Ultrastructural radioautographic analysis of neurogenesis in the hypothalamus of the adult frog, Rana temporaria, with special reference to physiological regeneration of the preoptic nucleus. II. Types of neuronal cells produced

Neurogenesis and pesticides: news of no new neurons

New hippocampal neurons are continuously generated in the adult human brain. Several studies have demonstrated that the proliferation of hippocampal cells is strongly influenced by a variety of stimuli, including pesticides exposure. These effects are particularly important because neurogenesis dysregulation could be associated with the decline of neuronal and cognitive functions and the possible development of neuropsychiatric disorders.

Adult neurogenesis in the telencephalon of the lizard Podarcis liolepis

In adult lizards, new neurons are generated from neural stem cells in the ventricular zone of the lateral ventricles. These new neurons migrate and integrate into the main telencephalic subdivisions. In this work we have studied adult neurogenesis in the lizard Podarcis liolepis (formerly Podarcis hispanica) by administering [³H]-thymidine and bromodeoxyuridine as proliferation markers and euthanizing the animals at different survival times to determine the identity of progenitor cells and to study their lineage derivatives. After short survival times, only type B cells are labeled, suggesting that they are neural stem cells. Three days after administration, some type A cells are labeled, corresponding to recently formed neuroblasts. Type A cells migrate to their final destinations, where they differentiate into mature neurons and integrate into functional circuits. Our results after long survival periods suggest that, in addition to actively dividing type B cells, there is also a type B subpopulation with low proliferative activity. We also found that new neurons incorporated into the olfactory bulb are generated both in situ, in the walls of the anterior extension of the lateral ventricle of the olfactory bulbs, but also at more caudal levels, most likely in anterior levels of the sulcus ventralis/terminalis. These cells follow a tangential migration toward the olfactory bulbs where they integrate. We hypothesized that at least part of the newly generated neurons would undergo a specialization process over time. In support of this prediction, we found two neuronal populations in the cellular layer of the medial cortex, which we named type I and II neurons. At intermediate survival times (1 month) only type II neurons were labeled with [³H]-thymidine, while at longer survival times (3, 6, or 12 months) both type I and type II neurons were labeled. This study sheds light on the ultrastructural characteristics of the ventricular zone of P. liolepis as a neurogenic niche, and adds to our knowledge of the processes whereby newly generated neurons in the adult brain migrate and integrate into their final destinations.

Characterisation and origins of melanin-concentrating hormone immunoreactive fibres of the posterior lobe of the pituitary and median eminence during lactation in the Long-Evans rat

Although the melanin-concentrating hormone (MCH) and its coding mRNA are predominantly found in the tuberal hypothalamus, there is detectable synthesis of MCH in the preoptic hypothalamus exclusively in lactating dams, suggesting a participation of MCH in the alterations that take place after parturition. Also implicated in the dam physiology is oxytocin, a neurohormone released from the posterior pituitary that is necessary for milk ejection. Because the projection fields from oxytocin-immunoreactive (-IR) neurones and the mediobasal preoptic hypothalamus overlap and MCH-IR neurones are found in proximity to oxytocin neurones, we investigated the spatial relationship between MCH and oxytocin fibres. Accordingly, we employed multiple immunohistochemistry labelling for MCH and oxytocin for light and electron microscopy techniques, in addition to i.v. tracer injection combined with in situ hybridisation to identify MCH neurones that project to neurosecretory areas. As described for other strains, lactating Long-Evans dams also display immunoreactivity for MCH in the preoptic hypothalamus on days 12 and 19 of lactation. The appearance of these neurones is contemporaneous with an increase in MCH-IR fibres in both the internal layer of the median eminence and the posterior pituitary. In both regions, MCH- and oxytocin-IR fibres were found in great proximity, although there was no evidence for synaptic interaction between these two populations at the ultrastructural level. The tracer injection revealed that only mediobasal preoptic MCH neurones project to the posterior pituitary, suggesting a neuroendocrine-modulatory role for this population. When taken together, the results obtained in the present study indicate that neuroplasticity events at the mediobasal preoptic hypothalamus that occur during late lactation may be part of a neuroendocrinology control loop involving both MCH and oxytocin.

Comparative Views of Adult Neurogenesis

Traditional views maintain that the generation of neurons within the mammalian brain is restricted to a discrete developmental period. This perspective has undergone significant revision during the later half of this century, culminating recently with the demonstration of neurogenesis in the brains of adult primates, including humans. Although it is becoming increasingly clear that adult neurogenesis represents an important mode of structural modification for the adult brain, its functional significance has not been determined. The production and survival of new neurons in the adult mammalian brain is regulated by both experiential and neuroendocrine factors, suggesting that adult-generated neurons may serve as a substrate by which these cues influence normal brain function. This article reviews significant advances that have led to the discovery of neurogenesis in adult mammals and examines comparative data suggesting that adult neurogenesis may play a role in certain forms of learning. Neural activity associated with behavioral experience is known to result in changes in brain structure and connectivity, for example, by modifying synapse number, axonal sprouting, dendrite length and branching, or synaptic strength. In the case of adult neurogenesis, experience may shape neural networks by directing the production and connectivity of whole cell populations.

Steroid modulation of neurogenesis: Focus on radial glial cells in zebrafish

Estrogens are known as steroid hormones affecting the brain in many different ways and a wealth of data now document effects on neurogenesis. Estrogens are provided by the periphery but can also be locally produced within the brain itself due to local aromatization of circulating androgens. Adult neurogenesis is described in all vertebrate species examined so far, but comparative investigations have brought to light differences between vertebrate groups. In teleost fishes, the neurogenic activity is spectacular and adult stem cells maintain their mitogenic activity in many proliferative areas within the brain. Fish are also quite unique because brain aromatase expression is limited to radial glia cells, the progenitor cells of adult fish brain. The zebrafish has emerged as an interesting vertebrate model to elucidate the cellular and molecular mechanisms of adult neurogenesis, and notably its modulation by steroids. The main objective of this review is to summarize data related to the functional link between estrogens production in the brain and neurogenesis in fish. First, we will demonstrate that the brain of zebrafish is an endogenous source of steroids and is directly targeted by local and/or peripheral steroids. Then, we will present data demonstrating the progenitor nature of radial glial cells in the brain of adult fish. Next, we will emphasize the role of estrogens in constitutive neurogenesis and its potential contribution to the regenerative neurogenesis. Finally, the negative impacts on neurogenesis of synthetic hormones used in contraceptive pills production and released in the aquatic environment will be discussed.

Restricted nature of adult neural stem cells: re-evaluation of their potential for brain repair

Neural stem cells (NSCs) in the walls of the lateral ventricles continue to produce new neurons and oligodendrocytes throughout life. The identification of NSCs, long-range neuronal migration, and the integration of new neurons into fully formed mature neural circuits-all in the juvenile or adult brain-has dramatically changed concepts in neurodevelopment and suggests new strategies for brain repair. Yet, the latter has to be seen in perspective: NSCs in the adult are heterogeneous and highly regionally specified; young neurons derived from these primary progenitors migrate and integrate in specific brain regions. Neurogenesis appears to have a function in brain plasticity rather than brain repair. If similar processes could be induced in regions of the brain that are normally not a target of new neurons, therapeutic neuronal replacement may one day reinstate neural circuit plasticity and possibly repair broken neural circuits.

Evolution of oxytocin pathways in the brain of vertebrates

The central oxytocin system transformed tremendously during the evolution, thereby adapting to the expanding properties of species. In more basal vertebrates (paraphyletic taxon Anamnia, which includes agnathans, fish and amphibians), magnocellular neurosecretory neurons producing homologs of oxytocin reside in the wall of the third ventricle of the hypothalamus composing a single hypothalamic structure, the preoptic nucleus. This nucleus further diverged in advanced vertebrates (monophyletic taxon Amniota, which includes reptiles, birds, and mammals) into the paraventricular and supraoptic nuclei with accessory nuclei (AN) between them. The individual magnocellular neurons underwent a process of transformation from primitive uni- or bipolar neurons into highly differentiated neurons. Due to these microanatomical and cytological changes, the ancient release modes of oxytocin into the cerebrospinal fluid were largely replaced by vascular release. However, the most fascinating feature of the progressive transformations of the oxytocin system has been the expansion of oxytocin axonal projections to forebrain regions. In the present review we provide a background on these evolutionary advancements. Furthermore, we draw attention to the non-synaptic axonal release in small and defined brain regions with the aim to clearly distinguish this way of oxytocin action from the classical synaptic transmission on one side and from dendritic release followed by a global diffusion on the other side. Finally, we will summarize the effects of oxytocin and its homologs on pro-social reproductive behaviors in representatives of the phylogenetic tree and will propose anatomically plausible pathways of oxytocin release contributing to these behaviors in basal vertebrates and amniots.

Hypothalamic neurogenesis in the adult brain

Adult-born new neurons are continuously added to the hippocampus and the olfactory bulb to serve aspects of learning and perceptual functions. Recent evidence establishes a third neurogenic niche in the ventral hypothalamic parenchyma surrounding the third ventricle that ensures the plasticity of specific brain circuits to stabilize physiological functions such as the energy-balance regulatory system. Hypothalamic lesion studies have demonstrated that regions associated with reproduction-related functions are also capable of recruiting newborn neurons to restore physiological functions and courtship behavior. Induced by lesion or other stimulation, elevated neurotrophic factors trigger neurogenic cascades that contribute to remodeling of certain neural circuits to meet specific transient functions. This insight raises the possibility that event-specific changes, such as increased GnRH, may be mediated by courtship-sensitive neurotrophic factors. We will discuss the potentially integral and ubiquitous roles of neurogenesis in physiological and biological phenomena, roles that await future experimental exploration.

Comparative aspects of adult neural stem cell activity in vertebrates

At birth or after hatching from the egg, vertebrate brains still contain neural stem cells which reside in specialized niches. In some cases, these stem cells are deployed for further postnatal development of parts of the brain until the final structure is reached. In other cases, postnatal neurogenesis continues as constitutive neurogenesis into adulthood leading to a net increase of the number of neurons with age. Yet, in other cases, stem cells fuel neuronal turnover. An example is protracted development of the cerebellar granular layer in mammals and birds, where neurogenesis continues for a few weeks postnatally until the granular layer has reached its definitive size and stem cells are used up. Cerebellar growth also provides an example of continued neurogenesis during adulthood in teleosts. Again, it is the granular layer that grows as neurogenesis continues and no definite adult cerebellar size is reached. Neuronal turnover is most clearly seen in the telencephalon of male canaries, where projection neurons are replaced in nucleus high vocal centre each year before the start of a new mating season--circuitry reconstruction to achieve changes of the song repertoire in these birds? In this review, we describe these and other examples of adult neurogenesis in different vertebrate taxa. We also compare the structure of the stem cell niches to find common themes in their organization despite different functions adult neurogenesis serves in different species. Finally, we report on regeneration of the zebrafish telencephalon after injury to highlight similarities and differences of constitutive neurogenesis and neuronal regeneration.

Adult neural stem cells bridge their niche

Major developments in the neural stem cell (NSC) field in recent years provide new insights into the nature of the NSC niche. In this perspective, we integrate recent anatomical data on the organization of the two main neurogenic niches in the adult brain, the ventricular-subventricular zone (V-SVZ) and the subgranular zone (SGZ), with signaling pathways that control the behavior of NSCs. NSCs in the adult brain stretch into physiologically distinct compartments of their niche. We propose how adult NSCs' morphology may allow these cells to integrate multiple signaling pathways arising from unique locations of their niche.

Hypothesis

Circannual rhythms are innately timed long-term (tau ≈ 12 months) cycles of physiology and behavior, crucial for life in habitats ranging from the equator to the Poles. Here the authors propose that circannual rhythm generation depends on tissue-autonomous, reiterated cycles of cell division, functional differentiation, and cell death. They see the feedback control influencing localized stem cell niches as crucial to this cyclical histogenesis hypothesis. Analogous to multi-oscillator circadian organization, circannual rhythm generation occurs in multiple tissues with hypothalamic and pituitary sites serving as central pacemakers. Signals including day length, nutrition, and social factors can synchronize circannual rhythms through hormonal influences, notably via the thyroid and glucocorticoid axes, which have profound effects on histogenesis. The authors offer 4 arguments in support of this hypothesis: (1) Cyclical histogenesis is a prevalent process in seasonal remodeling of physiology. It operates over long time domains and exhibits tissue autonomy in its regulation. (2) Experiments in which selected peripheral endocrine signals are held constant indicate that circannual rhythms are not primarily the product of interacting hormonal feedback loops. (3) Hormones known to control cell proliferation, differentiation, and organogenesis profoundly affect circannual rhythm expression. (4) The convergence point between photoperiodic input pathways and circannual rhythm expression occurs in histogenic regions of the hypothalamus and pituitary. In this review, the authors discuss how testing this hypothesis will depend on the use of cellular/molecular tools and animal models borrowed from developmental biology and neural stem cell research.

Sex-specific modulation of cell proliferation by socially relevant stimuli in the adult green treefrog brain (Hyla cinerea)

Social experience plays an important role in regulating the neural, physiological and hormonal changes that accompany the expression of reproductive behavior in vertebrates. This suite of functions is sexually dimorphic, with different neural control areas preeminent in males and females. In anuran amphibians, social experience comes in the form of acoustic communication, which is central to their reproductive behavior. We sought to determine whether acoustic cues regulate cell proliferation in the brain of adult green treefrogs (Hyla cinerea). Our results show that both male and female treefrogs that heard a conspecific chorus during the breeding season exhibited increased brain cell proliferation compared to animals that heard random tones. Increased cell proliferation, as assessed by the number of 5-bromo-2'-deoxyuridine-immunoreactive (BrdU+) cells, were found near the ventricles of acoustically sensitive brain regions such as the preoptic area (POA) and the infundibular hypothalamus (IF). Sex differences emerged in the location of this socially modulated cell proliferation: increases occurred primarily in the male POA and the female IF. In addition, gonadal steroid hormones might have played a role in the social modulation of cell proliferation: by statistically control- ling for hormone level, we revealed that androgens might influence socially induced increases in BrdU+ cells in the male POA, but estrogen did not contribute to socially induced increases in the female IF. These results indicate that the reception of social cues increases cell proliferation in brain regions mediating sexual behavior and endocrine regulation, and moreover that social modulation of cell proliferation occurs in a sexually differentiated fashion.

Cell proliferation and death in the brain of active and hibernating frogs

'Binomial' cell proliferation and cell death have been studied in only a few non-mammalian vertebrates, such as fish. We thought it of interest to map cell proliferation/apoptosis in the brain of the frog (Rana esculenta L.) as this animal species undergoes, during the annual cycle, physiological events that could be associated with central nervous system damage. Therefore, we compared the active period and the deep underground hibernation of the frog. Using western blot analysis for proliferating cell nuclear antigen (PCNA), we revealed a positive 36 kDa band in all samples and found higher optical density values in the hibernating frogs than in active frogs. In both active and hibernating frogs, we found regional differences in PCNA-immunoreactive cells and terminal transferase dUTP nick-end labelling apoptotic cells in the ventricular zones and parenchyma areas of the main encephalon subdivisions. During the active period of the frogs, the highest concentration of PCNA-immunoreactive cells was found in the ventricle dorsal zone of the cerebral hemispheres but only some of the cells were apoptotic. By contrast, the tectal and cerebellar ventricular zones had a small or medium amount of PCNA-immunoreactive cells, respectively, and a higher number of apoptotic cells. During hibernation, an increased PCNA-immunoreactive cell number was observed in both the brain ventricles and parenchyma compared with active frogs. This increase was primarily evident in the lateral ventricles, a region known to be a proliferation 'hot spot'. Although differences existed among the brain areas, a general increase of apoptotic cell death was found in hibernating frogs, with the highest number of apoptotic cells being detected in the parenchyma of the cerebral hemispheres and optic tectum. In particular, the increased number of apoptotic cells in the hibernating frogs compared with active frogs in the parenchyma of these brain areas occurred when cell proliferation was higher in the corresponding ventricular zones. We suggest that the high number of dying cells found in the parenchymal regions of hibernating frogs might provide the stimulus for the ventricular zones to proliferate. Hibernating frogs could utilize an increased cell proliferation in the brain areas as a neuroprotective strategy to face cell death and the onset of neurological damages. Therefore, the hibernator promises to be a valuable model for studying the mechanisms naturally carried out by the central nervous system in order to adapt itself or survive adverse conditions.

Neurogenic and neuroendocrine effects of goldfish pheromones

Goldfish (Carassius auratus) use reproductive hormones as endocrine signals to synchronize sexual behavior with gamete maturation and as exogenous signals (pheromones) to mediate spawning interactions between conspecifics. We examined the differential effects of two hormonal pheromones, prostaglandin F(2alpha) (PGF(2alpha)) and 17alpha,20beta-dihydroxy-4-pregnen-3-one (17,20beta-P) on neurogenesis, neurotransmission, and neuronal activities, and on plasma androstenedione (AD) levels. Exposure to waterborne PGF(2alpha) induced a multitude of changes in male goldfish brain. Histological examination indicated an increase in the number of dividing cells in male diencephalon (p < 0.05, Kruskal-Wallis test). Real-time quantitative PCR tests showed elevated levels of transcripts for the salmon gonadotropin-releasing hormone (GnRH) in the male telencephalon and cerebellum (p < 0.005, one-way ANOVA) and for ChAT (choline acetyltransferase) in the male vagal lobe and the brainstem underneath the vagal lobe (p < 0.05, one-way ANOVA). Therefore, PGF(2alpha) seemed to modulate male brain plasticity that coincided with behavioral changes during spawning season. Exposure to waterborne 17,20beta-P, however, increased circulatory levels of immunoreactive AD in males and the transcripts of androgen receptor and cGnRH-II (chicken-II GnRH) in the female cerebellum (p < 0.05, one-way ANOVA). PGF(2alpha) and 17,20beta-P thereby seemed to act through distinct pathways to elicit different responses in the neuroendocrine system. This is the first finding that links a specific pheromone molecule (PGF(2alpha)) to neurogenesis in a vertebrate animal.

Cell proliferation in the Drosophila adult brain revealed by clonal analysis and bromodeoxyuridine labelling

BACKGROUND

The production of new neurons during adulthood and their subsequent integration into a mature central nervous system have been shown to occur in all vertebrate species examined to date. However, the situation in insects is less clear and, in particular, it has been reported that there is no proliferation in the Drosophila adult brain.

RESULTS

We report here, using clonal analysis and 5'-bromo-2'-deoxyuridine (BrdU) labelling, that cell proliferation does occur in the Drosophila adult brain. The majority of clones cluster on the ventrolateral side of the antennal lobes, as do the BrdU-positive cells. Of the BrdU-labelled cells, 86% express the glial gene reversed polarity (repo), and 14% are repo negative.

CONCLUSION

We have observed cell proliferation in the Drosophila adult brain. The dividing cells may be adult stem cells, generating glial and/or non-glial cell types.

Adult hippocampal neurogenesis and sudden unexpected death in epilepsy: reality or just an attractive history?

Neurogenesis persists throughout life in the adult mammalian dentate gyrus and is regulated by several environmental, physiological, and molecular factors. Seizure activity also influences dentate granule cell neurogenesis. In these lines, studies of neurogenesis have demonstrated the presence of hilar-ectopic dentate granule cells after status epilepticus induced experimentally and that these cells are migrate aberrantly, abnormally integrated and hyperexcitable, contributing with this to seizure generation and/or propagation. As we know, epilepsy is the most common serious neurological condition and sudden unexpected death in epilepsy (SUDEP) is the most important direct epilepsy-related cause of death. Information concerning risk factors for SUDEP is conflicting, but high seizure frequency is a potential risk factor. Additionally, potential pathomechanisms for SUDEP are unknown, but it is very probable that cardiac arrhythmias during and between seizures or transmission of epileptic activity to the heart via the autonomic nervous system potentially play a role. Based on these facts, in this paper we postulate that aberrant neurogenesis could influence negatively the cardiovascular system of the patient with epilepsy leading to cardiac abnormalities and hence SUDEP.

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.

Neurogenesis in the amygdala: A new etiologic hypothesis of autism?

Neurogenesis studies had an increased development after BrdU (5-bromo-3'-deoxyuridine), a marker of cell proliferation. Today, several studies have showed the relevance of neurogenesis in the hippocampal formation. Notwithstanding, other brains areas have been described presenting neurogenesis, including the amygdala. This key structure is a complex cerebral region which has been associated with social behaviors and the emotional significance of the daily experiences. Several studies have associated the amygdala to the autism, a severe neurodevelopmental disorder. In this paper, we discuss the hypothesis of neurogenesis in the amygdala as a contributing cause of autism. The social skills require competent new neuronal connections, including efficient plasticity synaptic rearranging. Interestingly, emotional context cannot be imprinting in mature neurons in the presence of GABA, a neurotransmitter release during new environments experiences. However, it is known that new neurons are not well responsive to GABA stimulation, allowing the long-term potentiation necessary for the learning process. Based on these evidence it is tantalizing to hypothesize that the sociability impairment seen in some individuals with autism may partly be assigned to impaired regulation of the GABAergic system and to the impact of this impairment on the adequate functioning of the amygdala and on its capacity to store new experiences and to modulate the plasticity of the corticostriatal connections.

Adult neurogenesis in non-mammalian vertebrates

Adult neurogenesis is an exciting and rapidly advancing field of research. It addresses basic biological questions, such as the how and why of de novo neuronal production during adulthood, as well as medically relevant issues, including the potential link between adult neural stem cells and psychiatric disorders, or how stem cell manipulation might be used as a strategy for neuronal replacement. Current research mainly focuses on rodents, but we review here recent examination of non-mammalian vertebrates, which demonstrates that bona fide adult neural stem cells exist in these species. Importantly, especially in teleost fish, these cells can be abundant and located in various brain areas. Hence, non-mammalian vertebrate species provide invaluable comparative material for extracting core mechanisms of adult neural stem cell maintenance and fate.

Cell proliferation in the forebrain and midbrain of the adult bullfrog, Rana catesbeiana

The distribution of proliferating cells in the midbrain, thalamus, and telencephalon of adult bullfrogs (Rana catesbeiana) was examined using immunohistochemistry for the thymidine analog 5-bromo-2'-deoxyuridine (BrdU) and DNA dot-blotting. At all time points examined (2 to 28 days post-injection), BrdU-labeled cells were located in ventricular zones at all levels of the neuraxis, but with relatively more label around the telencephalic ventricles. Labeled cells, some showing profiles indicative of dividing and migrating cells, were present in brain parenchyma from 7 to 28 days post-injection. These labeled cells were particularly numerous in the dorsal and ventral hypothalamus, preoptic area, optic tectum, and laminar and principal nuclei of the torus semicircularis, with label also present, but at qualitatively reduced levels, in thalamic and telencephalic nuclei. Double-label immunohistochemistry using glial and early neural markers indicated that gliogenesis and neurogenesis both occurred, with new neurons observed particularly in the hypothalamus, optic tectum, and torus semicircularis. In all brain areas, many cells not labeled with BrdU were nonetheless labeled with the early neural marker TOAD-64, indicating that these cells were postmitotic. Incorporation of DNA measured by dot-blotting confirms the presence of DNA synthesis in the forebrain and brainstem at all time points measured. The pattern of BrdU label confirms previous experiments based on labeling with (3)H-thymidine and proliferating cell nuclear antigen showing cell proliferation in the adult ranid brain, particularly in hypothalamic nuclei. The consistent appearance of new cells in the hypothalamus of adult frogs suggests that proliferative activity may be important in mediating reproductive behaviors in these animals.

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.

Generation and long-term persistence of new neurons in the adult zebrafish brain: A quantitative analysis

Zebrafish, like other teleosts, are distinguished by their enormous potential to produce new neurons in many parts of the adult brain. By labeling S-phase cells with the thymidine analog 5-bromo-2'-deoxyuridine (BrdU), quantitative analysis demonstrated that, on average, 6000 new cells were generated in the entire adult brain within any 30 min period. This corresponds to roughly 0.06% of the total number of brain cells. Part of these cells underwent a second round of cell division a few days after their generation so that 10 days post-BrdU administration, when the cells have exited the mitotic cycle, approximately 10,000 BrdU-labeled cells were present in the entire brain. At post-BrdU survival times of 446-656 days, on average 4600 BrdU-labeled cells were found, suggesting that approximately 46% of the cells present at 10 days persisted in the adult zebrafish brain. Combination of BrdU-labeling of mitotic cells with immunostaining against Hu showed that roughly 47% of the BrdU-labeled cells that persisted in the brain expressed this neuronal marker protein. Taken together, the results of this investigation demonstrate that at least half of the cells generated in the adult zebrafish brain develop into neurons and are likely to persist for the rest of the fish's life.

Hypoxia tolerance in reptiles, amphibians, and fishes: life with variable oxygen availability

The ability of fishes, amphibians, and reptiles to survive extremes of oxygen availability derives from a core triad of adaptations: profound metabolic suppression, tolerance of ionic and pH disturbances, and mechanisms for avoiding free-radical injury during reoxygenation. For long-term anoxic survival, enhanced storage of glycogen in critical tissues is also necessary. The diversity of body morphologies and habitats and the utilization of dormancy have resulted in a broad array of adaptations to hypoxia in lower vertebrates. For example, the most anoxia-tolerant vertebrates, painted turtles and crucian carp, meet the challenge of variable oxygen in fundamentally different ways: Turtles undergo near-suspended animation, whereas carp remain active and responsive in the absence of oxygen. Although the mechanisms of survival in both of these cases include large stores of glycogen and drastically decreased metabolism, other mechanisms, such as regulation of ion channels in excitable membranes, are apparently divergent. Common themes in the regulatory adjustments to hypoxia involve control of metabolism and ion channel conductance by protein phosphorylation. Tolerance of decreased energy charge and accumulating anaerobic end products as well as enhanced antioxidant defenses and regenerative capacities are also key to hypoxia survival in lower vertebrates.

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.

Myosin16b: The COOH-tail region directs localization to the nucleus and overexpression delays S-phase progression

Rat Myo16a and Myo16b comprise the founding members of class XVI myosin and are characterized by an N-terminal ankyrin repeat domain thought to mediate an association with protein phosphatase 1 catalytic subunits 1alpha and 1gamma. Myo16b is the principal isoform and reveals predominant expression in developing neural tissue. Here, we use COS-7 cells as a model system to develop an understanding of Myo16b function. We find that Myo16b displays predominant localization in the nucleus of cells transitioning through interphase, but is not associated with processes of mitosis. Using a panel of EGFP-Myo16b-expression plasmids in transient transfection studies, we identified the COOH-terminal residues 1616-1912 as necessary and solely sufficient to target Myo16b to the nucleus. We show that the Myo16b-tail region directs localization to a nuclear compartment containing profilin and polymerized actin, which appears to form a three-dimensional meshwork through the depth of the nucleus. Further, we demonstrate that this compartment localizes within euchromatic regions of the genome and contains proliferating cell nuclear antigen (PCNA) and cyclin A, both markers of S-phase of the cell cycle. Cells transiently expressing Myo16b or Myo16b-tail region show limited incorporation of BrdU, delayed progression through S-phase of the cell cycle, and curtailed cellular proliferation.

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.

Isolation, cultivation, and differentiation of neural stem cells from adult fish brain

In contrast to mammals, teleost fish are distinct in their ability to continuously produce a tremendous number of new neurons in many regions of the adult brain. In the present study, we have isolated intrinsic stem cells from the telencephalon, corpus cerebelli, and valvula cerebelli of the teleost Apteronotus leptorhynchus and examined their properties in vitro. After 3-4 days in culture, neurospheres developed that grew through cell proliferation and reached diameters of up to 140 microm within 3 weeks. An increase in the number of developing neurospheres could be promoted by addition of epidermal growth factor or basic fibroblast growth factor, but no additive effect was observed after combined treatment. The number of neurospheres could furthermore be enhanced by seeding brain cells at densities of approximately 1 x 10(6). Differentiation conditions were optimal by exposing neurospheres to 10% fetal bovine serum and laminin as coating substrate. Neurosphere cells gave rise to both neurons, immunopositive for Hu-C/D or MAP2 (2a + 2b), and glial cells, immunopositive for glial fibrillary acidic protein or vimentin. Since, in addition to their multipotency, the cells isolated from the adult teleostean brain exhibited the ability for self-renewal, we hypothesize that they are true stem cells.

Proliferative activity in the frog brain: a PCNA-immunohistochemistry analysis

By means proliferating cell nuclear antigen (PCNA) immunohistochemistry, we have provided a detailed neuroanatomical mapping of proliferative activity during development and adulthood in the frog (Rana esculenta) brain. Western blot analysis confirmed the presence of this protein in brain extracts from adults and tadpoles. Proliferative activity was observed in the ventricular and subventricular zones throughout the brain. The present study provides details as to which of the morphologically distinguishable brain region(s) has a long-lasting proliferative activity and in which region this activity undergoes a progressive decrease during development. In the subventricular zones of the third ventricle, PCNA-labeled cells were particularly abundant in the magnocellular preoptic nucleus and the ventromedial thalamic nucleus. It was observed that proliferation zones are present practically in all major subdivisions of the forebrain, midbrain and hindbrain, including the cerebellum in which PCNA-labeled cells were located in the outer granular layer and the inner molecular layer. The habenulae, epiphysis and isthmic nuclei never showed the presence of PCNA-immunoreactive nuclei. The widespread proliferative activity implies that the frog brain has a great potential for neurogenesis/gliogenesis not only during larval development but also in the adulthood.

Plasticity of nonapeptidergic neurosecretory cells in fish hypothalamus and neurohypophysis

The structure and function of nonapeptidergic neurosecretory cells (NP-NSC) are considered in terms of comparative morphology. Among NSC of different ergicity for NP-NSC the most characteristic involve massive accumulation and storage of neurohormonal products. Only in NP-NSC are the secretory cycles of functioning clearly expressed. Their highest reactivity is established during experimental and physiological stresses. In contrast, liberinergic, statinergic, and monoaminergic NSC, unlike NP-NSC, are characterized even in the "norm" by a constantly high level of extrusion processes. As signs of maximum NP-NSC plasticity, we consider the largest size of elementary neurosecretory granules, the diversity of secretion forms, and the maximum development of Herring bodies-clear manifestations of secretory cycles of functioning. In particular, phases of massive storage of neurosecretory granules in the extrusion cycle of NP-NSC neurosecretory terminals express accumulation of neurosecretory products. It is concluded that a particularly high degree of plasticity of NP-NSC is provided by their capability for functional reversion. This reversion is manifested first in the form of the restoration of the initial moderate level of functioning and especially in the accumulation of neurosecretory products. The reversion is considered an important mechanism providing a high degree of NSC plasticity. This degree turns out to be sufficient for participation of NP-NSC in the integration of fish reproduction. It is shown that NP-NSC are organized by the principle of a triad of the balanced system. This system consists of two alternative states: accumulation and release of neurosecretory products and the center of control of dynamics of their interrelations, the self-regulating center. In the latter, the key role is probably played by the Golgi complex.

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.

Neurogênese e depressão: etiologia ou nova ilusão?

Novos neurônios continuam sendo gerados no cérebro adulto de diversas espécies animais. Muitos estudos têm demonstrado que diversos fatores ambientais, inclusive o estresse, influenciam a proliferação de células hipocampais. Nesse sentido, a diminuição da neurogênese induzida pelo estresse parece ser um importante fator na etiologia da depressão. Nessa revisão, a relação entre neurogênese e depressão é enfatizada.

Neurogenesis in the ependymal layer of the adult rat 3rd ventricle

Neurogenesis has been described in limited regions of the adult mammalian brain. In this study, we showed that the ependymal layer of the 3rd ventricle is a neurogenic region in the adult rat brain. DiI labeling of the 3rd ventricle revealed that neural progenitor cells were derived from cells at the ependymal layer of the adult 3rd ventricle. The mitosis of these progenitor cells at the ependymal layer was promoted by bFGF administration. Combination of BrdU administration, nestin/GFAP immunohistochemistry, and labeling by GFP-recombinant adenoviral infection (vGFP) indicated that at least some tanycytes might be neural progenitor cells in the ependymal layer of the 3rd ventricle. Tracing by vGFP indicated that neural progenitor cells may have migrated from the 3rd ventricle to the hypothalamic parenchyma, where they were integrated into neural networks by forming synapses. In addition, some BrdU(+) neurons had immunoreactivity for orexin A in the hypothalamus. These results indicate that neural progenitor cells exist in the ependymal layer of the adult rat 3rd ventricle and that they may differentiate into neurons functioning in the hypothalamus.

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.

The proliferative ventricular zone in adult vertebrates: a comparative study using reptiles, birds, and mammals

Although evidence accumulated during the last decades has advanced our understanding of adult neurogenesis in the vertebrate brain, many aspects of this intriguing phenomenon remain controversial. Here we review the organization and cellular composition of the ventricular wall of reptiles, birds, and mammals in an effort to identify differences and commonalities among these vertebrate classes. Three major cell types have been identified in the ventricular zone of reptiles and birds: migrating (Type A) cells, radial glial (Type B) cells, and ependymal (Type E) cells. Cells similar anatomically and functionally to Types A, B, and E have also been described in the ventricular wall of mammals, which contains an additional cell type (Type C) not found in reptiles or birds. The bulk of the evidence points to a role of Type B cells as primary neural precursors (stem cells) in the three classes of living amniotic vertebrates. This finding may have implications for the development of strategies for the possible treatment of human neurological disorders.

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.

Chasing fate and function of new neurons in adult brains

Neuron production, migration and differentiation are major developmental events that continue, on a smaller scale, into adult life in a wide range of species from insects to mammals. Recent reports of adult neurogenesis in primates, including humans, have led to explosive scientific and public attention. During the last two years, significant discoveries have revealed that the generation, recruitment and survival of new neurons in adult brains are governed by principles similar to those that shape the developing brain, such as neuronal death, sensory experience, activity levels, and learning. Similarly, many factors implicated in embryonic neurogenesis are increasingly found to regulate adult neurogenesis and survival as well. These findings now allow the first manipulations of the numbers of adult-generated neurons to address their potential behavioral function.

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.

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.

Reactive neurogenesis during regeneration of the lesioned medial cerebral cortex of lizards

This study reports that lesion of the adult lizard medial cortex (lizard hippocampal fascia dentata) induces a short period of intensive neurogenesis which we have termed reactive neurogenesis; a cell proliferation event that occurs in the subjacent ependyma. Specific lesion of the medial cortex was achieved by intraperitoneal injection of the neurotoxin 3-acetylpyridine and proliferating cells were detected using tritiated thymidine or 5-bromodeoxiuridine pulse labelling. After lesion, granule neurons in the lizard medial cortex cell layer appeared pyknotic and died; they were then removed and progressively replaced by a set of new neurons. These neurons were mostly generated from the second to the seventh day post-lesion. A dramatic temporal increment of labelled ependymal cells was detected when either tritiated thymidine or 5-bromodeoxiuridine pulses were delivered in that period. The maximum of about five thousand labelled cells per hemisphere was reached by the fourth day after the lesion. Beyond the seventh day post-lesion, the numbers of labelled cells returned to a level of about four hundred per hemisphere, similar to that of the control specimens. Electron microscopy revealed that the recently generated cells were neuroblasts or immature neurons with a characteristic pattern of chromatin condensation and a high number of ribonucleic granules.

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.

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.

Development of vasotocin pathways in the bullfrog brain

The brain of adult bullfrogs (Rana catesbeiana) contains six populations of cells which are immunoreactive for the neurohypophysial peptide arginine vasotocin (AVT). It is unknown when some of these cell populations first appear during development and when the sexual differences in AVT distribution first become apparent. We therefore used immunocytochemistry to examine development of AVT pathways in developing bullfrog tadpoles and in newly metamorphosed froglets of both sexes. AVT-immunoreactive (AVT-ir) cells were already present in the three diencephalic areas (magnocellular preoptic nucleus, suprachiasmatic nucleus and hypothalamus) at stage III (Taylor and Kollros stages), the earliest stage examined. Cell size in the magnocellular nucleus was not bimodally distributed in either tadpoles or froglets. AVT-ir cells in the telencephalic septal nucleus and amygdala did not appear until stage VI. There was no sexual difference in the density of AVT-ir cells or fibers in the amygdala of tadpoles or froglets. Finally, cells in the hindbrain pretrigeminal nucleus appeared much later--after stage XX. Thus, different populations of neurons begin to express AVT at unique times during development. The sexual dimorphism in AVT content observed in the amygdala of adult bullfrogs must appear during juvenile development or at adulthood.