Postnatal neurogenesis in the olfactory bulbs of a lizard. A tritiated thymidine autoradiographic study

Broadening the functional and evolutionary understanding of postnatal neurogenesis using reptilian models

The production of new neurons in the brains of adult animals was first identified by Altman and Das in 1965, but it was not until the late 20th century when methods for visualizing new neuron production improved that there was a dramatic increase in research on neurogenesis in the adult brain. We now know that adult neurogenesis is a ubiquitous process that occurs across a wide range of taxonomic groups. This process has largely been studied in mammals; however, there are notable differences between mammals and other taxonomic groups in how, why and where new neuron production occurs. This Review will begin by describing the processes of adult neurogenesis in reptiles and identifying the similarities and differences in these processes between reptiles and model rodent species. Further, this Review underscores the importance of appreciating how wild-caught animals vary in neurogenic properties compared with laboratory-reared animals and how this can be used to broaden the functional and evolutionary understanding of why and how new neurons are produced in the adult brain. Studying variation in neural processes across taxonomic groups provides an evolutionary context to adult neurogenesis while also advancing our overall understanding of neurogenesis and brain plasticity.

Adult Hippocampal Neurogenesis in Different Taxonomic Groups: Possible Functional Similarities and Striking Controversies

Adult neurogenesis occurs in many species, from fish to mammals, with an apparent reduction in the number of both neurogenic zones and new neurons inserted into established circuits with increasing brain complexity. Although the absolute number of new neurons is high in some species, the ratio of these cells to those already existing in the circuit is low. Continuous replacement/addition plays a role in spatial navigation (migration) and other cognitive processes in birds and rodents, but none of the literature relates adult neurogenesis to spatial navigation and memory in primates and humans. Some models developed by computational neuroscience attribute a high weight to hippocampal adult neurogenesis in learning and memory processes, with greater relevance to pattern separation. In contrast to theories involving neurogenesis in cognitive processes, absence/rarity of neurogenesis in the hippocampus of primates and adult humans was recently suggested and is under intense debate. Although the learning process is supported by plasticity, the retention of memories requires a certain degree of consolidated circuitry structures, otherwise the consolidation process would be hampered. Here, we compare and discuss hippocampal adult neurogenesis in different species and the inherent paradoxical aspects.

New neurons for injured brains? The emergence of new genetic model organisms to study brain regeneration

Neuronal circuits in the adult brain have long been viewed as static and stable. However, research in the past 20 years has shown that specialized regions of the adult brain, which harbor adult neural stem cells, continue to produce new neurons in a wide range of species. Brain plasticity is also observed after injury. Depending on the extent and permissive environment of neurogenic regions, different organisms show great variability in their capacity to replace lost neurons by endogenous neurogenesis. In Zebrafish and Drosophila, the formation of new neurons from progenitor cells in the adult brain was only discovered recently. Here, we compare properties of adult neural stem cells, their niches and regenerative responses from mammals to flies. Current models of brain injury have revealed that specific injury-induced genetic programs and comparison of neuronal fitness are implicated in brain repair. We highlight the potential of these recently implemented models of brain regeneration to identify novel regulators of stem cell activation and regenerative neurogenesis.

Tangential migratory pathways of subpallial origin in the embryonic telencephalon of sharks: evolutionary implications

Tangential neuronal migration occurs along different axes from the axis demarcated by radial glia and it is thought to have evolved as a mechanism to increase the diversity of cell types in brain areas, which in turn resulted in increased complexity of functional networks. In the telencephalon of amniotes, different embryonic tangential pathways have been characterized. However, little is known about the exact routes of migrations in basal vertebrates. Cartilaginous fishes occupy a key phylogenetic position to assess the ancestral condition of vertebrate brain organization. In order to identify putative subpallial-derived tangential migratory pathways in the telencephalon of sharks, we performed a detailed analysis of the distribution pattern of GAD and Dlx2, two reliable markers of tangentially migrating interneurons of subpallial origin in the developing forebrain. We propose the existence of five tangential routes directed toward different telencephalic regions. We conclude that four of the five routes might have emerged in the common ancestor of jawed vertebrates. We have paid special attention to the characterization of the proposed migratory pathway directed towards the olfactory bulbs. Our results suggest that it may be equivalent to the "rostral migratory stream" of mammals and led us to propose a hypothesis about its evolution. The analysis of the final destinations of two other streams allowed us to identify the putative dorsal and medial pallium of sharks, the regions from which the neocortex and hippocampus might have, respectively, evolved. Derived features were also reported and served to explain some distinctive traits in the morphology of the telencephalon of cartilaginous fishes.

How neurogenesis finds its place in a hardwired sensory system

So far most studies on adult neurogenesis aimed to unravel mechanisms and molecules regulating the integration of newly generated neurons in the mature brain parenchyma. The exceedingly abundant amount of results that followed, rather than being beneficial in the perspective of brain repair, provided a clear evidence that adult neurogenesis constitutes a necessary feature to the correct functioning of the hosting brain regions. In particular, the rodent olfactory system represents a privileged model to study how neuronal plasticity and neurogenesis interact with sensory functions. Until recently, the vomeronasal system (VNS) has been commonly described as being specialized in the detection of innate chemosignals. Accordingly, its circuitry has been considered necessarily stable, if not hard-wired, in order to allow stereotyped behavioral responses. However, both first and second order projections of the rodent VNS continuously change their synaptic connectivity due to ongoing postnatal and adult neurogenesis. How the functional integrity of a neuronal circuit is maintained while newborn neurons are continuously added—or lost—is a fundamental question for both basic and applied neuroscience. The VNS is proposed as an alternative model to answer such question. Hereby the underlying motivations will be reviewed.

From pattern to purpose: how comparative studies contribute to understanding the function of adult neurogenesis

The study of adult neurogenesis has had an explosion of fruitful growth. Yet numerous uncertainties and challenges persist. Our review begins with a survey of species that show evidence of adult neurogenesis. We then discuss how neurogenesis varies across brain regions and point out that regional specializations can indicate functional adaptations. Lifespan and aging are key life-history traits. Whereas 'adult neurogenesis' is the common term in the literature, it does not reflect the reality of neurogenesis being primarily a 'juvenile' phenomenon. We discuss the sharp decline with age as a universal trait of neurogenesis with inevitable functional consequences. Finally, the main body of the review focuses on the function of neurogenesis in birds and mammals. Selected examples illustrate how our understanding of avian and mammalian neurogenesis can complement each other. It is clear that although the two phyla have some common features, the function of adult neurogenesis may not be similar between them and filling the gaps will help us understand neurogenesis as an evolutionarily conserved trait to meet particular ecological pressures.

Proliferation, neurogenesis and regeneration in the non-mammalian vertebrate brain

Post-embryonic neurogenesis is a fundamental feature of the vertebrate brain. However, the level of adult neurogenesis decreases significantly with phylogeny. In the first part of this review, a comparative analysis of adult neurogenesis and its putative roles in vertebrates are discussed. Adult neurogenesis in mammals is restricted to two telencephalic constitutively active zones. On the contrary, non-mammalian vertebrates display a considerable amount of adult neurogenesis in many brain regions. The phylogenetic differences in adult neurogenesis are poorly understood. However, a common feature of vertebrates (fish, amphibians and reptiles) that display a widespread adult neurogenesis is the substantial post-embryonic brain growth in contrast to birds and mammals. It is probable that the adult neurogenesis in fish, frogs and reptiles is related to the coordinated growth of sensory systems and corresponding sensory brain regions. Likewise, neurons are substantially added to the olfactory bulb in smell-oriented mammals in contrast to more visually oriented primates and songbirds, where much fewer neurons are added to the olfactory bulb. The second part of this review focuses on the differences in brain plasticity and regeneration in vertebrates. Interestingly, several recent studies show that neurogenesis is suppressed in the adult mammalian brain. In mammals, neurogenesis can be induced in the constitutively neurogenic brain regions as well as ectopically in response to injury, disease or experimental manipulations. Furthermore, multipotent progenitor cells can be isolated and differentiated in vitro from several otherwise silent regions of the mammalian brain. This indicates that the potential to recruit or generate neurons in non-neurogenic brain areas is not completely lost in mammals. The level of adult neurogenesis in vertebrates correlates with the capacity to regenerate injury, for example fish and amphibians exhibit the most widespread adult neurogenesis and also the greatest capacity to regenerate central nervous system injuries. Studying these phenomena in non-mammalian vertebrates may greatly increase our understanding of the mechanisms underlying regeneration and adult neurogenesis. Understanding mechanisms that regulate endogenous proliferation and neurogenic permissiveness in the adult brain is of great significance in therapeutical approaches for brain injury and disease.

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.

Comparative aspects of cerebral cortical development

This review aims to provide examples of how both comparative and genetic analyses contribute to our understanding of the rules for cortical development and evolution. Genetic studies have helped us to realize the evolutionary rules of telencephalic organization in vertebrates. The control of the establishment of conserved telencephalic subdivisions and the formation of boundaries between these subdivisions has been examined and the very specific alterations at the striatocortical junction have been revealed. Comparative studies and genetic analyses both demonstrate the differential origin and migratory pattern of the two basic neuron types of the cerebral cortex. GABAergic interneurons are mostly generated in the subpallium and a common mechanism governs their migration to the dorsal cortex in both mammals and sauropsids. The pyramidal neurons are generated within the cortical germinal zone and migrate radially, the earliest generated cell layers comprising preplate cells. Reelin-positive Cajal-Retzius cells are a general feature of all vertebrates studied so far; however, there is a considerable amplification of the Reelin signalling with cortical complexity, which might have contributed to the establishment of the basic mammalian pattern of cortical development. Based on numerous recent observations we shall present the argument that specialization of the mitotic compartments may constitute a major drive behind the evolution of the mammalian cortex. Comparative developmental studies have revealed distinct features in the early compartments of the developing macaque brain, drawing our attention to the limitations of some of the current model systems for understanding human developmental abnormalities of the cortex. Comparative and genetic aspects of cortical development both reveal the workings of evolution.

Regeneration of granule neurons after lesioning of hippocampal dentate gyrus: evaluation using adult mice treated with trimethyltin chloride as a model

The hippocampal dentate gyrus in adult animals is known to contain neural progenitors that proliferate and differentiate into neurons in response to brain injury. Little has been observed, however, on regeneration of the granule cell layer of the dentate gyrus that has been directly injured. Using trimethyltin (TMT)-treated mice as an in vivo model, we evaluated the ability of this layer to regenerate after injury. The administration of TMT induced neuronal death in the dentate gyrus selectively 2 days later, with recovery of granule neurons on day 14 and thereafter. At an early stage (days 2-5) after the damage by TMT treatment, 5-bromo-2'-deoxyuridine (BrdU) incorporation into at least two different types of cells was facilitated in the dentate gyrus: BrdU-positive/neuronal nuclear antigen (NeuN)-negative cells were found predominantly in the subgranular zone and granule cell layer, whereas BrdU-positive/NeuN-positive cells were numerous in the dentate molecular layer and hilus. In addition, expression of proliferating cell nuclear antigen, nestin, NeuroD3, and doublecortin, which are markers for proliferating cells and neural progenitors/neuronal precursors, was extremely enhanced in the dentate gyrus at the early stage after treatment. Double staining revealed that BrdU was colocalized with nestin and doublecortin in the subgranular zone. Behavioral analysis revealed that TMT-induced cognition impairment was ameliorated by day 14 after the treatment. Taken together, our data indicate that the hippocampal dentate gyrus itself is capable of regenerating the neuronal cell layer through rapid enhancement of neurogenesis after injury.

Neurogenesis and neuronal regeneration in the adult fish brain

Fish are distinctive in their enormous potential to continuously produce new neurons in the adult brain, whereas in mammals adult neurogenesis is restricted to the olfactory bulb and the hippocampus. In fish new neurons are not only generated in structures homologous to those two regions, but also in dozens of other brain areas. In some regions of the fish brain, such as the optic tectum, the new cells remain near the proliferation zones in the course of their further development. In others, as in most subdivisions of the cerebellum, they migrate, often guided by radial glial fibers, to specific target areas. Approximately 50% of the young cells undergo apoptotic cell death, whereas the others survive for the rest of the fish's life. A large number of the surviving cells differentiate into neurons. Two key factors enabling highly efficient brain repair in fish after injuries involve the elimination of damaged cells by apoptosis (instead of necrosis, the dominant type of cell death in mammals) and the replacement of cells lost to injury by newly generated ones. Proteome analysis has suggested well over 100 proteins, including two dozen identified ones, to be involved in the individual steps of this phenomenon of neuronal regeneration.

Proliferation, migration, neuronal differentiation, and long-term survival of new cells in the adult zebrafish brain

In contrast to mammals, fish exhibit an enormous potential to produce new cells in the adult brain. By labeling mitotically dividing cells with 5-bromo-2'-deoxyuridine (BrdU), we have characterized the development of these cells in the zebrafish (Danio rerio). Proliferation zones were located in specific regions of the olfactory bulb, dorsal telencephalon (including a region presumably homologous to the mammalian hippocampus), preoptic area, dorsal zone of the periventricular hypothalamus, optic tectum, torus longitudinalis, vagal lobe, parenchyma near the rhombencephalic ventricle, and in a region of the medulla oblongata lateral to the vagal motor nucleus, as well as in all three subdivisions of the cerebellum, the valvula cerebelli, the corpus cerebelli, and the lobus caudalis cerebelli. In the valvula cerebelli and the corpus cerebelli, the young cells migrated from their site of origin in the molecular layers to the corresponding granule cell layers. By contrast, in the lobus caudalis cerebelli and optic tectum, no indication of a migration of the newly generated cells over wider distances could be obtained. BrdU-labeled cells remained present in the brain over at least 292 days post-BrdU administration, indicating a long-term survival of a significant portion of the newly generated cells. The combination of BrdU immunohistochemistry with immunolabeling against the neural marker protein Hu, or with retrograde tracing, suggested a neuronal differentiation in a large portion of the young cells.

Unpacking the cognitive map: the parallel map theory of hippocampal function

In the parallel map theory, the hippocampus encodes space with 2 mapping systems. The bearing map is constructed primarily in the dentate gyrus from directional cues such as stimulus gradients. The sketch map is constructed within the hippocampus proper from positional cues. The integrated map emerges when data from the bearing and sketch maps are combined. Because the component maps work in parallel, the impairment of one can reveal residual learning by the other. Such parallel function may explain paradoxes of spatial learning, such as learning after partial hippocampal lesions, taxonomic and sex differences in spatial learning, and the function of hippocampal neurogenesis. By integrating evidence from physiology to phylogeny, the parallel map theory offers a unified explanation for hippocampal function.

Neurogenesis and gliogenesis in the spinal cord of turtles

A 5-bromo-3'-deoxyuridine (BrdU) pulse administered to juvenile turtles resulted in cell labeling throughout the gray matter (GM) and white matter (WM) of the spinal cord. One and twenty-four hours postinjection, larger densities of BrdU-labeled nuclei (LN) occurred within the GM, with a density peak localized in the central region (CR). Seven days later, density differences between GM and WM disappeared, accompanying a more uniform distribution of LN in the GM (absence of the central peak). Multiple injection experiments also showed similar evolution in the distribution of LN. Morphometric studies revealed that the size of LN had undergone time-related increments: Larger nuclei appeared at protracted fixation time points. Double-labeling experiments indicated that BrdU-labeled cells expressed neuroactive substances, such as gamma-aminobutyric acid (GABA), neuron-specific nuclear protein (NeuN), and the cytoplasmic early postmitotic neuronal marker (TUC-4). Other BrdU-labeled cells expressed the glial-specific protein (GFAP). GABA-BrdU, TUC-4-BrdU, and GFAP-BrdU double-labeled cells were recognized 6 days after the first BrdU injection. NeuN-BrdU double-labeled cells were found at 50 days postinjection. Three-dimensional transmission electron microscopy revealed the presence of synapses and typical kinocilia in putative immature nerve cells. Kinocilia were also found in putative immature glial cells. In consideration of the scattered distribution pattern of BrdU-labeled cells, in animals fixed 1 hour postinjection, the existence of a single proliferating center was discarded. The CR, including the ependymal epithelium, showed the highest density of LN.

The common properties of neurogenesis in the adult brain: from invertebrates to vertebrates

Until recently, it was believed that adult brains were unable to generate any new neurons. However, it is now commonly known that stem cells remain in the adult central nervous system and that adult vertebrates as well as adult invertebrates are currently adding new neurons in some specialized structures of their central nervous system. In vertebrates, the subventricular zone and the dentate gyrus of the hippocampus are the sites of neuronal precursor proliferation. In some insects, persistent neurogenesis occurs in the mushroom bodies, which are brain structures involved in learning and memory and considered as functional analogues of the hippocampus. In both vertebrates and invertebrates, secondary neurogenesis (including neuroblast proliferation and neuron differentiation) appears to be regulated by hormones, transmitters, growth factors and environmental cues. The functional implications of adult neurogenesis have not yet been clearly demonstrated and comparative study of the various model systems could contribute to better understand this phenomenon. Here, we review and discuss the common characteristics of adult neurogenesis in the various animal models studied so far.

Investigating the use of primary adult subventricular zone neural precursor cells for neuronal replacement therapies

With the relatively recent discovery that neurogenesis persists throughout life in restricted regions of the adult mammalian brain, including those of human beings, there has been great interest in the use of adult-derived neural stem cells for neuronal replacement. There are many great hurdles that must be overcome in order for such replacement strategies to succeed. In this review, we outline some of these hurdles and discuss recent experiments that investigate the potential of using neural precursor cells found in the subventricular zone of the adult brain for brain repair.

Neurogenesis in the olfactory bulb of adult zebrafish

Cell genesis in the adult brain of zebrafish, with specific reference to the olfactory bulbs, was examined using bromodeoxyuridine immunocytochemistry. Mature fish were exposed to a 1% solution of the thymidine analog 5-bromo-2'-deoxyuridine for 1 h and then killed after short (4-h) or long (3-4-week) survival periods. A monoclonal antibody to bromodeoxyuridine allowed visualization of cells that incorporated the drug during the S phase of mitosis. Four hours after administration of the drug, antibody-labeled cells were found almost exclusively in the proliferative zones around the ventricles and in the cerebellum. Very few labeled nuclei were seen in other locations in the brain, indicating that cell genesis occurs in discrete regions in adults. The few labeled profiles in the olfactory bulbs were located in the olfactory nerve layer; these profiles had the morphology of glial nuclei and did not stain with a neuronal marker, the Hu antibody. After longer survival times, labeled cells were present throughout the layers of the olfactory bulb, and many of the immunoreactive profiles in the internal cell layer were also labeled with the Hu antibody, indicating that they are likely adult-formed interneurons. Thus, neurogenesis continues in the olfactory bulb of adult zebrafish. Understanding the process of the generation of new neurons in the brain of adult animals can lead to important insights into neural regeneration and adult plasticity.

Astrocytes give rise to new neurons in the adult mammalian hippocampus

Neurogenesis in the dentate gyrus of the hippocampus persists throughout life in many vertebrates, including humans. The progenitors of these new neurons reside in the subgranular layer (SGL) of the dentate gyrus. Although stem cells that can self-renew and generate new neurons and glia have been cultured from the adult mammalian hippocampus, the in vivo primary precursors for the formation of new neurons have not been identified. Here we show that SGL cells, which express glial fibrillary acidic protein and have the characteristics of astrocytes, divide and generate new neurons under normal conditions or after the chemical removal of actively dividing cells. We also describe a population of small electron-dense SGL cells, which we call type D cells and are derived from the astrocytes and probably function as a transient precursor in the formation of new neurons. These results reveal the origins of new neurons in the adult hippocampus.

Postnatal neurogenesis in the telencephalon of turtles: evidence for nonradial migration of new neurons from distant proliferative ventricular zones to the olfactory bulbs

Postnatal neurogenesis in the the turtle telencephalon was investigated by using bromodeoxyuridine immunocytochemistry and [3H]thymidine autoradiography. Red-eared slider turtles Trachemys scripta elegans (Cryptodira, Emydidae) 2-3 months old were injected with the thymidine analogue 5'-bromodeoxyuridine (BrdU) and allowed to survive for 7, 30, 90, and 180 days. Results indicate that cells in the walls of the lateral ventricles continue to proliferate postnatally. Shortly after BrdU treatment (seven days) most labelled cells were found in the walls of the lateral ventricles (ventricular zone: VZ). Labelled cells were particularly abundant in and around the ventricular sulci. The same pattern of labelling was found in the telencephalon of juvenile turtles (> two years old) injected with BrdU and killed seven day later, suggesting that the proliferative activity continues in the telencephalic VZ of turtles during juvenile stages of life and possibly into adulthood. With longer survival periods after BrdU administration (30, 90, and 180 days), the VZ of the telencephalon showed a similar pattern of labelling to that found at seven days. Furthermore, with survival periods of 90 and 180 days labelled cells resembling neurons were found in most telencephalic regions. The largest numbers of these putative neurons were found in the olfactory bulbs. By using [3H]thymidine autoradiography combined with electron microscopy these postnatally generated cells were confirmed as neurons. We conclude that postnatal neurogenesis occurs in the turtle telencephalon. This process is most prominent in the olfactory bulbs. From the pattern of proliferation of neuronal precursors in the VZ we infer that neurons recruited postnatally into the olfactory bulbs come from distant proliferative VZs in the walls of the lateral ventricles.

3-Acetylpyridine-induced degeneration and regeneration in the adult lizard brain: a qualitative and quantitative analysis

The neurotoxin 3-acetylpyridine (3AP) produces highly selective neuronal damage in specific areas of the lizard brain. Following 3AP intoxication, proliferation and migration of replacement neurons born in the ventricular walls lead to regeneration of the lesioned areas. Earlier studies established the time course of 3AP-induced degeneration and subsequent regeneration in the medial cerebral cortex of adult lizards (Font, E., García-Verdugo, J.M., Alcántara, S. and Lopez-García, C., Neuron regeneration reverses 3-acetylpyridine-induced cell loss in the cerebral cortex of adult lizards, Brain Res., 551 (1991) 230-235 [13]). Complementary to our previous studies, we now provide a qualitative and quantitative account of the extent and distribution of neurotoxic damage in the brain as a whole of lizards treated with 3AP using Nissl and Golgi stains, a degeneration-sensitive reduced-silver method, and electron microscopy. Additionally, [3H]thymidine autoradiography was used to assess changes in the rate of neurogenesis caused by the 3AP treatment. Single doses of 3AP caused degenerative changes in all the cortical areas, anterior dorsal ventricular ridge, deep layers of the lateral cortex, lateral amygdaloid nucleus, and nucleus sphericus, while sparing other brain areas. The most frequent neuropathic change after 3AP treatment was clumping of the nuclear chromatin with formation of pyknotic nuclei. Occasionally, a second type of injury was observed in neurons of the cell layer of the dorsomedial cortex (DMC). 3AP also caused a conspicuous loss of dendritic spines in bipyramidal neurons of the dorsomedial and dorsal cortices possibly representing transneuronal degeneration. Numbers of [3H]thymidine-labeled cells were higher in lizards previously treated with 3AP than in controls. These results demonstrate that the neurotoxic lesion is capable of inducing an increase in the normal rate of adult neurogenesis. Whereas regeneration in the remaining areas was morphologically and histologically complete, in some animals, cell proliferation in the DMC resulted in formation of an abnormal cell plate.

Neurogenesis in the olfactory bulb of the frog Xenopus laevis shows unique patterns during embryonic development and metamorphosis

We determined the time of origin of neurons in the olfactory bulb of the South African clawed frog, Xenopus laevis. Tritiated thymidine injections were administered to frog embryos and tadpoles from gastrulation (stage 11/12) through metamorphosis (stage 65), paraffin sections were processed for autoradiography, and the distribution of heavily and lightly labeled cells was examined. In the ventral olfactory bulb, we observed that the mitral cells were born as early as stage 11/12 and continued to be generated through the end of metamorphosis. Interneurons (periglomerular and granule cells) were not born in the ventral bulb until stage 41, and birth of these cells also continued through metamorphosis. Labeled cells were observed in the accessory olfactory bulb, beginning at stage 41. In contrast, the cells of the dorsal olfactory bulb were not born until the onset of metamorphosis (stage 54); at this stage in the dorsal bulb, the genesis of mitral cells, interneurons, and glial cells completely overlapped. The results indicate that olfactory axon innervation is not necessary to induce early stages of neurogenesis in the ventral olfactory bulb. On the other hand, the results on the dorsal olfactory bulb are consistent with the hypothesis that innervation from new or transformed sensory neurons in the principal cavity induces neurogenesis in the dorsal bulb.

Adult neurogenesis in the telencephalon of a lizard: a [3H]thymidine autoradiographic and bromodeoxyuridine immunocytochemical study

A qualitative and quantitative evaluation of adult neurogenesis in the telencephalon of a lizard was conducted using [3H]thymidine autoradiography and immunocytochemical detection of 5'-bromodeoxyuridine (BrdU). The sites of cell proliferation, the fate, and the phenotype of cells born in adulthood were determined by short and long survival experiments (7 and 30 days). Adult specimens of the Moorish gecko, Tarentola mauritanica (Sauria, Gekkonidae), received subcutaneous injections of either BrdU or [3H]thymidine. Seven days after BrdU administration most labelled cells were in the walls (ventricular zone; VZ) of the lateral ventricles, particularly close to the sulcal zones. Labelled cells in the VZ of other ventricles were practically absent. Following 30 days, labelled cells outside the VZ were seen in most telencephalic regions: olfactory bulbs (OB), medial, dorsal, and lateral cortices (MC, DC, LC), anterior dorsal ventricular ridge (ADVR), nucleus sphericus (NS), and striatum (St). Electron microscopic examination of [3H]thymidine labelled cells found in these areas revealed their neuronal identity. No labelled neurons were detected in the dorsomedial cerebral cortex (DMC), septum (Sp), or elsewhere in the telencephalon. We conclude that neurogenesis occur in most regions of the adult lizard telencephalon. The most intense neuronal production occurs in the MC. Other telencephalic regions, such as the OB, ADVR, and NS, also show a considerable rate of adult neurogenesis. We infer from our results that the new neurons are born in the walls of the lateral ventricles and then migrate to their destinations. Furthermore, we report some evidence suggesting that neurons formed in adulthood in the olfactory bulbs are generated in the distant proliferative VZ. The functional significance of neurogenesis in the telencephalon of adult lizards is unclear, but it seems to be restricted to areas that have been implicated in olfactory plasticity, learning, and memory.

The postembryonic development of somatostatin immunoreactivity in the central posterior/prepacemaker nucleus of weakly electric fish, Apteronotus leptorhynchus: a double-labelling study

The neuropeptide somatostatin (SS) is widely distributed in both the central and peripheral nervous system of vertebrates. Its widespread distribution is paralleled by a large variety of diverse functions. While embryonic and perinatal development of SS-like immunoreactivity have been well examined, little is known about the postnatal development of this neuropeptide. Since, in teleosts, neurogenesis persists in many brain regions during adulthood, these vertebrates are well suited to investigate this phenomenon. In the present study, we have, therefore, examined the development of somatostatinergic cells born during adulthood in the central posterior/prepacemaker nucleus (CP/PPn) of Apteronotus leptorhynchus, a weakly electric gymnotiform fish. This was achieved by labelling proliferating cells with the thymidine analogue 5-bromo-2'-deoxyuridine (BrdU) and by simultaneous immunocytochemical detection of SS-like immunoreactivity. SS-like immunoreactivity is adopted in a period between 2 days and 3.5 days after birth. While the number of BrdU-labelled cells in the CP/PPn decreases 10 days after birth, the percentage of double-labelled cells among the BrdU-labelled cells remains with 1.0-7.6% in the period between 3.5 days and 100 days after birth rather constant. This percentage matches well the fraction of SS-positive cells in the total population of cells present in the CP/PPn.

Expression of polysialylated N-CAM in the central nervous system of adult canaries and its possible relation to function

Polysialylated neuronal cell adhesion molecule (PSA-N-CAM) is a cell surface molecule associated with neurons that undergo changes in configuration or spatial translocation. In both cases, this molecule is thought to reduce the adhesivity of these cells or of their processes, which can thereby insinuate themselves into the existing parenchyma. We used a monoclonal antibody specific to PSA to offer what we believe is the first account of the distribution of PSA-N-CAM in the adult songbird brain. This antibody stained a diversity of cell classes and processes, as follows: 1) a subset of ventricular zone cells; 2) migrating cells thought to be neuroblasts; 3) a subset of differentiated neurons; 4) some brain surface astrocytes; 5) some tanycytes; 6) the neuropil of some regions; 7) some axonal fibers; and 8) possibly some synapses. Our results demonstrate also, for the first time, the wide distribution of a very numerous population of migrating cells in the telencephalon and the seasonal regulation of PSA-N-CAM expression in a part of the adult brain known to undergo seasonal changes in cell recruitment and function. However, we did not find PSA-N-CAM associated with young migrating cells in the high vocal center (HVC), nor was there PSA-N-CAM in the robust nucleus of the archistriatum (RA), which is known to receive new axonal endings from HVC. In these instances spatial translocation may occur with the assistance of other surface molecules.

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.

Proliferation zones in the brain of adult gymnotiform fish: a quantitative mapping study

Whereas in mammals postnatal neurogenesis, gliogenesis, and angiogenesis appear to be kept at low rates, in fish the capability for the production of new brain cells during adulthood is very pronounced. Many of the newly generated cells originate from germinal layers that maintain their proliferative activity during adulthood. By employing incorporation of the thymidine analogue 5-bromo-2'-deoxyuridine (BrdU) into mitotic active cells, we have quantitatively mapped such proliferation zones in the brain of adult Apteronotus leptorhynchus (Gymnotiformes, Teleostei). In the telencephalon, diencephalon, mesencephalon, and rhombencephalon, the total number of BrdU-labelled cells was low, making up approximately 25% of all mitotic active cells in the brain. Many of these cells were scattered over wide areas. Otherwise, zones of high proliferative activity were typically located at or near the surface of ventricular, paraventricular, and cisternal systems. Approximately 75% of all BrdU-labelled cells found in the brain of adult Apteronotus leptorhynchus were situated in the cerebellum. Zones displaying proliferative activity were restricted to small areas, such as narrow stripes around the midline of corpus cerebelli and valvula cerebelli, the boundary between corpus and valvula, and a large portion of the area covered by the eminentia granularis medialis. Counts indicate that, on average, 100,000 cells, corresponding to approximately 0.2% of the total population of cells in the brain of adult Apteronotus leptorhynchus, are in S-phase within a period of 2 hours. At least part of these newly generated cells is added to the population of already existing cells. This leads to a permanent growth of the brain with increasing size of the fish, a process that appears to slow down only in individuals of relatively advanced age.

Neurogenesis persists in the subependymal layer of the adult mouse brain

Neurogenesis of olfactory bulb granule cells is known to persist in adult rats where, in some strains, the bulbs grow throughout life. In mice, bulb growth ceases early in adulthood and here we ask if granule cell neurogenesis persists after the bulbs have stopped growing. By injecting adult mice with bromodeoxyuridine (BrdU) and allowing short and long survival times, we found that new cells form in the subependymal layer and that they migrate subsequently into the olfactory bulbs where they acquire the nuclear morphology of granule cells and express neuron-specific markers. Using [3H]thymidine, we found that most of these adult-generated granule neurons persist within the bulbs for at least 16 weeks. This shows the persistence of neurogenesis and neuronal migration in adult animals in which the olfactory bulbs have stopped growing.

Neuron regeneration reverses 3-acetylpyridine-induced cell loss in the cerebral cortex of adult lizards

Systemic administration of the neurotoxin 3-acetylpyridine to adult lizards results in extensive loss of neurons in the medial cerebral cortex, other brain areas remaining largely unaffected. After the neurotoxic trauma, new cells are produced by mitotic division of cells in the ventricular wall. The new cells migrate along radial glial fibers and replace lost neurons in the medial cortex. Electron microscopic examination of cells labeled with [3H]thymidine confirms that the newly generated cells are neurons. Thus, neuron regeneration can occur in the cerebral cortex of adult lizards.

Incorporation of 3H-thymidine in telencephalic structures of the vomeronasal and olfactory systems of embryonic garter snakes

Previous studies have shown that the vomeronasal and, possibly, olfactory systems are functional in newborn garter snakes. However, little is known about neurogenesis in these chemosensory pathways. In the companion paper, we describe the embryonic growth of the sensory epithelia for both the vomeronasal and olfactory systems. In the present study, we examine neurogenesis in the telencephalic structures of these chemosensory systems by using 3H-thymidine autoradiography (ARG). The majority of neurogenesis appears to occur before birth in the accessory and main olfactory bulbs and their principal projection sites, the nucleus sphericus and lateral cortex, respectively. The data suggest that some postnatal neurogenesis may occur in the accessory and main olfactory bulbs and in the nucleus sphericus. Although the neuronal constituents of the accessory and main olfactory bulbs appear to mature concurrently, those of the lateral cortex appear to mature before those of the nucleus sphericus. Along with previous findings, this latter result supports the hypothesis that the olfactory system develops before the vomeronasal system in garter snakes. There appears to be a rostral to caudal gradient of neurogenesis within the mural layer of the nucleus sphericus. However, an "outside to inside" gradient of neurogenesis was not observed in the mantle layer of the lateral cortex, as described for other reptiles. Similarities and differences observed by other investigators in other reptilian species and mammals are discussed.

Mechanism of neurogenesis in adult avian brain

Adult neurogenesis in birds offers unique opportunities to study basic questions addressing the birth, migration and differentiation of neurons. Neurons in adult canaries originate from discrete proliferative regions on the walls of the lateral ventricles. They migrate away from their site of birth, initially at high rates, along the processes of radical cells. The rates of dispersal diminish as the young neurons invade regions devoid of radial fibers, probably under the guidance of other cues. The discrete sites of birth in the ventricular zone generate neurons that end up differentiating throughout the telencephalon. New neurons may become interneurons or projection neurons; the latter connect two song control nuclei between neostriatum and archistriatum. Radial cells, that in mammals disappear as neurogenesis comes to an end, persist in the adult avian brain. The presence of radial cells may be key to adult neurogenesis. Not only do they serve as guides for initial dispersal, they also divide and may be the progenitors of new neurons.

Postnatal neurogenesis in the nucleus sphericus of the lizard, Podarcis hispanica

Autoradiography was used to demonstrate the genesis and migration of cells in the nucleus sphericus of perinatal, juvenile and adult lizards, Podarcis hispanica (Sauria, Lacertidae). Following intraperitoneal injections of [3H]thymidine (5 microCi/g b. w.) and survival times of 1, 7, 18 and 28 days, labelled cells were found in the ependyma, marginal layer and mural layer of the nucleus sphericus in specimens of all ages. After short survival times, most labelled cells were located in the ependymal layer. Longer survival times resulted in labelling of cells mainly in the mural layer. At intermediate survival times, a substantial number of labelled cells was also found in the marginal layer. The time course of labelling suggests that neuronal proliferation takes place in the ependyma surrounding the ventricle. Subsequently, the newly formed neurons migrate centripetally and are recruited into the mural layer.