Neurogenesis in adult insect mushroom bodies

Adult brain neurogenesis does not account for behavioral differences between solitary and social bees

In many taxa, increasing attention is being paid to how group living shapes the expression of brain plasticity and behavioural flexibility. In eusocial insects, the lifelong commitment of workers and queens to a reproductive or non-reproductive caste is accompanied by a loss of behavioural totipotency, and often, by the expression of a limited behavioural repertoire in workers due to their specialisation. On the other hand, individuals of solitary species have a broader behavioural repertoire as they have to perform all the tasks themselves. This raises the question of whether solitary and social insects differ in their levels of brain plasticity. One mechanism found in both invertebrates and vertebrates to contribute to brain plasticity is adult neurogenesis. It is a mechanism by which adult-born neurons are generated, differentiated and functionally integrated in the brain circuits during adulthood. In this study, we compared the solitary bee Osmia bicornis and the eusocial bee Apis mellifera. We focused on the mushroom bodies which are higher-order integration centres in the insect brain. Based on their known behavioural repertoire, our prediction was that both solitary and social bees would exhibit neurogenesis in the brain until the pupal stage, but that this capacity would persist only in adult solitary bees. However, our results do not validate this prediction, as they indicate that no cells are produced in the mushroom bodies or other areas of the adult solitary bee brain.

A modified method to analyse cell proliferation using EdU labelling in large insect brains

The study of neurogenesis is critical to understanding of the evolution of nervous systems. Within invertebrates, this process has been extensively studied in Drosophila melanogaster, which is the predominant model thanks to the availability of advanced genetic tools. However, insect nervous systems are extremely diverse, and by studying a range of taxa we can gain additional information about how nervous systems and their development evolve. One example of the high diversity of insect nervous system diversity is provided by the mushroom bodies. Mushroom bodies have critical roles in learning and memory and vary dramatically across species in relative size and the type(s) of sensory information they process. Heliconiini butterflies provide a useful snapshot of this diversity within a closely related clade. Within Heliconiini, the genus Heliconius contains species where mushroom bodies are 3-4 times larger than other closely related genera, relative to the rest of the brain. This variation in size is largely explained by increases in the number of Kenyon cells, the intrinsic neurons which form the mushroom body. Hence, variation in mushroom body size is the product of changes in cell proliferation during Kenyon cell neurogenesis. Studying this variation requires adapting labelling techniques for use in less commonly studied organisms, as methods developed for common laboratory insects often do not work. Here, we present a modified protocol for EdU staining to examine neurogenesis in large-brained insects, using Heliconiini butterflies as our primary case, but also demonstrating applicability to cockroaches, another large-brained insect.

Adult neurogenesis does not explain the extensive post-eclosion growth of Heliconius mushroom bodies

Among butterflies, Heliconius have a unique behavioural profile, being the sole genus to actively feed on pollen. Heliconius learn the location of pollen resources, and have enhanced visual memories and expanded mushroom bodies, an insect learning and memory centre, relative to related genera. These structures also show extensive post-eclosion growth and developmental sensitivity to environmental conditions. However, whether this reflects plasticity in neurite growth, or an extension of neurogenesis into the adult stage, is unknown. Adult neurogenesis has been described in some Lepidoptera, and could provide one route to the increased neuron number observed in Heliconius. Here, we compare volumetric changes in the mushroom bodies of freshly eclosed and aged Heliconius erato and Dryas iulia, and estimate the number of intrinsic mushroom body neurons using a new and validated automated method to count nuclei. Despite extensive volumetric variation associated with age, our data show that neuron number is remarkably constant in both species, suggesting a lack of adult neurogenesis in the mushroom bodies. We support this conclusion with assays of mitotic cells, which reveal very low levels of post-eclosion cell division. Our analyses provide an insight into the evolution of neural plasticity, and can serve as a basis for continued exploration of the potential mechanisms behind brain development and maturation.

In-vivo egfp expression in the honeybee Apis mellifera induced by electroporation and viral expression vector

In this study we describe egfp expression induced by two techniques: in vivo electroporation and viral transduction in several cell types of the adult honeybee brain. Non-neuronal and neuronal cell types were identified and the expression persisted at least during three days. Kenyon cells, optic lobe neurons and protocerebral lobe neurons were electroporated. Astrocyte-like glia cells, fibrous lamellar glia cells and cortex glia cells were identified. Viral transduction targeted one specific type of glia cells that could not be identified. EGFP positive cells types were rather variable after electroporation, and viral transduction resulted in more homogenous groups of positive cells. We propose that these techniques remain a good alternative to transgenic animals because they potentially target only somatic cells.

The adverse effects of injected functionalized multi-walled carbon nanotube (f-MWCNT) on in vivo neurosecretory brain cells of Jamaican field cricket, Gryllus assimilis

Carbon nanotubes (CNTs) have been increasingly more prevalent due to their use in product technology owing to their exceptional electrical and thermal conductivity and tensile strength because of their nanostructure and strength of the bonds among carbon atoms. The potential increase of CNTs in the environment is a concern, and studies to assess the toxic effects of these nanomaterials (NMs) are needed. However, so far, most of the studies are focused on aquatic species and much less is understood about the effects of NM in terrestrial organisms. This investigation used a functionalized multi-walled carbon nanotube (f-MWCNT) and the Jamaican cricket Gryllus assimilis to assess the effects of this NM. Cricket nymphs were injected with f-MWCNT suspension-at three different concentrations. The insecticide Fipronil was used as a positive control. Survival was monitored, and histological analysis was made in the brains. Pyknotic cells were quantified in two brain regions, a neurosecretory called Pars intercerebralis (PI), and an associative region called mushroom body (MB). No mortality was observed in any f-MWCNT concentration tested. A significant increase in pyknotic cells was observed as sub-lethal effect for the intermediate concentration of f-MWCNT, at PI, while any significant change was observed at the Kenyon cells of the MB. These results are discussed in the context of agglomeration and dispersion of the f-MWCNT at different concentrations, and availability of the f-MWCNT on the circulatory system, as well as the natural decay of pyknotic cells with time and different patterns of adult cricket neurogenesis. Our results showed that f-MWCNT had negative effects in the neurosecretory region of the brain.

A crabs' high-order brain center resolved as a mushroom body-like structure

The hypothesis of a common origin for high-order memory centers in bilateral animals presents the question of how different brain structures, such as the vertebrate hippocampus and the arthropod mushroom bodies, are both structurally and functionally comparable. Obtaining evidence to support the hypothesis that crustaceans possess structures equivalent to the mushroom bodies that play a role in associative memories has proved challenging. Structural evidence supports that the hemiellipsoid bodies of hermit crabs, crayfish and lobsters, spiny lobsters, and shrimps are homologous to insect mushroom bodies. Although a preliminary description and functional evidence supporting such homology in true crabs (Brachyura) has recently been shown, other authors consider the identification of a possible mushroom body homolog in Brachyura as problematic. Here we present morphological and immunohistochemical data in Neohelice granulata supporting that crabs possess well-developed hemiellipsoid bodies that are resolved as mushroom bodies-like structures. Neohelice exhibits a peduncle-like tract, from which processes project into proximal and distal domains with different neuronal specializations. The proximal domains exhibit spines and en passant-like processes and are proposed here as regions mainly receiving inputs. The distal domains exhibit a "trauben"-like compartmentalized structure with bulky terminal specializations and are proposed here as output regions. In addition, we found microglomeruli-like complexes, adult neurogenesis, aminergic innervation, and elevated expression of proteins necessary for memory processes. Finally, in vivo calcium imaging suggests that, as in insect mushroom bodies, the output regions exhibit stimulus-specific activity. Our results support the shared organization of memory centers across crustaceans and insects.

A Toll-receptor map underlies structural brain plasticity

Experience alters brain structure, but the underlying mechanism remained unknown. Structural plasticity reveals that brain function is encoded in generative changes to cells that compete with destructive processes driving neurodegeneration. At an adult critical period, experience increases fiber number and brain size in Drosophila. Here, we asked if Toll receptors are involved. Tolls demarcate a map of brain anatomical domains. Focusing on Toll-2, loss of function caused apoptosis, neurite atrophy and impaired behaviour. Toll-2 gain of function and neuronal activity at the critical period increased cell number. Toll-2 induced cycling of adult progenitor cells via a novel pathway, that antagonized MyD88-dependent quiescence, and engaged Weckle and Yorkie downstream. Constant knock-down of multiple Tolls synergistically reduced brain size. Conditional over-expression of Toll-2 and wek at the adult critical period increased brain size. Through their topographic distribution, Toll receptors regulate neuronal number and brain size, modulating structural plasticity in the adult brain.

Everything that you experience leaves its mark on your brain. When you learn something new, the neurons involved in the learning episode grow new projections and form new connections. Your brain may even produce new neurons. Physical exercise can induce similar changes, as can taking antidepressants. By contrast, stress, depression, ageing and disease can have the opposite effect, triggering neurons to break down and even die. The ability of the brain to change in response to experience is known as structural plasticity, and it is in a tug-of-war with processes that drive neurodegeneration.

Structural plasticity occurs in other species too: for example, it was described in the fruit fly more than a quarter of a century ago. Yet, the molecular mechanisms underlying structural plasticity remain unclear. Li et al. now show that, in fruit flies, this plasticity involves Toll receptors, a family of proteins present in the brain but best known for their role in the immune system.

Fruit flies have nine different Toll receptors, the most abundant being Toll-2. When activated, these proteins can trigger a series of molecular events in a cell. Li et al. show that increasing the amount of Toll-2 in the fly brain makes the brain produce new neurons. Activating neurons in a brain region has the same effect, and this increase in neuron number also depends on Toll-2. By contrast, reducing the amount of Toll-2 causes neurons to lose their projections and connections, and to die, and impairs fly behaviour.

Li et al. also show that each Toll receptor has a unique distribution across the fly brain. Different types of experiences activate different brain regions, and therefore different Toll receptors. These go on to trigger a common molecular cascade, but they modulate it such as to result in distinct outcomes. By working together in different combinations, Toll receptors can promote either the death or survival of neurons, and they can also drive specific brain cells to remain dormant or to produce new neurons.

By revealing how experience changes the brain, Li et al. provide clues to the way neurons work and form; these findings may also help to find new treatments for disorders that change brain structure, such as certain psychiatric conditions. Toll-like receptors in humans could thus represent a promising new target for drug discovery.

Adult neurogenesis in the mushroom bodies of red flour beetles (Tribolium castaneum, HERBST) is influenced by the olfactory environment

Several studies showed adult persisting neurogenesis in insects, including the red flour beetle Tribolium castaneum, while it is absent in honeybees, carpenter ants, and vinegar flies. In our study, we focus on cell proliferation in the adult mushroom bodies of T. castaneum. We reliably labelled the progenies of the adult persisting mushroom body neuroblasts and determined the proliferation rate under several olfactory conditions within the first week after adult eclosion. We found at least two phases of Kenyon cell proliferation in the early adult beetle. Our results suggest that the generation of Kenyon cells during the first three days after adult eclosion is mainly genetically predetermined and a continuation of the developmental processes (nature), whereas from day four on proliferation seems to be mainly dependent on the odour environment (nurture). Considering that the mushroom bodies are linked to learning and memory, neurogenesis in the mushroom bodies is part of the remodelling of neuronal circuits leading to the adaption to the environment and optimization of behaviour.

Characterization of plexinA and two distinct semaphorin1a transcripts in the developing and adult cricket Gryllus bimaculatus

Guidance cues act during development to guide growth cones to their proper targets in both the central and peripheral nervous systems. Experiments in many species indicate that guidance molecules also play important roles after development, though less is understood about their functions in the adult. The Semaphorin family of guidance cues, signaling through Plexin receptors, influences the development of both axons and dendrites in invertebrates. Semaphorin functions have been extensively explored in Drosophila melanogaster and some other Dipteran species, but little is known about their function in hemimetabolous insects. Here, we characterize sema1a and plexA in the cricket Gryllus bimaculatus. In fact, we found two distinct predicted Sema1a proteins in this species, Sema1a.1 and Sema1a.2, which shared only 48% identity at the amino acid level. We include a phylogenetic analysis that predicted that many other insect species, both holometabolous and hemimetabolous, express two Sema1a proteins as well. Finally, we used in situ hybridization to show that sema1a.1 and sema1a.2 expression patterns were spatially distinct in the embryo, and both roughly overlap with plexA. All three transcripts were also expressed in the adult brain, mainly in the mushroom bodies, though sema1a.2 was expressed most robustly. sema1a.2 was also expressed strongly in the adult thoracic ganglia while sema1a.1 was only weakly expressed and plexA was undetectable.

Magnitude Assessment of Adult Neurogenesis in the Octopus vulgaris Brain Using a Flow Cytometry-Based Technique

Adult neurogenesis is widespread among metazoans, it occurs in animals with a network nervous system, as cnidarians, and in animals with a complex and centralized brain, such as mammals, non-mammalian vertebrates, ecdysozoans, and a lophotrochozoan, Octopus vulgaris. Nevertheless, there are important differences among taxa, especially in the number of the regions involved and in cell proliferation rate during the life-cycle. The comparative evaluation of adult neurogenesis among different brain regions is an arduous task to achieve with only stereological techniques. However, in Octopus vulgaris we recently confirmed the presence of active proliferation in the learning-memory centers, multisensory integration centers, and the motor centers of the adult brain. Here, using a flow cytometry technique, we provide a method to quantify the active proliferation in octopus nervous system using a BrdU in vitro administration without exposing the animals to stress or painful injections usually used. This method is in line with the current animal welfare regulations regarding cephalopods, and the flow cytometry-based technique enabled us to measure adult neurogenesis more quickly and reliably than histological techniques, with the additional advantage of processing multiple samples in parallel. Flow cytometry is thus an appropriate technique for measuring and comparing adult neurogenesis in animals that are in a different physiological and/or environmental contexts. A BrdU immunoreactivity distribution, to define the neurogenic areas, and the effective penetration in vitro of the BrdU is also provided.

A Cold-Blooded View on Adult Neurogenesis

During brain development, highly complex and interconnected neural circuits are established. This intricate wiring needs to be robust to faithfully perform adult brain function throughout life, but at the same time offer room for plasticity to integrate new information. In the mammalian brain, adult-born neurons are produced in restricted niches harboring neural stem cells. In the fruit fly Drosophila, low-level adult neurogenesis arising from a dispersed population of neural progenitors has recently been detected in the optic lobes. Strikingly, these normally quiescent neural stem cells proliferate upon brain injury and produce new neurons for brain regeneration. Here, we review adult neurogenesis in crustaceans and insects and highlight that neurogenesis in the visual system is prominent in arthropods, but its role and underlying mechanisms are unclear. Moreover, we discuss how the study of damage-responsive progenitor cells in Drosophila may help to understand robust regenerative neurogenesis and open new avenues to enhance brain repair after injury or stroke in humans.

Context-dependent memory traces in the crab's mushroom bodies: Functional support for a common origin of high-order memory centers

The hypothesis of a common origin for the high-order memory centers in bilateral animals is based on the evidence that several key features, including gene expression and neuronal network patterns, are shared across several phyla. Central to this hypothesis is the assumption that the arthropods' higher order neuropils of the forebrain [the mushroom bodies (MBs) of insects and the hemiellipsoid bodies (HBs) of crustaceans] are homologous structures. However, even though involvement in memory processes has been repeatedly demonstrated for the MBs, direct proof of such a role in HBs is lacking. Here, through neuroanatomical and immunohistochemical analysis, we identified, in the crab Neohelice granulata, HBs that resemble the calyxless MBs found in several insects. Using in vivo calcium imaging, we revealed training-dependent changes in neuronal responses of vertical and medial lobes of the HBs. These changes were stimulus-specific, and, like in the hippocampus and MBs, the changes reflected the context attribute of the memory trace, which has been envisioned as an essential feature for the HBs. The present study constitutes functional evidence in favor of a role for the HBs in memory processes, and provides key physiological evidence supporting a common origin of the arthropods' high-order memory centers.

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.

The 'ventral organs' of Pycnogonida (Arthropoda) are neurogenic niches of late embryonic and post-embryonic nervous system development

Early neurogenesis in arthropods has been in the focus of numerous studies, its cellular basis, spatio-temporal dynamics and underlying genetic network being by now comparably well characterized for representatives of chelicerates, myriapods, hexapods and crustaceans. By contrast, neurogenesis during late embryonic and/or post-embryonic development has received less attention, especially in myriapods and chelicerates. Here, we apply (i) immunolabeling, (ii) histology and (iii) scanning electron microscopy to study post-embryonic ventral nerve cord development in Pseudopallene sp., a representative of the sea spiders (Pycnogonida), the presumable sister group of the remaining chelicerates. During early post-embryonic development, large neural stem cells give rise to additional ganglion cell material in segmentally paired invaginations in the ventral ectoderm. These ectodermal cell regions - traditionally designated as 'ventral organs' - detach from the surface into the interior and persist as apical cell clusters on the ventral ganglion side. Each cluster is a post-embryonic neurogenic niche that features a tiny central cavity and initially still houses larger neural stem cells. The cluster stays connected to the underlying ganglionic somata cortex via an anterior and a posterior cell stream. Cell proliferation remains restricted to the cluster and streams, and migration of newly produced cells along the streams seems to account for increasing ganglion cell numbers in the cortex. The pycnogonid cluster-stream-systems show striking similarities to the life-long neurogenic system of decapod crustaceans, and due to their close vicinity to glomerulus-like neuropils, we consider their possible involvement in post-embryonic (perhaps even adult) replenishment of olfactory neurons - as in decapods. An instance of a potentially similar post-embryonic/adult neurogenic system in the arthropod outgroup Onychophora is discussed. Additionally, we document two transient posterior ganglia in the ventral nerve cord of Pseudopallene sp. and evaluate this finding in light of the often discussed reduction of a segmented 'opisthosoma' during pycnogonid evolution.

Timelines in the insect brain: fates of identified neural stem cells generating the central complex in the grasshopper Schistocerca gregaria

This study employs labels for cell proliferation and cell death, as well as classical histology to examine the fates of all eight neural stem cells (neuroblasts) whose progeny generate the central complex of the grasshopper brain during embryogenesis. These neuroblasts delaminate from the neuroectoderm between 25 and 30 % of embryogenesis and form a linear array running from ventral (neuroblasts Z, Y, X, and W) to dorsal (neuroblasts 1-2, 1-3, 1-4, and 1-5) along the medial border of each protocerebral hemisphere. Their stereotypic location within the array, characteristic size, and nuclear morphologies, identify these neuroblasts up to about 70 % of embryogenesis after which cell shrinkage and shape changes render progressively more cells histologically unrecognizable. Molecular labels show all neuroblasts in the array are proliferative up to 70 % of embryogenesis, but subsequently first the more ventral cells (72-75 %), and then the dorsal ones (77-80 %), cease proliferation. By contrast, neuroblasts elsewhere in the brain and optic lobe remain proliferative. Apoptosis markers label the more ventral neuroblasts first (70-72 %), then the dorsal cells (77 %), and the absence of any labeling thereafter confirms that central complex neuroblasts have exited the cell cycle via programmed cell death. Our data reveal appearance, proliferation, and cell death proceeding as successive waves from ventral to dorsal along the array of neuroblasts. The resulting timelines offer a temporal blueprint for building the neuroarchitecture of the various modules of the central complex.

Gliogenesis in the embryonic brain of the grasshopper Schistocerca gregaria with particular focus on the protocerebrum prior to mid-embryogenesis

I investigate the pattern of gliogenesis in the brain of the grasshopper Schistocerca gregaria prior to mid-embryogenesis, with particular focus on the protocerebrum. Using the glia-specific marker Repo and the neuron-specific marker HRP, I identify three types of glia with respect to their respective positions in the brain: surface glia form the outmost cell layer ensheathing the brain; cortex glia are intermingled with neuronal somata forming the brain cortex; and neuropil glia are associated with brain neuropils. The ontogeny of each glial type has also been studied. At 24% of embryogenesis, a few glia are observed in each hemisphere of the proto-, deuto- and tritocerebrum. In each protocerebral hemisphere, such glia form a cluster that expands rapidly during later development. Closer examination reveals proliferative glia in such clusters at ages spanning from 24 to 36% of embryogenesis, indicating that glial proliferation may account for the expansion of the clusters. Data derived from 33-39% of embryogenesis suggest that, in the protocerebrum, each type of glia is likely to be generated by its respective progenitor-forming clusters. Moreover, the glial cluster located at the anterior end of the brain can give rise to both surface glia and cortex glia that populate the protocerebrum via subsequent migration. Proliferation is observed for all three glial types, indicating a possible source for the glia.

Glia associated with central complex lineages in the embryonic brain of the grasshopper Schistocerca gregaria

We have investigated the pattern of glia associated with central complex lineages in the embryonic brain of the grasshopper Schistocerca gregaria. Using the glia-specific marker Repo, we identified glia associated externally with such lineages, termed lineage-extrinsic glia, and glia located internally within the lineages, termed lineage-intrinsic glia. Populations of both glial types increase up to 60 % of embryogenesis, and thereafter decrease. Extrinsic glia change their locations over time, while intrinsic ones are consistently found in the more apical part of a lineage. Apoptosis is not observed for either glial type, suggesting migration is a likely mechanism accounting for changes in glial number. Proliferative glia are present both within and without individual lineages and two glial clusters associated with the lineages, one apically and the other basally, may represent sources of glia.

Postembryonic development of astrocyte-like glia of the central complex in the grasshopper Schistocerca gregaria

Central complex modules in the postembryonic brain of the grasshopper Schistocerca gregaria are enveloped by Repo-positive/glutamine-synthetase-positive astrocyte-like glia. Such cells constitute Rind-Neuropil Interface glia. We have investigated the postembryonic development of these glia and their anatomical relationship to axons originating from the w, x, y, z tract system of the pars intercerebralis. Based on glutamine synthetase immunolabeling, we have identified four morphological types of cells: bipolar type 1 glia delimit the central body but only innervate its neuropil superficially; monopolar type 2 glia have a more columnar morphology and direct numerous gliopodia into the neuropil where they arborize extensively; monopolar type 3 glia are found predominantly in the region between the noduli and the central body and have a dendritic morphology and their gliopodia project deeply into the central body neuropil where they arborize extensively; multipolar type 4 glia link the central body neuropil with neighboring neuropils of the protocerebrum. These glia occupy type-specific distributions around the central body. Their gliopodia develop late in embryogenesis, elongate and generally become denser during subsequent postembryonic development. Gliopodia from putatively type 3 glia within the central body have been shown to lie closely apposed to individual axons of identified columnar fiber bundles from the w, x, y, z tract system of the central complex. This anatomical association might offer a substrate for neuron/glia interactions mediating postembryonic maturation of the central complex.

New neurons for 'survival of the fittest'

Adult neurogenesis is often considered an archaic trait that has undergone a 'phylogenetic reduction' from amphibian ancestors to humans. However, adult neurogenesis in the hippocampal dentate gyrus might actually be a late-evolved trait. In non-mammals, adult hippocampal neurogenesis is not restricted to the equivalents of the dentate gyrus, which also show different connectivity and functionality compared to their mammalian counterpart. Moving actively in a changing world and dealing with novelty and complexity regulate adult neurogenesis. New neurons might thus provide the cognitive adaptability to conquer ecological niches rich with challenging stimuli.

Developmental expression of cell recognition molecules in the mushroom body and antennal lobe of the locust Locusta migratoria

We examined the development of olfactory neuropils in the hemimetabolous insect Locusta migratoria with an emphasis on the mushroom bodies, protocerebral integration centers implicated in memory formation. Using a marker of the cyclic adenosine monophosphate (cAMP) signaling cascade and lipophilic dye labeling, we obtained new insights into mushroom body organization by resolving previously unrecognized accessory lobelets arising from Class III Kenyon cells. We utilized antibodies against axonal guidance cues, such as the cell surface glycoproteins Semaphorin 1a (Sema 1a) and Fasciclin I (Fas I), as embryonic markers to compile a comprehensive atlas of mushroom body development. During embryogenesis, all neuropils of the olfactory pathway transiently expressed Sema 1a. The immunoreactivity was particularly strong in developing mushroom bodies. During late embryonic stages, Sema 1a expression in the mushroom bodies became restricted to a subset of Kenyon cells in the core region of the peduncle. Sema 1a was differentially sorted to the Kenyon cell axons and absent in the dendrites. In contrast to Drosophila, locust mushroom bodies and antennal lobes expressed Fas I, but not Fas II. While Fas I immunoreactivity was widely distributed in the midbrain during embryogenesis, labeling persisted into adulthood only in the mushroom bodies and antennal lobes. Kenyon cells proliferated throughout the larval stages. Their neurites retained the embryonic expression pattern of Sema 1a and Fas I, suggesting a role for these molecules in developmental mushroom body plasticity. Our study serves as an initial step toward functional analyses of Sema 1a and Fas I expression during locust mushroom body formation.

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

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

Influence of two methods of dietary restriction on life history features and aging of the cricket Acheta domesticus

Studying aging is constrained using vertebrates by their longevity, size, ethical restrictions, and expense. The key insect model, Drosophila melanogaster, is holometabolous. Larvae feed on yeast in moist media and adults sponge food. Most aging studies are restricted to adults. Another key model, the nematode Caenorhabditis elegans, feeds on bacteria in moist media. For either invertebrate refreshing test materials, preventing degradation and obtaining accurate dosing are difficult even with synthetic media. The cricket Acheta domesticus has a short lifespan (∼120 days at 30°C) and is omnivorous. Age-matched cohorts are easily obtained from eggs. The life cycle is hemimetabolous and nymphs eat the same foods as adults. Growth is easily monitored, gender can be differentiated before maturity, and maturation is indicated by wings and mature genitalia. Crickets can be reared in large numbers at low cost. Test materials can be mixed into food and ingestion rates or mass budgets easily assessed. Here, we validate the cricket as a model of aging by testing two fundamental methods of restricting food intake: time-restricted access to food and dietary dilution. Growth, maturation, survivorship, and longevity varied with treatments and genders. Intermittent feeding (which is ineffective in flies) significantly extended longevity of crickets. Dietary dilution also extended longevity via remarkable prolongation of the juvenile period.

Cytoarchitecture and ultrastructure of neural stem cell niches and neurogenic complexes maintaining adult neurogenesis in the olfactory midbrain of spiny lobsters, Panulirus argus

New interneurons are continuously generated in small proliferation zones within neuronal somata clusters in the olfactory deutocerebrum of adult decapod crustaceans. Each proliferation zone is connected to a clump of cells containing one neural stem cell (i.e., adult neuroblast), thus forming a "neurogenic complex." Here we provide a detailed analysis of the cytoarchitecture of neurogenic complexes in adult spiny lobsters, Panulirus argus, based on transmission electron microscopy and labeling with cell-type-selective markers. The clump of cells is composed of unique bipolar clump-forming cells that collectively completely envelop the adult neuroblast and are themselves ensheathed by a layer of processes of multipolar cell body glia. An arteriole is attached to the clump of cells, but dye perfusion experiments show that hemolymph has no access to the interior of the clump of cells. Thus, the clump of cells fulfills morphological criteria of a protective stem cell niche, with clump-forming cells constituting the adult neuroblast's microenvironment together with the cell body glia processes separating it from other tissue components. Bromodeoxyuridine pulse-chase experiments with short survival times suggest that adult neuroblasts are not quiescent but rather cycle actively during daytime. We propose a cell lineage model in which an asymmetrically dividing adult neuroblast repopulates the pool of neuronal progenitor cells in the associated proliferation zone. In conclusion, as in mammalian brains, adult neurogenesis in crustacean brains is fueled by neural stem cells that are maintained by stem cell niches that preserve elements of the embryonic microenvironment and contain glial and vascular elements.

A subpopulation of mushroom body intrinsic neurons is generated by protocerebral neuroblasts in the tobacco hornworm moth, Manduca sexta (Sphingidae, Lepidoptera)

Subpopulations of Kenyon cells, the intrinsic neurons of the insect mushroom bodies, are typically sequentially generated by dedicated neuroblasts that begin proliferating during embryogenesis. When present, Class III Kenyon cells are thought to be the first born population of neurons by virtue of the location of their cell somata, farthest from the position of the mushroom body neuroblasts. In the adult tobacco hornworm moth Manduca sexta, the axons of Class III Kenyon cells form a separate Y tract and dorsal and ventral lobelet; surprisingly, these distinctive structures are absent from the larval Manduca mushroom bodies. BrdU labeling and immunohistochemical staining reveal that Class III Kenyon cells are in fact born in the mid-larval through adult stages. The peripheral position of their cell bodies is due to their genesis from two previously undescribed protocerebral neuroblasts distinct from the mushroom body neuroblasts that generate the other Kenyon cell types. These findings challenge the notion that all Kenyon cells are produced solely by the mushroom body neuroblasts, and may explain why Class III Kenyon cells are found sporadically across the insects, suggesting that when present, they may arise through de novo recruitment of neuroblasts outside of the mushroom bodies. In addition, lifelong neurogenesis by both the Class III neuroblasts and the mushroom body neuroblasts was observed, raising the possibility that adult neurogenesis may play a role in mushroom body function in Manduca.

Adult neurogenesis: examples from the decapod crustaceans and comparisons with mammals

Defining evolutionary origins is a means of understanding an organism's position within the integrated web of living beings, and not only to trace characteristics back in time, but also to project forward in an attempt to reveal relationships with more recently evolved forms. Both the vertebrates and arthropods possess condensed nervous systems, but this is dorsal in the vertebrates and ventral in the arthropods. Also, whereas the nervous system in the vertebrates develops from a neural tube in the embryo, that of the arthropods comes from an ectodermal plate. Despite these apparently fundamental differences, it is now generally accepted that life-long neurogenesis, the generation of functionally integrated neurons from progenitor cells, is a common feature of the adult brains of a variety of organisms, ranging from insects and crustaceans to birds and mammals. Among decapod crustaceans, there is evidence for adult neurogenesis in basal species of the Dendrobranchiata, as well as in more recent terrestrial, marine and fresh-water species. The widespread nature of this phenomenon in decapod species may relate to the importance of the adult-born neurons, although their functional contribution is not yet known. The many similarities between the systems generating neurons in the adult brains of decapod crustaceans and mammals, reviewed in this paper, suggest that adult neurogenesis is governed by common ancestral mechanisms that have been retained in a phylogenetically broad group of species.

The functional organisation of glia in the adult brain of Drosophila and other insects

This review annotates and categorises the glia of adult Drosophila and other model insects and analyses the developmental origins of these in the Drosophila optic lobe. The functions of glia in the adult vary depending upon their sub-type and location in the brain. The task of annotating glia is essentially complete only for the glia of the fly's lamina, which comprise: two types of surface glia-the pseudocartridge and fenestrated glia; two types of cortex glia-the distal and proximal satellite glia; and two types of neuropile glia-the epithelial and marginal glia. We advocate that the term subretinal glia, as used to refer to both pseudocartridge and fenestrated glia, be abandoned. Other neuropiles contain similar glial subtypes, but other than the antennal lobes these have not been described in detail. Surface glia form the blood brain barrier, regulating the flow of substances into and out of the nervous system, both for the brain as a whole and the optic neuropiles in particular. Cortex glia provide a second level of barrier, wrapping axon fascicles and isolating neuronal cell bodies both from neighbouring brain regions and from their underlying neuropiles. Neuropile glia can be generated in the adult and a subtype, ensheathing glia, are responsible for cleaning up cellular debris during Wallerian degeneration. Both the neuropile ensheathing and astrocyte-like glia may be involved in clearing neurotransmitters from the extracellular space, thus modifying the levels of histamine, glutamate and possibly dopamine at the synapse to ultimately affect behaviour.

Brain plasticity in Diptera and Hymenoptera

To mediate different types of behaviour, nervous systems need to coordinate the proper operation of their neural circuits as well as short- and long-term alterations that occur within those circuits. The latter ultimately devolve upon specific changes in neuronal structures, membrane properties and synaptic connections that are all examples of plasticity. This reorganization of the adult nervous system is shaped by internal and external influences both during development and adult maturation. In adults, behavioural experience is a major driving force of neuronal plasticity studied particularly in sensory systems. The range of adaptation depends on features that are important to a particular species, and is therefore specific, so that learning is essential for foraging in honeybees, while regenerative capacities are important in hemimetabolous insects with long appendages. Experience is usually effective during a critical period in early adult life, when neural function becomes tuned to future conditions in an insect's life. Tuning occur at all levels, in synaptic circuits, neuropile volumes, and behaviour. There are many examples, and this review incorporates only a select few, mainly those from Diptera and Hymenoptera.

Agonistic behavior enhances adult neurogenesis in male Acheta domesticus crickets

We examined the effect of agonistic behavior on cell proliferation and neurogenesis in the central nervous system (CNS) of adult male Acheta domesticus crickets. We combined 5-bromo,2'deoxyuridine (BrdU)-labeling of dividing cells with immunocytochemical detection of the neuronal marker horseradish peroxidase to examine the proliferation of progenitor cells and the survival of newborn neurons. In crickets, the mushroom bodies of the brain contain clusters of proliferative cells that divide and generate new neurons in adulthood. Pairs of male crickets were allowed to fight and establish social rank and were then injected with BrdU. Proliferation of mushroom body neurogenic cluster cells was unaffected by agonistic interactions; 24 h after a fight, the number of BrdU positive cells in fought and un-fought males did not significantly differ. However, agonistic interactions did influence cell survival. Two weeks after an agonistic interaction, fought males had more newborn neurons than males that did not fight. There was also a rank-specific effect because dominant males had significantly more new neurons than subordinates. We also report for the first time that neurogenesis in adult crickets can occur in other regions of the brain and in other CNS ganglia, including the terminal abdominal ganglion (TAG). Agonistic interactions enhanced the proliferation of these distributed precursor cells but did not increase the survival of the newborn neurons generated by these cells.

Cellular basis of neurogenesis in the brain of crayfish, Procambarus clarkii: Neurogenic complex in the olfactory midbrain from hatchlings to adults

Neurogenesis in the central olfactory pathway of decapod crustaceans persists throughout life. Here we describe the structural basis of neurogenesis within the olfactory deutocerebrum of the crayfish Procambarus clarkii from hatchlings to adults. Using a proliferation marker and immunostaining, we found that throughout development each hemibrain contains a neurogenic complex consisting of five parts: two proliferation zones, each within the neuronal soma clusters containing local or projection interneurons, a tail of proliferating cells extending from each proliferation zone, and an elongated clump of cells where the two tails meet. The clump of cells comprises two subdivisions joined at a nucleus-free central area. Each subdivision consists of a dense group of clump cells with small, spindle-shaped nuclei and is connected to one of the proliferation zones by a strand of fibrous material encompassing the tail of proliferating cells extending from it. We identify one proliferating cell with a large nucleus in each subdivision as a putative neuroblast. Its daughter cells migrate through the strands to the associated proliferation zones, but in the strand leading to the soma cluster of local interneurons this is masked by local proliferation. We conclude that neurogenesis in the olfactory deutocerebrum of juvenile and adult P. clarkii is based on a few neuroblasts that are associated with unique clumps of cells likely representing stem cell niches.

Metamorphosis and adult development of the mushroom bodies of the red flour beetle, Tribolium castaneum

The insect mushroom bodies play important roles in a number of higher processing functions such as sensory integration, higher level olfactory processing, and spatial and associative learning and memory. These functions have been established through studies in a handful of tractable model systems, of which only the fruit fly Drosophila melanogaster has been readily amenable to genetic manipulations. The red flour beetle Tribolium castaneum has a sequenced genome and has been subject to the development of molecular tools for the ready manipulation of gene expression; however, little is known about the development and organization of the mushroom bodies of this insect. The present account bridges this gap by demonstrating that the organization of the Tribolium mushroom bodies is strikingly like that of the fruit fly, with the significant exception that the timeline of neurogenesis is shifted so that the last population of Kenyon cells is born entirely after adult eclosion. Tribolium Kenyon cells are generated by two large neuroblasts per hemisphere and segregate into an early-born delta lobe subpopulation followed by clear homologs of the Drosophila gamma, alpha'/beta' and alpha/beta lobe subpopulations, with the larval-born cohorts undergoing dendritic reorganization during metamorphosis. BrdU labeling and immunohistochemical staining also reveal that a proportion of individual Tribolium have variable numbers of mushroom body neuroblasts. If heritable, this variation represents a unique opportunity for further studies of the genetic control of brain region size through the control of neuroblast number and cell cycle dynamics.

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.

Neuronal programmed cell death induces glial cell division in the adult Drosophila brain

Although mechanisms that lead to programmed cell death (PCD) in neurons have been analysed extensively, little is known about how surrounding cells coordinate with it. Here we show that neuronal PCD in the Drosophila brain induces glial cell division. We identified PCD in neurons and cell division in glia occurring in a consistent spatiotemporal manner in adult flies shortly after eclosion. Glial division was suppressed when neuronal PCD was inhibited by ectopic expression of the caspase inhibitor gene p35, indicating their causal relationship. Glia also responded to neural injury in a similar manner: both stab injury and degeneration of sensory axons in the brain caused by antennal ablation induced glial division. Eiger, a tumour necrosis factor superfamily ligand, appears to be a link between developmental PCD/neural injury and glial division, as glial division was attenuated in eiger mutant flies. Whereas PCD soon after eclosion occurred in eiger mutants as in the wild type, we observed excess neuronal PCD 2 days later, suggesting a protective function for Eiger or the resulting glial division against the endogenous PCD. In older flies, between 6 and 50 days after adult eclosion, glial division was scarcely observed in the intact brain. Moreover, 8 days after adult eclosion, glial cells no longer responded to brain injury. These results suggest that the life of an adult fly can be divided into two phases: the first week, as a critical period for neuronal cell death-associated glial division, and the remainder.

Regulators of adult neurogenesis in the healthy and diseased brain

1. In recent decades evidence has accumulated demonstrating the birth and functional integration of new neurons in specific regions of the adult mammalian brain, including the dentate gyrus of the hippocampus and the subventricular zone. 2. Studies in a variety of models have revealed genetic, environmental and pharmacological factors that regulate adult neurogenesis. The present review examines some of the molecular and cellular mechanisms that could be mediating these regulatory effects in both the normal and dysfunctional brain. 3. The dysregulation of adult neurogenesis may contribute to the pathogenesis of neurodegenerative disorders, such as Huntington's, Alzheimer's and Parkinson's disease, as well as psychiatric disorders such as depression. Recent evidence supports this idea and, furthermore, also indicates that factors promoting neurogenesis can modify the onset and progression of specific brain disorders, including Huntington's disease and depression.

Identification of putative neuroblasts at the base of adult neurogenesis in the olfactory midbrain of the spiny lobster, Panulirus argus

Continuous neurogenesis persists during adulthood in the olfactory midbrain of decapod crustaceans, including spiny lobsters, Panulirus argus. This encompasses generation of projection and local interneurons, whose somata are in the lateral soma cluster (LC) and medial soma cluster (MC), respectively. Both neuronal types originate from immediate precursors labeled by a single injection of BrdU and located in a small proliferation zone within each cluster. The aim of this study was to identify neuroblasts as a source of the dividing cells by multiple injections of BrdU over 2 days. All animals receiving multiple injections had one or a few 'extra' BrdU-positive nuclei near the proliferation zones, and these nuclei were significantly larger than nuclei of neurons or BrdU-positive cells in the proliferation zones. Since the defining morphological feature of neuroblasts in preadult neurogenesis in arthropods is being larger than their progeny, these large extra BrdU-positive nuclei represent "putative adult neuroblasts." Multiple BrdU-injections revealed a clump of small cells enclosing the putative adult neuroblasts in LC and MC, and these cells shared morphological characteristics with newly identified putative glial cells in the soma clusters and perivascular cells in the walls of arterioles. These results on P. argus suggest that adult neurogenesis is based on one adult neuroblast per soma cluster, adult neurogenesis appears to be a continuation of embryonic and larval neurogenesis, and the newly identified clumps of cells surrounding the putative adult neuroblasts might provide them with specific microenvironments necessary for their unusual lifelong proliferative and self-renewal capacity.

Understanding the regulation and function of adult neurogenesis: contribution from an insect model, the house cricket

Since the discovery of adult neurogenesis, a major issue is the role of newborn neurons and the function-dependent regulation of adult neurogenesis. We decided to use an animal model with a relatively simple brain to address these questions. In the adult cricket brain as in mammals, new neurons are produced throughout life. This neurogenesis occurs in the main integrative centers of the insect brain, the mushroom bodies (MBs), where the neuroblasts responsible for their formation persist after the imaginal molt. The rate of production of new neurons is controlled not only by internal cues such as morphogenetic hormones but also by external environmental cues. Adult crickets reared in an enriched sensory environment experienced an increase in neuroblast proliferation as compared with crickets reared in an impoverished environment. In addition, unilateral sensory deprivation led to reduced neurogenesis in the MB ipsilateral to the lesion. In search of a functional role for the new cells, we specifically ablated MB neuroblasts in young adults using brain-focused gamma ray irradiation. We developed a learning paradigm adapted to the cricket, which we call the "escape paradigm." Using this operant associative learning test, we showed that crickets lacking neurogenesis exhibited delayed learning and reduced memory retention of the task when olfactory cues were used. Our results suggest that environmental cues are able to influence adult neurogenesis and that, in turn, newly generated neurons participate in olfactory integration, optimizing learning abilities of the animal, and thus its adaptation to its environment. Nevertheless, odor learning in adult insects cannot always be attributed to newly born neurons because neurogenesis is completed earlier in development in many insect species. In addition, many of the irradiated crickets performed significantly better than chance on the operant learning task.

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

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

Adult neurogenesis: a common strategy across diverse species

Adult neurogenesis, the generation of new neurons from adult precursor cells, occurs in the brains of a phylogenetically diverse array of animals. In the higher (amniotic) vertebrates, these precursor cells are glial cells that reside within specialized regions, known as neurogenic niches, the elements of which both support and regulate neurogenesis. The in vivo identity and location of the precursor cells responsible for adult neurogenesis in nonvertebrate taxa, however, remain largely unknown. Among the invertebrates, adult neurogenesis has been particularly well characterized in freshwater crayfish (Arthropoda, Crustacea), although the identity of the precursor cells sustaining continuous neuronal proliferation in these animals has yet to be established. Here we provide evidence suggesting that, as in the higher vertebrates, the precursor cells maintaining adult neurogenesis in the crayfish Procambarus clarkii are glial cells. These precursor cells reside within a specialized region, or niche, on the ventral surface of the brain, and their progeny migrate from this niche along glial fibers and then proliferate to form new neurons in the central olfactory pathway. The niche in which these precursor cells reside has many features in common with the neurogenic niches of higher vertebrates. These commonalities include: glial cells functioning as both precursor and support cells, directed migration, close association with the brain vasculature, and specialized basal laminae. The cellular machinery maintaining adult neurogenesis appears, therefore, to be shared by widely disparate taxa. These extensive structural and functional parallels suggest a common strategy for the generation of new neurons in adult brains.

Developmental and adult expression of semaphorin 2a in the cricketGryllus bimaculatus

Developmental guidance cues act to direct growth cones to their correct targets in the nervous system. Recent experiments also demonstrate that developmental cues are expressed in the adult mammalian nervous system, although their function in the brain is not yet clear. The semaphorin gene family has been implicated in the growth of dendrites and axons in a number of different species. While the expression of semaphorin and its influence on tibial pioneer neurons in the developing limb bud have been well characterized in the grasshopper, the expression of semaphorin 2a (sema2a) has not been explored in the adult insect. In this study we used polymerase chain reaction (PCR) with degenerate and gene-specific primers to clone part of the secreted form of sema2a from Gryllus bimaculatus. Using in situ hybridization and immunohistochemistry, we confirmed that sema2a mRNA and protein expression patterns in the embryonic cricket were similar to that seen in the grasshopper. We also showed that tibial neuron development in crickets was comparable to that described in grasshopper. An examination of both developing and adult cricket brains showed that sema2a mRNA and protein were expressed in the Kenyon cells in mushroom bodies, an area involved in learning and memory. Sema2a expression was most obvious near the apex of the mushroom body in a region surrounding the neurogenic tip, which produces neurons throughout the life of the cricket. We discuss the role of neurogenesis in learning and memory and the potential involvement of semaphorin in this process.

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.

Functional division of intrinsic neurons in the mushroom bodies of male Spodoptera littoralis revealed by antibodies against aspartate, taurine, FMRF-amide, Mas-allatotropin and DC0

The aim of this study was to further reveal the organization of Kenyon cells in the mushroom body calyx and lobes of the male moth Spodoptera littoralis, by using immunocytochemical labeling. Subdivisions of the mushroom bodies were identified employing antisera raised against the amino acids taurine and aspartate, the neuropeptides FMRF-amide and Mas-allatotropin, and against the protein kinase A catalytic subunit DC0. These antisera have previously been shown to label subsets of Kenyon cells in other species. The present results show that the organization of the mushroom body lobes into discrete divisions, described from standard neuroanatomical methods, is confirmed by immunocytology and shown to be further elaborated. Anti-taurine labels the accessory Y-tract, the gamma division of the lobes, and a thin subdivision of the most posterior component of the lobes. Aspartate antiserum labels the entire mushroom body. FMRF-amide-like immunolabeling is pronounced in the gamma division and in the anterior perimeter of the alpha/beta and alpha'/beta' divisions. Mas-allatotropin-like immunolabeling shows the opposite of FMRF-amide-like and taurine-like immunolabeling: the gamma division and the accessory Y-system is immunonegative whereas strong labeling is seen in both the alpha/beta and alpha'/beta' divisions. The present results agree with findings from other insects that mushroom bodies are anatomically divided into discrete parallel units. Functional and developmental implications of this organization are discussed.

A review of insect stem cell types

Insect stem cells have been described from both embryonic and adult tissues from a diversity of insect species, although much of the focus in insect stem cell research has been on Drosophila. Insects are a vast and diverse group and it is surprising that a critical aspect of their development like stem cells has not received more attention. In this review we discuss the current state of knowledge of insect stem cell types. We examine what stem cell types have been identified from insects, and briefly discuss what is known about their regulation.

Development-dependent and -independent ubiquitin expression in divisions of the cockroach mushroom body

It has been proposed that the alpha and beta divisions of the mushroom bodies support intermediate and long-term memory whereas the gamma lobes support short-term memory. Here we investigate developmentally dependent versus developmentally independent alterations of mushroom body structure with special emphasis on its lobes. We show that in the cockroach mushroom bodies there are two types of plastic remodeling. One is developmental, in which episodic addition of new circuitry to the alpha and beta lobes is accomplished by newly born Kenyon cells. The second is revealed as a persistent alteration of structure within the gamma lobe. In the alpha/beta lobes, newly generated Kenyon cell axons extend glutamate-immunoreactive collaterals across layers of the axons of mature Kenyon cells. At specific times in each developmental episode (instar) these collaterals express ubiquitin, undergo localized degeneration, and are scavenged by glial cells. In contrast, the mature Kenyon cells that comprise the gamma lobe express detectable ubiquitin throughout each developmental episode. This pattern of ubiquitin expression suggests that the gamma lobe circuitry undergoes continuous modification independent of development.

Hormonal and sensory inputs regulate distinct neuroblast cell cycle properties in adult cricket brain

From invertebrates to humans, it has been demonstrated that new neurons are added to specific brain structures throughout adult life. In the house cricket, adult neurogenesis occurs in the mushroom bodies, the main sensory integrative center of the brain, often considered an analogue of vertebrate hippocampus. We have previously shown that this neurogenesis can be modulated by hormones through the polyamine pathway and by environmental conditions through sensory inputs and the nitric oxide pathway. Environment-induced neurogenesis is independent of juvenile hormone levels, so we addressed the roles of sensory inputs and hormones in the control of neuroblast proliferation. Here, by using double labelling of cells specifically in S phase (5-bromo-2'-deoxyuridine) together with labelling of mitotically active cells in any phase (proliferating cell nuclear antigen), we show that juvenile hormone acts on progenitor cell proliferation by inducing quiescent neuroblasts to enter the cell cycle, whereas sensory inputs act by shortening the cell cycle. Thus, in the adult house cricket, regulation of neuroblast proliferation by hormonal and environmental cues occurs through two independent modes of action.