Membrane currents and morphological properties of neurons and glial cells in the spinal cord and filum terminale of the frog

Rapid Serum-Free Isolation of Oligodendrocyte Progenitor Cells from Adult Rat Spinal Cord

Oligodendrocyte progenitor cells (OPCs) play a pivotal role in both health and disease within the central nervous system, with oligodendrocytes, arising from resident OPCs, being the main myelinating cell type. Disruption in OPC numbers can lead to various deleterious health defects. Numerous studies have described techniques for isolating OPCs to obtain a better understanding of this cell type and to open doors for potential treatments of injury and disease. However, the techniques used in the majority of these studies involve several steps and are time consuming, with current culture protocols using serum and embryonic or postnatal cortical tissue as a source of isolation. We present a primary culture method for the direct isolation of functional adult rat OPCs, identified by neuron-glial antigen 2 (NG2) and platelet derived growth factor receptor alpha (PDGFrα) expression, which can be obtained from the adult spinal cord. Our method uses a simple serum-free cocktail of 3 growth factors - FGF₂, PDGF_AA, and IGF-I, to expand adult rat OPCs in vitro to 96% purity. Cultured cells can be expanded for at least 10 passages with very little manipulation and without losing their phenotypic progenitor cell properties, as shown by immunocytochemistry and RT-PCR. Cultured adult rat OPCs also maintain their ability to differentiate into GalC positive cells when incubated with factors known to stimulate their differentiation. This new isolation method provides a new source of easily accessible adult stem cells and a powerful tool for their expansion in vitro for studies aimed at central nervous system repair.

The Cellular Composition and Glia-Neuron Ratio in the Spinal Cord of a Human and a Nonhuman Primate: Comparison With Other Species and Brain Regions

The cellular composition of brains shows largely conserved, gradual evolutionary trends between species. In the primate spinal cord, however, the glia-neuron ratio was reported to be greatly increased over that in the rodent spinal cord. Here, we re-examined the cellular composition of the spinal cord of one human and one nonhuman primate species by employing two different counting methods, the isotropic fractionator and stereology. We also determined whether segmental differences in cellular composition, possibly reflecting increased fine motor control of the upper extremities, may explain a sharply increased glia-neuron ratio in primates. In the cynomolgus monkey spinal cord, the isotropic fractionator and stereology yielded 206-275 million cells, of which 13.3-25.1% were neurons (28-69 million). Stereological estimates yielded 21.1% endothelial cells and 65.5% glial cells (glia-neuron ratio of 4.9-5.6). In human spinal cords, the isotropic fractionator and stereology generated estimates of 1.5-1.7 billion cells and 197-222 million neurons (13.4% neurons, 12.2% endothelial cells, 74.8% glial cells), and a glia-neuron ratio of 5.6-7.1, with estimates of neuron numbers in the human spinal cord based on morphological criteria. The non-neuronal to neuron ratios in human and cynomolgus monkey spinal cords were 6.5 and 3.2, respectively, suggesting that previous reports overestimated this ratio. We did not find significant segmental differences in the cellular composition between cervical, thoracic and lumbar levels. When compared with brain regions, the spinal cord showed gradual increases of the glia-neuron ratio with increasing brain mass, similar to the cerebral cortex and the brainstem. Anat Rec, 301:697-710, 2018. © 2017 Wiley Periodicals, Inc.

Characterization of the Filum terminale as a neural progenitor cell niche in both rats and humans

Neural stem cells (NSCs) reside in a unique microenvironment within the central nervous system (CNS) called the NSC niche. Although they are relatively rare, niches have been previously characterized in both the brain and spinal cord of adult animals. Recently, another potential NSC niche has been identified in the filum terminale (FT), which is a thin band of tissue at the caudal end of the spinal cord. While previous studies have demonstrated that NSCs can be isolated from the FT, the in vivo architecture of this tissue and its relation to other NSC niches in the CNS has not yet been established. In this article we report a histological analysis of the FT NSC niche in postnatal rats and humans. Immunohistochemical characterization reveals that the FT is mitotically active and its cells express similar markers to those in other CNS niches. In addition, the organization of the FT most closely resembles that of the adult spinal cord niche. J. Comp. Neurol. 525:661–675, 2017. © 2016 Wiley Periodicals, Inc.

The shortened spinal cord in tetraodontiform fishes

In teleosts, the spinal cord generally extends along the entire vertebral canal. The Tetraodontiformes, in which the spinal cord is greatly reduced in length with a distinct long filum terminale and cauda equina, have been regarded as an aberration. The aims of this study are: 1) to elucidate whether the spinal cord in all tetraodontiform fishes shorten with the filum terminale, and 2) to describe the gross anatomical and histological differences in the spinal cord among all families of the Tetraodontiformes. Representative species from all families of the Tetraodontiformes, and for comparison the carp as a common teleost, were investigated. In the Triacanthodidae, Triacanthidae, and Triodontidae, which are the more ancestral taxa of the Tetraodontiformes, the spinal cord extends through the entire vertebral canal. In the Triacanthidae and Triodontidae, the caudal half or more spinal segments of the spinal cord, however, lack gray matter and consist largely of nerve fibers. In the other tetraodontiform families, the spinal cord is shortened forming a filum terminale with the cauda equina, which is prolonged as far as the last vertebra. The shortened spinal cord is divided into three groups. In the Ostraciidae and Molidae, the spinal cord tapers abruptly at the cranium or first vertebra forming a cord-like filum terminale. In the Monacanthidae, Tetraodontidae, and Diodontidae, it abruptly flattens at the rostral vertebrae forming a flat filum terminale. The spinal cord is relatively longer in the Monacanthidae than that in the other two families. It is suggested by histological features of the flat filum terminale that shortening of the spinal cord in this group progresses in order of the Monacanthidae, Tetraodontidae, and Diodontidae. In the Balistidae and Aracanidae, the cord is relatively long and then gradually decreased in dorso-ventral thickness.

Gross and microscopic study of the filum terminale: does the filum contain functional neural elements?

OBJECT

The filum terminale (FT) is considered a fibrous structure that extends from conus medullaris of the spinal cord to coccyx. Based on previous studies and from their own experience with intraoperative electrophysiological monitoring of the sacral nervous system, the authors postulate that the FT contains functional neural elements in some individuals.

METHODS

The FT was dissected from 13 fresh stillborn cadavers (7 male, 6 female; mean gestational age 36 weeks and 1 day). The gross anatomical features were recorded, and connections between the FT and the nerve roots of the cauda equina were noted. These connections, when present, were sectioned for histological studies. The fila (both interna and externa) were also sectioned for histological and immunohistochemical studies. In addition, FT specimens were obtained from 5 patients undergoing sectioning of the FT in an untethering surgical procedure.

RESULTS

There were 5 gross connections between the FT and nerve roots demonstrating nerve fibers that were positive for S100. The FT showed islands of cells that were positive for GFAP in 10 cases, synaptophysin in 3 cases, S100 in 11 cases, and nestin in 2 cases. The nerve fibers in the FT were myelinated in 2 cases. The conus ended at the L-1 or L-2 vertebral level in all 13 specimens. The dural sac terminated at the S-2 vertebral level in most of the specimens. The 5 FT specimens that were obtained from patients revealed nerve bundles that were positive for S100 in 4 cases and cells that were positive for GFAP in 3 cases.

CONCLUSIONS

There are gross anatomical connections between the FT and nerve roots that contain nerve fibers. Apart from fibrous stroma, the FT may contain nerve bundles and cells that stain positive for GFAP, synaptophysin, S100, and nestin. These microscopic findings and previous intraoperative electrophysiological studies suggest a probable functional role for the FT in some individuals. At birth, the conus ends at a higher vertebral level (lower L-1 or upper L-2) than L-3.

Isolation of human multipotent neural progenitors from adult filum terminale

Stem cells have been isolated from several CNS regions, including the spinal cord. However, the terminal end of the spinal cord, filum terminale, has been referred to as a fibrovascular tag without neurogenic potential and of no clinical significance. Recently, we were fortunate to acquire some samples of this tissue. We show for the first time that progenitor cells exhibiting the hallmarks of stem cells can be isolated from adult human filum terminale (FTNPs). More specifically, FTNPs self-renew and proliferate to form neurospheres, and exhibit tripotent differentiation into neurons, astrocytes, and oligodendrocytes. Equally important, FTNPs develop the electrophysiological profile of neurons and glia. Whole-cell patch-clamp recordings show beta-III-tubulin(+) neurons exhibiting overshooting action potentials, displaying both the fast inactivating TTX-sensitive sodium current as well as 4-AP and TEA sensitive potassium currents. To assess potency in vivo, FTNPs were transplanted into the posterior periventricular region of control or ischemic rat brains. Despite a vigorous immune response against the xenograft, FTNPs survived and were found not only in the graft area but had also migrated to the lesioned CA1 region. Notwithstanding the immune response, FTNPs differentiated into astrocytes, but no neuronal differentiation was observed in the transplant milieu tested. However, neuronal differentiation in vivo cannot be ruled out and assessment of the conditions necessary to promote neurogenesis in vivo requires more research. Significantly, no tumor formation or aberrant cell morphology was seen in or adjacent to the graft area. Thus, filum terminale provides a novel source of adult human neural progenitor cells that develop into functional neurons with possible clinical applications.

Intraoperative mapping of sacral nervous system (S2-4)

Electrophysiological mapping of the sacral nervous system was used during operations on 80 patients with conus and cauda equina lesions. At surgery, under controlled muscle relaxation, the sacral neural elements (S2-4) were mapped using direct mono-polar stimulation and recording of compound muscle action potentials (CMAPs) from the external anal sphincter (EAS). Responses were obtained in 86.25% (69/80) of the patients. In 33 (82.5%) out of 40 patients with preoperative deficits involving the S2-4 segments, CMAPs could be elicited. Identification of nerve roots was useful in dissection of lipomyelomeningocoeles, tumour excisions and untethering of filum terminale. In three patients, stimulation of the filum terminale elicited motor responses and, hence, it was not sectioned. Intraoperative mapping of the S2-4 nerve roots under controlled muscle relaxation is feasible in a majority of patients, including those with deficits involving S2-4. This method was useful in sparing viable nerve roots during surgery in conus and cauda equina regions, and identification of 'functional' filum terminale.

Cellular and spatial changes in the anuran superior olive across metamorphosis

In many vertebrate species, the superior olive in the auditory brainstem plays an essential role in sound source localization. Little is known, however, about the structural and functional changes in this nucleus during development when alterations in head size and shape as well as in inner ear projections are expected to affect the perception of binaural cues. Using stereological techniques, we investigated the changes in several cellular and spatial features of the bullfrog superior olive across metamorphosis, the time period during which the animal transforms from a totally aquatic larva to a semiterrestrial adult. The total number of cells shows a strongly linear increase from hatchling through late larval stages. The number of neurons decreases during metamorphic climax stages, and recovers to pre-metamorphic climax levels in the early post-metamorphic froglet stage. The number of glial cells increases during the early larval period, and remains relatively stable, with no systematic variation, from late larval to froglet stages. The volume of the superior olive increases rapidly in early larval stages, followed by a much-attenuated rate of growth between late larval and froglet stages. These morphological changes may provide a substrate for the functional restructuring of the bullfrog superior olive, shortly before the switch from aquatic to mostly atmospheric hearing.

Analysis of K+ accumulation reveals privileged extracellular region in the vicinity of glial cells in situ

Astrocytes and oligodendrocytes in rat and mouse spinal cord slices, characterized by passive membrane currents during de- and hyperpolarizing stimulation pulses, express a high resting K+ conductance. In contrast to the case for astrocytes, a depolarizing prepulse in oligodendrocytes produces a significant shift of reversal potential (Vrev) to positive values, arising from the larger accumulation of K+ in the vicinity of the oligodendrocyte membrane. As a result, oligodendrocytes express large tail currents (Itail) after a depolarizing prepulse due to the shift of K+ into the cell. In the present study, we used a mathematical model to calculate the volume of the extracellular space (ECS) in the vicinity of astrocytes and oligodendrocytes (ESVv), defined as the volume available for K+ accumulation during membrane depolarization. A mathematical analysis of membrane currents revealed no differences between glial cells from mouse (n = 59) or rat (n = 60) spinal cord slices. We found that the Vrev of a cell after a depolarizing pulse increases with increasing Itail, expressed as the ratio of the integral inward current (Qin) after the depolarizing pulse to the total integral outward current (Qout) during the pulse. In astrocytes with small Itail and Vrev ranging from -50 to -70 mV, the Qin was only 3-19% of Qout, whereas, in oligodendrocytes with large Itail and Vrev between -20 and 0 mV, Qin/Qout was 30-75%. On the other hand, ESVv decreased with increasing values of Vrev. In astrocytes, ESVv ranged from 2 to 50 microm3, and, in oligodendrocytes, it ranged from 0.1 to 2.0 microm3. Cell swelling evoked by the application of hypotonic solution shifted Vrev to more positive values by 17.2 +/- 1.8 mV and was accompanied by a decrease in ESVv of 3.6 +/- 1.3 microm3. Our mathematical analysis reveals a 10-100 times smaller region of the extracellular space available for K+ accumulation during cell depolarization in the vicinity of oligodendrocytes than in the vicinity of astrocytes. The presence of such privileged regions around cells in the CNS may affect the accumulation and diffusion of other neuroactive substances and alter communication between cells in the CNS.

The caudal end of the rat spinal cord: transformation to and ultrastructure of the filum terminale

Contrary to the current belief, the spinal cord of the rat does not terminate with the conus terminalis (CT), but its basic components (central canal, gray matter, white matter) continue in the filum terminale (FT). Proceeding caudally in the conus terminalis, first the motoneuron cell column discontinues in the ventral horn. More caudally the dorsal horns separate from the intermediate zone, and discontinue. The ensuing filum terminale consists of the slit-like central canal lined by ciliated ependymal cells, the periventricular gray matter and the peripheral white matter. Uniform small size neurons and glial cells populate the gray matter. Ultrastructural analysis revealed various types of axodendritic and axosomatic synapses as well as fine unmyelinated axons. The white matter consists mainly of myelinated nerve fibers. The neuronal components of the filum terminale, if they occur also in the human spinal cord, should be involved in the diagnosis and treatment of various diseases, e.g. tethered spinal cord syndrome, vascular malformations and disraphysm.

Microglia activation influences dye coupling and Cx43 expression of the astrocytic network

Under inflammatory conditions, activated microglia are capable of producing proinflammatory cytokines that are reported to influence cell-to-cell communication. The present study was performed to evaluate the influence of microglial activation on the coupling efficiency of the astroglial network. Primary astrocyte cultures of newborn rats were cocultured with either 5% (M5) or 30% (M30) microglia. Microglial activation (rounded phagocytotic phenotype) was investigated using the monoclonal anti-ED1 antibody, and immunofluorescence with a polyclonal anti-Cx43 antibody was used to study astroglial Cx43 expression and distribution. Functional coupling of astrocytes was evaluated by monitoring the transfer of microinjected Lucifer yellow into neighboring cells. The data obtained can be summarized as follows: astroglia/M30 cocultures contained significantly fewer resting microglia and significantly more activated microglia than the M5 cocultures; significantly reduced astroglial Cx43 staining was found in M30 cocultures concurrently with a reduced number of dye coupled astrocytes; and the positive correlation of percent activated microglia with reduced astroglial Cx43 expression was highly significant, indicating that the degree of intercellular communication in the astroglial network may be modulated by the activation of microglia under in vitro conditions.

Differential expression of nerve growth factor transcripts in glia and neurons and their regulation by transforming growth factor-beta1

Nerve growth factor (NGF) influences neuronal development, function, and response to injury. Using reverse transcriptase polymerase chain reaction, we find that mouse and rat cortex and spinal cord, and both neurons and glia in culture, express NGF mRNA. In the mouse, NGF is regulated by at least two promoters that govern synthesis of four different transcripts, A through D, that are all expressed in the mouse tissues and cells examined. In contrast, rat NGF expression varies with tissue and with cell type: transcript C is expressed strongly in brain but weakly in spinal cord, and transcript D is undetectable in rat central nervous system (CNS). In addition to species- and tissue-specific expression, NGF transcripts also exhibit cell type-specific expression: transcripts B, C and D are expressed in rat astrocytes but poorly or not at all in rat neurons, identifying glia as an important source of NGF in rat. NGF increases sharply after injury. TGF-beta1, which also increases immediately after injury, induces NGF mRNA and protein in rat and mouse glia but not in neurons. Furthermore, transcripts A, B and D, but not C, are upregulated by TGF-beta1 in mouse glia, whereas in rat glia, the major responsive transcript is C. Thus, there may be multiple TGF-beta1-responsive elements in the NGF promoters located upstream of exons 1 and 3 that may differ between mouse and rat. Moreover, NGF transcripts are differentially expressed in a species-, cell type-, and inducer-specific manner. These results have implications for the use of mice versus rats as models for the study of NGF regulation following CNS injury.

Effect of elevated K(+), hypotonic stress, and cortical spreading depression on astrocyte swelling in GFAP-deficient mice

Glial fibrillary acidic protein (GFAP) is the main component of intermediate filaments in astrocytes. To assess its function in astrocyte swelling, we compared astrocyte membrane properties and swelling in spinal cord slices of 8- to 10-day-old wild-type control (GFAP(+/+)) and GFAP-knockout (GFAP(-/-)) mice. Membrane currents and K(+) accumulation around astrocytes after a depolarizing pulse were studied using the whole-cell patch-clamp technique. In vivo cell swelling was studied in the cortex during spreading depression (SD) in 3 to 6-month-old animals. Swelling-induced changes of the extracellular space (ECS) diffusion parameters, i.e., volume fraction alpha and tortuosity lambda, were studied by the real-time iontophoretic tetramethylammonium (TMA(+)) method using TMA(+)-selective microelectrodes. Morphological analysis using confocal microscopy and quantification of xy intensity profiles in a confocal plane revealed a lower density of processes in GFAP(-/-) astrocytes than in GFAP(+/+) astrocytes. K(+) accumulation evoked by membrane depolarization was lower in the vicinity of GFAP(-/-) astrocytes than GFAP(+/+) astrocytes, suggesting the presence of a larger ECS around GFAP(-/-) astrocytes. Astrocyte swelling evoked by application of 50 mM K(+) or by hypotonic solution (HS) produced a larger increase in [K(+)](e) around GFAP(+/+) astrocytes than around GFAP(-/-) astrocytes. No differences in alpha and lambda in the spinal cord or cortex of GFAP(+/+) and GFAP(-/-) mice were found; however, the application of either 50 mM K(+) or HS in spinal cord, or SD in cortex, evoked a large decrease in alpha and an increase in lambda in GFAP(+/+) mice only. Slower swelling in GFAP(-/-) astrocytes indicates that GFAP and intermediate filaments play an important role in cell swelling during pathological states.