The sacral autonomic outflow is sympathetic.

A kinship between cranial and pelvic visceral nerves of vertebrates has been accepted for a century. Accordingly, sacral preganglionic neurons are considered parasympathetic, as are their targets in the pelvic ganglia that prominently control rectal, bladder, and genital functions. Here, we uncover 15 phenotypic and ontogenetic features that distinguish pre- and postganglionic neurons of the cranial parasympathetic outflow from those of the thoracolumbar sympathetic outflow in mice. By every single one, the sacral outflow is indistinguishable from the thoracolumbar outflow. Thus, the parasympathetic nervous system receives input from cranial nerves exclusively and the sympathetic nervous system from spinal nerves, thoracic to sacral inclusively. This simplified, bipartite architecture offers a new framework to understand pelvic neurophysiology as well as development and evolution of the autonomic nervous system.

Oligodendrocyte progenitors: adult stem cells of the central nervous system?

Oligodendrocyte progenitors (OPs) are a major proliferating cell population within the adult CNS. In response to myelin loss or increasing demand, OPs have the capacity to differentiate into mature, myelinating oligodendrocytes. The name 'oligodendrocyte progenitor' suggests restriction to the oligodendrocyte cell lineage. However, with growing evidence of the lineage plasticity of OPs both in vitro and in vivo, we discuss whether they have potential beyond that expected of dedicated progenitor cells, and hence may justify categorization as adult stem cells.

Ventral neurogenesis and the neuron-glial switch.

In the developing spinal cord, neuroepithelial precursors at different positions along the dorsal-ventral axis generate distinct neuronal and glial subtypes. For example, one group of ventral precursors generates neurons followed by oligodendrocytes. A spate of recent articles, including several in this issue of Neuron, are devoted to the mechanisms governing neuronal and glial subtype specification in the ventral cord. We review these studies and discuss the nature of the ventral neuron-oligodendrocyte switch.

Hedgehog-dependent oligodendrocyte lineage specification in the telencephalon.

In the caudal neural tube, oligodendrocyte progenitors (OLPs) originate in the ventral neuroepithelium under the influence of Sonic hedgehog (SHH), then migrate throughout the spinal cord and brainstem before differentiating into myelin-forming cells. We present evidence that oligodendrogenesis in the anterior neural tube follows a similar pattern. We show that OLPs in the embryonic mouse forebrain express platelet-derived growth factor alpha-receptors (PDGFRA), as they do in more caudal regions. They first appear within a region of anterior hypothalamic neuroepithelium that co-expresses mRNA encoding SHH, its receptor PTC1 (PTCH) and the transcription factors OLIG1, OLIG2 and SOX10. Pdgfra-positive progenitors later spread through the forebrain into areas where Shh is not expressed, including the cerebral cortex. Cyclopamine inhibited OLP development in cultures of mouse basal forebrain, suggesting that hedgehog (HH) signalling is obligatory for oligodendrogenesis in the ventral telencephalon. Moreover, Pdgfra-positive progenitors did not appear on schedule in the ventral forebrains of Nkx2.1 null mice, which lack the telencephalic domain of Shh expression. However, OLPs did develop in cultures of Nkx2.1(-/-) basal forebrain and this was blocked by cyclopamine. OLPs also developed in neocortical cultures, even though Shh transcripts could not be detected in the embryonic cortex. Here, too, the appearance of OLPs was suppressed by cyclopamine. In keeping with these findings, we detected mRNA encoding SHH and Indian hedgehog (IHH) in both Nkx2.1(-/-) basal forebrain cultures and neocortical cultures. Overall, the data are consistent with the idea that OLPs in the telencephalon, possibly even some of those in the cortex, develop under the influence of SHH in the ventral forebrain.

Oligodendrocyte development in the spinal cord and telencephalon: common themes and new perspectives.

There are clear parallels between oligodendrocyte development in the spinal cord and forebrain. However, there is new evidence that in both of these regions oligodendrocyte lineage development may be more complex than we earlier thought. This stems from the recent identification of three new transcription factor genes, Olig1, Olig2 and Sox10, that are expressed from the early stages of oligodendrocyte lineage development. In this article, we highlight the common themes underlying specification and early development of oligodendrocytes in the spinal cord and telencephalon. Then, we discuss recent studies of Sox10 and the Olig genes and their implications for oligodendrocyte specification. We conclude that although the mechanisms of oligodendrogenesis appear to be fundamentally similar at different rostro-caudal levels of the neuraxis, there are still many unanswered questions about the details of oligodendrocyte specification.

Control of progenitor cell number by mitogen supply and demand.

BACKGROUND: Much is known about how cell proliferation is controlled at the single cell level, but much less about the control of cell numbers in developing populations. Cell number might be determined by an intracellular division limiter or, alternatively, by the availability of mitogens or other factors outside the cell. We investigated the relative importance of intracellular and extracellular controls for one well-defined population of neural precursor cells, namely the glial progenitors that give rise to oligodendrocytes in the mouse spinal cord. RESULTS: We found by cumulative BrdU labeling in vivo that the progenitor cell division cycle slows down markedly as their numbers increase during embryogenesis. When cultured in saturating PDGF, the main mitogen for these cells, their cell cycle accelerated and was independent of their prior rate of division in vivo. This shows that mitogens are limiting in vivo, and suggests that division normally slows down because the PDGF concentration declines. In PDGF-transgenic mice, cell number was proportional to the PDGF supply and apparently unsaturable; at ten times the normal rate of supply, cell number was still increasing but the animals were no longer viable. CONCLUSIONS: Progenitor cell proliferation in the embryo is limited by environmental factors, not a cell-intrinsic mechanism. The linear relationship between PDGF supply and final cell number strongly suggests that cells deplete the mitogenic activity in their environment at a rate proportional to the total number of cells. The cells might simply consume the available PDGF or they might secrete autocrine inhibitors, or both.

Platelet-derived growth factor is constitutively secreted from neuronal cell bodies but not from axons.

Neurons synthesise and secrete many growth and survival factors but it is not usually clear whether they are released locally at the cell body or further afield from axons or axon terminals. Without this information, we cannot predict the site(s) of action or the biological functions of many neuron-derived factors. For example, can neuronal platelet-derived growth factor (PDGF) be secreted from axons and reach glial cells in nerve-fibre (white-matter) tracts? To address this question, we expressed PDGF-A in retinal ganglion neurons in transgenic mice and tested for release of PDGF from cell bodies in the retina and from axons in the optic nerve. In both the retina and optic nerve, there are glial cells that express PDGF receptor alpha (PDGFR alpha) [1] and divide in response to PDGF [2-5], so we could detect functional PDGF indirectly through the mitogenic response of glia at both locations. Expressing PDGF-A in neurons under the control of the neuron-specific enolase promoter (NSE-PDGF-A) resulted in a striking hyperplasia of retinal astrocytes, demonstrating that PDGF is secreted from the cell bodies of neurons in the retina [4]. In contrast, glial proliferation in the optic nerve was unaffected, indicating that PDGF is not released from axons. When PDGF was expressed directly in the optic nerve under the control of an astrocyte-specific promoter (GFAP-PDGF-A), oligodendrocyte progenitors hyperproliferated, resulting in a hypertrophic optic nerve. We conclude that PDGF is constitutively secreted from neuronal cell bodies in vivo, but not from axons in white-matter tracts.

A human YAC transgene rescues craniofacial and neural tube development in PDGFRalpha knockout mice and uncovers a role for PDGFRalpha in prenatal lung growth.

The platelet-derived growth factor alpha-receptor (PDGFRalpha) plays a vital role in the development of vertebrate embryos, since mice lacking PDGFRalpha die in mid-gestation. PDGFRalpha is expressed in several types of migratory progenitor cells in the embryo including cranial neural crest cells, lung smooth muscle progenitors and oligodendrocyte progenitors. To study PDGFRalpha gene regulation and function during development, we generated transgenic mice by pronuclear injection of a 380 kb yeast artificial chromosome (YAC) containing the human PDGFRalpha gene. The YAC transgene was expressed in neural crest cells, rescued the profound craniofacial abnormalities and spina bifida observed in PDGFRalpha knockout mice and prolonged survival until birth. The ultimate cause of death was respiratory failure due to a defect in lung growth, stemming from failure of the transgene to be expressed correctly in lung smooth muscle progenitors. However, the YAC transgene was expressed faithfully in oligodendrocyte progenitors, which was not previously observed with plasmid-based transgenes containing only upstream PDGFRalpha control sequences. Our data illustrate the complexity of PDGFRalpha genetic control, provide clues to the location of critical regulatory elements and reveal a requirement for PDGF signalling in prenatal lung growth, which is distinct from the known requirement in postnatal alveogenesis. In addition, we found that the YAC transgene did not prolong survival of Patch mutant mice, indicating that genetic defects outside the PDGFRalpha locus contribute to the early embryonic lethality of Patch mice.

Oligodendrocyte lineage and the motor neuron connection.

One of the more surprising recent discoveries in glial biology has been that oligodendrocytes (OLs) originate from very restricted regions of the embryonic neural tube. This was surprising because myelinating OLs are widespread in the mature central nervous system, so there was no reason to suspect that their precursors should be restricted. What we now know about early OL development suggests that they might have as much (or more) in common with ventral neurons-specifically motor neurons (MNs)-as with other types of glia. This has implications for the way we think about glial development, function, and evolution. In this article we review the evidence for a shared MN-OL lineage and debate whether this is the only lineage that generates OLs. We decide in favour of a single embryonic lineage with regional variations along the anterior-posterior neuraxis.

Defective oligodendrocyte development and severe hypomyelination in PDGF-A knockout mice.

There is a class of oligodendrocyte progenitors, called O-2A progenitors, that is characterized by expression of platelet-derived growth factor &agr;-receptors (PDGFR(&agr;)). It is not known whether all oligodendrocytes are derived from these PDGFRalpha-progenitors or whether a subset(s) of oligodendrocytes develops from a different, PDGFR alpha-negative lineage(s). We investigated the relationship between PDGF and oligodendrogenesis by examining mice that lack either PDGF-A or PDGF-B. PDGF-A null mice had many fewer PDGFR alpha-progenitors than either wild-type or PDGF-B null mice, demonstrating that proliferation of these cells relies heavily (though not exclusively) on PDGF-AA homodimers. PDGF-A-deficient mice also had reduced numbers of oligodendrocytes and a dysmyelinating phenotype (tremor). Not all parts of the central nervous system (CNS) were equally affected in the knockout. For example, there were profound reductions in the numbers of PDGFR alpha-progenitors and oligodendrocytes in the spinal cord and cerebellum, but less severe reductions of both cell types in the medulla. This correlation suggests a close link between PDGFRalpha-progenitors and oligodendrogenesis in most or all parts of the CNS. We also provide evidence that myelin proteolipid protein (PLP/DM-20)-positive cells in the late embryonic brainstem are non-dividing cells, presumably immature oligodendrocytes, and not proliferating precursors.

Pax6 influences the time and site of origin of glial precursors in the ventral neural tube.

Neuroepithelial precursors in the ventral ventricular zone (VZ) of the spinal cord generate motor neurons (MNs) and interneurons, and then a subset of precursors starts to produce oligodendrocyte progenitors (OLPs). We show that OLPs originate in the ventral-most part of the Pax6-positive VZ, which at earlier times generates somatic (Isl2/Lim3-positive) MNs. In Small eye (Pax6-deficient) mice, the origin of OLPs is shifted dorsally and both OLPs and Isl2/Lim3 MNs are delayed. We suggest that somatic MNs and OLPs are generated sequentially from a common set of MN-OL precursors whose position in the VZ is influenced by Pax6. Neuron-glia fate switching might be a preprogrammed property of these precursors or a response to feedback from newly generated neurons. OLs developed normally in explants of Isl1(-/-) spinal cords, which lack MNs, arguing against feedback control and suggesting that the neuron-glia switch is an intrinsic developmental program in a specific subset of neural precursors.

Oligodendrocyte population dynamics and the role of PDGF in vivo.

Oligodendrocyte progenitors originate near the floor plate of the spinal cord, then proliferate and migrate throughout the cord before giving rise to oligodendrocytes. Progenitor cell proliferation stops before birth because the cell cycle slows down, linked to an increase in differentiation and death. Experiments with transgenic mice show that platelet-derived growth factor (PDGF) drives progenitor cell division and suggest that slowing of and exit from the cycle reflects a decline in PDGF signaling. Overexpressing PDGF induces hyperproliferation of progenitor cells and excessive, ectopic production of oligodendrocytes. However, the superfluous oligodendrocytes die at an immature stage of differentiation, leaving a normal complement of myelin-forming cells. Therefore, cell survival controls override proliferation controls for determining the final number and distribution of mature oligodendrocytes.

Dorsal spinal cord neuroepithelium generates astrocytes but not oligodendrocytes.

There is evidence that oligodendrocytes in the spinal cord are derived from a restricted part of the ventricular zone near the floor plate. An alternative view is that oligodendrocytes are generated from all parts of the ventricular zone. We reinvestigated glial origins by constructing chick-quail chimeras in which dorsal or ventral segments of the embryonic chick neural tube were replaced with equivalent segments of quail neural tube. Ventral grafts gave rise to both oligodendrocytes and astrocytes. In contrast, dorsal grafts produced astrocytes but not oligodendrocytes. In mixed cultures of ventral and dorsal cells, only ventral cells generated oligodendrocytes, whereas both ventral and dorsal cells generated astrocytes. Therefore, oligodendrocytes are derived specifically from ventral neuroepithelium, and astrocytes from both dorsal and ventral.

Normal temporal and spatial distribution of oligodendrocyte progenitors in the myelin-deficient (md) rat.

A point mutation in exon 3 of the proteolipid protein (PLP) gene of the myelin-deficient (md) rat leads to a failure of oligodendrocyte maturation and early death of oligodendrocytes, resulting in dysmyelination. It has been suggested that an alternative-splice isoform of PLP, known as DM-20, might be expressed in oligodendrocyte progenitors in the embryonic central nervous system (CNS), raising the possibility that early development of the oligodendrocyte lineage might also be affected in the md rat. To test this suggestion, we visualized oligodendrocyte progenitors in the embryonic md rat spinal cord and brain by in situ hybridization with a probe to the platelet-derived growth factor alpha receptor (PDGFR). We could detect no abnormalities in the time of first appearance of oligodendrocyte precursors, nor in their subsequent proliferation and dispersal throughout the CNS. These data strongly suggest that the PLP mutation in the md rat primarily or exclusively affects the later stages of oligodendrocyte lineage.

Origins of spinal cord oligodendrocytes: possible developmental and evolutionary relationships with motor neurons.

Spinal cord oligodendrocytes develop from migratory glial progenitor cells that are generated by a small subset of neuroepithelial cells in the ventral part of the neural tube. Specification of these neuroepithelial oligodendrocyte precursors, in common with other ventral cells such as motor neurons, depends on morphogenetic signals from the notochord and/or floor plate. The ventrally derived signals can be mimicked in vitro by purified Sonic hedgehog (Shh) protein. Oligodendrocytes and motor neurons are induced over the same range of concentrations of Shh, consistent with the idea that Shh might specify a common precursor of motor neurons and oligodendrocytes. A lineage relationship between motor neurons and oligodendrocytes has previously been suggested by clonal analysis in the embryonic chick spinal cord. We propose a lineage diagram that connects oligodendrocytes and motor neurons and that takes into account the fact that motor neurons and oligodendrocyte precursors are generated at different times during development. Oligodendrocytes might originally have evolved to ensheath motor axons and facilitate a rapid escape response. If so, oligodendrocyte ontogeny and phylogeny might share a common basis.

PDGF mediates a neuron-astrocyte interaction in the developing retina.

Astrocytes invade the developing retina from the optic nerve head, over the axons of retinal ganglion cells (RGCs). RGCs express the platelet-derived growth factor A-chain (PDGF-A) and retinal astrocytes the PDGF alpha-receptor (PDGFR alpha), suggesting that PDGF mediates a paracrine interaction between these cells. To test this, we inhibited PDGF signaling in the eye with a neutralizing anti-PDGFR alpha antibody or a soluble extracellular fragment of PDGFR alpha. These treatments inhibited development of the astrocyte network. We also generated transgenic mice that overexpress PDGF-A in RGCs. This resulted in hyperproliferation of astrocytes, which in turn induced excessive vasculogenesis. Thus, PDGF appears to be a link in the chain of cell-cell interactions responsible for matching numbers of neurons, astrocytes, and blood vessels during retinal development.

Spinal cord oligodendrocytes develop from ventrally derived progenitor cells that express PDGF alpha-receptors.

Platelet-derived growth factor alpha-receptors (PDGFR alpha) are expressed by a subset of neuroepithelial cells in the ventral half of the embryonic day 14 (E14) rat spinal cord. The progeny of these cells subsequently proliferate and migrate into the dorsal parts of the cord after E16. Here, we show that E14 ventral cells are able to generate oligodendrocytes in culture but that dorsal cells acquire this ability only after E16, coinciding with the appearance of PDGFR alpha-immunoreactive cells in the starting population. PDGFR alpha-positive cells in optic nerve and spinal cord cultures co-labelled with antibody markers of oligodendrocyte progenitors. When PDGFR alpha-positive cells were purified from embryonic rat spinal cords by immunoselection and cultured in defined medium, they all differentiated into oligodendrocytes. Very few oligodendrocytes developed in cultures of embryonic spinal cord cells that had been depleted of PDGFR alpha-expressing cells by antibody-mediated complement lysis. These data demonstrate that all PDGFR alpha-positive cells in the embryonic rat spinal cord are oligodendrocyte progenitors and that most or all early-developing oligodendrocytes are derived from these ventrally-derived precursors.

Determination of neuroepithelial cell fate: induction of the oligodendrocyte lineage by ventral midline cells and sonic hedgehog.

Near the floor plate of the embryonic neural tube there is a group of neuroepithelial precursor cells that are specialized for production of the oligodendrocyte lineage. We performed experiments to test whether specification of these neuroepithelial oligodendrocyte precursors, like other ventral neural cell types, depends on signals from the notochord and/or floor plate. We analyzed heterozygous Danforth's short tail (Sd/+) mutant mice, which lack a notochord and floor plate in caudal regions of the neural tube, and found that oligodendrocyte precursors did not appear at the ventricular surface where there was no floor plate. Moreover, oligodendrocytes did not develop in explant cultures of Sd/+ spinal cord in the absence of a floor plate. When a second notochord was grafted into an ectopic position dorsolateral to the endogenous notochord of a chicken embryo, an additional floor plate was induced along with an ectopic focus of oligodendrocyte precursors at the ventricular surface. Oligodendrocytes developed in explants of intermediate neural tube only when they were cocultured with fragments of notochord or in the presence of purified Sonic hedgehog (Shh) protein. Thus, signals from the notochord/floor plate, possibly involving Shh, are necessary and sufficient to induce the development of ventrally derived oligodendroglia. These signals appear to act by specifying the future fate(s) of neuroepithelial cells at the ventricular surface rather than by influencing the proliferation or differentiation of prespecified progenitor cells in the parenchyma of the cord.

Embryonic expression of myelin genes: evidence for a focal source of oligodendrocyte precursors in the ventricular zone of the neural tube.

2',3'-cyclic-nucleotide 3'-phosphodiesterase (CNP) is an abundant protein of myelinating oligodendrocytes. We report that one of the alternatively spliced CNP mRNAs is also expressed in cultured oligodendrocyte progenitor cells. In situ hybridization revealed a thin longitudinal column of CNP-positive cells in the ventral ventricular zone of the embryonic day 14 rat spinal cord, coincident in time and space with cells that express the platelet-derived growth factor alpha receptor, another putative marker of the oligodendrocyte lineage. These data support the hypothesis that the oligodendrocyte lineage originates at a discrete location in the ventral ventricular zone of the embryonic day 14 rat spinal cord. We further report that transcripts encoding the myelin proteolipid protein (PLP/DM-20) are expressed in an unidentified population of neural progenitors in the ventricular zone abutting the floor plate. Our results support the idea that the ventricular zone is a mosaic of specialized progenitor cells.

Acute spinal ligament disruption: MR imaging with anatomic correlation.

Disruption of spinal ligaments can lead to instability that jeopardizes the spinal cord and nerve roots. Magnetic resonance (MR) imaging can directly image spinal ligaments; however, the sensitivity with which this modality demonstrates ligament injury has, to the authors' knowledge, not been reported. On a biomechanical testing machine, 28 cadaveric spines were subjected to controlled injury that resulted in ligament tears. The spines were then imaged with plain radiography, computed tomography, and MR imaging (1.5 T). The images were analyzed for evidence of ligament injury before dissection of the specimen. Forty-one of 52 (79%) ligament tears of various types were correctly identified at MR imaging. Disruptions of the anterior and posterior longitudinal ligaments were most conspicuous and were detected in all seven cases in which they were present (no false-positive or false-negative results); disruptions of the ligamentum flavum, capsular ligaments, and interspinous ligaments could also be identified but less reliably (three false-positive and 11 false-negative results). That MR imaging can reliably and directly allow assessment of spinal ligament disruption in this in vitro model suggests its potential utility for this assessment in patients.