Adult neurogenesis: From canaries to the clinic

Estrogen and adult neurogenesis in the amygdala and hypothalamus

In mammals, adult neurogenesis has been extensively studied in the dentate gyrus of the hippocampus and subventricular zone. However, newly proliferated neurons have also been documented in other brain regions, including the amygdala and hypothalamus. In this review, we will examine the evidence for new neurons in the adult amygdala and hypothalamus and then discuss how environmental influences can alter cell proliferation. As some of these environmental effects may be attributed to changes in the levels of circulating hormones, we will provide evidence for estrogen-mediated cell proliferation among different species and between sexes. Finally, we will review recent data suggesting that new neurons may become functionally significant in adulthood.

Regional distribution and migration of proliferating cell populations in the adult brain of Hyla cinerea (Anura, Amphibia)

We examined the distribution of adult cell proliferation throughout the brain of an anuran amphibian using 5-bromo-2'-deoxyuridine (BrdU). BrdU, a thymidine analog, is a commonly used cellular marker that is incorporated into actively dividing progenitor cells. Adult green treefrogs, Hyla cinerea, received injections of BrdU and were sacrificed 2 h, 2 days, 2 weeks, or 30 days later. Immunohistochemistry revealed BrdU-immunopositive (BrdU+) cells to be distributed in ventricular zones throughout the brain. The heaviest concentrations of cells were located in the telencephalon, primarily in the ventrolateral region of the lateral ventricles, and the ventricles of olfactory bulbs. Numerous BrdU+ cells were located around the preoptic and hypothalamic recesses and few around the third ventricle in the diencephalon. Proceeding caudally towards the midbrain, there was a marked decrease in BrdU labeling and few BrdU+ cells were found in the hindbrain. Consistent with previous studies in ectothermic vertebrates, BrdU+ cells were found predominantly in the ventricular zone (VZ) and immediately adjacent to the VZ; at later time points (i.e., 30 days), the cells appeared to have migrated into parenchymal regions. The extent of cellular proliferation in anurans is similar to that of fishes and reptiles and thus is more widespread compared to mammals.

Cellular localization of mGluR3 and mGluR5 mRNAs in normal and injured rat brain

In order to understand the role of metabotropic glutamate receptors (mGluRs) in the brain, it is important to know how the mGluRs are differentially expressed among the different cell types. At present, the cellular expression of mGluR3 and mGluR5 has been mostly studied in terms of proteins with observations suggesting the expression of both mGluR3 and mGluR5 in neuronal and in glial cells. In order to verify the brain cell type-expressing mGluR3 and mGluR5 mRNAs, both in normal and injured brain, we performed a double labeling analysis, by in situ hybridization for mGluR3 or mGluR5 mRNA and immunohistochemistry for specific cellular markers. This approach allowed us to find mGluR3 mRNA expressed in neurons (NeuN-positive cells), and in glial cells, such as astrocytes (GFAP-positive cells) and oligodendrocytes (CNPase-positive cells). The same analysis showed that only NeuN-positive cells express mGluR5 mRNA. The time course of mGluR3 mRNA expression in two models of hippocampal formation lesion, kainate-induced seizures or ibotenic acid injection, showed an increased expression of mGluR3 in the area of lesion. This effect appears 1 week after the injury and was localized in GFAP- and CNPase-positive cells. In contrast, mGluR5 was not found expressed in the area of lesion. The present results contribute to extend available data on cell type-expressing mGluR3 and mGluR5 in normal and injured brain and could be relevant to understand the mechanisms that drive neuron-glial cells interaction both in normal and repairing processes.

Adult neurogenesis and cellular brain repair with neural progenitors, precursors and stem cells

Recent work in neuroscience has shown that the adult central nervous system (CNS) contains neural progenitors, precursors and stem cells that are capable of generating new neurons, astrocytes and oligodendrocytes. While challenging the previous dogma that no new neurons are born in the adult mammalian CNS, these findings bring with them the future possibilities for development of novel neural repair strategies. The purpose of this review is to present the current knowledge about constitutively occurring adult mammalian neurogenesis, highlight the critical differences between 'neurogenic' and 'non-neurogenic' regions in the adult brain, and describe the cardinal features of two well-described neurogenic regions-the subventricular zone/olfactory bulb system and the dentate gyrus of the hippocampus. We also provide an overview of presently used models for studying neural precursors in vitro, mention some precursor transplantation models and emphasize that, in this rapidly growing field of neuroscience, one must be cautious with respect to a variety of methodological considerations for studying neural precursor cells both in vitro and in vivo. The possibility of repairing neural circuitry by manipulating neurogenesis is an intriguing one, and, therefore, we also review recent efforts to understand the conditions under which neurogenesis can be induced in non-neurogenic regions of the adult CNS. This work aims towards molecular and cellular manipulation of endogenous neural precursors in situ, without transplantation. We conclude this review with a discussion of what might be the function of newly generated neurons in the adult brain, and provide a summary of present thinking about the consequences of disturbed adult neurogenesis and the reaction of neurogenic regions to disease.

Glial progenitor-based repair of demyelinating neurological diseases

Demyelinating diseases of the brain and spinal cord affect more than one-quarter million of Americans, with numbers reaching more than two million across the world. These patients experience not only the vascular, traumatic, and inflammatory demyelinations of adulthood but the congenital and childhood dysmyelinating syndromes of the pediatric leukodystrophies. Several disease-modifying strategies have been developed that slow disease progression, especially in the inflammatory demyelinations and in multiple sclerosis in particular. Yet, currently available disease modifiers typically influence the immune system and are neither intended to nor competent to reverse the structural neurologic damage attending acquired demyelination. Fortunately, however, the disorders of myelin lend themselves well to attempts at structural repair, because central oligodendrocytes are the primary, and often sole, victims of the underlying disease process. Given the relative availability and homogeneity of human oligodendrocyte progenitor cells, the disorders of myelin formation and maintenance may be especially compelling targets for cell-based neurologic therapy.

The olfactory pathway of decapod crustaceans--an invertebrate model for life-long neurogenesis

The first part of this review includes a short description of the cellular and morphological organization of the olfactory pathway of decapod crustaceans, followed by an overview of adult neurogenesis in this pathway focusing on the olfactory lobe (OL), the first synaptic relay in the brain. Adult neurogenesis in the central olfactory pathway has the following characteristics. 1) It is present in all the diverse species of decapod crustaceans so far studied. 2) In all these species, projection neurons (PNs), which have multiglomerular dendritic arborizations, are generated. 3) Neurons are generated by one round of symmetrical cell divisions of a small population of immediate precursor cells that are located in small proliferation zones at the inner margin of the respective soma clusters. 4) The immediate precursor cells in each soma cluster appear to be generated by repeated cell divisions of one or few neuronal stem cells that are located outside of the proliferation zone. 5) These neuronal stem cells are enclosed in a highly structured clump of small glial-like cells, which likely establishes a specific microenvironment and thus can be regarded as a stem cell niche. 6) Diverse internal and external factors, such as presence of olfactory afferents, age, season of the year, and living under constant and deprived conditions modulate the generation and/or survival of new neurons. In the second part of this review, I address the question why in decapod crustaceans adult neurogenesis persists in the visual and olfactory pathways of the brain but is lacking in all other mechanosensory-chemosensory pathways. Due to the indeterminate growth of most adult decapod crustaceans, new sensory neurons of all modalities (olfaction and chemo-, mechano-, and photoreception) are continuously added during adulthood and provide an ever-increasing sensory input to all primary sensory neuropils of the central nervous system. From these facts, I conclude that adult neurogenesis in the brain cannot simply be a mechanism to accommodate increasing sensory input and propose instead that it is causally linked to the specific "topographic logic" of information processing implemented in the sensory neuropils serving different modalities. For the presumptive odotopic type of information processing in the OL, new multiglomerular PNs allow interconnection of novel combinations of spatially unrelated input channels (glomeruli), whose simultaneous activation by specific odorants is the basis of odor coding. Thus, adult neurogenesis could provide a unique way to increase the resolution of odorant quality coding and allow adaptation of the olfactory system of these long-lived animals to ever-changing odor environments.

Adult Neurogenesis in the Hippocampus of Long-Lived Mice During Aging

Ames dwarf mice live considerably longer than normal animals, exhibit apparently normal cognitive functions, and maintain them into advanced age. Neurogenesis occurs throughout adult life span in the dentate gyrus of mammalian hippocampus and has been suggested to play an important role in cognitive function. We now report that the total number of bromodeoxyuridine (BrdU)-labeled cells in this brain region in aged Ames dwarf mice was not different from that in aged normal mice, whereas the fraction of newly generated neurons was significantly increased by monitoring BrdU labeling and cell marker expression. Evidence of activation of anti-apoptosis signal transduction cascade was also found in the hippocampus of aged dwarf mice. Together with previous findings, the results may suggest that an increase in hippocampal insulin-like growth factor-I protein expression and subsequent activation of antiapoptotic signaling might contribute to survival of newly born neurons and subsequently to the delay of cognitive loss during aging in these long-lived dwarf mice.

Acute intermittent nicotine treatment induces fibroblast growth factor-2 in the subventricular zone of the adult rat brain and enhances neuronal precursor cell proliferation

Over the past years, evidence has accumulated that stem cells are present in the adult brain, and generate neurons and/or glia from two active germinal zones: the subventricular zone (SVZ) of the lateral ventricles and the subgranular zone (SGZ) of the dentate gyrus of the hippocampus. This study shows that acute intermittent nicotine treatment significantly enhances neuronal precursor cell proliferation in the SVZ of adult rat brain, but not in the SGZ of the hippocampus, and pre-treatment with mecamylamine, a nonselective nAChR antagonist, blocks the enhanced precursor proliferation by nicotine. This effect is supported by up-regulation of fibroblast growth factor-2 (FGF-2) mRNA in the SVZ and the expression of its receptor FGFR-1 in cells of SVZ showing precursor cells profile. It is also demonstrated that the nicotine effect on neuronal precursor proliferation is mediated by FGF-2 via fibroblast growth factor receptor 1 (FGFR-1) activation by showing that i.c.v. pre-treatment with anti-FGF-2 antibodies or with FGFR-1 inhibitor 3-[(3-(2-carboxyethyl)-4-methylpyrrol-2-yl)methylene]-2-indolinone (SU5402) blocks nicotine-induced precursor cell proliferation. This nicotine enhancement of neuronal precursor cell proliferation was not accompanied by an increase in the number of apoptotic cells. Taken together the present findings revealed the existence in the SVZ of the adult rat brain of a trophic mechanism mediated by FGF-2 and its receptor and regulated by nAchR activation. This possibility of in vivo regulation of neurogenesis in the adult brain by exogenous factors may aid to develop treatments stimulating neurogenesis with potential therapeutic implications.

A comparative framework for understanding the biological principles of adult neurogenesis

Adult neurogenesis has been identified in all vertebrate species examined thus far. However, an evolutionary trend towards a reduction in both the number of proliferation zones and the overall number of newborn cells has been revealed in more recent lineages of vertebrates, such as mammals. Adult neurogenesis, and in particular the characterization of adult neural stem cells in mammals has been the focus of intense research with the goal of developing new cell-based regenerative treatments for neurodegenerative diseases, spinal cord injury, and acute damage due to stroke. Conversely, most other vertebrate classes, which display widespread production of adult neurons, are not typically used as model systems in this context. A more profound understanding of the structural composition and the mechanisms that support proliferation zones in the mature brain have become critical for revealing how adult neural stem cells are maintained in these regions and how they regulate neurogenesis. In this review we argue that comprehensive analyses of adult neurogenesis in various vertebrate and invertebrate species will lead to a more complete understanding of the fundamental biology and evolution of adult neurogenesis and provide a better framework for testing hypotheses regarding the functional significance of this trait.

Cell replacement therapy in neurological disease

Diseases of the brain and spinal cord represent especially daunting challenges for cell-based strategies of repair, given the multiplicity of cell types within the adult central nervous system, and the precision with which they must interact in both space and time. Nonetheless, a number of diseases are especially appropriate for cell-based therapy, in particular those in which single phenotypes are lost, and in which the re-establishment of vectorially specific connections is not entirely requisite for therapeutic benefit. We review here a set of potential therapeutic indications that meet these criteria as potentially benefiting from the transplantation of neural stem and progenitor cells. These include: (i) transplantation of phenotypically restricted neuronal progenitor cells into diseases of a single neuronal phenotype, such as Parkinson's disease; (ii) implantation of mixed progenitor pools into diseases characterized by the loss of a limited number of discrete phenotypes, such as spinal cord injury and the motor neuronopathies; (iii) transplantation of glial and nominally oligodendrocytic progenitor cells as a means of treating disorders of myelin; and (iv) transplantation of neural stem cells as a means of treating lysosomal storage disorders and other diseases of enzymatic deficiency. Among the diseases potentially approachable by these strategies, the myelin disorders, including the paediatric leucodystrophies as well as adult traumatic and inflammatory demyelinations, may present the most compelling targets for cell-based neurological therapy.

Hippocampal IGF-1 expression, neurogenesis and slowed aging: clues to longevity from mutant mice

Recent studies point out the important role of IGF and insulin-related signaling pathways in the control of longevity of laboratory animals. The Ames dwarf mouse is a murine model of circulating GH and IGF-1 deficiency that exhibits dwarf phenotype characteristics and significantly extends lifespan. It is interesting to know that Ames dwarf mice do not experience an age-related decline in cognitive function when compared to their young counterparts. In this study, the most recent works on local GH and IGF-1 expression in the hippocampus of Ames mice are briefly reviewed.

Conserved and acquired features of adult neurogenesis in the zebrafish telencephalon

Our understanding of the cellular and molecular mechanisms underlying the adult neural stem cell state remains fragmentary. To provide new models on this issue, we searched for stem cells in the adult brain of the zebrafish. Using BrdU tracing and immunodetection of cell-type-specific markers, we demonstrate that the adult zebrafish telencephalon contains self-renewing progenitors, which show features of adult mammalian neural stem cells but distribute along the entire dorso-ventral extent of the telencephalic ventricular zone. These progenitors give rise to newborn neurons settling close to the ventricular zone within the telencephalon proper. They have no equivalent in mammals and therefore constitute a new model of adult telencephalic neural stem cells. In addition, progenitors from the ventral subpallium generate rapidly dividing progenitors and neuroblasts that reach the olfactory bulb (OB) via a rostral migratory stream and differentiate into GABAergic and TH-positive neurons. These ventral progenitors are comparable to the mammalian neural stem cells of the subependymal zone. Interestingly, dorsal and ventral progenitors in the adult telencephalon express a different combination of transcription factors than their embryonic counterparts. In the case of neurogenin1, this is due to the usage of different enhancer elements. Together, our results highlight the conserved and unique phylogenic and ontogenic features of adult neurogenesis in the zebrafish telencephalon and open the way to the identification of adult neural stem cell characters in cross-species comparative studies.

Localization of Id2 mRNA in the adult mouse brain

Id proteins are negative regulators of basic helix-loop-helix transcription factors and are involved in cellular differentiation and proliferation. Four members of the Id gene family exhibit closely related but distinct expression patterns in various mammalian organs of not only embryos but also adults. Among them, Id2 is known to be expressed in Purkinje cells and neurons in the cortical layers of the adult mouse brain, suggesting that Id2 is involved in some neural functions in the adult. To get insight into the role of Id2 in the nervous system, we investigated the localization of Id2 mRNA-expressing cells in the adult mouse brain in detail by in situ hybridization with the radiolabeled antisense probe and compared it with the localization of other Id gene family members. The results indicated that Id2 mRNA is detected in more varied brain regions than previously reported. These regions include the amygdaloid complex, caudate putamen, globus pallidus, substantia nigra pars reticulata, suprachiasmatic nucleus, and the anterior part of the subventricular zone. These results suggest the possibility that Id2 plays a role in the neural activity and cognitive functions. On the other hand, Id1 was barely detectable. Although moderate or low expression of Id3 was observed diffusely, high expression was observed in some specific regions including the molecular layer of the dentate gyrus and the external capsule. Id4 mRNA was detected in the regions such as the caudate putamen and the lateral amygdaloid nucleus. Thus, the expression pattern of Id2 is distinct from those of other Id gene family members.

Adult neurogenesis and the ischemic forebrain

The recent identification of endogenous neural stem cells and persistent neuronal production in the adult brain suggests a previously unrecognized capacity for self-repair after brain injury. Neurogenesis not only continues in discrete regions of the adult mammalian brain, but new evidence also suggests that neural progenitors form new neurons that integrate into existing circuitry after certain forms of brain injury in the adult. Experimental stroke in adult rodents and primates increases neurogenesis in the persistent forebrain subventricular and hippocampal dentate gyrus germinative zones. Of greater relevance for regenerative potential, ischemic insults stimulate endogenous neural progenitors to migrate to areas of damage and form neurons in otherwise dormant forebrain regions, such as the neostriatum and hippocampal pyramidal cell layer, of the mature brain. This review summarizes the current understanding of adult neurogenesis and its regulation in vivo, and describes evidence for stroke-induced neurogenesis and neuronal replacement in the adult. Current strategies used to modify endogenous neurogenesis after ischemic brain injury also will be discussed, as well as future research directions with potential for achieving regeneration after stroke and other brain insults.

Therapeutic potential of neurotrophic factors in neurodegenerative diseases

There is a vast amount of evidence indicating that neurotrophic factors play a major role in the development, maintenance, and survival of neurons and neuron-supporting cells such as glia and oligodendrocytes. In addition, it is well known that alterations in levels of neurotrophic factors or their receptors can lead to neuronal death and contribute to the pathogenesis of neurodegenerative diseases such as Parkinson disease, Alzheimer disease, Huntington disease, amyotrophic lateral sclerosis, and also aging. Although various treatments alleviate the symptoms of neurodegenerative diseases, none of them prevent or halt the neurodegenerative process. The high potency of neurotrophic factors, as shown by many experimental studies, makes them a rational candidate co-therapeutic agent in neurodegenerative disease. However, in practice, their clinical use is limited because of difficulties in protein delivery and pharmacokinetics in the central nervous system. To overcome these disadvantages and to facilitate the development of drugs with improved pharmacotherapeutic profiles, research is underway on neurotrophic factors and their receptors, and the molecular mechanisms by which they work, together with the development of new technologies for their delivery into the brain.

Cellular proliferation and migration following a controlled cortical impact in the mouse

Neurogenesis following neural degeneration has been demonstrated in many models of disease and injury. The present study further examines the early proliferative and migratory response of the brain to a controlled cortical impact (CCI) model of traumatic brain injury. The CCI was centered over the forelimb sensorimotor cortex, unilaterally, in the adult mouse. To examine proliferation, bromo-deoxyuridine (BrdU) was injected i.p. immediately post-injury and on post-injury days 1, 2, and 3. To assess migration, we labeled SVZ cells with inert latex microspheres immediately post-injury. By combining microsphere labeling with BrdU, we determined if migrating cells had gone through the S-phase of the cell cycle after the lesion. In addition, we used a marker of neurogenesis and migration, doublecortin, to further characterize the response of the SVZ to the injury. Lastly, we determined whether subregions of the SVZ respond differentially to injury. The current study demonstrates that 3 days following CCI cellular proliferation is seen around the cortex, in the SVZ, corpus callosum, and subcortical areas anatomically connected to, but not directly damaged by the impact. It delineates that an increase in proliferation occurs in the dorsal-most aspect of the ipsilateral SVZ following impact. Lastly, it demonstrates that proliferating cells migrate from the SVZ to cortical and subcortical structures affected by the injury and that some of these cells are migrating neuroblasts.

Stem and progenitor cell-based therapy of the human central nervous system

Multipotent neural stem cells, capable of giving rise to both neurons and glia, line the cerebral ventricles of all adult animals, including humans. In addition, distinct populations of nominally glial progenitor cells, which also have the capacity to generate several cell types, are dispersed throughout the subcortical white matter and cortex. A number of approaches have evolved for using neural progenitor cells in cell therapy. Four strategies are especially attractive for clinical translation: first, transplantation of oligodendrocyte progenitor cells as a means of treating the disorders of myelin; second, transplantation of phenotypically restricted neuronal progenitor cells to treat diseases of discrete loss of a single neuronal phenotype, such as Parkinson disease; third, implantation of mixed progenitor pools to treat diseases characterized by the loss of several discrete phenotypes, such as spinal cord injury; and fourth, mobilization of endogenous neural progenitor cells to restore neurons lost as a result of neurodegenerative diseases, in particular Huntington disease. Together, these may present the most compelling strategies and near-term disease targets for cell-based neurological therapy.

Estrogen regulation of cell proliferation and distribution of estrogen receptor-alpha in the brains of adult female prairie and meadow voles

Adult female prairie (Microtus ochrogaster) and meadow (M. pennsylvanicus) voles were compared to examine neural cell proliferation and the effects of estrogen manipulation on cell proliferation in the amygdala, ventromedial hypothalamus (VMH), and dentate gyrus of the hippocampus (DG). Unlike prior studies, our study focused on the amygdala and VMH, because they are involved in social behaviors and may underlie behavioral differences between the species. Meadow voles had a higher density of cells labeled with the cell proliferation marker 5-bromo-2'-deoxyuridine (BrdU) in the amygdala and DG than did prairie voles. Treatment with estradiol benzoate (EB) for 3 days increased the density of BrdU-labeled cells in the amygdala, particularly in the posterior cortical (pCorA) and medial (pMeA) nuclei, in meadow, but not prairie, voles. Furthermore, the majority of the BrdU-labeled cells in the pCorA and pMeA displayed either a neuronal or a glial progenitor phenotype, but no species or treatment differences were found in the percentage of neuronal or glial progenitor cells. To understand better estrogen's effects on adult neurogenesis, we also examined estrogen receptor-alpha (ERalpha) distribution. Meadow voles had more ERalpha-labeled cells in the pCorA and VMH, but not in the pMeA or DG, than did prairie voles. In addition, more than one-half of the BrdU-labeled cells in the amygdala of both species coexpressed ERalpha labeling. Together, these data indicate that estrogen alters cell proliferation in a species- and region-specific manner, and some of these effects may lie in the specific localization of estrogen receptors in the adult vole brain.

Adult neurogenesis and repair of the adult CNS with neural progenitors, precursors, and stem cells

Recent work in neuroscience has shown that the adult central nervous system contains neural progenitors, precursors, and stem cells that are capable of generating new neurons, astrocytes, and oligodendrocytes. While challenging previous dogma that no new neurons are born in the adult mammalian CNS, these findings bring with them future possibilities for the development of novel neural repair strategies. The purpose of this review is to present current knowledge about constitutively occurring adult mammalian neurogenesis, to highlight the critical differences between "neurogenic" and "non-neurogenic" regions in the adult brain, and to describe the cardinal features of two well-described neurogenic regions-the subventricular zone/olfactory bulb system, and the dentate gyrus of the hippocampus. We also provide an overview of currently used models for studying neural precursors in vitro, mention some precursor transplantation models, and emphasize that, in this rapidly growing field of neuroscience, one must take caution with respect to a variety of methodological considerations for studying neural precursor cells both in vitro and in vivo. The possibility of repairing neural circuitry by manipulating neurogenesis is an intriguing one, and, therefore, we also review recent efforts to understand the conditions under which neurogenesis can be induced in non-neurogenic regions of the adult CNS. This work aims toward molecular and cellular manipulation of endogenous neural precursors in situ, without transplantation. We conclude this review with a discussion of what the function might be of newly generated neurons in the adult brain and provide a summary of current thinking about the consequences of disturbed adult neurogenesis and the reaction of neurogenic regions to disease.

Biotechnology, the brain and the future

Several new approaches to illness, inspired by recent advances in molecular biology, informatics and nanoscience, are readily applicable to diseases of the central nervous system. Novel classes of drugs will widen the scope of therapeutic action beyond merely modifying transmitter function and stem cell and gene therapies could offer an even more selective mode of targeting. Further into the future, nanotechnology has the potential to allow development of new medicines and novel access routes via miniaturized monitoring and screening devices: these systems, together with increasing use of carbon-silicon interfacing, will challenge traditional neuropharmacology. As the 21(st) century unfolds, the structure and function of the brain, which is incomparable with any other organ, will present unique technological and ethical questions.

The neuronal primary cilium--an extrasynaptic signaling device

Many, but likely most, neurons in the central nervous system have a nonmotile "primary" cilium extending like an antenna or finger from one of the pair of centrioles in the cell's centrosome into the extracellular space. Since their discovery over 100 years ago, these organelles have been either dismissed as functionless relicts of a bygone era or more often simply ignored. However, it has long been known that the photoreceptor-bearing outer segments of retinal rods and cones are modified primary cilia and it has recently been found that kidney cells' primary cilia are sensitive flowmeters the disabling of which causes polycystic kidney disease. It has also been recently shown that somatostatin sst3 receptors and serotonin 5-HT(6) receptors are selectively sited on neurons in various parts of the rat brain. It seems likely that these selectively receptored neuronal primary cilia will turn out to be the forerunners of a family of cell-signaling devices that help drive various brain functions by sending signals into their own cells and into adjacent cells through gap junctions and via conventional chemical synapses.

Neuron addition and loss in the song system: regulation and function

Neurons continue to be produced and replaced throughout life in songbirds. Proliferation in the walls of the lateral ventricles gives rise to neurons that migrate long distances to populate many diverse telencephalic regions, including nuclei dedicated to the perception and production of song, a learned behavior. Many projection neurons are incorporated into the efferent motor pathway for song control. Replacement of these neurons is regulated, in part, by neuron death. Underlying mechanisms include gonadal steroids and BDNF, but are likely to involve other trophic factors as well. The functional significance of neuronal replacement remains unclear. However, recent experiments suggest a link between cell turnover and one or more specific attributes of song learning and production. Several hypotheses are critically examined, including the possibility that neuronal replacement provides motor flexibility to allow for error correction-a capacity needed for juvenile and adult song learning, but also likely to be important for the maintenance of song stereotypy. We highlight important gaps in our knowledge and discuss future directions that may bring us closer to solving the riddle of why neurons are produced and replaced in adulthood.

Involvement of highly polysialylated neural cell adhesion molecule (PSA-NCAM)-positive granule cells in the amygdaloid-kindling-induced sprouting of a hippocampal mossy fiber trajectory

The mossy fiber system in the hippocampus of amygdaloid-kindled rats was examined by using highly polysialylated neural cell adhesion molecule (PSA-NCAM) as a marker for immunohistochemical detection of immature dentate granule cells and mossy fibers in combination with bromodeoxyuridine (BrdU) labeling of newly generated granule cells. Statistically significant increases in BrdU-labeled cells and PSA-NCAM-positive cells occurred in the dentate gyrus following kindling. The increase in PSA-NCAM-immunoreactive neurites was confined to the entire stratum lucidum of CA3. Immunoelectron-microscopic examination also revealed that PSA-NCAM-positive immature synaptic terminals of the sprouting mossy fibers increased in the stratum lucidum of CA3 in the kindled rats. The increase in the numbers of PSA-NCAM-positive granule cells correlated well with the increase in the immunopositive neurites and synaptic terminals on the mossy fiber trajectory. The increase in these PSA-NCAM-immunopositive structures is thought to reflect the enhancement of sprouting and synaptogenesis of mossy fibers by a subset of granule cells newly generated during amygdaloid-kindling and suggests that the reorganization of the mossy fiber system on the normal trajectory at least in part contributes to the acquisition and maintenance of an epileptogenic state.

Radial glia express aromatase in the injured zebra finch brain

Estrogens have neurotrophic and neuroprotective properties. The synthesis of estrogen occurs via the expression of aromatase. Previous studies have shown that injury to the vertebrate brain results in a rapid and dramatic up-regulation of aromatase expression in astrocytes around the lesion. As part of experiments examining injury-induced glial aromatization, we identified aromatase in radial glia of the zebra finch brain. Adult female zebra finches received a penetrating injury to the right hippocampus. Twenty-four hours after lesioning, birds were administered bromodeoxyuridine (BrdU) and sacrificed 2 hours, 1 day, or 7 days later. We determined the distribution of aromatase and BrdU labeling by using immunocytochemistry. Radial aromatase was localized to cells lining the lateral ventricle adjacent to the lesioned hippocampus. Injury also induced a dramatic accumulation of newly generated cells labeled with BrdU around the lesion. BrdU labeling was strongly associated with aromatase-positive radial fibers, suggesting the migration of newly generated cells along these fibers. In the songbird brain, estrogen supports neuronal recruitment and promotes the survival and addition of new neurons. The presence of aromatase in radial glia provides a mechanism of estrogen delivery to postmitotic cells. Radial aromatization may be a key feature in the repair of the vertebrate brain following neural injury.

Anatomy and physiology of chronic pain

Although much has been accomplished in the past several decades, treatment of chronic pain remains imperfect. This article presents the anatomy and physiology of the pain system along with the neurobiologic changes that occur in the establishment and maintenance of chronic pain states.

Differential patterns of glial fibrillary acidic protein-immunolabeling in the brain of adult lizards

The present study describes by means of immunohistochemistry the comparative distribution of glial fibrillary acidic protein (GFAP)-positive cells in the forebrain and midbrain of three species of lizards: Eumeces algeriensis, Scincoidae; Agama impalearis, Agamidae; Tarentola mauritanica, Gekkonidae. In the species studied, the different types and proportions of glial cells expressing GFAP showed considerable variation. These cells include radial glia, oval cells, tanycytes, ependymocytes, glia limitans, and astrocytes. In Eumeces, astrocytes are particularly abundant and their processes form numerous perivascular end-feet; in addition well-differentiated ependymal cells and glia limitans express GFAP. These mature glial features are concordant with the relatively advanced phylogenetic level of Eumeces. In Tarentola, relatively few GFAP-expressing glial cells are observed, consisting mainly of radial glia and tanycytes. These features indicate a relatively immature state of the glial cell populations in this species. In Agama, GFAP-immunostained cells are confined to the periventricular and subpial brain areas; the ventricular lining contains numerous GFAP-immunopositive tanycytes and well-differentiated glia limitans. This pattern indicates that the glial cell profile in Agama exhibits characteristics intermediate between Eumeces and Tarentola, a feature which is discordant with the relatively primitive phylogenetic level of Agamidae compared to Gekkonidae. Together, the results of the present study provide novel data on the characterization of GFAP-expressing cell populations in different species of lizards. We suggest that the different glial patterns observed in the lizard brain correlates with developmental and functional aspects.

Postnatal neurogenesis in the vasopressin and oxytocin-containing nucleus of the pig hypothalamus

The vasopressin and oxytocin-containing nucleus (VON) of the pig hypothalamus demonstrates dramatic postnatal growth in nucleus size, both volume and neuron number, during puberty, and continues to increase in size in the adult sexually mature female pig throughout its reproductive prime. This study was designed to show that postnatal neurogenesis is responsible for the VON growth that occurs between adolescence and maturity. Recently divided neurosecretory cells of the hypothalamus were identified in adolescent and mature non-lactating female pigs using a sequential immunohistochemistry double-labeling technique with monoclonal mouse antibodies to detect vasopressin and proliferating cell nuclear antigen (PCNA), a protein associated with the S phase of the cell cycle. A computer-assisted image-analysis system was used to assess nucleus volume and neuron counts. The VON of the mature dry sows was significantly larger in volume and number of vasopressin neurons than the VON of the adolescent pigs. Double-labeled cells were noted in the VON of both adolescent and mature dry sows, but the number and proportion of double-labeled cells was significantly higher in adolescent pigs. Our results indicate the presence of neurons containing PCNA in the VON of the pig hypothalamus. This suggests that mitosis of neurogenic precursors plays a role in the growth of the nucleus.

Recent advances in stem cell neurobiology

1. Neural stem cells can be cultured from the CNS of different mammalian species at many stages of development. They have an extensive capacity for self-renewal and will proliferate ex vivo in response to mitogenic growth factors or following genetic modification with immortalising oncogenes. Neural stem cells are multipotent since their differentiating progeny will give rise to the principal cellular phenotypes comprising the mature CNS: neurons, astrocytes and oligodendrocytes. 2. Neural stem cells can also be derived from more primitive embryonic stem (ES) cells cultured from the blastocyst. ES cells are considered to be pluripotent since they can give rise to the full cellular spectrum and will, therefore, contribute to all three of the embryonic germ layers: endoderm, mesoderm and ectoderm. However, pluripotent cells have also been derived from germ cells and teratocarcinomas (embryonal carcinomas) and their progeny may also give rise to the multiple cellular phenotypes contributing to the CNS. In a recent development, ES cells have also been isolated and grown from human blastocysts, thus raising the possibility of growing autologous stem cells when combined with nuclear transfer technology. 3. There is now an emerging recognition that the adult mammalian brain, including that of primates and humans, harbours stem cell populations suggesting the existence of a previously unrecognised neural plasticity to the mature CNS, and thereby raising the possibility of promoting endogenous neural reconstruction. 4. Such reports have fuelled expectations for the clinical exploitation of neural stem cells in cell replacement or recruitment strategies for the treatment of a variety of human neurological conditions including Parkinson's disease (PD), Huntington's disease, multiple sclerosis and ischaemic brain injury. Owing to their migratory capacity within the CNS, neural stem cells may also find potential clinical application as cellular vectors for widespread gene delivery and the expression of therapeutic proteins. In this regard, they may be eminently suitable for the correction of genetically-determined CNS disorders and in the management of certain tumors responsive to cytokines. Since large numbers of stem cells can be generated efficiently in culture, they may obviate some of the technical and ethical limitations associated with the use of fresh (primary) embryonic neural tissue in current transplantation strategies. 5. While considerable recent progress has been made in terms of developing new techniques allowing for the long-term culture of human stem cells, the successful clinical application of these cells is presently limited by our understanding of both (i) the intrinsic and extrinsic regulators of stem cell proliferation and (ii) those factors controlling cell lineage determination and differentiation. Although such cells may also provide accessible model systems for studying neural development, progress in the field has been further limited by the lack of suitable markers needed for the identification and selection of cells within proliferating heterogeneous populations of precursor cells. There is a further need to distinguish between the committed fate (defined during normal development) and the potential specification (implying flexibility of fate through manipulation of its environment) of stem cells undergoing differentiation. 6. With these challenges lying ahead, it is the opinion of the authors that stem-cell therapy is likely to remain within the experimental arena for the foreseeable future. In this regard, few (if any) of the in vivo studies employing neural stem cell grafts have shown convincingly that behavioural recovery can be achieved in the various model paradigms. Moreover, issues relating to the quality control of cultured cells and their safety following transplantation have only begun to be addressed. 7. While on the one hand cell biotechnologists have been quick to realise the potential commercial value, human stem cell research and its clinical applications has been the subject of intense ethical and legislative considerations. The present chapter aims to review some recent aspects of stem cell research applicable to developmental neurobiology and the potential applications in clinical neuroscience.

Primitive neuroectodermal tumors in the central nervous system following cranial irradiation: a report of four cases

BACKGROUND

Radiation induced intracranial neoplasms are uncommon but well described and include gliomas, meningiomas, and sarcomas. The development of primitive neuroectodermal tumors (PNETs) following prophylactic craniospinal irradiation has been infrequently reported previously. The authors present four additional cases of PNETs that developed after previous cranial irradiation.

METHODS

Four patients who had previously been irradiated were determined to have PNETs of the central nervous system characterized by histopathologic and immunohistochemical features. The average patient age at diagnosis of the initial tumors and cranial irradiation was 17 years. The PNETs developed 5, 11, 11, and 18 years after completion of radiation.

RESULTS

Three patients had posterior fossa tumors, one pilocytic astrocytoma, one low grade astrocytoma, and one malignant ependymoma, which had been diagnosed and treated in childhood. Two of those patients developed supratentorial PNETs and the third a cerebellar hemispheric PNET. The fourth patient developed a posterior fossa PNET following irradiation for a temporal astrocytoma, which was initially diagnosed and resected at 37 years of age. Mean survival was 12 months after diagnosis.

CONCLUSIONS

The development of PNETs after cranial irradiation may be more common than thought previously and should be considered in the differential diagnosis of irradiation induced neoplasms. Survival after diagnosis of these radiation induced PNETs was short, and this may reflect an inability to provide standard therapy used for primary PNETs.

Proliferation and apoptosis in the developing human neocortex

The cell kinetics of the developing central nervous system (CNS) is determined by both proliferation and apoptosis. In the human neocortex at week 6 of gestation, proliferation is confined to the ventricular zone, where mitotic figures and nuclear immunoreactivity for proliferating cell nuclear antigen (PCNA) are detectable. Cell division is symmetric, with both daughter cells reentering mitosis. At week 7, the subventricular zone, a secondary proliferative zone, appears. It mainly gives rise to local circuit neurons and glial cells. Around week 12, the ventricular and subventricular zones are thickest, and the nuclear PCNA label is strongest, indicating that proliferation peaks at this stage. Thereafter, asymmetric division becomes the predominant mode of proliferation, with one daughter cell reentering mitosis and the other one migrating out. Towards late gestation, the ventricular and subventricular zones almost completely disappear and proliferation shifts towards the intermediate and subplate zones, where mainly glial cells are generated. A remnant of the subventricular zone with proliferative activity persists into adulthood. In general, proliferation follows a latero-medial gradient in the neocortex lasting longer in its lateral parts. Apoptotic nuclei have been detected around week 5, occurring in low numbers in the ventricular zone at this stage. Apoptotic cell death increases around midgestation and then spreads throughout all cortical layers, with most dying cells located in the ventricular and subventricular zones. This spatial distribution of apoptosis extends into late gestation. During the early postnatal period, most apoptotic cells are still located in the subcortical layers. During early embryonic development, proliferation and apoptosis are closely related, and are probably regulated by common regulators. In the late fetal and early postnatal periods, when proliferation has considerably declined in all cortical layers, apoptosis may occur in neurons whose sprouting axons do not find their targets.

Effects of excess thyroid hormone on cell death, cell proliferation, and new neuron incorporation in the adult zebra finch telencephalon

Widespread telencephalic neuronal replacement occurs throughout life in birds. We explored the potential relationship between thyroxine (T4) and cell turnover in the adult male zebra finch. We found that many cells in the zebra finch brain, including long-projection neurons in the high vocal center (HVC), stained positively with an antibody to thyroid hormone receptors (TR). Labeling was generally weak in the ventricular zone (VZ) that gives rise to new neurons but some proliferative VZ cells and/or their progeny, identified by [3H]-thymidine labeling, co-labeled with anti-TR antibody. Acute T4 treatment dramatically increased the number of pyknotic and TUNEL-positive cells in HVC and other telencephalic regions. In contrast, degenerating cells were never observed in the archistriatum or sub-telencephalic regions, suggesting that excess T4 augments cell death selectively in regions that show naturally occurring neuronal turnover. VZ mitotic activity was not altered shortly after acute T4 treatment at a dosage that stimulated cell death, although [3H]-labeling intensity per cell was slightly reduced. Moreover, the incorporation rates for neurons formed shortly before or after acute hormone treatment were no different from control values. Chronic T4 treatment resulted in a reduction in the total number of HVC neurons. Thus, hyperthyroidism augmented neuronal death, which was not compensated for by neuronal replacement. Collectively, these results indicate that excess T4 affects adult neuronal turnover in birds, and raises the possibility that thyroxine plays an important role in the postnatal development of the avian brain and vocal behavior.

Coordinated interaction of neurogenesis and angiogenesis in the adult songbird brain

Neurogenesis proceeds throughout life in the higher vocal center (HVC) of the adult songbird neostriatum. Testosterone induces neuronal addition and endothelial division in HVC. We asked if testosterone-induced angiogenesis might contribute importantly to HVC neuronal recruitment. Testosterone upregulated both VEGF and its endothelial receptor, VEGF-R2/Quek1/KDR, in HVC. This yielded a burst in local HVC angiogenesis. FACS-isolated HVC endothelial cells produced BDNF in a testosterone-dependent manner. In vivo, HVC BDNF rose by the third week after testosterone, lagging by over a week the rise in VEGF and VEGF-R2. In situ hybridization revealed that much of this induced BDNF mRNA was endothelial. In vivo, both angiogenesis and neuronal addition to HVC were substantially diminished by inhibition of VEGF-R2 tyrosine kinase. These findings suggest a causal interaction between testosterone-induced angiogenesis and neurogenesis in the adult forebrain.

Exogenous growth factors induce the production of ganglion cells at the retinal margin

Neural progenitors at the retinal margin of the post-hatch chicken normally produce amacrine and bipolar cells, but not photoreceptor or ganglion cells. The purpose of this study was to test whether exogenous growth factors influence the types of cells produced by progenitors at the retinal margin. We injected insulin, FGF2 or a combination of insulin and FGF2 into the vitreous chamber of post-hatch chickens. To assay for growth factor-induced changes at the retinal margin, we used in situ hybridization and immunocytochemistry on cryosections. One day after the final injection, we found that insulin alone stimulated the addition of cells to the retinal margin, but this was not further increased when FGF2 was applied with insulin. Insulin alone increased the number of cells in the progenitor zone that expressed neurofilament, and this was further increased when FGF2 was applied with insulin. These neurofilament-expressing cells in the progenitor zone included differentiating neurons that expressed Islet1 or Hu. Four days after the final dose of growth factor, we found that the production of ganglion cells was induced by co-injection of insulin and FGF2, but not by either insulin or FGF2 alone. We conclude that the types of cells produced by progenitors at the retinal margin can be altered by exogenous growth factors and that normally the microenvironment imposes limitations on the types of neurons produced.

Recovery from spinal cord injury: a new transection model in the C57Bl/6 mouse

Spinal cord transections in mammalian animal models lead to loss of motor function. In this study, we show that functional recovery from complete transection of the adult mouse spinal cord can in fact occur without any intervention if dural injury along with displacement of the ends of the cut cord and fibroblastic infiltration is minimized. Underlying this function is the expression of GAP-43 in axonal growth cones, axonal extension and bridging of the injury site indicated by biocytin retrograde tracing and neuronal remodeling of both the white matter and the gray matter. Such studies suggest a new murine model for the study of spinal cord regeneration.

Cell proliferation and differentiation from ependymal, subependymal and choroid plexus cells in response to stroke in rats

We tested the hypothesis that populations of ependymal, subependymal and choroid plexus cells proliferate and differentiate after stroke in adult rats. Rats were subjected to 2 h of middle cerebral artery occlusion (n=70) and euthanized at 1, 2, 4, 7, 14, 21 and 28 days (10 per time point). Hematoxylin and eosin staining and immunostaining were performed using antibodies against bromodeoxyuridine, neuronal nuclear antigen and glial fibrillary acidic protein after stroke. In normal nonischemic rats (n=10), single layers of ependymal and choroid plexus cells do not react with bromodeoxyuridine, neuronal nuclear antigen or glial fibrillary acidic protein. Individual subependymal cells express glial fibrillary acidic protein and bromodeoxyuridine, but not neuronal nuclear antigen. After stroke, increased bromodeoxyuridine reactivity was present in multiple layers of ependymal cells and nodules of subependymal cells and in scattered choroid plexus cells from 2 to 28 days and peaked at 7 days. Bromodeoxyuridine immunoreactivity colocalized with neural phenotypes of neuronal nuclear antigen (approximately 0.1-3.5%) and glial fibrillary acidic protein (approximately 8.6%) immunoreactivity in cells in the ventricular zone and the subventricular zone, as well as in the choroid plexus of the ischemia affected hemisphere. Our data suggest that ependymal, subependymal and choroid plexus cells are potential neural precursor cells in the adult mammalian brain.

Neural stem cells and regulation of cell number

Normal CNS development involves the sequential differentiation of multipotent stem cells. Alteration of the numbers of stem cells, their self-renewal ability, or their proliferative capacity will have major effects on the appropriate development of the nervous system. In this review, we discuss different mechanisms that regulate neural stem cell differentiation. Proliferation signals and cell cycle regulators may regulate cell kinetics or total number of cell divisions. Loss of trophic support and cytokine receptor activation may differentially contribute to the induction of cell death at specific stages of development. Signaling from differentiated progeny or asymmetric distribution of specific molecules may alter the self-renewal characteristics of stem cells. We conclude that the final decision of a cell to self-renew, differentiate or remain quiescent is dependent on an integration of multiple signaling pathways and at each instant will depend on cell density, metabolic state, ligand availability, type and levels of receptor expression, and downstream cross-talk between distinct signaling pathways.

Adult-generated hippocampal and neocortical neurons in macaques have a transient existence

Previously we reported that new neurons are added to the hippocampus and neocortex of adult macaque monkeys. Here we compare the production and survival of adult-generated neurons and glia in the dentate gyrus, prefrontal cortex, and inferior temporal cortex. Twelve adult macaques were injected with the thymidine analogue BrdUrd, and the phenotypes of labeled cells were examined after 2 h, 24 h, 2 wk, 5 wk, 9 wk, and 12 wk by using the following immunocytochemical markers: for immature and mature neurons, class III beta-tubulin (TuJ1); for mature neurons, neuronal nuclei; for astrocytes, glial fibrillary acidic protein; and for oligodendrocytes, 2',3'-cyclic nucleotide 3' phosphodiesterase. We found that the dentate gyrus had many more BrdUrd-labeled cells than either neocortical area. Furthermore, a greater percentage of BrdUrd-labeled cells expressed a neuronal marker in the dentate gyrus than in either neocortical area. The number of new cells in all three areas declined by 9 wk after BrdUrd labeling, suggesting that some of the new cells have a transient existence. BrdUrd-labeled cells also were found in the subventricular zone and in the white matter between the lateral ventricle and neocortex; some of the latter cells were double-labeled for BrdUrd and TuJ1. Adult neocortical neurogenesis is not restricted to primates. Five adult rats were injected with BrdUrd, and after a 3-wk survival time, there were cells double-labeled for BrdUrd and either TuJ1 or neuronal nuclei in the anterior neocortex as well as the dentate gyrus.

Expression of polysialylated neural cell adhesion molecule in rat brain after transient middle cerebral artery occlusion

The highly polysialylated form of neural cell adhesion molecule (PSA-NCAM) is important for neurite outgrowth. With this molecule as a marker of plastic change in neurons, we investigated its temporal expression in rat brain after transient middle cerebral artery (MCA) occlusion. In sham-control brain, only subependymal neurons showed a positive immunoreactivity for PSA-NCAM. After 90 min of transient MCA occlusion, neurons in the piriform cortex began to be positively stained at 1 h, while neurons in the cortex and caudate of the MCA territory became positive after 8 h. The stainings persisted for 1 and 3 days after reperfusion. The present results indicate that neurons in the cerebral cortex and caudate have the capability of plastic change in the adult brain, and that those in the piriform cortex rapidly undergo plastic change probably in response to transneuronal injury.

Reproductive status influences cell proliferation and cell survival in the dentate gyrus of adult female meadow voles: a possible regulatory role for estradiol

Galea and McEwen [Galea and McEwan (1999) Neuroscience 89, 955-964] found that cell proliferation was suppressed in female meadow voles trapped during the breeding season relative to females trapped during the non-breeding season. We investigated the effect of reproductive status and estradiol level on cell proliferation and cell survival in adult laboratory-reared female meadow voles to control for the variables of age, experience and pregnancy that could confound the results derived from a wild sample. Voles were housed in either a long- or short-photoperiod to simulate season and a male or female cage partner was introduced to influence reproductive status. Because females are reflex ovulators, exposure to a male rapidly induces behavioural estrous and high levels of estradiol. Forty-eight hours after introducing a cage partner, we injected either bromodeoxyuridine or [3H]thymidine to mark cell synthesis and then examined labelled cells 2h (cell proliferation) or five weeks (cell survival) later, respectively. To determine whether estradiol mimicked the effect of reproductive status, groups of reproductively inactive females were given a single injection of estradiol benzoate (10 microg) either four or 48h prior to bromodeoxyuridine labelling. The density of proliferating cells in the granule cell layer and the hilus was elevated in reproductively inactive females compared to reproductively active females and was correlated negatively with serum estradiol level. Exposure to estradiol benzoate initially increased cell proliferation (within 4h) but subsequently suppressed cell proliferation (within 48h). In addition, the density of surviving cells was greater in reproductively inactive females relative to reproductively active females but reproductively active females had a greater rate of cell survival than did reproductively inactive females. Reproductive status did not influence the number of pyknotic cells in the dentate gyrus (at either 2h or five weeks).We conclude that reproductive status regulates cell proliferation in adult female meadow voles, possibly via an estradiol-regulated mechanism. The results from the present study showed that reproductively active female meadow voles have suppressed rates of cell proliferation in the dentate gyrus relative compared with reproductively inactive female meadow voles. Administering estradiol initially (within 4h) elevates the cell proliferation within the dentate gyrus of adult females but subsequently (within 48h) suppresses cell proliferation. However, more new cells survived in females with high endogenous levels of estradiol (reproductively active females). In conclusion, reproductive status regulates the level of cell proliferation and survival through a complex estradiol regulated mechanism(s).

A unified hypothesis on the lineage of neural stem cells

For many years, it was assumed that neurons and glia in the central nervous system were produced from two distinct precursor pools that diverged early during embryonic development. This theory was partially based on the idea that neurogenesis and gliogenesis occurred during different periods of development, and that neurogenesis ceased perinatally. However, there is now abundant evidence that neural stem cells persist in the adult brain and support ongoing neurogenesis in restricted regions of the central nervous system. Surprisingly, these stem cells have the characteristics of fully differentiated glia. Neuroepithelial stem cells in the embryonic neural tube do not show glial characteristics, raising questions about the putative lineage from embryonic to adult stem cells. In the developing brain, radial glia have long been known to produce cortical astrocytes, but recent data indicate that radial glia might also divide asymmetrically to produce cortical neurons. Here we review these new developments and propose that the stem cells in the central nervous system are contained within the neuroepithelial --> radial glia --> astrocyte lineage.

Neuroprotection by estradiol

This review highlights recent evidence from clinical and basic science studies supporting a role for estrogen in neuroprotection. Accumulated clinical evidence suggests that estrogen exposure decreases the risk and delays the onset and progression of Alzheimer's disease and schizophrenia, and may also enhance recovery from traumatic neurological injury such as stroke. Recent basic science studies show that not only does exogenous estradiol decrease the response to various forms of insult, but the brain itself upregulates both estrogen synthesis and estrogen receptor expression at sites of injury. Thus, our view of the role of estrogen in neural function must be broadened to include not only its function in neuroendocrine regulation and reproductive behaviors, but also to include a direct protective role in response to degenerative disease or injury. Estrogen may play this protective role through several routes. Key among these are estrogen dependent alterations in cell survival, axonal sprouting, regenerative responses, enhanced synaptic transmission and enhanced neurogenesis. Some of the mechanisms underlying these effects are independent of the classically defined nuclear estrogen receptors and involve unidentified membrane receptors, direct modulation of neurotransmitter receptor function, or the known anti-oxidant activities of estrogen. Other neuroprotective effects of estrogen do depend on the classical nuclear estrogen receptor, through which estrogen alters expression of estrogen responsive genes that play a role in apoptosis, axonal regeneration, or general trophic support. Yet another possibility is that estrogen receptors in the membrane or cytoplasm alter phosphorylation cascades through direct interactions with protein kinases or that estrogen receptor signaling may converge with signaling by other trophic molecules to confer resistance to injury. Although there is clear evidence that estradiol exposure can be deleterious to some neuronal populations, the potential clinical benefits of estrogen treatment for enhancing cognitive function may outweigh the associated central and peripheral risks. Exciting and important avenues for future investigation into the protective effects of estrogen include the optimal ligand and doses that can be used clinically to confer benefit without undue risk, modulation of neurotrophin and neurotrophin receptor expression, interaction of estrogen with regulated cofactors and coactivators that couple estrogen receptors to basal transcriptional machinery, interactions of estrogen with other survival and regeneration promoting factors, potential estrogenic effects on neuronal replenishment, and modulation of phenotypic choices by neural stem cells.

EGF infusion stimulates the proliferation and migration of embryonic progenitor cells transplanted in the adult rat striatum

Immature progenitor cells (generated by in vitro propagation) may provide a useful alternative to primary cells (from dissected embryonic tissue) for transplantation if their migratory and proliferative and differentiation properties can be controlled and directed in vivo. In this study E15 murine EGF-responsive progenitor cells were transplanted to the striatum of adult rats. Simultaneously, these animals received continuous infusion of either epidermal growth factor (EGF) or vehicle, to the lateral ventricle, for 8 days. In animals that received EGF, the transplanted progenitors migrated toward the lateral ventricle and proliferated, as evidenced by bromodeoxyuridine incorporation. Progenitor cells transplanted to rats that received vehicle infusions showed neither of these responses. In all animals, transplanted progenitors expressed an immature astrocyte or oligodendrocyte phenotype, the majority of cells being astrocytes. We conclude that EGF stimulates the migration and proliferation of murine progenitor cells in vivo, either directly or indirectly, but does not influence their phenotypic differentiation.