The subependymal layer and neighboring region in the brain of the young rat

Neuronal progenitor-like cells expressing polysialylated neural cell adhesion molecule are present on the ventricular surface of the adult rat brain and spinal cord

In the adult rodent brain, it is now well established that neurons are continuously generated from proliferating neuronal progenitor cells located in the subventricular zone of the lateral ventricle (SVZ) and the dentate gyrus of the hippocampus. Recently, it has been shown that neurons can also be generated in vitro from various regions of the adult brain and spinal cord ventricular neuroaxis. As the highly polysialylated neural cell adhesion molecule (PSA-NCAM) has been shown to be specifically expressed by neuronal progenitor cells of the SVZ and the hippocampus, the present study was designed to determine whether cells expressing this molecule could be detected in the vicinity of the ventricular system of the adult rat brain and spinal cord. After double or triple immunostaining for different neuronal and glial markers, confocal microscopy was used to examine the surface of the ventricular neuroaxis in either 40- to 50-microm-thick transverse vibratome sections cut through different brain regions, or in 200- to 300-microm-thick tissue slices including the intact surface of the brain ventricles or of the spinal cord central canal. In untreated rats, PSA-NCAM, microtubule associated protein 2 (MAP2) and class III-beta-tubulin were found to be associated with a number of neuron-like cells located on the surface of the third and fourth ventricles and of the spinal cord central canal. The proliferation of the PSA-NCAM-immunoreactive (IR) neuron-like cells detected on the surface of the third and fourth ventricles was not affected by injection of epidermal growth factor (EGF) or basic fibroblast growth factor (bFGF) into these ventricles, but was stimulated by the combined injection of EGF + bFGF. These data indicate that cells exhibiting features of neuronal progenitors are present on the ependymal surface of the adult rat brain and spinal cord ventricular axis.

Metabolic activation of the subependymal zone after cortical injury

The subependymal zone (SEZ) of the adult mammalian forebrain contains a population of progenitor cells that proliferate in response to brain injury. This study examined the effect of cortical injury on metabolic activity in the SEZ using quantitative histochemistry of cytochrome oxidase. The SEZ showed significantly enhanced cytochrome oxidase activity in rats with electrolytic cortical injuries relative to sham-operated controls, while other brain regions showed no such changes. The results indicate that the SEZ had increased oxidative energy demands, and thus provide metabolic evidence that SEZ cells are activated in response to brain injury.

Migrational analysis of the constitutively proliferating subependyma population in adult mouse forebrain

Initial experiments to evaluate the in vivo fate(s) of constitutively proliferating subependymal cells determined that, following in vivo labeling of this population by infection with a retrovirus containing a beta-galactosidase reporter gene, there was a progressive and eventually complete loss of histochemically beta-galactosidase-positive cells within the lateral ventricle subependyma with increasing survival times of up to 28 days after retroviral infection. Subsequent experiments were designed to ascertain the potential contributions of: (i) the migration of subependymal cells away from the forebrain lateral ventricles; and (ii) the down-regulation of the retroviral reporter gene expression. Retroviral lineage tracing experiments demonstrate that a major in vivo fate for constitutively proliferating subependymal cells is their rostral migration away from the walls of the lateral ventricle to the olfactory bulb. Although down-regulation of retroviral reporter gene expression does not contribute to the loss of detection of beta-galactosidase-labeled cells from the lateral ventricle subependyma, it does result in an underestimation of the absolute number of retrovirally labeled cells in the olfactory bulb at longer survival times. Furthermore, a temporal decrease in the double labeling of beta-galactosidase-labeled cells with [3H]thymidine was observed, indicating that only a subpopulation of the migratory subependymal-derived cells continue to actively proliferate en route to the olfactory bulb. These two events may contribute to the lack of a significant increase in the total number of retrovirally labeled subependymal cells during rostral migration. Evidence from separately published studies suggests that cell death is also an important regulator of the size of the constitutively proliferating subependymal population. In summary, in vivo studies utilizing retroviral reporter gene labeling demonstrate that constitutively proliferating subependymal cells born in the lateral ventricle migrate rostrally to the olfactory bulb. Loss of proliferation potential and retroviral reporter gene down-regulation contribute to the lack of any significant increase in the total number of labeled cells recovered in the olfactory bulb.

The subependymal layer in rodents: a site of structural plasticity and cell migration in the adult mammalian brain

The persistence of neurogenesis and structural plasticity was believed until recently to be restricted to lower vertebrates and songbirds. Nevertheless, it has now been ascertained that these phenomena can occur in the adult mammalian nervous system, at least in three distinct sites: the olfactory neuroepithelium of the nasal mucosa and two brain regions, namely, the hippocampal dentate gyrus and the olfactory bulb. The newly generated cells of the olfactory bulb originate from the subependymal layer, a remnant of the primitive subventricular zone persisting in the adult forebrain. Besides being characterized by high rates of cell proliferation, the subependymal layer is a site of long-distance tangential cell migration, wherein migrating cells form chains enwrapped by a particular type of astrocytes. These glial cells give rise to channels (glial tubes) that separate single chains from the surrounding mature tissue. The cellular composition and the pattern of cell migration in the mammalian subependymal layer appear to be quite different in neonatal and adult animals, changing strikingly in the postnatal period. Other features of uniqueness involve the capability of neuronal precursors to divide while undergoing migration and the presence of multipotent stem cells. Thus, the subependymal layer is an area of the adult mammalian brain endowed with a cohort of phenomena proper of neural development, persisting into (and adapted to) the fully mature nervous tissue. Such features make this system an optimal model to unravel mechanisms permitting highly dynamic structural plasticity during adulthood, in the perspective of providing strategies for possible brain repair.

Tangential migration of young neurons arising from the subventricular zone of adult rats is impaired by surgical lesions passing through their natural migratory pathway

In the brain of adult rodents, young neurons arising from the subventricular zone (SVZ) of the lateral ventricle migrate tangentially along the rostral migratory stream (RMS) toward the olfactory bulb. The aim of this study was to determine whether surgical lesions placed through the RMS could affect the rostral migration of these newly formed neurons. Confocal and electron microscopy were used to characterize their anatomical organization within the intact and lesioned forebrains. As soon as 7 days and up to 45 days after placing a surgical lesion through the proximal portions of the RMS, numerous cells immunostained for polysialylated neural cell adhesion molecule (PSA-NCAM) were detected both (1) throughout the lesional cavity extending from the cortex to the anterior commissura, and (2) within the tissue located caudal to the lesion. In both regions, these PSA-NCAM-immunostained cells were labeled for neuronal markers but were negative for glial fibrillary acidic protein (GFAP). After administration of the proliferation marker bromodeoxyuridine (BrdU), nuclear labeling was associated with cells immunostained for PSA-NCAM but GFAP-negative, that accumulated within the lesional cavity and in the tissue caudal to the lesion. For the longest postlesional delays, a number of the PSA-NCAM-immunostained neurons located in various portions of the lesional cavity exhibited intense immunostaining for gamma-aminobutyric acid, whereas only a few of them exhibited faint immunostaining for tyrosine hydroxylase. These data indicate that surgical lesions placed through the RMS of adult rats impede the migration toward the olfactory bulb of the neuroblasts arising from the SVZ, inducing their accumulation and their partial differentiation in forebrain regions caudal to the lesion.

Identification and characterization of early glial progenitors using a transgenic selection strategy

To define the spatiotemporal development of and simultaneously select for oligodendrocytes (OLs) and Schwann cells (SCs), transgenic mice were generated that expressed a bacterial beta-galactosidase (beta-gal) and neomycin phosphotransferase fusion protein (betageo) under the control of murine 2'3'-cyclic nucleotide 3'-phosphodiesterase (muCNP) promoters I and II. Transgenic beta-gal activity was detected at embryonic day 12.5 in the ventral region of the rhombencephalon and spinal cord and in the neural crest. When cells from the rhombencephalon were cultured in the presence of G418, surviving cells differentiated into OLs, indicating that during development this brain region provides one source of OL progenitors. Postnatally, robust beta-gal activity was localized to OLs throughout the brain and was absent from astrocytes, neurons, and microglia or monocytes. In the sciatic nerve beta-gal activity was localized exclusively to SCs. Cultures from postnatal day 10 brain or sciatic nerve were grown in the presence of G418, and within 8-9 d exposure to antibiotic, 99% of all surviving cells were beta-gal-positive OLs or SCs. These studies demonstrate that the muCNP-betageo transgenic mice are useful for identifying OLs and SCs beginning at early stages of the glial cell lineage and throughout their development. This novel approach definitively establishes that the beta-gal-positive cells identified in vivo are glial progenitors, as defined by their ability to survive antibiotic selection and differentiate into OLs or SCs in vitro. Moreover, this experimental paradigm facilitates the rapid and efficient selection of pure populations of mouse OLs and SCs and further underscores the use of cell-specific promoters in the purification of distinct cell types.

Architecture and cell types of the adult subventricular zone: in search of the stem cells

Neural stem cells are maintained in the subventricular zone (SVZ) of the adult mammalian brain. Here, we review the cellular organization of this germinal layer and propose lineage relationships of the three main cell types found in this area. The majority of cells in the adult SVZ are migrating neuroblasts (type A cells) that continue to proliferate. These cells form an extensive network of tangentially oriented pathways throughout the lateral wall of the lateral ventricle. Type A cells move long distances through this network at high speeds by means of chain migration. Cells in the SVZ network enter the rostral migratory stream (RMS) and migrate anteriorly into the olfactory bulb, where they differentiate into interneurons. The chains of type A cells are ensheathed by slowly proliferating astrocytes (type B cells), the second most common cell type in this germinal layer. The most actively proliferating cells in the SVZ, type C, form small clusters dispersed throughout the network. These foci of proliferating type C cells are in close proximity to chains of type A cells. We discuss possible lineage relationships among these cells and hypothesize which are the neural stem cells in the adult SVZ. In addition, we suggest that interactions between type A, B, and C cells may regulate proliferation and initial differentiation within this germinal layer.

Intraventricular administration of BDNF increases the number of newly generated neurons in the adult olfactory bulb

We have previously demonstrated that the most rostral part of the subventricular zone (SVZ) is a source of neuronal progenitor cells whose progeny are destined to become interneurons of the olfactory bulb. To determine whether the number of newly generated neurons in the adult olfactory bulb could be increased by the administration of an exogenous factor, brain-derived neurotrophic factor (BDNF) was infused for 12 days into the right lateral ventricle of adult rat brains. The production of new cells was monitored by either the intraventricular infusion or intraperitoneal injection of the cell proliferation marker BrdU. In both experimental paradigms we observed significantly more BrdU-labeled cells in the olfactory bulbs on the BDNF-infused side than in the olfactory bulb of PBS-infused animals. Analysis of the BDNF-infused brains of animals injected intraperitoneally with BrdU demonstrated a 100% increase in the number of BrdU-labeled cells in the bulb, the preponderance ( approximately 90%) of which were double-labeled with a neuron-specific antibody. These results demonstrate that the generation and/or survival of new neurons in the adult brain can be increased substantially by an exogenous factor. Furthermore, the SVZ, and in particular the rostral part, may constitute a reserve pool of progenitor cells available for neuronal replacement in the diseased or damaged brain.

Morphological analysis of the early stages of oligodendrocyte development in the vertebrate central nervous system

Oligodendrocytes are responsible for myelin formation in the vertebrate central nervous system. While in vitro analysis have provided critical information on the cellular properties of oligodendrocyte precursors, they provide limited information on the morphological development of these cells in the intact CNS. Recent studies have begun to provide insights into when and where oligodendrocyte precursors arise in the neural tube. In the chick CNS, the monoclonal antibody O4 selectively labels oligodendrocyte lineage cells both in vitro and in vivo and here we discuss the characteristics of O4+ oligodendrocyte precursors during development of the chick CNS. The earliest oligodendrocytes initially develop in restricted locations in the CNS. In the spinal cord, for example, oligodendrocyte precursors arise in the ventral ventricular zone dorsal to the floor plate. These early oligodendrocyte precursors are integral components of the lining of the central canal and have an embryonic neuroepithelial cell morphology, suggesting that commitment to the oligodendrocyte lineage occurs in the ventricular zone. With maturation these early oligodendrocyte precursors lose their ventricular connection, adopt a uni- or-bipolar morphology, and migrate throughout the CNS. When these cells reach presumptive white matter, they stop migrating, become multiprocessed, and differentiate into immature oligodendrocytes. The maturation of these newly formed oligodendrocytes results in the upregulation of expression of a variety of myelin specific genes such as MBP and PLP and the subsequent elaboration of the myelin organelle. In the developing optic nerve, the onset of myelination occurs several days after oligodendrocyte precursors populate the nerve, suggesting that additional signals are required to induce myelin formation.

Expression of NOS, PSA-N-CAM and S100 protein in the granule cell migration pathway of the adult guinea pig forebrain

To investigate the possible role of nitric oxide (NO) in adult neurogenesis and neuron-glial migration in the rostral migratory stream (RMS), we used a double-labeled immunofluorescence technique together with confocal laser scanning microscopy, and examined the localization of nitric oxide synthase (NOS), the highly polysialylated isoform of neural cell adhesion molecule (PSA-N-CAM), and the astroglial marker in brain, S100 protein (S100), throughout the length of the subependymal layer (SEL) to olfactory bulb (OB) pathway of the adult guinea pig forebrain. Blast-like, beaded, clustered immature cellular elements stained for PSA-N-CAM and those having a typical astrocytic phenotypes positive for S100 protein were densely interlaced throughout the entire length of the SEL. Some S100 positive ependymoglial cells (tanycytes) gave off their basal projections into the closely packed PSA-N-CAM immunopositive clusters in the rostral extension of the subependymal zone (SEZre). The SEL was devoid of NOS immunoreactivity. A dense network of punctate, fenestrated and radially oriented immature cellular elements positive both for NOS and PSA-N-CAM intermingled and overlapped in the inner part of the internal granular layer (IGr), whereas in the outer part, PSA-N-CAM expression gradually diminished and the cells shifted to mature bipolar, spherical or spindle-shaped granule cells with uniform cellular contours, which were exclusively immunopositive for NOS. Radially oriented astroglial phenotypes were intertwined with PSA-N-CAM neuronal clusters in the SEL, and were closely apposed to NOS neuronal elements in the IGr. In summary, these results showed a distinct separation of neurons and glia as revealed by PSA-N-CAM and S100 protein immunostaining, and an inverse spatio-temporal correlation of expression between PSA-N-CAM (immature neuroblasts) and NOS (mature neurons) in the adult guinea pig RMS.

Proliferation and phenotype regulation in the subventricular zone during experimental allergic encephalomyelitis: in vivo evidence of a role for nerve growth factor

Proliferating cells in the subventricular zone (SVZ) of adult rat brain could provide a source of cells for repair attempts during degenerative diseases. However, very few reports dealt with the spontaneous regulation of this cell population during experimental conditions. In this paper, we describe an increase in the proliferation activity in the SVZ during experimental allergic encephalomyelitis, a demyelinating disease widely used as an experimental model for human multiple sclerosis. Moreover, p75(LNGFR)-immunoreactive elements in the SVZ were larger in experimental allergic encephalomyelitis compared with control groups, and they also showed multiple and branched elongations. Finally, a selective uptake of 125I-nerve growth factor was observed in the SVZ in neonatal rats, and positive elements migrated in the corpus callosum within a few days. These data indicate that cell populations in the SVZ are regulated during inflammatory conditions and degenerative diseases involving oligodendrocytes and neurotrophins, including nerve growth factor, could participate in these phenomena.

Proliferation of differentiated glial cells in the brain stem

Classical studies of macroglial proliferation in muride rodents have provided conflicting evidence concerning the proliferating capabilities of oligodendrocytes and microglia. Furthermore, little information has been obtained in other mammalian orders and very little is known about glial cell proliferation and differentiation in the subclass Metatheria although valuable knowledge may be obtained from the protracted period of central nervous system maturation in these forms. Thus, we have studied the proliferative capacity of phenotypically identified brain stem oligodendrocytes by tritiated thymidine radioautography and have compared it with known features of oligodendroglial differentiation as well as with proliferation of microglia in the opossum Didelphis marsupialis. We have detected a previously undescribed ephemeral, regionally heterogeneous proliferation of oligodendrocytes expressing the actin-binding, ensheathment-related protein 2'3'-cyclic nucleotide 3'-phosphodiesterase (CNPase), that is not necessarily related to the known regional and temporal heterogeneity of expression of CNPase in cell bodies. On the other hand, proliferation of microglia tagged by the binding of Griffonia simplicifolia B4 isolectin, which recognizes an alpha-D-galactosyl-bearing glycoprotein of the plasma membrane of macrophages/microglia, is known to be long lasting, showing no regional heterogeneity and being found amongst both ameboid and differentiated ramified cells, although at different rates. The functional significance of the proliferative behavior of these differentiated cells is unknown but may provide a low-grade cell renewal in the normal brain and may be augmented under pathological conditions.

Cells of the oligodendrocyte lineage proliferate following cortical stab wounds: an in vitro analysis

We have previously shown that a cortical stab wound induces the proliferation of microglia and astrocytes in situ, but no evidence was obtained for proliferation of cells of the oligodendrocyte lineage (Amat et al., 1996). To study further the properties of cells involved in repair following brain injury, groups of adult rats received either sham operations or bilateral stab wounds. Proliferating cells were labeled in vivo 3 days later with [3H]-thymidine (Thy) and sacrificed the same day. Oligodendrocyte-enriched preparations were isolated, cultured, and analyzed. The fate and antigenic phenotype of the proliferating cells was analyzed using three-color immunofluorescence combined with autoradiography at 1, 2, 3, 5, and 10 days in vitro (DIV). Cells were immunostained for ganglioside GD3 (glial stem cells), O4 antigen (cells of the oligodendrocyte lineage), galactosyl ceramide (GC, differentiated oligodendrocytes), and GFAP (astrocytes). Thymidine-labeled O4+/GC- cells were found only in cultures from wounded animals and most of them differentiated in vitro as mature oligodendrocytes, but no Thy+/O4+/GC+ oligodendrocytes were seen at 1, 2, or 3 DIV. There was also a marked increase in the number of Thy+/GD3+ cells in the experimental cultures. In both experimental and control groups the total number of Thy+ and Thy- GD3+ cells declined with time in culture concomitant with an increase in total number of both Thy+ and Thy- GFAP+ astrocytes, and without any significant change in the Thy+ cell fraction of O4+ oligodendrocytes in the experimental cultures. Therefore most of the GD3+/O4- cells apparently differentiated as GFAP+ astrocytes, not as oligodendrocytes. We conclude that O4+/GC- oligodendrocyte precursor cells, but not differentiated oligodendrocytes, proliferate in response to brain injury. These cells proliferate slowly or not at all in normal adult animals and constitute a phenotypically and kinetically distinct group from the GD3+ glial precursors. This result is consistent with the existence within the adult CNS of a quiescent premyelinating oligodendrocyte. We propose that these immature committed oligodendrocytes are induced to proliferate at the wound site and serve as a source of new oligodendrocytes.

Postnatal mouse subventricular zone neuronal precursors can migrate and differentiate within multiple levels of the developing neuraxis

The mammalian subventricular zone (SVZ) of the lateral wall of the forebrain ventricle retains a population of proliferating neuronal precursors throughout life. Neuronal precursors born in the postnatal and adult SVZ migrate to the olfactory bulb where they differentiate into interneurons. Here we tested the potential of mouse postnatal SVZ precursors in the environment of the embryonic brain: (i) a ubiquitous genetic marker, (ii) a neuron-specific transgene, and (iii) a lipophilic-dye were used to follow the fate of postnatal day 5-10 SVZ cells grafted into embryonic mouse brain ventricles at day 15 of gestation. Graft-derived cells were found at multiple levels of the neuraxis, including septum, thalamus, hypothalamus, and in large numbers in the midbrain inferior colliculus. We observed no integration into the cortex. Neuronal differentiation of graft derived cells was demonstrated by double-staining with neuron-specific beta-tubulin antibodies, expression of the neuron-specific transgene, and the dendritic arbors revealed by the lipophilic dye. We conclude that postnatal SVZ cells can migrate through and differentiate into neurons within multiple embryonic brain regions other than the olfactory bulb.

FGF-2 is sufficient to isolate progenitors found in the adult mammalian spinal cord

The adult rat brain contains progenitor cells that can be induced to proliferate in vitro in response to FGF-2. In the present study we explored whether similar progenitor cells can be cultured from different levels (cervical, thoracic, lumbar, and sacral) of adult rat spinal cord and whether they give rise to neurons and glia as well as spinal cord-specific neurons (e.g., motoneurons). Cervical, thoracic, lumbar, and sacral areas of adult rat spinal cord (>3 months old) were microdissected and neural progenitors were isolated and cultured in serum-free medium containing FGF-2 (20 ng/ml) through multiple passages. Although all areas generated rapidly proliferating cells, the cultures were heterogeneous in nature and cell morphology varied within a given area as well as between areas. A percentage of cells from all areas of the spinal cord differentiate into cells displaying antigenic properties of neuronal, astroglial, and oligodendroglial lineages; however, the majority of cells from all regions expressed the immature proliferating progenitor marker vimentin. In established multipassage cultures, a few large, neuron-like cells expressed immunoreactivity for p75NGFr and did not express GFAP. These cells may be motoneurons. These results demonstrate that FGF-2 is mitogenic for progenitor cells from adult rat spinal cord that have the potential to give rise to glia and neurons including motoneurons.

Expression of neural RNA-binding proteins in the postnatal CNS: implications of their roles in neuronal and glial cell development

There is an increasing interest in the role of RNA-binding proteins during neural development. Mouse-Musashi-1 (m-Msi-1) is a mouse neural RNA-binding protein with sequence similarity to Drosophila musashi (d-msi), which is essential for neural development. m-Msi-1 is highly enriched in neural precursor cells that are capable of generating both neurons and glia during embryonic CNS development. The present study characterized m-Msi-1-expressing cells in the postnatal and adult CNS. Postnatally, m-Msi-1 was expressed in proliferative neuronal precursors in the external granule cell layer of the cerebellum and in the anterior corner of the subventricular zone of the lateral ventricles. In gliogenesis, the persistent expression of m-Msi-1 was observed in cells of the astrocyte lineage ranging from proliferative glial precursors in the subventricular zone (SVZ) to differentiated astrocytes in the parenchyma. In addition, we showed that m-Msi-1 was still expressed in proliferating cells in the adult SVZ, which may contain neural precursor or stem cells. Another neural RNA-binding protein Hu (the mammalian homolog of a Drosophila neuronal RNA-binding protein Elav) was present in postmitotic neurons throughout the development of the CNS, and its pattern of expression was compared with that of m-Msi-1. These observations imply that these two RNA-binding proteins may be involved in the development of neurons and glia by regulating gene expression at the post-transcriptional level.

Transforming growth factor-alpha null and senescent mice show decreased neural progenitor cell proliferation in the forebrain subependyma

The adult mammalian forebrain subependyma contains neural stem cells and their progeny, the constitutively proliferating progenitor cells. Using bromodeoxyuridine labeling to detect mitotically active cells, we demonstrate that the endogenous expression of transforming growth factor-alpha (TGFalpha) is necessary for the full proliferation of progenitor cells localized to the dorsolateral corner of the subependyma and the full production of the neuronal progenitors that migrate to the olfactory bulbs. Proliferation of these progenitor cells also is diminished with age (in 23- to 25-months-old compared with 2- to 4-months-old mice), likely because of a lengthening of the cell cycle. Senescence or the absence of endogenous TGFalpha does not affect the numbers of neural stem cells isolated in vitro in the presence of epidermal growth factor. These results suggest that endogenous TGFalpha and the effects of senescence may regulate the proliferation of progenitor cells in the adult subependyma, but that the number of neural stem cells is maintained throughout life.

Epidermal growth factor and fibroblast growth factor-2 have different effects on neural progenitors in the adult rat brain

Neurons and glia are generated throughout adulthood from proliferating cells in two regions of the rat brain, the subventricular zone (SVZ) and the hippocampus. This study shows that exogenous basic fibroblast growth factor (FGF-2) and epidermal growth factor (EGF) have differential and site-specific effects on progenitor cells in vivo. Both growth factors expanded the SVZ progenitor population after 2 weeks of intracerebroventricular administration, but only FGF-2 induced an increase in the number of newborn cells, most prominently neurons, in the olfactory bulb, the normal destination for neuronal progenitors migrating from the SVZ. EGF, on the other hand, reduced the total number of newborn neurons reaching the olfactory bulb and substantially enhanced the generation of astrocytes in the olfactory bulb. Moreover, EGF increased the number of newborn cells in the striatum either by migration of SVZ cells or by stimulation of local progenitor cells. No evidence of neuronal differentiation of newborn striatal cells was found by three-dimensional confocal analysis, although many of these newborn cells were associated closely with striatal neurons. The proliferation of hippocampal progenitors was not affected by either growth factor. However, EGF increased the number of newborn glia and reduced the number of newborn neurons, similar to the effects seen in the olfactory bulb. These findings may be useful for elucidating the in vivo role of growth factors in neurogenesis in the adult CNS and may aid development of neuronal replacement strategies after brain damage.

Cellular composition and three-dimensional organization of the subventricular germinal zone in the adult mammalian brain

The adult mammalian subventricular zone (SVZ) contains stem cells that give rise to neurons and glia. In vivo, SVZ progeny migrate 3-8 mm to the olfactory bulb, where they form neurons. We show here that the SVZ of the lateral wall of the lateral ventricles in adult mice is composed of neuroblasts, glial cells, and a novel putative precursor cell. The topographical organization of these cells suggests how neurogenesis and migration are integrated in this region. Type A cells had the ultrastructure of migrating neuronal precursors. These cells were arranged as chains parallel to the walls of the ventricle and were polysialylated neural adhesion cell molecule- (PSA-NCAM), TuJ1- (beta-tubulin), and nestin-positive but GFAP- and vimentin-negative. Chains of Type A cells were ensheathed by two ultrastructurally distinct astrocytes (Type B1 and B2) that were GFAP-, vimentin-, and nestin-positive but PSA-NCAM- and TuJ1-negative. Type A and B2 (but not B1) cells incorporated [3H]thymidine. The most actively dividing cell in the SVZ corresponded to Type C cells, which had immature ultrastructural characteristics and were nestin-positive but negative to the other markers. Type C cells formed focal clusters closely associated with chains of Type A cells. Whereas Type C cells were present throughout the SVZ, they were not found in the rostral migratory stream that links the SVZ with the olfactory bulb. These results suggest that chains of migrating neuroblasts in the SVZ may be derived from Type C cells. Our results provide a topographical model for the adult SVZ and should serve as a basis for the in vivo identification of stem cells in the adult mammalian brain.

Selective expression of dopamine D3 receptor mRNA in proliferative zones during embryonic development of the rat brain

We studied by in situ hybridization histochemistry the expression of D3 receptor (D3R) mRNA at various stages of rat brain development. The first expression of D3R mRNA was detected at embryonic day 14 (E14) in the striatal and rhinencephalic neuroepithelia and throughout the tectal neuroepithelium. From E16 to E19 D3R mRNA expression extended along a rostrocaudal axis to additional proliferative ventricular zones of the basal forebrain, including the neuroepithelia of the olfactory bulb, nucleus accumbens, septum, and amygdala, whereas D1 and D2 receptor (D1R and D2R) mRNAs were expressed predominantly by migrating neuroblasts and/or differentiating striatal neurons. Only a few neuroblasts, migrating in the lateral cortical stream or developing as cerebellar Purkinje cells, expressed D3R mRNA from E18. At birth D3R expression mRNA appeared in differentiating neuronal fields of the nucleus accumbens and medial mamillary body primordia and on P5 reached a distribution similar to that found in adult. In addition, a transient upregulation was detected on P5 in the medial mamillary bodies, parietofrontal cortex, and olfactory tubercle. In the adult brain D3R gene expression continued in the striatal proliferative subventricular zone. The late expression D3R mRNA in neurons, after achievement of dopamine innervation, supports the existence of a regulating factor released from dopamine neurons, as suggested by denervation studies in the adult. The sustained and abundant D3R gene expression, predominantly in germinative neuroepithelial zones actively involved in neurogenesis of most basal forebrain structures, supports the hypothesis of a neurogenetic but minor morphogenetic modulatory role for the D3R during CNS development.

Glial lineages and myelination in the central nervous system

Oligodendrocytes, derived from stem cell precursors which arise in subventricular zones of the developing central nervous system, have as their specialist role the synthesis and maintenance of myelin. Astrocytes contribute to the cellular architecture of the central nervous system and act as a source of growth factors and cytokines; microglia are bone-marrow derived macrophages which function as primary immunocompetent cells in the central nervous system. Myelination depends on the establishment of stable relationships between each differentiated oligodendrocyte and short segments of several neighbouring axons. There is growing evidence, especially from studies of glial cell implantation, that oligodendrocyte precursors persist in the adult nervous system and provide a limited capacity for the restoration of structure and function in myelinated pathways damaged by injury or disease.

Glial tubes in the rostral migratory stream of the adult rat

In the central nervous system cell migration is usually restricted to developmental periods and occurs mainly radially, following the radial glia. Nevertheless, in the subependymal layer of the adult rodent forebrain tangential migration of newly generated neuronal precursors directed to the olfactory bulb, which follow a well-defined pathway without dispersion, has been recently demonstrated. In the present study, by using light microscopic immunocytochemistry for glia-associated antigens (glial fibrillary acidic protein, S-100 and vimentin), and conventional electron microscopy, we observed a dense mesh-work of astrocytic cells and processes throughout the subependymal layer of the adult rat. These cells were organized to form tangentially oriented glial tubes in the subependymal layer of the lateral ventricle and in its rostral extension to the olfactory bulb. Glial tubes were particularly evident within the rostral extension and were widely intercommunicating. Using markers for the proliferating/ migrating cells of the rostral migratory stream (5-bromo-2'-deoxyuridine, PSA-NCAM, class III beta-tubulin), we provide evidence that long chains of PSA-NCAM/beta-tubulin-positive, newly generated cells are consistently observed inside the glial tubes. These results demonstrate the existence of a peculiar glial organization within the subependymal layer of the adult rat, consisting of long astrocytic tubes that likely represent a new type of glial guidance, accounting for the tangential migration of a high number of cells along their restricted pathway, to the olfactory bulb.

Network of tangential pathways for neuronal migration in adult mammalian brain

Cells in the brains of adult mammals continue to proliferate in the subventricular zone (SVZ) throughout the lateral wall of the lateral ventricle. Here we show, using whole mount dissections of this wall from adult mice, that the SVZ is organized as an extensive network of chains of neuronal precursors. These chains are immunopositive to the polysialylated form of NCAM, a molecule present at sites of plasticity, and TuJ1, an early neuronal marker. The majority of the chains are oriented along the rostrocaudal axis and many join the rostral migratory stream that terminates in the olfactory bulb. Using focal microinjections of DII and transplantation of SVZ cells carrying a neuron-specific reporter gene, we demonstrate that cells originating at different rostrocaudal levels of the SVZ migrate rostrally and reach the olfactory bulb where they differentiate into neurons. Our results reveal an extensive network of pathways for the tangential chain migration of neuronal precursors throughout the lateral wall of the lateral ventricle in the adult mammalian brain.

Bone morphogenetic proteins promote astroglial lineage commitment by mammalian subventricular zone progenitor cells

The epigenetic signals that regulate lineage development in the embryonic mammalian brain are poorly understood. Here we demonstrate that a specific subclass of the transforming growth factor beta superfamily, the bone morphogenetic proteins (BMPs), cause the selective, dose-dependent elaboration of the astroglial lineage from murine embryonic subventricular zone (SVZ) multipotent progenitor cells. The astroglial inductive effect is characterized by enhanced morphological complexity and expression of glial fibrillary acidic protein, with concurrent suppression of neuronal and oligodendroglial cell fates. SVZ progenitor cells express transcripts for the appropriate BMP-specific type I and II receptor subunits and selective BMP ligands, suggesting the presence of paracrine or autocrine developmental signaling pathways (or both). These observations suggest that the BMPs have a selective role in determining the cell fate of SVZ multipotent progenitor cells or their more developmentally restricted progeny.

Cortical lesions induce an increase in cell number and PSA-NCAM expression in the subventricular zone of adult rats

The subventricular zone (SVZ) bordering the lateral ventricle is one of the few regions of adult brain that contains dividing cells. These cells can differentiate into neurons in vivo after migration into the olfactory bulb and in vitro in the presence of appropriate growth factors. Little is known, however, about the fate of these cells in vivo after brain injury in adults. We examined cell number and expression of differentiation markers in the SVZ of adult rats after cortical lesions. Aspiration lesions of the sensorimotor cortex in adult rats induced a transient doubling of the number of cells in the SVZ at the level of the striatum without consistent increases in bromodeoxyuridine-labeled cells. Immunoreactivity to the polysialylated neural cell adhesion molecule, expressed by the majority of cells of the SVZ during development, increased dramatically after lesion. In contrast, immunolabeling for molecules found in mature neurons and glia did not increase in the SVZ after lesion, and immunoreactivity for growth factors that induce differentiation of SVZ cells in vitro decreased or remained undetectable, suggesting that lack of appropriate growth factor expression may contribute to the lack of differentiation of the newly accumulated cells in vivo. The data reveal that cells of the SVZ are capable of plasticity in the adult rat after brain injury in vivo and that the newly accumulated cells retain characteristics seen during development.

Young neurons from the adult subependymal zone proliferate and migrate along an astrocyte, extracellular matrix-rich pathway

The subependymal zone (SEZ) of the lateral ventricle of adult rodents has long been known to be mitotically active. There has been increased interest in the SEZ, since it has been demonstrated that neuroepithelial stem cells residing there generate neurons in addition to glia in vitro. In the present study, we have examined parasagittal sections of the adult mouse brain using immunocytochemistry for extracellular matrix (ECM) molecules (tenascin and chondroitin sulfate-containing proteoglycans), glial fibrillary acidic protein (GFAP, a cytoskeletal protein prominently expressed by immature and reactive astrocytes), RC-2 (a radial glial and immature astrocyte cytoskeletal marker), TuJ1 (a class III beta-tubulin isoform expressed solely by postmitotic and adult neurons), nestin (a cytoskeletal protein associated with stem cells), neuron-specific enolase, and bromodeoxyuridine (BrdU, which is taken up by dividing cells). Our results demonstrate that a population of young neurons reside within an ECM-rich, GFAP-positive astrocyte pathway from the rostral SEZ all the way into the olfactory bulb. Furthermore, BrdU labeling studies indicate that there is a high level of cell division along the entire length of this path, and double-labeling studies indicate that neurons committed to a neuronal lineage (i.e., TuJ1+) take up BrdU (suggesting they are in the DNA synthesis phase of the cell cycle), again along the entire length of the SEZ "migratory pathway." Thus, the SEZ appears to retain the ability to produce neurons and glia throughout the life of the animal, functioning as a type of "brain marrow." The implications of these findings are discussed in relation to the role that such a glial/ ECM-rich boundary (as seen in the embryonic cortical subplate and other developing areas) may play in: confining the migratory populations and maintaining them in a persistent state of immaturity; facilitating their migration to the olfactory bulb, where they are incorporated into established adult circuitries; and potentially altering SEZ cell cycle dynamics that eventually lead to cell death.

Co-localization of NG2 proteoglycan and PDGF alpha-receptor on O2A progenitor cells in the developing rat brain

A detailed comparison in the developing rat central nervous system between the distribution of the NG2 proteoglycan and the alpha-receptor for platelet-derived growth factor (PDGF) shows that these two molecules are co-expressed by glial progenitor cells of the O2A lineage and can serve as reliable markers for identification of O2A cells in vivo. Our mapping experiments indicate that NG2-positive, PDGF alpha-receptor positive O2A cells are abundant throughout the developing central nervous system in both white and gray matter. The earliest cells immunoreactive for either of the two markers are found adjacent to the central canal of the embryonic day 15 (E15) spinal cord. These cells express only PDGF alpha-receptor and not NG2. By E17, process-bearing cells expressing both NG2 and PDGF alpha-receptor in a highly co-localized fashion are found throughout the central nervous system. The first postnatal week marks the peak in the number of NG2 and PDGF alpha-receptor immunoreactive cells, as well as the peak in the level of expression and the extent of co-localization of the two molecules. After the first week, the level of expression of both NG2 and PDGF alpha-receptor declines, although both molecules continue to be expressed in the adult brain. On O2A cells in the mature brain, NG2 and PDGF alpha-receptor are not as well co-localized at the subcellular level as they are on O2A cells in the younger brain. The functional consequences of co-localization and subsequent dissociation of NG2 and PDGF alpha-receptor on maturing O2A progenitors are investigated in the accompanying paper (Nishiyama et al.: J Neurosci Res 43:315-330, 1996).

Cell and molecular analysis of the developing and adult mouse subventricular zone of the cerebral hemispheres

The subventricular zone (SVZ) of the lateral ventricle remains mitotically active in the adult mammalian central nervous system (CNS). Recent studies have suggested that this region may contain neuronal precursors (neural stem cells) in adult rodents. A variety of neuronal and glial markers as well as three extracellular matrix (ECM) markers were examined with the hope of understanding factors that may affect the growth and migration of neurons from this region throughout development and in the adult. This study has characterized the subventricular zone of late embryonic, postnatal, and adult mice using several neuronal markers [TuJ1, nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d), neuron-specific enolase (NSE)], glial markers [RC-2, vimentin, glial fibrillary acidic protein (GFAP), galactocerebroside (Gal-C)], ECM markers [tenascin-C (TN-C), chondroitin sulfate, a chondroitin sulfate proteoglycan termed dermatan sulfate-dependent proteoglycan-1 (DSD-1-PG)], stem-cell marker (nestin), and proliferation-specific marker [bromodeoxyuridine (BrdU)]. TuJ1+ and nestin+ cells (neurons and stem cells, respectively) persist in the region into adulthood, although the numbers of these cells become more sparse as the animal develops, and they appear to be immature compared to the cells in surrounding forebrain structures (e.g., not expressing NSE and having few, if any, processes). Likewise, NADPH-d+ cells are found in and around the SVZ during early postnatal development but become more sparse in the proliferative zone through maturity, and, by adulthood, only a few labeled cells can be found at the border between the SVZ and surrounding forebrain structures (e.g., the striatum), and even smaller numbers of positive cells can be found within the adult SVZ proper. BrdU labeling also seems to decrease significantly after the first postnatal week, but it still persists in the SVZ of adult animals. The disappearance of RC-2+ (radial) glia during postnatal development and the persistence of glial-derived ECM molecules such as tenascin and chondroitin sulfate proteoglycans (as well as other "boundary" molecules) in the adult SVZ may be associated with a persistence of immaturity, cell death, and a lack of cell emigration from the SVZ in the adult.

Neuronal stem cells in the brain of adult vertebrates

It is generally assumed that neurogenesis in the central nervous system ceases before or soon after birth. In the last three decades, however, several studies have reported that new neurons continue to be added into the brain of adult fish, frogs, reptiles, birds and mammals. The precursor cells that give rise to the neurons generated in adulthood are generally located in the walls of the brain ventricles. From these proliferative regions, neuronal precursors migrate toward their final targets where they differentiate; they often traverse long distances through complex brain parenchyma. The identity of the neuronal precursors in the brains of adult animals is still unknown. Experiments in adult birds suggest that proliferating radial cells may be the neuronal precursors. In adult mice, cells present in the subventricular zone can generated neurons in vivo and in vitro. These neuronal precursors can be induced to proliferate in vitro when exposed to growth factors and retain their potential to differentiate into neurons and glia. Whether these putative neural stem cells can differentiate into multiple neuronal types remains to be determined. The neuronal precursors of the adult brain could be used as a source of cells for neuronal transplantation. In addition, these cells could be manipulated in vivo or in vitro to introduce genes into the brain. Adult neurogenesis offers new experimental opportunities to study neuronal birth, migration and differentiation and for the treatment of neurological diseases.

Lineage, migration, and fate determination of postnatal subventricular zone cells in the mammalian CNS

We have been examining the developmental fates and migrational patterns of the immature cells in the subventricular zone (SVZ) of the mammalian forebrain by labeling postnatal rat SVZ cells by stereotactic injection of replication-deficient murine retroviruses bearing reporter genes. SVZ cells migrate into adjacent white matter, cortex, and striatum, and differentiate into astrocytes and oligodendrocytes. In white matter, they largely differentiate into oligodendrocytes, whereas in gray matter, they differentiate into both oligodendrocytes and astrocytes. In vitro, SVZ cells are multipotential, able to generate both types of glia, as well as neurons. We infer that developmental fates are in part controlled by important environmental cues that the cells encounter during their migration.

Expression of polysialylated neural cell adhesion molecule by proliferating cells in the subependymal layer of the adult rat, in its rostral extension and in the olfactory bulb

The highly sialylated isoform of the neural cell adhesion molecule is thought to be expressed predominantly in the developing nervous system, where it is implicated in a variety of dynamic events linked to neural morphogenesis. It has become increasingly evident, however, that this "embryonic" neural cell adhesion molecule isoform continues to be expressed in certain adult neuronal systems, and in particular, in those that can undergo structural plasticity. In the present study, we performed light microscopic immunocytochemistry with an antibody specific for polysialylated neural cell adhesion molecule and confirmed our earlier observations [Bonfanti L. et al. (1992) Neuroscience 49, 419-436] showing polysialylated neural cell adhesion molecule-immunoreactive cells in the subependymal layer of the lateral ventricle of the adult rat, a region where cell proliferation continues into the postnatal period. In addition, we used an antibody raised against the proliferating cell nuclear antigen and found that proliferating cells continue to be visible in this area, even in the adult. Double immunolabeling showed that many of these newly generated cells displayed high polysialylated neural cell adhesion molecule immunoreactivity. Cells from a portion of the subependymal layer migrate to the olfactory bulb and contribute to the continual replacement of its granule neurons [Luskin M. B. (1993) Neuron 11, 173-189]. We found polysialylated neural cell adhesion molecule-immunoreactive cells all along the pathway purported to be followed by the newly generated cells to their final destination and in neurons corresponding to granular and periglomerular cells in the olfactory bulb. Our present observations thus support the contention that polysialylation is a feature of neurons capable of dynamic change and may contribute to the molecular mechanisms permitting cell proliferation and migration not only during development but also in the adult.

Long-distance neuronal migration in the adult mammalian brain

During the development of the mammalian brain, neuronal precursors migrate to their final destination from their site of birth in the ventricular and subventricular zones (VZ and SVZ, respectively). SVZ cells in the walls of the lateral ventricle continue to proliferate in the brain of adult mice and can generate neurons in vitro, but their fate in vivo is unknown. Here SVZ cells from adult mice that carry a neuronal-specific transgene were grafted into the brain of adult recipients. In addition, the fate of endogenous SVZ cells was examined by microinjection of tritiated thymidine or a vital dye that labeled a discrete population of SVZ cells. Grafted and endogenous SVZ cells in the lateral ventricle of adult mice migrate long distances and differentiate into neurons in the olfactory bulb.

Age-dependent changes in the glial cell nests of the canine rhinencephalic allocortex. A morphometric study

In the present study, the morphological features as well as the age-dependent changes of the glial cell nests (GCN) within the rhinencephalic allocortex of the dog are described. A combination of two stereological methods, i.e., Cavalieri's principle and the optical disector, was used to obtain unbiased estimates of the volumes of the whole brain, the allocortex, and the GCN. Furthermore, the numerical densities and total number of the two prevailing populations of undifferentiated cells within these nests were determined. Cells with medium-sized dark nuclei (CMD) and cells with large pale nuclei (CLP) were distinguished. The volume of the GCN in relation to the volume of the allocortex decreased with increasing age. The numerical density and the total number of all cells of the GCN and of the CMD were reduced with age, whereas the numerical density and number of the CLP increased with advancing age. Similar morphological features as well as age-dependent changes have already been described of the cell populations in the subependymal layer. Therefore, in analogy, we presume that the glial cells of the GCN have emigrated from the subependymal layer. The significance of these age-dependent changes remains as obscure as does the function of the GCN.

Altered postnatal development of the visual cortex in trisomy 19 mice

The development of the visual cortex was studied in 30 Trisomy 19 (Ts19) mice aged 1-16 days postpartum and their euploid littermates. Morphogenesis of the Ts19 visual cortex, though delayed in development, followed the regular sequence observed in control littermates. Early morphogenetic events, such as obliteration of the ventricular lumen, disappearance of the ventricular zone, formation of a visible apical dendrite, as well as disappearance of both migrating neurons and the columnar organization of bipolar preneurons were delayed by 1 day; maturation steps occurring later such as appearance and disappearance of perikaryal basophilia were delayed by 2 days. Myelination of the white matter was similarly retarded by 2 days. The fronto-occipital length of the cerebral hemispheres and the thickness of the visual cortex were decreased by about 20%, consistent with a hypoplasia of the Ts19 neocortex. Unlike in the cerebral cortex of human Ts21, morphometric analysis of the visual cortex of Ts19 mice did not give any indication of a selective deficit in a particular neuron population; the increased cell density and the reduced nuclear volume observed during early postnatal development are attributable to a maturational delay. The relevance of these results with respect to the mechanisms underlying neuropathological alterations in human Ts21 is discussed.

The migrational patterns and developmental fates of glial precursors in the rat subventricular zone are temporally regulated

Postnatal gliogenesis in the rodent forebrain was studied by infecting subventricular zone cells of either neonates or juvenile rats with replication-deficient retroviruses that encode reporter enzymes, enabling the migration and fate of these germinal zone cells to be traced over the ensuing several weeks. Neither neonatal nor juvenile subventricular zone cells migrated substantially along the rostral-caudal axis. Neonatal subventricular zone cells migrated dorsally and laterally into hemispheric gray and white matter and became both astrocytes and oligodendrocytes. Juvenile subventricular zone cells migrated into more medial areas of the subcortical white matter and on occasion appeared in the white matter of the contralateral hemisphere, but rarely migrated into the neocortex. Juvenile subventricular zone cells almost exclusively differentiated into oligodendrocytes. Thus, the migratory patterns and the developmental fates of subventricular zone cells change during the first 2 weeks of life. When either neonatal or juvenile subventricular zone cells were labeled in vivo and then removed and cultured, some generated homogeneous clones that contained either astrocytes with a ‘type 1′ phenotype or oligodendrocytes, but some generated heterogeneous clones that contained both glial types. These results provide additional evidence for a common progenitor for astrocytes and oligodendrocytes and strongly suggest that temporally and spatially regulated environmental signals control the destiny of glial progenitors during postnatal development.

Differential expression of insulin-like growth factor binding proteins (IGFBP) 4 and 5 mRNA in the rat brain after transient hypoxic-ischemic injury

Recent studies suggest a role for the insulin-like growth factor (IGF) system in the repair of damaged tissue following hypoxic-ischemic injury in the infant rat brain. We have used a unilateral model of hypoxic-ischemic injury to assess the possible involvement of two IGF binding proteins (IGFBPs), IGFBP-4 and IGFBP-5, in the post-asphyxial response. Ligation of the right carotid artery of 21-day-old rats was followed by either 15 min or 60 min exposure to 8% oxygen to produce moderate and severe damage respectively. Using in situ hybridization, the distribution of IGFBP-4 and IGFBP-5 mRNA was determined in brains collected over 10 days following the insult. In the control brains (no damage), both IGFBPs were expressed in distinct regions. IGFBP-4 mRNA was detected in limited areas of the hippocampus and in several cortical layers, while IGFBP-5 mRNA was found primarily in the thalamus. In response to hypoxic-ischemic injury, IGFBP-4 mRNA expression was reduced in regions of neuronal loss, suggesting a neuronal origin for IGFBP-4. The expression of IGFBP-5 mRNA was not altered by the 15 min insult, but was heavily induced from 3 days following the 60 min insult, particularly in the subependymal layer and adjacent white matter on the ligated hemisphere. This suggests that IGFBP-5 may be involved in recovery from severe hypoxic-ischemic injury and may be important in the regeneration of oligodendrocytes.

The reconstruction of an astrocytic environment in glia-deficient areas of white matter

Injection of ethidium bromide into X-irradiated spinal cord white matter produces a lesion in which demyelinated axons reside in an environment that is permanently depleted of glial cells. By transplanting defined populations of glial cells into this lesion it is possible to recreate normal or novel glial environments. In this study we have transplanted cultures of astrocytes into the X-irradiated ethidium bromide lesion in order to (1) assess the ability of these cells to relate to components within the lesion environment and thereby contribute to tissue reconstruction and (2) establish an astrocytic environment around demyelinated axons that resembles pathological states such as the chronic demyelinated plaques of multiple sclerosis. In order to focus attention on the interactions between astrocytes and demyelinated axons we developed a protocol for depleting astrocyte cultures of oligodendrocyte lineage cells and Schwann cells based on complement-mediated immunocytolysis and in vitro X-irradiation. In addition to establishing the ability of transplanted astrocytes to form an astrocytic matrix around demyelinated axons, this study has also revealed the diversity of cell types present within neonatal forebrain cultures.

Neuropathological changes caused by hydrocephalus

The medical literature concerning neuropathological changes caused by hydrocephalus is reviewed. In both humans and experimental animals the ependyma suffers focal destruction, cerebral blood vessels are distorted and capillaries collapse, there is damage to axons and myelin in the periventricular white matter, and occasionally neurons suffer injury. The damage appears to result from mechanical distortion of the brain combined with impaired cerebral blood flow. If ventriculomegaly develops very early, foci of cortical dysgenesis may be the result. The character and distribution of pathological changes are dependent on the age at which hydrocephalus develops, the rate and magnitude of ventricular enlargement, and the duration of hydrocephalus. Diversionary shunting of cerebrospinal fluid can only incompletely reverse the damage and the potential for reversal diminishes as the duration of hydrocephalus increases.

Proliferating subventricular zone cells in the adult mammalian forebrain can differentiate into neurons and glia

Subventricular zone (SVZ) cells proliferate spontaneously in vivo in the telencephalon of adult mammals. Several studies suggest that SVZ cells do not differentiate after mitosis into neurons or glia but die. In the present work, we show that SVZ cells labeled in the brains of adult mice with [3H]thymidine differentiate directly into neurons and glia in explant cultures. In vitro labeling with [3H]thymidine shows that 98% of the neurons that differentiate from the SVZ explants are derived from precursor cells that underwent their last division in vivo. This report identifies the SVZ cells as neuronal precursors in an adult mammalian brain.

Ultrastructural radioautographic analysis of neurogenesis in the hypothalamus of the adult frog, Rana temporaria, with special reference to physiological regeneration of the preoptic nucleus. I. Ventricular zone cell proliferation

The localization and fine structure of proliferating cells in the hypothalamic preoptic area were studied by light- and electron-microscopic radioautography 1-2 h following single application of 3H-thymidine to adult Rana temporaria taken from their natural habitat in the spring and autumn. 3H-thymidine uptake by proliferating cells was much more pronounced in frogs caught in May/June, i.e., a month after the breeding period (labeled cells represent about 10% of the total ventricular zone cell population), compared to animals caught in mid-September, when it was very low. In both 3H-thymidine treatment groups the vast majority of labeled cells are found exclusively within the preoptic recess ventricular zone. With regard to ultrastructure, it contained proliferating cells of at least 4 types, ranging from immature forms (bipolar stem cells) to more differentiated elements (tanycyte-like ependymoblasts, "classical" ependymoblasts). All of them showed label over their nuclei indicating that these cells are capable of DNA synthesis and mitosis. The possible role of the preoptic recess ventricular zone as a source of precursor cells for new peptidergic neurosecretory cells, conventional neurons and glial cells in the hypothalamic preoptic area of the adult frog is discussed.

Extracellular somatostatin measured by microdialysis in the hippocampus of freely moving rats: evidence for neuronal release

Intracerebral microdialysis combined with a sensitive and specific radioimmunoassay was used to monitor the neuronal release of somatostatin (somatostatin-like immunoreactivity, SLI) in the dorsal hippocampus of freely moving rats. The sensitivity of the radioimmunoassay was optimized to detect < 1 fmol/ml. The basal concentration of SLI in 20-min dialysate fractions (5 microliters/min) collected 24 h after probe implantation was stable over at least 200 min. The spontaneous efflux dropped by 54 +/- 6.4% (p < 0.05) when Ca2+ was omitted and 1 mM EGTA added to the Krebs-Ringer solution and by 65.5 +/- 3.2% (p < 0.05) in the presence of 1 microM tetrodotoxin. Depolarizing concentrations of the Na+ channel opener veratridine (6.25, 25, 100 microM) induced 11 +/- 2 (p < 0.05), 17 +/- 2 (p < 0.05), and 21 +/- 5 (p < 0.01) fold increase in SLI concentration, respectively, during the first 20 min of perfusion. The effect of 100 microM veratridine was blocked by coperfusion with 5 microM tetrodotoxin (p < 0.01) and reduced by 79% (p < 0.01) in the virtual absence of Ca2+. Neuronal depolarization by 20 min of perfusion with Krebs-Ringer solution containing 25 and 50 mM KCl and proportionally lowered Na+ increased the dialysate SLI 4.4 +/- 1 (p < 0.05) and 17 +/- 3 (p < 0.01) fold baseline, respectively. Ten micromolar ouabain, a blocker of Na+,K(+)-ATPase, increased the dialysate SLI 15-fold baseline, on average (p < 0.05), during 80 min of perfusion. The results demonstrate the suitability of brain microdialysis for monitoring the neuronal release of SLI and for studying its role in synaptic transmission.

Localization of Pi class glutathione-S-transferase in the forebrains of neonatal and young rats: evidence for separation of astrocytic and oligodendrocytic lineages

The Yp isoform (Pi class) of glutathione-S-transferase has recently been localized in oligodendrocytes in the brains of mature rats. To examine at what postnatal age Pi first appears in oligodendrocytes or precursor cells, antibodies against Pi were used to immunostain tissue sections from the forebrains of neonatal rats and young rats up to 17 days of age. In the brains of neonates Pi immunofluorescence was observed in ovoid cells in the subependymal layer, and in ovoid cells and cells bearing short, thick processes in the corpus callosum and cingulum. These cells did not immunostain for vimentin. During the first postnatal week Pi-positive cells showed positive immunostaining for ganglioside GD3, which is characteristic of oligodendrocyte precursors, and process-bearing Pi-positive cells appeared in the cingulum and at the lateral borders of the corpus callosum in increasing numbers. During the second postnatal week the cytoplasm of Pi-positive cells became more compact, and the processes thinner, and the Pi-positive cells and their processes began to immunostain for 2',3'-cyclic nucleotide-3'-phosphohydrolase, which is characteristic of immature and mature oligodendrocytes and myelin sheaths. By age 17 days Pi was observed in relatively mature oligodendrocytes. The observations suggest that Pi occurs in oligodendrocyte precursors, immature oligodendrocytes, and mature oligodendrocytes in the postnatal through 17 day old rat forebrain. In the accompanying paper (Cammer and Zhang, '92)--if references are permitted in the Abstract a different glutathione-S-transferase isoform, Yb (Mu class), was localized in cells of the astrocyte lineage, beginning in the forebrains of neonatal rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Fine structural organization of the ependymal region of the paraventricular nucleus of the rat thalamus and its relation with projection neurons

The ependymal lining of the diencephalic third ventricle is known to exhibit significant variation in zonal architecture and the relations of neurites to the ventricular surface in different regions remains obscure. The present study explores the fine structural organization of the ependymal region of the thalamic paraventricular nucleus. Methodology was developed for tracing neurites of cells retrogradely labelled with horseradish peroxidase based on our recent observation that paraventricular neurons projecting to the amygdaloid complex cluster near the ventricle and emit numerous dendrites extending toward the ependymal surface. A relatively uniform population of cuboidal 'pale' ependymocytes dominates the ventricular lining of the thalamic paraventricular nucleus, although a few 'dark' ependymocytes are interspersed. The subependymal region displays a variety of glial elements. Dendrites of thalamic paraventricular projection neurons terminate in proximity to the ependymal layer from which they are generally separated by thin cytoplasmic processes of putative astrocytes, and few indent the basal portion of ependymal cells. Thin 'terminal' (i.e., serially traced) horseradish peroxidase-labelled dendrites filled with lipid and lysosome-like dense bodies were often enveloped by astrocyte membrane whorls. This feature may constitute a reactive glial response in horseradish peroxidase-labelled dendritic terminals. A distinctive arrangement of tortuous astrocyte leaflets was insinuated between the basal portion of ependymocytes in a zone exhibiting numerous caveolae, apparently isolating neurites from direct contact with the cerebrospinal fluid. These findings indicate that the ependymal region of the thalamic paraventricular nucleus is not characterized by those features of the basal third ventricle suspected to confer neuroendocrine interaction between neurons and cerebrospinal fluid. The structural arrangement between ependymocytes and thalamic paraventricular projection cells indicate a specialized relation of these neurons with the ependymal interface, but apparently not directly with the overlying cerebrospinal fluid.

Expression of IGF-1 mRNA in the murine subventricular zone during postnatal development

Insulin-like growth factor-1 (IGF-1) stimulates the proliferation and maturation of neuroglia in vitro. To further investigate its role in gliogenesis, in situ hybridization was utilized to determine whether IGF-1 mRNA was expressed in the subventricular zone (SVZ) of the postnatal mouse forebrain. The SVZ is a transient germinal zone and in the neonate is the principle source of oligodendroglia for myelinating fiber tracts of the forebrain. Strong hybridization signal was detected over cells in the SVZ at postnatal day (PND) 4, the earliest time point examined. Positive signal persisted in the SVZ at PND 8, however, the number of IGF-1-labeled cells declined rapidly during the second postnatal week. IGF-1 mRNA was not uniformly distributed throughout the SVZ and the majority of labeled cells were located within its so-called 'border' region. In contrast to the SVZ, IGF-1 mRNA-expressing cells were only rarely found in forebrain fiber tracts. IGF-1 transcripts were not detected in ependymal lining or choroid plexus of the lateral ventricle. In light of its known gliotrophic activity, the localization of IGF-1 mRNA in the SVZ suggests that locally produced IGF-1 may act as a mitogen or differentiation-inducing agent during gliogenesis.