Newborn cells in the adult crayfish brain differentiate into distinct neuronal types

Mitotically active regions persist in the brains of decapod crustaceans throughout their lifetimes, as they do in many vertebrates. The most well-studied of these regions in decapods occurs within a soma cluster, known as cluster 10, located in the deutocerebrum. Cluster 10 in crayfish and lobsters is composed of the somata of two anatomically and functionally distinct classes of projection neurons: olfactory lobe (OL) projection neurons and accessory lobe (AL) projection neurons. While adult-generated cells in cluster 10 survive for at least a year, their final phenotypes remain unknown. To address this question, we combined BrdU labeling of proliferating cells with specific neuronal and glial markers and tracers to examine the differentiation of newborn cells in cluster 10 of the crayfish, Cherax destructor. Our results show that large numbers of adult-generated cells in cluster 10 differentiate into neurons expressing the neuropeptide crustacean-SIFamide. No evidence was obtained suggesting that cells differentiate into glia. The functional phenotypes of newborn neurons in cluster 10 were examined by combining BrdU immunocytochemistry with the application of dextran dyes to different brain neuropils. These studies showed that while the majority of cells born during the early postembryonic development of C. destructor differentiate in AL projection neurons, neurogenesis in adult crayfish is characterized by the addition of both OL and AL projection neurons. In addition to our examination of neurogenesis in the olfactory pathway, we provide the first evidence that adult neurogenesis is also a characteristic feature of the optic neuropils of decapod crustaceans.     

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

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

A new look at embryonic development of the visual system in decapod crustaceans: neuropil formation, neurogenesis, and apoptotic cell death

In recent years, comparing the structure and development of the central nervous system in crustaceans has provided new insights into the phylogenetic relationships of arthropods. Furthermore, the structural evolution of the compound eyes and optic ganglia of adult arthropods has been discussed, but it was not possible to compare the ontogeny of arthropod visual systems, owing to the lack of data on species other than insects. In the present report, we studied the development of the crustacean visual system by examining neurogenesis, neuropil formation, and apoptotic cell death in embryos of the American lobster, Homarus americanus, the spider crab, Hyas araneus, and the caridean shrimp, Palaemonetes argentinus, and compare these processes with those found in insects. Our results on the patterns of stem cell proliferation provide evidence that in decapod crustaceans and hemimetabolous insects, there exist considerable similarities in the mechanisms by which accretion of the compound eyes and growth of the optic lobes is achieved, suggesting an evolutionary conservation of these mechanisms.     

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

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

Adult neurogenesis and brain regeneration in zebrafish

Adult neurogenesis is a widespread trait of vertebrates; however, the degree of this ability and the underlying activity of the adult neural stem cells differ vastly among species. In contrast to mammals that have limited neurogenesis in their adult brains,zebrafish can constitutively produce new neurons along the whole rostrocaudal brain axis throughout its life.This feature of adult zebrafish brain relies on the presence of stem/progenitor cells that continuously proliferate,and the permissive environment of zebrafish brain for neurogenesis. Zebrafish has also an extensive regenerative capacity, which manifests itself in responding to central nervous system injuries by producing new neurons to replenish the lost ones. This ability makes zebrafish a useful model organism for understanding the stem cell activity in the brain, and the molecular programs required for central nervous system regeneration.In this review, we will discuss the current knowledge on the stem cell niches, the characteristics of the stem/progenitor cells, how they are regulated and their involvement in the regeneration response of the adult zebrafish brain. We will also emphasize the open questions that may help guide the future research.     

Introduction to ‘Homology and convergence in nervous system evolution’

The origin of brains and central nervous systems (CNSs) is thought to have occurred before the Palaeozoic era 540 Ma. Yet in the absence of tangible evidence, there has been continued debate whether today''s brains and nervous systems derive from one ancestral origin or whether similarities among them are due to convergent evolution. With the advent of molecular developmental genetics and genomics, it has become clear that homology is a concept that applies not only to morphologies, but also to genes, developmental processes, as well as to behaviours. Comparative studies in phyla ranging from annelids and arthropods to mammals are providing evidence that corresponding developmental genetic mechanisms act not only in dorso–ventral and anterior–posterior axis specification but also in segmentation, neurogenesis, axogenesis and eye/photoreceptor cell formation that appear to be conserved throughout the animal kingdom. These data are supported by recent studies which identified Mid-Cambrian fossils with preserved soft body parts that present segmental arrangements in brains typical of modern arthropods, and similarly organized brain centres and circuits across phyla that may reflect genealogical correspondence and control similar behavioural manifestations. Moreover, congruence between genetic and geological fossil records support the notion that by the ‘Cambrian explosion’ arthropods and chordates shared similarities in brain and nervous system organization. However, these similarities are strikingly absent in several sister- and outgroups of arthropods and chordates which raises several questions, foremost among them: what kind of natural laws and mechanisms underlie the convergent evolution of such similarities? And, vice versa: what are the selection pressures and genetic mechanisms underlying the possible loss or reduction of brains and CNSs in multiple lineages during the course of evolution? These questions were addressed at a Royal Society meeting to discuss homology and convergence in nervous system evolution. By integrating knowledge ranging from evolutionary theory and palaeontology to comparative developmental genetics and phylogenomics, the meeting covered disparities in nervous system origins as well as correspondences of neural circuit organization and behaviours, all of which allow evidence-based debates for and against the proposition that the nervous systems and brains of animals might derive from a common ancestor.     

Adult neurogenesis and cell cycle regulation in the crustacean olfactory pathway: from glial precursors to differentiated neurons

Adult neurogenesis is a characteristic feature of the olfactory pathways of decapod crustaceans. In crayfish and clawed lobsters, adult-born neurons are the progeny of precursor cells with glial characteristics located in a neurogenic niche on the ventral surface of the brain. The daughters of these precursor cells migrate during S and G₂ stages of the cell cycle along glial fibers to lateral (cluster 10) and medial (cluster 9) proliferation zones. Here, they divide (M phase) producing offspring that differentiate into olfactory interneurons. The complete lineage of cells producing neurons in these animals, therefore, is arranged along the migratory stream according to cell cycle stage. We have exploited this model to examine the influence of environmental and endogenous factors on adult neurogenesis. We find that increased levels of serotonin upregulate neuronal production, as does maintaining animals in an enriched (versus deprived) environment or augmenting their diet with omega-3 fatty acids; increased levels of nitric oxide, on the other hand, decrease the rate of neurogenesis. The features of the neurogenic niche and migratory streams, and the fact that these continue to function in vitro, provide opportunities unavailable in other organisms to explore the sequence of cellular and molecular events leading to the production of new neurons in adult brains.     

Chasing fate and function of new neurons in adult brains

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

Pharmacological activities of fusaric acid (5-butylpicolinic acid).

This review article aims at summarizing research findings on the various pharmacological activities of fusaric acid (5-butylpicolinic acid), a mycotoxin produced by several Fusarium species which commonly infect cereal grains and other agricultural commodities. The actions of the toxin on mammals, birds, arthropods, crustaceans and plants are covered. The effects on mammals are diverse and are apparent in the nervous, cardiovascular and immune systems. Fusaric acid is toxic to some mammalian tumor cell lines.     

Immunocytochemical studies on the naupliar nervous system of Balanus improvisus (Crustacea, Cirripedia, Thecostraca)

The nervous system of nauplii of the crustacean taxon Cirripedia was analysed in the species Balanus improvisus Darwin, 1854 using for the first time immunocytochemical staining against serotonin, RFamide and α-tubulin in combination with confocal laser scanning microscopy. This approach revealed a circumoesophageal neuropil ring with nerves extending to the first and second antennae and to the mandibles, all features typical for Crustacea. In addition, RFamidergic structures are present in the region of the thoraco-abdomen. A pair of posterior nerves and a pair of lateral nerves run in anterior-posterior direction and are connected by a thoracic nerve ring and a more posteriorly situated commissure. A median nerve is situated along the ventral side of the thoraco-abdomen. The innervation of frontolateral horns and the frontal filaments are α-tubulin-positive. Several pairs of large neurons in the protocerebrum, along the circumoesophageal connectives and in the mandibular ganglion stain only for serotonin. Due to the almost complete absence of comparable data on the neuroanatomy of early (naupliar) stages in other Crustacea, we include immunocytochemical data on the larvae of the branchiopod, Artemia franciscana Kellogg, 1906 in our analysis. We describe several characteristic neurons in the brains of the nauplius larvae of both species which are also found in decapod larvae and in adult brains of other crustaceans. Furthermore, our data reveal that the naupliar brain of cirripedes is more complex than the adult brain. It is concluded that this ontogenetic brain reduction is related to the sessile life style of adult Cirripedia.     

A developmental study of serotonin-immunoreactive neurons in the embryonic brain of the Marbled Crayfish and the Migratory Locust: Evidence for a homologous protocerebral group of neurons

It is well established that the brains of adult malacostracan crustaceans and winged insects display distinct homologies down to the level of single neuropils such as the central complex and the optic neuropils. We wanted to know if developing insect and crustacean brains also share similarities and therefore have explored how neurotransmitter systems arise during arthropod embryogenesis. Previously, Sintoni et al. (2007) had already reported a homology of an individually identified cluster of neurons in the embryonic crayfish and insect brain, the secondary head spot cells that express the Engrailed protein. In the present study, we have documented the ontogeny of the serotonergic system in embryonic brains of the Marbled Crayfish in comparison to Migratory Locust embryos using immunohistochemical methods combined with confocal laser-scan microscopy. In both species, we found a cluster of early emerging serotonin-immunoreactive neurons in the protocerebrum with neurites that cross to the contralateral brain hemisphere in a characteristic commissure suggesting a homology of this cell cluster. Our study is a first step towards a phylogenetic analysis of neurotransmitter system development and shows that, as for the ventral nerve cord, traits related to neurogenesis in the brain can provide valuable hints for resolving the much debated question of arthropod phylogeny.     

Patterns of neurogenesis in the midbrain of embryonic lobsters differ from proliferation in the insect and the crustacean ventral nerve cord

Neurogenesis persists throughout life in the olfactory pathway of many decapod crustaceans. However, the relationships between precursor cells and the temporal characteristics of mitotic events in these midbrain regions have not been examined. We have conducted studies aimed at characterizing the sequence of proliferative events that leads to the production of new deutocerebral projection neurons in embryos of the American lobster, Homarus americanus. In vivo bromodeoxyuridine (BrdU) labeling patterns show that three distinct cell types are involved in neurogenesis in this region. Quantitative and temporal analyses suggest that the clearing time for BrdU is 2-3 days in lobster embryos, and that the sequence of proliferative events in the midbrain is significantly different from the stereotypical pattern for the generation of neurons in the ventral nerve cord ganglia of insects and crustaceans. The unusual pattern of proliferation in the crustacean midbrain may be related to the persistence of neurogenesis throughout life in these regions.     

Arthropod evolution: great brains, beautiful bodies

While arthropod phylogeny remains controversial, comparative studies of the genetic control of segmentation and of the nervous system have begun to throw light on how mandibulate arthropods (myriapods, crustaceans and insects) reached their current level of morphological and behavioural complexity. Insects and crustaceans show remarkable similarities in the construction of their brains, suggesting that their common ancestor had typically arthropod behaviour, while developmental genetic studies are consistent with this ancestor having had distinct head, trunk and tail regions. This conclusion contrasts with the influential view, drawn from comparative embryology and functional anatomy, that insects and crustaceans evolved independently from a simple worm-like organism, perhaps resembling an annelid.     

Comparative aspects of adult neural stem cell activity in vertebrates

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

Acoustic detection and communication by decapod crustaceans

This paper reviews behavioral, physiological, anatomical, and ecological aspects of sound and vibration detection by decapod crustaceans. Our intent is to demonstrate that despite very limited work in this area in the past 20 years, evidence suggests that at least some decapod crustaceans are able to detect and use sounds in ways that parallel detection and processing mechanisms in aquatic and terrestrial vertebrates. Some aquatic decapod crustaceans produce sounds, and many are able to detect substrate vibration at sensitivities sufficient to tell of the proximity of mates, competitors, or predators. Some semi-terrestrial crabs produce and use sounds for communication. These species detect acoustic stimuli as either air- or substrate-borne energies, socially interact in acoustic "choruses," and probably use "calls" to attract mates.     

Neurogenesis of the cholinergic medial septum in female and male C57BL/6J mice

Sex differences exist in the structure and function of the cholinergic septo-hippocampal system throughout the lifespan of mammals. How and when these sex differences originate is unclear. Because estrogen modulates sexual differentiation of several brain regions during development and influences neurogenesis in adult mammals, we hypothesized that sexual dimorphism of the cholinergic septo-hippocampal system would extend to its neurogenesis. A birthdating agent 5'-bromo-2'-deoxyuridine (BrdU) was injected into pregnant dams on one of eight gestational days, ranging from embryonic day (E)10 to E17. The offspring were euthanized at 2 months of age, and brains were processed for BrdU and choline acetyltransferase (ChAT) immunoreactivity to label cholinergic neurons that became postmitotic on a given embryonic day and survived to adulthood. Unbiased stereology was used to compare the number of double-labeled neurons in the medial septum (MS) of female and male offspring. Cholinergic neurons in the MS were generated primarily between E11 and E14, similar to other published reports. We found sex differences in the pattern of peak neurogenesis but not in the length of neurogenesis, or in total number of neurons generated in the MS. Additionally, in adult female and male mice, we estimated the total number of cholinergic neurons using unbiased stereology and found no sex differences in the number of cholinergic neurons or in the volume of the MS in adulthood. These results suggest that sex differences noted in the function of the postnatal cholinergic septo-hippocampal system may originate from its neurogenesis.