pathways (Dev Neurosci 2012; 34:68-81). However, it remains unknown if TGF-β1 acts in RG-astrocyte differentiation in vivo. Here, we addressed the astrogliogenesis induced by TGF-β1 by using the intraventricular in utero injection in vivo approach. We show that injection of TGF-β1 in the lateral ventricles of E14,5 mice embryos resulted in RG fibers disorganization and premature gliogenesis, evidenced by appearance of GFAP positive cells in the cortical wall. These events were followed by decreased numbers of neurons in the cortical plate (CP). Together, we also described that TGF-β1 actions are region-dependent, once RG cells from dorsal region of the cerebral cortex demonstrated to be more responsive to this cytokine compared with RG from lateral cortex either in vitro as well as in vivo. Our work demonstrated that TGF-β1 is a critical cytokine that regulates RG fate decision and differentiation into astrocytes in vitro and in vivo. We also suggest that RG cells are heterogeneous population that acts as distinct targets of TGF-β1 during cerebral cortex development.

Postnatal developmental profile of neurons and glia in motor nuclei of the brainstem and spinal cord, and its comparison with organotypic slice cultures

In vitro preparations of the neonatal rat spinal cord or brainstem are useful to investigate the organization of motor networks and their dysfunction in neurological disease models. Long-term spinal cord organotypic cultures can extend our understanding of such pathophysiological processes over longer times. It is, however, surprising that detailed descriptions of the type (and number) of neurons and glia in such preparations are currently unavailable to evaluate cell-selectivity of experimental damage. The focus of the present immunohistochemical study is the novel characterization of the cell population in the lumbar locomotor region of the rat spinal cord and in the brainstem motor nucleus hypoglossus at 0-4 postnatal days, and its comparison with spinal organotypic cultures at 2-22 days in vitro. In the nucleus hypoglossus, neurons were 40% of all cells and 80% of these were motoneurons. Astrocytes (35% of total cells) were the main glial cells, while microglia was <10%. In the spinal gray matter, the highest neuronal density was in the dorsal horn (>80%) and the lowest in the ventral horn (≤57%) with inverse astroglia numbers and few microglia. The number of neurons (including motoneurons) and astrocytes was stable after birth. Like in the spinal cord, motoneurons in organotypic spinal culture were <10% of ventral horn cells, with neurons <40%, and the rest made up by glia. The present report indicates a comparable degree of neuronal and glial maturation in brainstem and spinal motor nuclei, and that this condition is also observed in 3-week-old organotypic cultures.

Myelinated, synapsing cultures of murine spinal cord--validation as an in vitro model of the central nervous system

Research in central nervous system (CNS) biology and pathology requires in vitro models, which, to recapitulate the CNS in vivo, must have extensive myelin and synapse formation under serum-free (defined) conditions. However, finding such a model has proven difficult. The technique described here produces dense cultures of myelinated axons, with abundant synapses and nodes of Ranvier, that are suitable for both morphological and biochemical analysis. Cellular and molecular events were easily visualised using conventional microscopy. Ultrastructurally, myelin sheaths were of the appropriate thickness relative to axonal diameter (G-ratio). Production of myelinated axons in these cultures was consistent and repeatable, as shown by statistical analysis of multiple experimental repeats. Myelinated axons were so abundant that from one litter of embryonic mice, myelin was produced in amounts sufficient for bulk biochemical analysis. This culture method was assessed for its ability to generate an in vitro model of the CNS that could be used for both neurobiological and neuropathological research. Myelin protein kinetics were investigated using a myelin fraction isolated from the cultures. This fraction was found to be superior, quantitatively and qualitatively, to the fraction recovered from standard cultures of dissociated oligodendrocytes, or from brain slices. The model was also used to investigate the roles of specific molecules in the pathogenesis of inflammatory CNS diseases. Using the defined conditions offered by this culture system, dose-specific, inhibitory effects of inflammatory cytokines on myelin formation were demonstrated, unequivocally. The method is technically quick, easy and reliable, and should have wide application to CNS research.

Murine spinal cord explants: A model for evaluating axonal growth and myelination in vitro

In vitro models of myelinating central nervous system axons have mainly been of two types, organotypic or dissociated. In organotypic cultures, the tissue fragment is thick and usually requires sectioning (physically or optically) before visual examination. In dissociated cultures, tissue is dispersed across the culture surface, making it difficult to measure the extent of myelinated fiber growth. We aimed to develop a method of culturing myelinated CNS fibers in defined medium that could be 1) studied by standard immunofluorescence microscopy (i.e., monolayer type culture), 2) used to measure axonal growth, and 3) used to evaluate the effect of substrate and media components on axonal growth and myelination. We used 120-micro m slices of embryonic murine spinal cord as a focal source of CNS tissue from which myelinated axons could extend in a virtual monolayer. Explants were cultured on both poly-L-lysine and astrocytes. The latter were used because they are the scaffold on which axonal growth and myelination occurs during normal development. Outgrowth from the explant and myelination of axons was poor on poly-L-lysine but was promoted by an astrocyte bed layer. The best myelin formation occurred in defined media based on DMEM using N2 mix; it was not promoted by Sato mix or Neurobasal medium with B27 supplement. Neuronal survival was poor in serum-containing medium. This tissue culture model should facilitate the study of factors involved in promoting outgrowth of CNS axons and their myelination. As such it is relevant to studies on myelination and spinal cord repair.

Differentiation of glial cells and motor neurons during the formation of neuromuscular junctions in cocultures of rat spinal cord explant and human muscle

Motor axons extending from embryonic rat spinal cord explants form fully mature neuromuscular junctions with cocultured human muscle. This degree of maturation is not observed in muscle innervated by dissociated motor neurons. Glial cells present in the spinal cord explants seem to be, besides remaining interneurons, the major difference between the two culture systems. In light of this observation and the well documented role of glia in neuronal development, it can be hypothesized that differentiated and long-lived neuromuscular junctions form in vitro only if their formation is accompanied by codifferentiation of neuronal and glial cells and if this codifferentiation follows the spatial and temporal pattern observed in vivo. Investigation of this hypothesis necessitates the characterization of neuronal and glial cell development in spinal cord explant-muscle cocultures. No such study has been reported, although these cocultures have been used in numerous studies of neuromuscular junction formation. The aim of this work was therefore to investigate the temporal relationship between neuromuscular junction formation and the differentiation of neuronal and glial cells during the first 3 weeks of coculture, when formation and development of the neuromuscular junction occurs in vitro. The expression of stage-specific markers of neuronal and glial differentiation in these cocultures was characterized by immunocytochemical and biochemical analyses. Differentiation of astrocytes, Schwann cells, and oligodendrocytes proceeded in concert with the differentiation of motor neurons and neuromuscular junction formation. The temporal coincidence between maturation of the neuromuscular junction and lineage progression of neurons and glial cells was similar to that observed in vivo. These findings support the hypothesis that glial cells are a major contributor to maturity of the neuromuscular junction formed in vitro in spinal cord explant-muscle cocultures.

Control of myelination, axonal growth, and synapse formation in spinal cord explants by ion channels and electrical activity

The involvement of axonal electrical activity and ion channels as mediators of neuron-glial communication during myelin formation has been tested in explant culture. Transverse slices of embryonic mouse spinal cord were maintained under conditions normally leading to extensive myelination. Axonal conduction was measured optically through the use of a voltage-sensitive dye. Glial development was at a very early stage at the time of plating, and oligodendrocyte precursor cells had not yet appeared. Spontaneous electrical activity was blocked either by tetrodotoxin or by elevation of external K+ concentrations. Myelin development was unaffected by tetrodotoxin and was also present, though quantitatively reduced, in elevated K+. Tetraethylammonium ion (TEA+), a blocker of many K+ channels, almost entirely eliminated myelination at a concentration of 1 mM, but axonal growth and conduction were unaffected. Synapse formation was followed both morphologically and functionally, and was altered neither by conduction block nor by 1 mM TEA+. It is concluded that in the spinal cord oligodendrocyte development and myelination can proceed in the absence of axonal action potentials, but ion channels, possibly in glial membranes, play an important role in these events.

Development of spinal cord in the isolated CNS of a neonatal mammal (the opossum Monodelphis domestica) maintained in longterm culture

The CNS of the newly born opossum removed in its entirety survives and maintains its electrical excitability in suitable culture media for up to ten days at 25 degrees C. The structure of the developing neonatal spinal cord has been studied in the intact animal and in the cultured CNS. The differentiation and survival of individual cells and subcellular structures were followed at the light and electron microscopic level. The expression of cell markers in neuronal and glial cells was studied immunocytochemically using commercially available antibodies. Both mono- and polyclonal antibodies raised against antigens from several other species cross-reacted with Monodelphis antigens. The spinal cord of preparations removed from three-day-old-animals showed many neuron specific enolase-positive large neurons in the ventral horn as well as vimentin- and glial fibrillary acidic protein-positive radial glial cells and numerous small diameter unmyelinated axons, abundant dendrites and synaptic structures. From post natal day 5 to post natal day 8 continued differentiation of neurons and differentiation of radial glial cells into astrocytes were apparent. Radial glial fibres and astrocytes reacted positively to antibodies against glial fibrillary acidic protein. Myelin had not appeared at 8 days. A comparison of material obtained from postnatal day 3-postnatal day 4 preparations fixed immediately after dissection and from postnatal day 3-postnatal day 4 preparations fixed after 5 days in culture showed growth with continued mitotic activity of the neuroepithelial cells and further neuronal and glial maturation in the spinal cord especially in the more rostral end. In successful experiments in vitro, the preservation of individual cells, organelles, membranes and synapses was similar in the freshly dissected and cultured preparations apart from a distinct loss of the youngest and some of the oldest neurons in the spinal cord. Also the main fibre tracts (dorsal, lateral and ventromedial funiculus) survived. Virtually all preparations that had not been damaged or injured showed these results. Possible reasons for the death or survival of individual neuronal or glial cell populations in these preparations are discussed.

Perineuronal satellitosis in the human hippocampal formation

A previously unreported example of perineuronal satellitosis in the medial CA1 and adjacent subiculum in the human hippocampal formation is described. This phenomenon is characterized by a clustering of glial cells in relation to the perikarya of a subpopulation of neurons in the deep pyramidal layer and around most neurons scattered in the stratum oriens and subcortical white matter. Most of the perineuronal satellite glia were identified as oligodendrocytes based on their nuclear chromatin patterns and antigenic properties. Satellite oligodendrocytes were mostly of the medium dense variety. A type of satellite glia with nuclear features of the dark oligodendrocyte could not be identified unequivocally using the antigenic criteria employed in this study.

A clonal analysis of glial lineages in neonatal forebrain development in vitro

Retrovirus-mediated gene transfer combined with triple immunostaining for astro- and oligodendroglial markers (antibodies to glial fibrillary acidic protein, GD3 ganglioside, and galactocerebroside, and the O4 antibody) was used to study clonal aspects of glial lineage in primary cultures of the neonatal rat striatum. We found two major clonal populations: astrocyte clones containing GFAP+, but GD3-, O4-, and GC- cells, and oligodendrocyte clones containing cells expressing various combinations of GD3, O4, and GC, with rare GFAP+ cells. These results indicate that astrocytes and oligodendrocytes belong to separate lineages in forebrain postnatal development.

A rapid procedure for the preparation of oligodendrocyte-enriched cultures from rat spinal cord

Spinal cords and cerebra from 7-day-old rat pups were compared as tissue sources for the isolation of oligodendrocytes and for studies on the development of these cells in culture. After 1 day in culture the serum-containing medium was replaced by a chemically-defined medium, which contained a cocktail of hormones that stimulated oligodendrocyte development. The cultures were characterized with various immunocytochemical markers; monoclonal A2B5 for bipotential glial progenitor cells, anti-galactocerebroside (GC) serum for oligodendrocytes, and anti-glial fibrillary acidic protein (GFAP) serum for astrocytes. The number of positive cells was counted and expressed as a percentage of total cells. At 1 day in culture the cell cultures from spinal cord contained 30% GC+ cells, increasing to 90% after 7 days in culture. In cultures derived from cerebra the percentage of GC+ cells was always lower than in cultures from spinal cord. In cerebral cultures GFAP+ cells increased from 15% at 1 day in culture to 30% at 7 days in culture, whereas it remained low in spinal cord cultures. The activity of oligodendroglial marker enzyme 2',3'-cyclic-nucleotide 3'-phosphodiesterase was followed during development in culture. The specific activity increased rapidly in both types of culture but was more than threefold higher in cultures derived from spinal cord. This procedure yields, within one week and without subculture, primary glial cultures from rat spinal cord, that are highly enriched in oligodendrocytes (greater than or equal to 90%; 3.10(5) oligodendrocytes per rat pup).

In vitro experiments on neuronal and glial cell lineages among the ventricular cells of the mouse neural plate

The proliferative ventricular cells of the early neural plate of the mouse are generally assumed to be pluripotent and equivalent to one another in their developmental capability. Ventricular cells from the rostral parts of the neural plates of mice (Theiler stages 11 and 12, embryonic days 71/2 and 8) were studied in tissue culture with respect to their potential to give rise to neurons or glial cells, or both. Autoradiographic and immunohistochemical analyses showed that ventricular cells developing into neuronal phenotypes stopped proliferating immediately upon transfer to cell culture. Using polyclonal anti-GFAP antibodies, a small proportion of immunoreactive cells could be detected after 4 days of culture. These cells retained their proliferative activity, displayed morphological characteristics of radial glial cells, and may have either developed from specific glial progenitor cells or have been induced to proceed along the glial differentiation pathway at the beginning of culture. Therefore, two distinct types of progenitor cells, committed either to neuronal or glial lineages, appear to co-exist among the cultured neural plate ventricular cells.

Demyelination in canine distemper encephalomyelitis: an ultrastructural analysis

A morphological study of selected white matter lesions was carried out in three dogs with canine distemper encephalomyelitis. Two dogs had experimental infections while the third was a spontaneous case. Two stages were identified in the process of demyelination. The earliest evidence of myelin injury was a ballooning change in myelin sheaths involving single or multiple axons. This was followed by a progressive stripping of compact sheaths by the cytoplasmic fingers of phagocytic cells which infiltrated and removed myelin lamellae. Some axonal necrosis also accompanied these changes. Where demyelination occurred, canine distemper viral nucleocapsids were found in astrocytes, macrophages, ependymal cells and infiltrating lymphocytes. In contrast, oligodendrocytes were conspicuous by their apparent lack of infection. Thus it seems that myelin loss cannot be ascribed to oligodendrocyte infection. Perturbed astrocyte function following canine distemper viral infection may cause oedema of myelin sheaths, leading to ballooning and primary demyelination. Cells which phagocytosed myelin were mainly identified as microglial cells with lesser involvement by astrocytes. Rarely, oligodendrocytes also acted as macrophages. Myelin debris was engulfed in bulk or as small droplets into coated pits. Remyelination was present in established plaques although not in great abundance, perhaps due to the diminished oligodendrocyte numbers and a relative increase in immature forms of these cells. These observations are compared to similar changes observed in other demyelinating diseases of animals and man.

Gliogenesis in organotypic tissue culture of the spinal cord of the embryonic mouse. II. Autoradiographic studies

Organotypic cultures of the spinal cord of the embryonic mouse were subjected to pulses of tritiated thymidine at various times between explanation and 42 days in vitro (DIV). Autoradiography was performed both on cultures fixed immediately at the end of the pulse and on cultures maintained in radioactive-free medium for various periods after the pulse. Quantitative light autoradiographic studies showed a single peak of glial cell proliferation at 9 DIV equivalent to that demonstrated in vivo. The growth rate of glial cells (related to time in culture) decreased along an exponential decay type curve. All these observations were statistically significant when tested against the corresponding null hypothesis. Ultrastructural autoradiography shows that at early stages of the culture, radial glial cells and immature glial cells divided and eventually gave rise to astrocytes and oligodendrocytes. During the period of maximal cell proliferation, tritiated thymidine was incorporated by differentiated astrocytes and ultrastructurally recognizable immature oligodendrocytes. Oligodendrocytes did not divide beyond the stage of active oligodendrocytes (the cells initiating myelination). They were capable of producing dark oligodendrocytes within a week following the last division. These observations emphasize the similarity of the proliferation during development in organotypic culture to that in vivo, modified by the trauma of explantation and the culture conditions.