Brain size and limits to adult neurogenesis

Neuraminidase inhibition promotes the collective migration of neurons and recovery of brain function

In the injured brain, new neurons produced from endogenous neural stem cells form chains and migrate to injured areas and contribute to the regeneration of lost neurons. However, this endogenous regenerative capacity of the brain has not yet been leveraged for the treatment of brain injury. Here, we show that in healthy brain chains of migrating new neurons maintain unexpectedly large non-adherent areas between neighboring cells, allowing for efficient migration. In instances of brain injury, neuraminidase reduces polysialic acid levels, which negatively regulates adhesion, leading to increased cell-cell adhesion and reduced migration efficiency. The administration of zanamivir, a neuraminidase inhibitor used for influenza treatment, promotes neuronal migration toward damaged regions, fosters neuronal regeneration, and facilitates functional recovery. Together, these findings shed light on a new mechanism governing efficient neuronal migration in the adult brain under physiological conditions, pinpoint the disruption of this mechanism during brain injury, and propose a promising therapeutic avenue for brain injury through drug repositioning.

Fetal influence on the human brain through the lifespan

Human fetal development has been associated with brain health at later stages. It is unknown whether growth in utero, as indexed by birth weight (BW), relates consistently to lifespan brain characteristics and changes, and to what extent these influences are of a genetic or environmental nature. Here we show remarkably stable and lifelong positive associations between BW and cortical surface area and volume across and within developmental, aging and lifespan longitudinal samples (N = 5794, 4-82 y of age, w/386 monozygotic twins, followed for up to 8.3 y w/12,088 brain MRIs). In contrast, no consistent effect of BW on brain changes was observed. Partly environmental effects were indicated by analysis of twin BW discordance. In conclusion, the influence of prenatal growth on cortical topography is stable and reliable through the lifespan. This early-life factor appears to influence the brain by association of brain reserve, rather than brain maintenance. Thus, fetal influences appear omnipresent in the spacetime of the human brain throughout the human lifespan. Optimizing fetal growth may increase brain reserve for life, also in aging.

Delayed maturation and migration of excitatory neurons in the juvenile mouse paralaminar amygdala

The human amygdala paralaminar nucleus (PL) contains many immature excitatory neurons that undergo prolonged maturation from birth to adulthood. We describe a previously unidentified homologous PL region in mice that contains immature excitatory neurons and has previously been considered part of the amygdala intercalated cell clusters or ventral endopiriform cortex. Mouse PL neurons are born embryonically, not from postnatal neurogenesis, despite a subset retaining immature molecular and morphological features in adults. During juvenile-adolescent ages (P21-P35), the majority of PL neurons undergo molecular, structural, and physiological maturation, and a subset of excitatory PL neurons migrate into the adjacent endopiriform cortex. Alongside these changes, PL neurons develop responses to aversive and appetitive olfactory stimuli. The presence of this homologous region in both humans and mice points to the significance of this conserved mechanism of neuronal maturation and migration during adolescence, a key time period for amygdala circuit maturation and related behavioral changes.

Protracted neuronal recruitment in the temporal lobes of young children

The temporal lobe of the human brain contains the entorhinal cortex (EC). This region of the brain is a highly interconnected integrative hub for sensory and spatial information; it also has a key role in episodic memory formation and is the main source of cortical hippocampal inputs1-4. The human EC continues to develop during childhood5, but neurogenesis and neuronal migration to the EC are widely considered to be complete by birth. Here we show that the human temporal lobe contains many young neurons migrating into the postnatal EC and adjacent regions, with a large tangential stream persisting until the age of around one year and radial dispersal continuing until around two to three years of age. By contrast, we found no equivalent postnatal migration in rhesus macaques (Macaca mulatta). Immunostaining and single-nucleus RNA sequencing of ganglionic eminence germinal zones, the EC stream and the postnatal EC revealed that most migrating cells in the EC stream are derived from the caudal ganglionic eminence and become LAMP5+RELN+ inhibitory interneurons. These late-arriving interneurons could continue to shape the processing of sensory and spatial information well into postnatal life, when children are actively interacting with their environment. The EC is one of the first regions of the brain to be affected in Alzheimer's disease, and previous work has linked cognitive decline to the loss of LAMP5+RELN+ cells6,7. Our investigation reveals that many of these cells arrive in the EC through a major postnatal migratory stream in early childhood.

Neuron and Brain Maturation 2.0

The mammalian central nervous system (CNS) is built up during embryogenesis by neural stem cells located in the periventricular germinal layers which undergo multiple division cycles [...].

Adult human neurogenesis: A view from two schools of thought

Are we truly losing neurons as we grow older? If yes, why, and how can the lost neurons be replaced or compensated for? Is so-called adult neurogenesis (ANG) still a controversial process, particularly in the human cerebral cortex? How do adult-born neurons -if proven to exist- contribute to brain functions? Is adult neurogenesis a disease-relevant process, meaning that neural progenitor cells are dormant in adulthood, but they may be reactivated, for example, following stroke? Is the earnest hope to cure neurological diseases justifying the readiness to accept ANG claim uncritically? These are all fundamental issues that have not yet been firmly explained. Although it is completely understandable that some researchers believe that we can add new neurons to our inevitably deteriorating brain, the brain regeneration process still possesses intellectually and experimentally diverting views, as until now, there has been significant confusion about the concept of ANG. This paper is not intended to be an extensively analytical review distilling all findings and conclusions presented in the ANG literature. Instead, it is an attempt to discuss the commonly entertained opinions and then present our reflective insight concerning the current status quo of the field, which might help redirect research questions, avoid marketing an exaggerated hope, and more importantly, save the ever-limited resources, namely, intellectuals' time, facilities, and grants.

Neurogenesis in primates versus rodents and the value of non-human primate models

Neurogenesis, the process of generating neurons from neural stem cells, occurs during both embryonic and adult stages, with each stage possessing distinct characteristics. Dysfunction in either stage can disrupt normal neural development, impair cognitive functions, and lead to various neurological disorders. Recent technological advancements in single-cell multiomics and gene-editing have facilitated investigations into primate neurogenesis. Here, we provide a comprehensive overview of neurogenesis across rodents, non-human primates, and humans, covering embryonic development to adulthood and focusing on the conservation and diversity among species. While non-human primates, especially monkeys, serve as valuable models with closer neural resemblance to humans, we highlight the potential impacts and limitations of non-human primate models on both physiological and pathological neurogenesis research.

Adult neurogenesis and "immature" neurons in mammals: an evolutionary trade-off in plasticity?

Neuronal plasticity can vary remarkably in its form and degree across animal species. Adult neurogenesis, namely the capacity to produce new neurons from neural stem cells through adulthood, appears widespread in non-mammalian vertebrates, whereas it is reduced in mammals. A growing body of comparative studies also report variation in the occurrence and activity of neural stem cell niches between mammals, with a general trend of reduction from small-brained to large-brained species. Conversely, recent studies have shown that large-brained mammals host large amounts of neurons expressing typical markers of neurogenesis in the absence of cell division. In layer II of the cerebral cortex, populations of prenatally generated, non-dividing neurons continue to express molecules indicative of immaturity throughout life (cortical immature neurons; cINs). After remaining in a dormant state for a very long time, these cINs retain the potential of differentiating into mature neurons that integrate within the preexisting neural circuits. They are restricted to the paleocortex in small-brained rodents, while extending into the widely expanded neocortex of highly gyrencephalic, large-brained species. The current hypothesis is that these populations of non-newly generated "immature" neurons might represent a reservoir of developmentally plastic cells for mammalian species that are characterized by reduced stem cell-driven adult neurogenesis. This indicates that there may be a trade-off between various forms of plasticity that coexist during brain evolution. This balance may be necessary to maintain a "reservoir of plasticity" in brain regions that have distinct roles in species-specific socioecological adaptations, such as the neocortex and olfactory structures.

Neuroestradiol and neuronal development: Not an exclusive male tale anymore

The brain synthesizes a variety of neurosteroids, including neuroestradiol. Inhibition of neuroestradiol synthesis results in alterations in basic neurodevelopmental processes, such as neurogenesis, neuroblast migration, neuritogenesis and synaptogenesis. Although the neurodevelopmental actions of neuroestradiol are exerted in both sexes, some of them are sex-specific, such as the well characterized effects of neuroestradiol derived from the metabolism of testicular testosterone during critical periods of male brain development. In addition, recent findings have shown sex-specific actions of neuroestradiol on neuroblast migration, neuritic growth and synaptogenesis in females. Among other factors, the epigenetic regulation exerted by X linked genes, such as Kdm6a/Utx, may determine sex-specific actions of neuroestradiol in the female brain. This review evidences the impact of neuroestradiol on brain formation in both sexes and highlights the interaction of neural steriodogenesis, hormones and sex chromosomes in sex-specific brain development.

Age-related changes in layer II immature neurons of the murine piriform cortex

The recent identification of a population of non-newly born, prenatally generated "immature" neurons in the layer II of the piriform cortex (cortical immature neurons, cINs), raises questions concerning their maintenance or depletion through the lifespan. Most forms of brain structural plasticity progressively decline with age, a feature that is particularly prominent in adult neurogenesis, due to stem cell depletion. By contrast, the entire population of the cINs is produced during embryogenesis. Then these cells simply retain immaturity in postnatal and adult stages, until they "awake" to complete their maturation and ultimately integrate into neural circuits. Hence, the question remains open whether the cINs, which are not dependent on stem cell division, might follow a similar pattern of age-related reduction, or in alternative, might leave a reservoir of young, undifferentiated cells in the adult and aging brain. Here, the number and features of cINs were analyzed in the mouse piriform cortex from postnatal to advanced ages, by using immunocytochemistry for the cytoskeletal marker doublecortin. The abundance and stage of maturation of cINs, along with the expression of other markers of maturity/immaturity were investigated. Despite a marked decrease in this neuronal population during juvenile stages, reminiscent of that observed in hippocampal neurogenesis, a small amount of highly immature cINs persisted up to advanced ages. Overall, albeit reducing in number with increasing age, we report that the cINs are present through the entire animal lifespan.

Shaping the cerebral cortex by cellular crosstalk

The cerebral cortex is the brain's outermost layer. It is responsible for processing motor and sensory information that support high-level cognitive abilities and shape personality. Its development and functional organization strongly rely on cell communication that is established via an intricate system of diffusible signals and physical contacts during development. Interfering with this cellular crosstalk can cause neurodevelopmental disorders. Here, we review how crosstalk between migrating cells and their environment influences cerebral cortex development, ranging from neurogenesis to synaptogenesis and assembly of cortical circuits.

Therapeutic strategies to recover ependymal barrier after inflammatory damage: relevance for recovering neurogenesis during development

The epithelium covering the surfaces of the cerebral ventricular system is known as the ependyma, and is essential for maintaining the physical and functional integrity of the central nervous system. Additionally, the ependyma plays an essential role in neurogenesis, neuroinflammatory modulation and neurodegenerative diseases. Ependyma barrier is severely affected by perinatal hemorrhages and infections that cross the blood brain barrier. The recovery and regeneration of ependyma after damage are key to stabilizing neuroinflammatory and neurodegenerative processes that are critical during early postnatal ages. Unfortunately, there are no effective therapies to regenerate this tissue in human patients. Here, the roles of the ependymal barrier in the context of neurogenesis and homeostasis are reviewed, and future research lines for development of actual therapeutic strategies are discussed.

Alzheimer's Disease and Its Possible Evolutionary Origin: Hypothesis

The enormous, 2-3-million-year evolutionary expansion of hominin neocortices to the current enormity enabled humans to take over the planet. However, there appears to have been a glitch, and it occurred without a compensatory expansion of the entorhinal cortical (EC) gateway to the hippocampal memory-encoding system needed to manage the processing of the increasing volume of neocortical data converging on it. The resulting age-dependent connectopathic glitch was unnoticed by the early short-lived populations. It has now surfaced as Alzheimer's disease (AD) in today's long-lived populations. With advancing age, processing of the converging neocortical data by the neurons of the relatively small lateral entorhinal cortex (LEC) inflicts persistent strain and high energy costs on these cells. This may result in their hyper-release of harmless Aβ_1-42 monomers into the interstitial fluid, where they seed the formation of toxic amyloid-β oligomers (AβOs) that initiate AD. At the core of connectopathic AD are the postsynaptic cellular prion protein (PrPC). Electrostatic binding of the negatively charged AβOs to the positively charged N-terminus of PrPC induces hyperphosphorylation of tau that destroys synapses. The spread of these accumulating AβOs from ground zero is supported by Aβ's own production mediated by target cells' Ca2+-sensing receptors (CaSRs). These data suggest that an early administration of a strongly positively charged, AβOs-interacting peptide or protein, plus an inhibitor of CaSR, might be an effective AD-arresting therapeutic combination.

The Adult Neurogenesis Theory of Alzheimer's Disease

Alzheimer's disease starts in neural stem cells (NSCs) in the niches of adult neurogenesis. All primary factors responsible for pathological tau hyperphosphorylation are inherent to adult neurogenesis and migration. However, when amyloid pathology is present, it strongly amplifies tau pathogenesis. Indeed, the progressive accumulation of extracellular amyloid-β deposits in the brain triggers a state of chronic inflammation by microglia. Microglial activation has a significant pro-neurogenic effect that fosters the process of adult neurogenesis and supports neuronal migration. Unfortunately, this "reactive" pro-neurogenic activity ultimately perturbs homeostatic equilibrium in the niches of adult neurogenesis by amplifying tau pathogenesis in AD. This scenario involves NSCs in the subgranular zone of the hippocampal dentate gyrus in late-onset AD (LOAD) and NSCs in the ventricular-subventricular zone along the lateral ventricles in early-onset AD (EOAD), including familial AD (FAD). Neuroblasts carrying the initial seed of tau pathology travel throughout the brain via neuronal migration driven by complex signals and convey the disease from the niches of adult neurogenesis to near (LOAD) or distant (EOAD) brain regions. In these locations, or in close proximity, a focus of degeneration begins to develop. Then, tau pathology spreads from the initial foci to large neuronal networks along neural connections through neuron-to-neuron transmission.

The Effects of Extrinsic and Intrinsic Factors on Neurogenesis

In the mammalian brain, neurogenesis is maintained throughout adulthood primarily in two typical niches, the subgranular zone (SGZ) of the dentate gyrus and the subventricular zone (SVZ) of the lateral ventricles and in other nonclassic neurogenic areas (e.g., the amygdala and striatum). During prenatal and early postnatal development, neural stem cells (NSCs) differentiate into neurons and migrate to appropriate areas such as the olfactory bulb where they integrate into existing neural networks; these phenomena constitute the multistep process of neurogenesis. Alterations in any of these processes impair neurogenesis and may even lead to brain dysfunction, including cognitive impairment and neurodegeneration. Here, we first summarize the main properties of mammalian neurogenic niches to describe the cellular and molecular mechanisms of neurogenesis. Accumulating evidence indicates that neurogenesis plays an integral role in neuronal plasticity in the brain and cognition in the postnatal period. Given that neurogenesis can be highly modulated by a number of extrinsic and intrinsic factors, we discuss the impact of extrinsic (e.g., alcohol) and intrinsic (e.g., hormones) modulators on neurogenesis. Additionally, we provide an overview of the contribution of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection to persistent neurological sequelae such as neurodegeneration, neurogenic defects and accelerated neuronal cell death. Together, our review provides a link between extrinsic/intrinsic factors and neurogenesis and explains the possible mechanisms of abnormal neurogenesis underlying neurological disorders.

Consistency and Variation in Doublecortin and Ki67 Antigen Detection in the Brain Tissue of Different Mammals, including Humans

Recently, a population of "immature" neurons generated prenatally, retaining immaturity for long periods and finally integrating in adult circuits has been described in the cerebral cortex. Moreover, comparative studies revealed differences in occurrence/rate of different forms of neurogenic plasticity across mammals, the "immature" neurons prevailing in gyrencephalic species. To extend experimentation from laboratory mice to large-brained mammals, including humans, it is important to detect cell markers of neurogenic plasticity in brain tissues obtained from different procedures (e.g., post-mortem/intraoperative specimens vs. intracardiac perfusion). This variability overlaps with species-specific differences in antigen distribution or antibody species specificity, making it difficult for proper comparison. In this work, we detect the presence of doublecortin and Ki67 antigen, markers for neuronal immaturity and cell division, in six mammals characterized by widely different brain size. We tested seven commercial antibodies in four selected brain regions known to host immature neurons (paleocortex, neocortex) and newly born neurons (hippocampus, subventricular zone). In selected human brains, we confirmed the specificity of DCX antibody by performing co-staining with fluorescent probe for DCX mRNA. Our results indicate that, in spite of various types of fixations, most differences were due to the use of different antibodies and the existence of real interspecies variation.

Morphological Features of Human Dendritic Spines

Dendritic spine features in human neurons follow the up-to-date knowledge presented in the previous chapters of this book. Human dendrites are notable for their heterogeneity in branching patterns and spatial distribution. These data relate to circuits and specialized functions. Spines enhance neuronal connectivity, modulate and integrate synaptic inputs, and provide additional plastic functions to microcircuits and large-scale networks. Spines present a continuum of shapes and sizes, whose number and distribution along the dendritic length are diverse in neurons and different areas. Indeed, human neurons vary from aspiny or "relatively aspiny" cells to neurons covered with a high density of intermingled pleomorphic spines on very long dendrites. In this chapter, we discuss the phylogenetic and ontogenetic development of human spines and describe the heterogeneous features of human spiny neurons along the spinal cord, brainstem, cerebellum, thalamus, basal ganglia, amygdala, hippocampal regions, and neocortical areas. Three-dimensional reconstructions of Golgi-impregnated dendritic spines and data from fluorescence microscopy are reviewed with ultrastructural findings to address the complex possibilities for synaptic processing and integration in humans. Pathological changes are also presented, for example, in Alzheimer's disease and schizophrenia. Basic morphological data can be linked to current techniques, and perspectives in this research field include the characterization of spines in human neurons with specific transcriptome features, molecular classification of cellular diversity, and electrophysiological identification of coexisting subpopulations of cells. These data would enlighten how cellular attributes determine neuron type-specific connectivity and brain wiring for our diverse aptitudes and behavior.

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.

Boosting Neurogenesis in the Adult Hippocampus Using Antidepressants and Mesenchymal Stem Cells

The hippocampus is one of the few privileged regions (neural stem cell niche) of the brain, where neural stem cells differentiate into new neurons throughout adulthood. However, dysregulation of hippocampal neurogenesis with aging, injury, depression and neurodegenerative disease leads to debilitating cognitive impacts. These debilitating symptoms deteriorate the quality of life in the afflicted individuals. Impaired hippocampal neurogenesis is especially difficult to rescue with increasing age and neurodegeneration. However, the potential to boost endogenous Wnt signaling by influencing pathway modulators such as receptors, agonists, and antagonists through drug and cell therapy-based interventions offers hope. Restoration and augmentation of hampered Wnt signaling to facilitate increased hippocampal neurogenesis would serve as an endogenous repair mechanism and contribute to hippocampal structural and functional plasticity. This review focuses on the possible interaction between neurogenesis and Wnt signaling under the control of antidepressants and mesenchymal stem cells (MSCs) to overcome debilitating symptoms caused by age, diseases, or environmental factors such as stress. It will also address some current limitations hindering the direct extrapolation of research from animal models to human application, and the technical challenges associated with the MSCs and their cellular products as potential therapeutic solutions.

Sex differences in inflammation in the hippocampus and amygdala across the lifespan in rats: associations with cognitive bias

Background

Cognitive symptoms of major depressive disorder, such as negative cognitive bias, are more prevalent in women than in men. Cognitive bias involves pattern separation which requires hippocampal neurogenesis and is modulated by inflammation in the brain. Previously, we found sex differences in the activation of the amygdala and the hippocampus in response to negative cognitive bias in rats that varied with age. Given the association of cognitive bias to neurogenesis and inflammation, we examined associations between cognitive bias, neurogenesis in the hippocampus, and cytokine and chemokine levels in the ventral hippocampus (HPC) and basolateral amygdala (BLA) of male and female rats across the lifespan.

Results

After cognitive bias testing, males had more IFN-γ, IL-1β, IL-4, IL-5, and IL-10 in the ventral HPC than females in adolescence. In young adulthood, females had more IFN-γ, IL-1β, IL-6, and IL-10 in the BLA than males. Middle-aged rats had more IL-13, TNF-α, and CXCL1 in both regions than younger groups. Adolescent male rats had higher hippocampal neurogenesis than adolescent females after cognitive bias testing and young rats that underwent cognitive bias testing had higher levels of hippocampal neurogenesis than controls. Neurogenesis in the dorsal hippocampus was negatively associated with negative cognitive bias in young adult males.

Conclusions

Overall, the association between negative cognitive bias, hippocampal neurogenesis, and inflammation in the brain differs by age and sex. Hippocampal neurogenesis and inflammation may play greater role in the cognitive bias of young males compared to a greater role of BLA inflammation in adult females. These findings lay the groundwork for the discovery of sex-specific novel therapeutics that target region-specific inflammation in the brain and hippocampal neurogenesis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12979-022-00299-4.

• Adolescent male rats had more hippocampal inflammation than females after cognitive bias testing.

• Adult female rats had more basolateral amygdalar inflammation than males after cognitive bias testing.

• HPC neurogenesis was negatively associated to cognitive bias in young adult male rats.

Animal welfare assessment after severe traumatic brain injury in rats

Severe traumatic brain injury (TBI) is a multifactorial injury process involving respiratory, cardiovascular and immune functions in addition to the brain. Thus, live animal models are needed to study the molecular, cellular and systemic mechanisms of TBI. The ethical use of laboratory animals requires that the benefits of approaches be carefully weighed against potential harm to animals. Welfare assessments adapted to severe TBI research are lacking. Here, we introduce a scoresheet to describe and monitor potential distress in animals, which includes general welfare (body weight, general appearance and spontaneous behaviour) and TBI-specific indices (respiratory function, pain, locomotor impairment, wound healing). Implementation of this scoresheet in Sprague-Dawley rats subjected to severe lateral fluid percussion TBI revealed a period of suffering limited to four days, followed by a recovery to normal welfare scores within 10-15 days, with females showing a worse impact than males. The scores indicate that animal suffering in this model is transitory compared with TBI consequences in humans. The scoresheet allows for the implementation of refinement measures including (1) analgesia during the initial period following TBI and (2) humane endpoints set (30% weight loss, score ≥90 and/or respiratory problems). This animal scoresheet tailored to TBI research provides a basis for further refinement of animal research paradigms aimed at understanding or treating the sequelae of severe TBI.

Immature excitatory neurons in the amygdala come of age during puberty

The human amygdala is critical for emotional learning, valence coding, and complex social interactions, all of which mature throughout childhood, puberty, and adolescence. Across these ages, the amygdala paralaminar nucleus (PL) undergoes significant structural changes including increased numbers of mature neurons. The PL contains a large population of immature excitatory neurons at birth, some of which may continue to be born from local progenitors. These progenitors disappear rapidly in infancy, but the immature neurons persist throughout childhood and adolescent ages, indicating that they develop on a protracted timeline. Many of these late-maturing neurons settle locally within the PL, though a small subset appear to migrate into neighboring amygdala subnuclei. Despite its prominent growth during postnatal life and possible contributions to multiple amygdala circuits, the function of the PL remains unknown. PL maturation occurs predominately during late childhood and into puberty when sex hormone levels change. Sex hormones can promote developmental processes such as neuron migration, dendritic outgrowth, and synaptic plasticity, which appear to be ongoing in late-maturing PL neurons. Collectively, we describe how the growth of late-maturing neurons occurs in the right time and place to be relevant for amygdala functions and neuropsychiatric conditions.

How Widespread Are the “Young” Neurons of the Mammalian Brain?

After the discovery of adult neurogenesis (stem cell-driven production of new neuronal elements), it is conceivable to find young, undifferentiated neurons mixed with mature neurons in the neural networks of the adult mammalian brain. This “canonical” neurogenesis is restricted to small stem cell niches persisting from embryonic germinal layers, yet, the genesis of new neurons has also been reported in various parenchymal brain regions. Whichever the process involved, several populations of “young” neurons can be found at different locations of the brain. Across the years, further complexity emerged: (i) molecules of immaturity can also be expressed by non-dividing cells born during embryogenesis, then maintaining immature features later on; (ii) remarkable interspecies differences exist concerning the types, location, amount of undifferentiated neurons; (iii) re-expression of immaturity can occur in aging (dematuration). These twists are introducing a somewhat different definition of neurogenesis than normally assumed, in which our knowledge of the “young” neurons is less sharp. In this emerging complexity, there is a need for complete mapping of the different “types” of young neurons, considering their role in postnatal development, plasticity, functioning, and interspecies differences. Several important aspects are at stake: the possible role(s) that the young neurons may play in maintaining brain efficiency and in prevention/repair of neurological disorders; nonetheless, the correct translation of results obtained from laboratory rodents. Hence, the open question is: how many types of undifferentiated neurons do exist in the brain, and how widespread are they?

The development and evolution of inhibitory neurons in primate cerebrum

Neuroanatomists have long speculated that expanded primate brains contain an increased morphological diversity of inhibitory neurons (INs)¹, and recent studies have identified primate-specific neuronal populations at the molecular level². However, we know little about the developmental mechanisms that specify evolutionarily novel cell types in the brain. Here, we reconstruct gene expression trajectories specifying INs generated throughout the neurogenic period in macaques and mice by analysing the transcriptomes of 250,181 cells. We find that the initial classes of INs generated prenatally are largely conserved among mammals. Nonetheless, we identify two contrasting developmental mechanisms for specifying evolutionarily novel cell types during prenatal development. First, we show that recently identified primate-specific TAC3 striatal INs are specified by a unique transcriptional programme in progenitors followed by induction of a distinct suite of neuropeptides and neurotransmitter receptors in new-born neurons. Second, we find that multiple classes of transcriptionally conserved olfactory bulb (OB)-bound precursors are redirected to expanded primate white matter and striatum. These classes include a novel peristriatal class of striatum laureatum neurons that resemble dopaminergic periglomerular cells of the OB. We propose an evolutionary model in which conserved initial classes of neurons supplying the smaller primate OB are reused in the enlarged striatum and cortex. Together, our results provide a unified developmental taxonomy of initial classes of mammalian INs and reveal multiple developmental mechanisms for neural cell type evolution.

Evolutionary modelling shows that an initial set of inhibitory neurons serving olfactory bulbs may have been repurposed to diversify the taxonomy of interneurons found in the expanded striata and cortices in primates.

Isolation and Culture of Neural Stem/Progenitor Cells from the Postnatal Periventricular Region

Due to the complexity of the neural stem cell (NSC) niche organization, the lack of specific NSC markers, and the difficulty of long-term tracking these cells and their progeny in vivo, the functional properties of the endogenous NSCs remain largely unexplored. These limitations have led to the development of methodologies to efficiently isolate, expand, and differentiate NSCs ex vivo. We describe here the peculiarities of the neurosphere assay (NSA) as a methodology that allows to efficiently isolate, expand, and differentiate somatic NSCs derived from the postnatal and adult forebrain periventricular region while preserving proliferation, self-renewal, and multipotency, the main attributes that provide their functional identification.

Steroid hormones and hippocampal neurogenesis in the adult mammalian brain

Hippocampal neurogenesis persists across the lifespan in many species, including rodents and humans, and is associated with cognitive performance and the pathogenesis of neurodegenerative disease and psychiatric disorders. Neurogenesis is modulated by steroid hormones that change across development and differ between the sexes in rodents and humans. Here, we discuss the effects of stress and glucocorticoid exposure from gestation to adulthood as well as the effects of androgens and estrogens in adulthood on neurogenesis in the hippocampus. Throughout the review we highlight sex differences in the effects of steroid hormones on neurogenesis and how they may relate to hippocampal function and disease. These data highlight the importance of examining age and sex when evaluating the effects of steroid hormones on hippocampal neurogenesis.

Computational benefits of structural plasticity, illustrated in songbirds

The plasticity of nervous systems allows animals to quickly adapt to a changing environment. In particular, the structural plasticity of brain networks is often critical to the development of the central nervous system and the acquisition of complex behaviors. As an example, structural plasticity is central to the development of song-related brain circuits and may be critical for song acquisition in juvenile songbirds. Here, we review current evidences for structural plasticity and their significance from a computational point of view. We start by reviewing evidence for structural plasticity across species and categorizing them along the spatial axes as well as the along the time course during development. We introduce the vocal learning circuitry in zebra finches, as a useful example of structural plasticity, and use this specific case to explore the possible contributions of structural plasticity to computational models. Finally, we discuss current modeling studies incorporating structural plasticity and unexplored questions which are raised by such models.

The PSA-NCAM-Positive "Immature" Neurons: An Old Discovery Providing New Vistas on Brain Structural Plasticity

Studies on brain plasticity have undertaken different roads, tackling a wide range of biological processes: from small synaptic changes affecting the contacts among neurons at the very tip of their processes, to birth, differentiation, and integration of new neurons (adult neurogenesis). Stem cell-driven adult neurogenesis is an exception in the substantially static mammalian brain, yet, it has dominated the research in neurodevelopmental biology during the last thirty years. Studies of comparative neuroplasticity have revealed that neurogenic processes are reduced in large-brained mammals, including humans. On the other hand, large-brained mammals, with respect to rodents, host large populations of special "immature" neurons that are generated prenatally but express immature markers in adulthood. The history of these "immature" neurons started from studies on adhesion molecules carried out at the beginning of the nineties. The identity of these neurons as "stand by" cells "frozen" in a state of immaturity remained un-detected for long time, because of their ill-defined features and because clouded by research ef-forts focused on adult neurogenesis. In this review article, the history of these cells will be reconstructed, and a series of nuances and confounding factors that have hindered the distinction between newly generated and "immature" neurons will be addressed.

Brain Plasticity in Humans and Model Systems: Advances, Challenges, and Future Directions

Plasticity, and in particular, neurogenesis, is a promising target to treat and prevent a wide variety of diseases (e.g., epilepsy, stroke, dementia). There are different types of plasticity, which vary with age, brain region, and species. These observations stress the importance of defining plasticity along temporal and spatial dimensions. We review recent studies focused on brain plasticity across the lifespan and in different species. One main theme to emerge from this work is that plasticity declines with age but that we have yet to map these different forms of plasticity across species. As part of this effort, we discuss our recent progress aimed to identify corresponding ages across species, and how this information can be used to map temporal variation in plasticity from model systems to humans.

Positive Controls in Adults and Children Support That Very Few, If Any, New Neurons Are Born in the Adult Human Hippocampus

Adult hippocampal neurogenesis was originally discovered in rodents. Subsequent studies identified the adult neural stem cells and found important links between adult neurogenesis and plasticity, behavior, and disease. However, whether new neurons are produced in the human dentate gyrus (DG) during healthy aging is still debated. We and others readily observe proliferating neural progenitors in the infant hippocampus near immature cells expressing doublecortin (DCX), but the number of such cells decreases in children and few, if any, are present in adults. Recent investigations using dual antigen retrieval find many cells stained by DCX antibodies in adult human DG. This has been interpreted as evidence for high rates of adult neurogenesis, even at older ages. However, most of these DCX-labeled cells have mature morphology. Furthermore, studies in the adult human DG have not found a germinal region containing dividing progenitor cells. In this Dual Perspectives article, we show that dual antigen retrieval is not required for the detection of DCX in multiple human brain regions of infants or adults. We review prior studies and present new data showing that DCX is not uniquely expressed by newly born neurons: DCX is present in adult amygdala, entorhinal and parahippocampal cortex neurons despite being absent in the neighboring DG. Analysis of available RNA-sequencing datasets supports the view that DG neurogenesis is rare or absent in the adult human brain. To resolve the conflicting interpretations in humans, it is necessary to identify and visualize dividing neuronal precursors or develop new methods to evaluate the age of a neuron at the single-cell level.

Histological Studies of the Ventricular-Subventricular Zone as Neural Stem Cell and Glioma Stem Cell Niche

The neural stem cell niche of the ventricular-subventricular zone supports the persistence of stem and progenitor cells in the mature brain. This niche has many notable cytoarchitectural features that affect the activity of stem cells and may also support the survival and growth of invading tumor cells. Histochemical studies of the niche have revealed many proteins that, in combination, can help to reveal stem-like cells in the normal or cancer context, although many caveats persist in the quest to consistently identify these cells in the human brain. Here, we explore the complex relationship between the persistent proliferative capacity of the neural stem cell niche and the malignant proliferation of brain tumors, with a special focus on histochemical identification of stem cells and stem-like tumor cells and an eye toward the potential application of high-dimensional imaging approaches to the field.

Effects of Brain Size on Adult Neurogenesis in Shrews

Shrews are small animals found in many different habitats. Like other mammals, adult neurogenesis occurs in the subventricular zone of the lateral ventricle (SVZ) and the dentate gyrus (DG) of the hippocampal formation. We asked whether the number of new generated cells in shrews depends on their brain size. We examined Crocidura russula and Neomys fodiens, weighing 10-22 g, and Crocidura olivieri and Suncus murinus that weigh three times more. We found that the density of proliferated cells in the SVZ was approximately at the same level in all species. These cells migrated from the SVZ through the rostral migratory stream to the olfactory bulb (OB). In this pathway, a low level of neurogenesis occurred in C. olivieri compared to three other species of shrews. In the DG, the rate of adult neurogenesis was regulated differently. Specifically, the lowest density of newly generated neurons was observed in C. russula, which had a substantial number of new neurons in the OB compared with C. olivieri. We suggest that the number of newly generated neurons in an adult shrew's brain is independent of the brain size, and molecular mechanisms of neurogenesis appeared to be different in two neurogenic structures.

Fundamentals of the Development of Connectivity in the Human Fetal Brain in Late Gestation: From 24 Weeks Gestational Age to Term

During the second half of gestation, the human cerebrum undergoes pivotal histogenetic events that underlie functional connectivity. These include the growth, guidance, selection of axonal pathways, and their first engagement in neuronal networks. Here, we characterize the spatiotemporal patterns of cerebral connectivity in extremely preterm (EPT), very preterm (VPT), preterm and term babies, focusing on magnetic resonance imaging (MRI) and histological data. In the EPT and VPT babies, thalamocortical axons enter into the cortical plate creating the electrical synapses. Additionally, the subplate zone gradually resolves in the preterm and term brain in conjunction with the growth of associative pathways leading to the activation of large-scale neural networks. We demonstrate that specific classes of axonal pathways within cerebral compartments are selectively vulnerable to temporally nested pathogenic factors. In particular, the radial distribution of axonal lesions, that is, radial vulnerability, is a robust predictor of clinical outcome. Furthermore, the subplate tangential nexus that we can visualize using MRI could be an additional marker as pivotal in the development of cortical connectivity. We suggest to direct future research toward the identification of sensitive markers of earlier lesions, the elucidation of genetic mechanisms underlying pathogenesis, and better long-term follow-up using structural and functional MRI.

Regeneration using endogenous neural stem cells following neonatal brain injury

Despite recent advancements in perinatal care, the incidence of neonatal brain injury has not decreased. No therapies are currently available to repair injured brain tissues. In the postnatal brain, neural stem cells reside in the ventricular-subventricular zone (V-SVZ) and continuously generate new immature neurons (neuroblasts). After brain injury in rodents, V-SVZ-derived neuroblasts migrate toward the injured area using blood vessels as a scaffold. Notably, the neonatal V-SVZ has a remarkable neurogenic capacity. Furthermore, compared with the adult brain, after neonatal brain injury, larger numbers of neuroblasts migrate toward the lesion, raising the possibility that the V-SVZ could be a source for endogenous neuronal regeneration after neonatal brain injury. We recently demonstrated that efficient migration of V-SVZ-derived neuroblasts toward a lesion is supported by neonatal radial glia via neural cadherin (N-cadherin)-mediated neuron-fiber contact, which promotes RhoA activity. Moreover, providing blood vessel- and radial glia-mimetic scaffolds for migrating neuroblasts promotes neuronal migration and improves functional gait behaviors after neonatal brain injury. In the V-SVZ, oligodendrocyte progenitor cells (OPCs) are also generated and migrate toward the surrounding white matter, where they differentiate and form myelin. After white matter injury in rodents, the production and subsequent migration of V-SVZ-derived OPCs are enhanced. In the neonatal period, administration of growth factors at a specific time promotes oligodendrocyte regeneration and functional recovery after brain injury. These findings suggest that activating the high regenerative capacity that is specific to the neonatal period could lead to the development of new therapeutic strategies for neonatal brain injury.

Population Dynamics and Neuronal Polyploidy in the Developing Neocortex

The mammalian neocortex is composed of different subtypes of projection neurons that are generated sequentially during embryogenesis by differentiation of neural progenitors. While molecular mechanisms that control neuronal production in the developing neocortex have been extensively studied, the dynamics and absolute numbers of the different progenitor and neuronal populations are still poorly characterized. Here, we describe a medium throughput approach based on flow cytometry and well-known identity markers of cortical subpopulations to collect quantitative data over the course of mouse neocortex development. We collected a complete dataset in a physiological developmental context on two progenitor and two neuron populations, including relative proportions and absolute numbers. Our study reveals unexpected total numbers of Tbr2+ progenitors. In addition, we show that polyploid neurons are present throughout neocortex development.

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.

Putative adult neurogenesis in palaeognathous birds: The common ostrich (Struthio camelus) and emu (Dromaius novaehollandiae)

In the current study, we examined adult neurogenesis throughout the brain of the common ostrich (Struthio camelus) and emu (Dromaius novaehollandiae) using immunohistochemistry for the endogenous markers PCNA which labels proliferating cells, and DCX, which stains immature and migrating neurons. The distribution of PCNA and DCX labelled cells was widespread throughout the brain of both species. The highest density of cells immunoreactive to both markers was observed in the olfactory bulbs and the telencephalon, especially the subventricular zone of the lateral ventricle. Proliferative hot spots, identified with strong PCNA and DCX immunolabelling, were identified in the dorsal and ventral poles of the rostral aspects of the lateral ventricles. The density of PCNA immunoreactive cells was less in the telencephalon of the emu compared to the common ostrich. Substantial numbers of PCNA immunoreactive cells were observed in the diencephalon and brainstem, but DCX immunoreactivity was weaker in these regions, preferentially staining axons and dendrites over cell bodies, except in the medial regions of the hypothalamus where distinct DCX immunoreactive cells and fibres were observed. PCNA and DCX immunoreactive cells were readily observed in moderate density in the cortical layers of the cerebellum of both species. The distribution of putative proliferating cells and immature neurons in the brain of the common ostrich and the emu is widespread, far more so than in mammals, and compares with the neognathous birds, and suggests that brain plasticity and neuronal turnover is an important aspect of cognitive brain functions in these birds.

Phylogenetic variation in cortical layer II immature neuron reservoir of mammals

The adult mammalian brain is mainly composed of mature neurons. A limited amount of stem cell-driven neurogenesis persists in postnatal life and is reduced in large-brained species. Another source of immature neurons in adult brains is cortical layer II. These cortical immature neurons (cINs) retain developmentally undifferentiated states in adulthood, though they are generated before birth. Here, the occurrence, distribution and cellular features of cINs were systematically studied in 12 diverse mammalian species spanning from small-lissencephalic to large-gyrencephalic brains. In spite of well-preserved morphological and molecular features, the distribution of cINs was highly heterogeneous, particularly in neocortex. While virtually absent in rodents, they are present in the entire neocortex of many other species and their linear density in cortical layer II generally increased with brain size. These findings suggest an evolutionary developmental mechanism for plasticity that varies among mammalian species, granting a reservoir of young cells for the cerebral cortex.

To acquire new skills or recover after injuries, the mammalian brain relies on plasticity, the ability for the brain to change its architecture and its connections during the lifetime of an animal.

Creating new nerve cells is one way to achieve plasticity, but this process is rarer in humans than it is in mammals with smaller brains. In particular, it is absent in the human cortex: this region is enlarged in species with large brains, where it carries out complex tasks such as learning and memory. Producing new cells in the cortex would threaten the stability of the structures that retain long-term memories.

Another route to plasticity is to reshape the connections between existing, mature nerve cells. This process takes place in the human brain during childhood and adolescence, as some connections are strengthened and others pruned away.

An alternative mechanism relies on keeping some nerve cells in an immature, ‘adolescent’ state. When needed, these nerve cells emerge from their state of arrested development and ‘grow up’, connecting with the appropriate brain circuits. This mechanism does not involve producing new nerve cells, and so it would be suitable to maintain plasticity in the cortex. Consistent with this idea, in mice some dormant nerve cells are present in a small, primitive part of the cortex.

La Rosa et al. therefore wanted to determine if the location and number of immature cells in the cortex differed between mammals, and if so, whether these differences depended on brain size. The study spanned 12 mammal species, from small-brained species like mice to larger-brained animals including sheep and non-human primates.

Microscopy imaging was used to identify immature nerve cells in brain samples, which revealed that the cortex in larger-brained species contained more adolescent cells than its mouse counterpart. The difference was greatest in a region called the neocortex, which has evolved most recently. This area is most pronounced in primates – especially humans – where it carries out high-level cognitive tasks.

These results identify immature nerve cells as a potential mechanism for plasticity in the cortex. La Rosa et al. hope that the work will inspire searches for similar reservoirs of young cells in humans, which could perhaps lead to new treatments for brain disorders like dementia.

P73 C-terminus is dispensable for multiciliogenesis

The p53 family transcriptional factor p73 plays a pivotal role in development. Ablation of p73 results in severe neurodevelopmental defects, chronic infections, inflammation and infertility. In addition to this, Trp73^-\- mice display severe alteration in the ciliated epithelial lining and the full-length N-terminal isoform TAp73 has been implicated in the control of multiciliogenesis transcriptional program. With our recently generated Trp73^Δ13/Δ13 mouse model, we interrogate the physiological role of p73 C-terminal isoforms in vivo. Trp73^Δ13/Δ13 mice lack exon 13 in Trp73 gene, producing an ectopic switch from the C-terminal isoforms p73α to p73β. Trp73^Δ13/Δ13 mice show a pattern of expression of TAp73 comparable to the wild-type littermates, indicating that the α to β switch does not significantly alter the expression of the gene in this cell type. Moreover, Trp73^Δ13/Δ13 do not display any significant alteration in the airway ciliated epithelium, suggesting that in this context p73β can fully substitute the function of the longer isoform p73α. Similarly, Trp73^Δ13/Δ13 ciliated epithelium of the brain ependyma also does appear defective. In this district however expression of TAp73 is not detectable, indicating that expression of the gene might be compensated by alternative mechanisms. Overall our work indicates that C-terminus p73 is dispensable for the multiciliogenesis program and suggests a possible tissue-specific effect of p73 alternative splicing.

Neurogenesis and neuronal migration in the postnatal ventricular-subventricular zone: Similarities and dissimilarities between rodents and primates

The ventricular-subventricular zone (V-SVZ) is located in the walls of the lateral ventricles and produces new neurons in the postnatal brain of mammals, including humans. Immature new neurons called "neuroblasts" generated by neural stem cells in the V-SVZ migrate toward their final destinations and contribute to brain development and plasticity. In this review, we describe recent progress in understanding the similarities and dissimilarities in postnatal neurogenesis and neuronal migration between rodents and primates. In rodents, most new V-SVZ-derived neurons migrate along the rostral migratory stream towards the olfactory bulb, where they differentiate into interneurons. In contrast, in humans, the extensive migration of new neurons towards the neocortex continues for several months after birth and might be involved in the development of the expanded neocortex. The mode of migration and the fate of neuroblasts seem to change depending on their environment, destination, and roles in the brain. A better understanding of these similarities and differences between rodents and primates will help translate important findings from animal models and may contribute to the development of clinical strategies for brain repair.

Tissue Engineering and Biomaterial Strategies to Elicit Endogenous Neuronal Replacement in the Brain

Neurogenesis in the postnatal mammalian brain is known to occur in the dentate gyrus of the hippocampus and the subventricular zone. These neurogenic niches serve as endogenous sources of neural precursor cells that could potentially replace neurons that have been lost or damaged throughout the brain. As an example, manipulation of the subventricular zone to augment neurogenesis has become a popular strategy for attempting to replace neurons that have been lost due to acute brain injury or neurodegenerative disease. In this review article, we describe current experimental strategies to enhance the regenerative potential of endogenous neural precursor cell sources by enhancing cell proliferation in neurogenic regions and/or redirecting migration, including pharmacological, biomaterial, and tissue engineering strategies. In particular, we discuss a novel replacement strategy based on exogenously biofabricated "living scaffolds" that could enhance and redirect endogenous neuroblast migration from the subventricular zone to specified regions throughout the brain. This approach utilizes the first implantable, biomimetic tissue-engineered rostral migratory stream, thereby leveraging the brain's natural mechanism for sustained neuronal replacement by replicating the structure and function of the native rostral migratory stream. Across all these strategies, we discuss several challenges that need to be overcome to successfully harness endogenous neural precursor cells to promote nervous system repair and functional restoration. With further development, the diverse and innovative tissue engineering and biomaterial strategies explored in this review have the potential to facilitate functional neuronal replacement to mitigate neurological and psychiatric symptoms caused by injury, developmental disorders, or neurodegenerative disease.

Brain Structural Plasticity: From Adult Neurogenesis to Immature Neurons

Brain structural plasticity is an extraordinary tool that allows the mature brain to adapt to environmental changes, to learn, to repair itself after lesions or disease, and to slow aging. A long history of neuroscience research led to fascinating discoveries of different types of plasticity, involving changes in the genetically determined structure of nervous tissue, up to the ultimate dream of neuronal replacement: a stem cell-driven “adult neurogenesis” (AN). Yet, this road does not seem a straight one, since mutable dogmas, conflicting results and conflicting interpretations continue to warm the field. As a result, after more than 10,000 papers published on AN, we still do not know its time course, rate or features with respect to other kinds of structural plasticity in our brain. The solution does not appear to be behind the next curve, as differences among mammals reveal a very complex landscape that cannot be easily understood from rodents models alone. By considering evolutionary aspects, some pitfalls in the interpretation of cell markers, and a novel population of undifferentiated cells that are not newly generated [immature neurons (INs)], we address some conflicting results and controversies in order to find the right road forward. We suggest that considering plasticity in a comparative framework might help assemble the evolutionary, anatomical and functional pieces of a very complex biological process with extraordinary translational potential.

Prolactin, Estradiol and Testosterone Differentially Impact Human Hippocampal Neurogenesis in an In Vitro Model

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Human hippocampal progenitor cells (HPCs) and tissue express classical sex hormone receptors.

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Prolactin does not impact human HPCs maintained in a proliferative state.

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Prolactin increases neuronal differentiation of human HPCs only in the short term.

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Estradiol and testosterone both increase the cell density of proliferating HPCs.

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Estradiol and testosterone have no observed effect on differentiating HPCs.

Previous studies have indicated that sex hormones such as prolactin, estradiol and testosterone may play a role in the modulation of adult hippocampal neurogenesis (AHN) in rodents and non-human primates, but so far there has been no investigation of their impact on human hippocampal neurogenesis. Here, we quantify the expression levels of the relevant receptors in human post-mortem hippocampal tissue and a human hippocampal progenitor cell (HPC) line. Secondly, we investigate how these hormones modulate hippocampal neurogenesis using a human in vitro cellular model. Human female HPCs were cultured with biologically relevant concentrations of either prolactin, estradiol or testosterone. Bromodeoxyuridine (BrdU) incorporation, immunocytochemistry (ICC) and high-throughput analyses were used to quantify markers determining cell fate after HPCs were either maintained in a proliferative state or allowed to differentiate in the presence of these hormones. In proliferating cells, estrogen and testosterone increased cell density but had no clear effect on markers of proliferation or cell death to account for this. In differentiating cells, a 3-day treatment of prolactin elicited a transient effect, whereby it increased the proportion of microtubule-associated protein 2 (MAP2)-positive and Doublecortin (DCX)-positive cells, but this effect was not apparent after 7-days. At this timepoint we instead observe a decrease in proliferation. Overall, our study demonstrates relatively minor, and possibly short-term effects of sex hormones on hippocampal neurogenesis in human cells. Further work will be needed to understand if our results differ to previous animal research due to species-specific differences, or whether it relates to limitations of our in vitro model.

Adult neurogenesis from reprogrammed astrocytes

The details of adult neurogenesis, including environmental triggers, region specificity, and species homology remain an area of intense investigation. Slowing or halting age-related cognitive dysfunction, or restoring neurons lost to disease or injury represent just a fraction of potential therapeutic applications. New neurons can derive from stem cells, pluripotent neural progenitor cells, or non-neuronal glial cells, such as astrocytes. Astrocytes must be epigenetically “reprogrammed” to become neurons, which can occur both naturally in vivo, and via artificial exogenous treatments. While neural progenitor cells are localized to a few neurogenic zones in the adult brain, astrocytes populate almost every brain structure. In this review, we will summarize recent research into neurogenesis that arises from conversion of post-mitotic astrocytes, detail the genetic and epigenetic pathways that regulate this process, and discuss the possible clinical relevance in supplementing stem-cell neurogenic therapies.

Novel insights into inner ear development and regeneration for targeted hearing loss therapies

Sensorineural hearing loss is the most common sensory deficit in humans. Despite the global scale of the problem, only limited treatment options are available today. The mammalian inner ear is a highly specialized postmitotic organ, which lacks proliferative or regenerative capacity. Since the discovery of hair cell regeneration in non-mammalian species however, much attention has been placed on identifying possible strategies to reactivate similar responses in humans. The development of successful regenerative approaches for hearing loss strongly depends on a detailed understanding of the mechanisms that control human inner ear cellular specification, differentiation and function, as well as on the development of robust in vitro cellular assays, based on human inner ear cells, to study these processes and optimize therapeutic interventions. We summarize here some aspects of inner ear development and strategies to induce regeneration that have been investigated in rodents. Moreover, we discuss recent findings in human inner ear development and compare the results with findings from animal models. Finally, we provide an overview of strategies for in vitro generation of human sensory cells from pluripotent and somatic progenitors that may provide a platform for drug development and validation of therapeutic strategies in vitro.

Adult neurogenesis in the mammalian dentate gyrus

Earlier observations in neuroscience suggested that no new neurons form in the mature central nervous system. Evidence now indicates that new neurons do form in the adult mammalian brain. Two regions of the mature mammalian brain generate new neurons: (a) the border of the lateral ventricles of the brain (subventricular zone) and (b) the subgranular zone (SGZ) of the dentate gyrus of the hippocampus. This review focuses only on new neuron formation in the dentate gyrus of the hippocampus. During normal prenatal and early postnatal development, neural stem cells (NSCs) give rise to differentiated neurons. NSCs persist in the dentate gyrus SGZ, undergoing cell division, with some daughter cells differentiating into functional neurons that participate in learning and memory and general cognition through integration into pre-existing neural networks. Axons, which emanate from neurons in the entorhinal cortex, synapse with dendrites of the granule cells (small neurons) of the dentate gyrus. Axons from granule cells synapse with pyramidal cells in the hippocampal CA3 region, which send axons to synapse with CA1 hippocampal pyramidal cells that send their axons out of the hippocampus proper. Adult neurogenesis includes proliferation, differentiation, migration, the death of some newly formed cells and final integration of surviving cells into neural networks. We summarise these processes in adult mammalian hippocampal neurogenesis and discuss the roles of major signalling molecules that influence neurogenesis, including neurotransmitters and some hormones. The recent controversy raised concerning whether or not adult neurogenesis occurs in humans also is discussed.

A balanced evaluation of the evidence for adult neurogenesis in humans: implication for neuropsychiatric disorders

There is a widespread belief that neurogenesis exists in adult human brain, especially in the dentate gyrus, and it is to be maintained and, if possible, augmented with different stimuli including exercise and certain drugs. Here, we examine the evidence for adult human neurogenesis and note important limitations of the methodologies used to study it. A balanced review of the literature and evaluation of the data indicate that adult neurogenesis in human brain is improbable. In fact, in several high-quality recent studies in adult human brain, unlike in adult brains of other species, neurogenesis was not detectable. These findings suggest that the human brain requires a permanent set of neurons to maintain acquired knowledge for decades, which is essential for complex high cognitive functions unique to humans. Thus, stimulation and/or injection of neural stem cells into human brains may not only disrupt brain homeostatic systems, but also disturb normal neuronal circuits. We propose that the focus of research should be the preservation of brain neurons by prevention of damage, not replacement.

Adult Neurogenesis Is Sustained by Symmetric Self-Renewal and Differentiation

Somatic stem cells have been identified in multiple adult tissues. Whether self-renewal occurs symmetrically or asymmetrically is key to understanding long-term stem cell maintenance and generation of progeny for cell replacement. In the adult mouse brain, neural stem cells (NSCs) (B1 cells) are retained in the walls of the lateral ventricles (ventricular-subventricular zone [V-SVZ]). The mechanism of B1 cell retention into adulthood for lifelong neurogenesis is unknown. Using multiple clonal labeling techniques, we show that the vast majority of B1 cells divide symmetrically. Whereas 20%-30% symmetrically self-renew and can remain in the niche for several months before generating neurons, 70%-80% undergo consuming divisions generating progeny, resulting in the depletion of B1 cells over time. This cellular mechanism decouples self-renewal from the generation of progeny. Limited rounds of symmetric self-renewal and consuming symmetric differentiation divisions can explain the levels of neurogenesis observed throughout life.