Birth,survival and differentiation of neurons in an adult crustacean brain

Life‐long neurogenesis is a characteristic feature of many vertebrate and invertebrate species. In decapod crustaceans, new neurons are added throughout life to two cell clusters containing local (cluster 9) and projection (cluster 10) interneurons in the olfactory pathway. Adult‐born neurons in clusters 9 and 10 in crayfish have the anatomical properties and chemistry of mature neurons by 6 months after birth. Here we use 5‐bromo‐2′‐deoxyuridine (BrdU) incorporation to pulse label mitotically active cells in these cell clusters, followed by a survival time of up to 8 months, during which crayfish (Cherax destructor) were sacrificed at intervals and the numbers of BrdU‐labeled cells quantified. We find a decrease in the numbers of BrdU‐labeled cells in cell cluster 10 between the first and second weeks following BrdU exposure, suggesting a period of cell death shortly after proliferation. Additional delayed cell divisions in both cell clusters are indicated by increases in labeled cells long after the BrdU clearing time. The differentiation time of these cells into neurons was defined by detection of the first immunoreactivity for the transmitter SIFamide in cluster 10 BrdU‐labeled cells, which begins at 4 weeks after BrdU labeling; the numbers of SIFamide‐labeled cells continues to increase over the following month. Experiments testing whether proliferation and survival of Cluster 10 cells are influenced by locomotor activity provided no evidence of a correlation between activity levels and cell proliferation, but suggest a strong influence of locomotor activity on cell survival. © 2013 Wiley Periodicals, Inc. Develop Neurobiol 74: 602–615, 2014     

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.     

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.     

Processing of antennular input in the brain of the spiny lobster, Panulirus argus

Neurons in the olfactory deutocerebrum of the spiny lobster, Panulirus argus, were recorded intracellularly and filled with biocytin. Recorded neurons arborized in the olfactory lobe (OL), a glomerular neuropil innervated by olfactory and some presumptive mechanosensory antennular afferents. The neurons responded to chemosensory input from the lateral antennular flagellum bearing the olfactory sensilla but not the medial flagellum bearing many non-olfactory chemosensory sensilla. Many neurons received additional mechanosensory input. Thus the OL integrates specifically olfactory with mechanosensory input. OL neurons had multiglomerular arborizations restricted to one or two of the three horizontal layers of the columnar glomeruli. OL local interneurons comprised core neurons with tree-like neurites and terminals in the base of the glomeruli and rim neurons with neurites surrounding the OL and terminals in the cap/subcap. The somata of OL local interneurons lay in the medial soma cluster (100000 somata). OL projection neurons arborized in the base of the glomeruli and ascended via the olfactory glomerular tract to the lateral protocerebrum. A parallel projection pathway is constituted by projection neurons of the accessory lobe, a glomerular neuropil without afferent innervation but intimate links to the OL. The projection neuron somata constituted the lateral soma cluster (200000 somata).Abbreviations AC anterior cluster (cluster 6,7) - AL accessory lobe - aMC anterior subcluster of medial cluster (cluster 9) - A _lNv main antenna I (antennular) nerve - A _lNM antenna I (antennular) motor nerve - A _llNv main antenna II (antennal) nerve - CB central body - CL central layer of accessory lobe - DC deutocerebral commissure - DCN deutocerebral commissure neuropil - dDUMC dorsal subcluster of dorsal unpaired median cluster (cluster 17) - dMC dorsal subcluster of medial cluster (cluster 11) - dVPALC dorsal subcluster of ventral paired anterolateral cluster (cluster 8) G glomerulus - IDUMC lateral subcluster of dorsal unpaired median cluster (cluster 16) - LC lateral cluster (cluster 10) - LF lateral flagellum of antenna I (antennule) - LL lateral layer of accessory lobe - MF medial flagellum of antenna I (antennule) - ML medial layer of accessory lobe - MPN anterior and posterior median protocerebral neuropils - OGT olfactory globular tract - OGTN olfactory globular tract neuropil - OL olfactory lobe - OLALT olfactory lobe-accessory lobe tract - PB protocerebral bridge - pMC posterior subcluster of medial cluster (cluster 9) - PT protocerebral tract - TNv tegumentary nerve - VPMC ventral paired medial cluster (cluster 12) - VUMC ventral unpaired medial cluster (cluster 13) - vVPALC ventral subcluster of ventral paired anterolateral cluster (cluster 8) - ASW artificial sea water - M3 mixture 3 - PRO L-proline - TM TetraMarin extract     

Neuronal differentiation and long‐term survival of newly generated cells in the olfactory midbrain of the adult spiny lobster,Panulirus argus

The fate of continuously generated cells in the soma clusters of the olfactory midbrain of adult spiny lobsters, Panulirus argus, was investigated by in vivo pulse‐chase experiments with the proliferation marker 5‐bromo‐2′‐deoxyuridine (BrdU) combined with immunostainings for neuropeptides of mature neurons. A BrdU injection after a survival time (ST) of 14 h labeled about 100 nuclei in the lateral soma clusters (LC), comprised of projection neurons, and about 30 nuclei in the medial soma clusters (MC), comprised of local interneurons. The BrdU‐positive nuclei were confined to small regions at the inside of these clusters, which also contain nuclei in different phases of mitosis and thus represent proliferative zones. After STs of 2 weeks or 3 months, the number of BrdU‐positive nuclei was doubled, indicating a mitosis of all originally labeled cells. Dependent on ST, the BrdU‐positive nuclei were translocated from the proliferative zones towards the outside of the clusters, where somata of mature neurons reside. Immunostainings with antibodies to the neuropeptides FMRFamide and substance P, both of which label a large portion of somata in the MC and a pair of giant neurons projecting into the LC, revealed that in both clusters the proliferative zones are surrounded by, but are themselves devoid of, labeling. In the MC, some BrdU‐positive somata were double‐labeled by the FMRFamide antibody after an ST of 3 months, and by the substance P antibody after STs of 6 and 11/14 months, but not after 3 months. In the LC, BrdU‐positive somata after an ST of 3 months partially and after 6 and 11/14 months widely overlapped with the arborizations of the giant neurons, indicating the establishment of synaptic input. The experiments show that cells generated in proliferative zones in the LC and MC of adult spiny lobsters after a final mitosis differentiate into neurons within months, survive for at least 1 year, and are integrated into the circuitry of the olfactory midbrain. A new hypothesis about the mechanism of adult neurogenesis in the central olfactory pathway of decapod crustaceans is developed, linking it to neurogenesis during embryonic and larval development. © 2001 John Wiley & Sons, Inc. J Neurobiol 48: 181–203, 2001     

Olfactory projection neuron pathways in two species of marine Isopoda (Peracarida,Malacostraca, Crustacea)

The neuroanatomy of the olfactory pathway has been intensely studied in many representatives of Malacostraca. Nevertheless, the knowledge about bilateral olfactory integration pathways is mainly based on Decapoda. Here, we investigated the olfactory projection neuron pathway of two marine isopod species, Saduria entomon and Idotea emarginata, by lipophilic dye injections into the olfactory neuropil. We show that both arms of the olfactory globular tract form a chiasm in the center of the brain, as known from several other crustaceans. Furthermore, the olfactory projection neurons innervate both the medulla terminalis and the hemiellipsoid body of the ipsi- and the contralateral hemisphere. Both protocerebral neuropils are innervated to a comparable extent. This is reminiscent of the situation in the basal decapod taxon Dendrobranchiata. Thus, we propose that an innervation by the olfactory globular tract of both the medulla terminalis and the hemiellipsoid body is characteristic of the decapod ground pattern, but also of the ground pattern of Caridoida.     

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.     

The peripheral and central antennular pathway of the Caribbean stomatopod crustacean Neogonodactylus oerstedii

Although stomatopod crustaceans use their chemical senses in many facets of behavior, little is known about their chemosensory neural pathways, especially in comparison to the better-studied decapod crustaceans. We examined the stomatopod Neogonodactylus oerstedii to determine organizational aspects of peripheral and central neural pathway of antennules, which is a major chemosensory organ. We describe the three flagella of the triramous antennule as the medial, dorsolateral, and ventrolateral flagella. The primary branch point is between the medial flagellum and lateral flagella, and the secondary branch point is at the junction of the dorsolateral and ventrolateral flagella. The antennule bears at least three types of setae, based on their external morphology. Simple setae are present only on the medial flagellum and ventrolateral flagellum, organized as a tuft of 10-15 setae on each flagellar annulus. Aesthetasc setae and asymmetric setae occur only on the distal annuli of the dorsolateral flagellum, with each annulus bearing a row of three aesthetascs and one asymmetric seta. DiI fills of the antennular nerve near the junction of the flagella show that sensory neurons in the antennular flagella project to two neuropils in the ipsilateral midbrain-the olfactory lobe (OL) and lateral antennular neuropil (LAN). The OL is glomerular and has rich serotonergic innervation, a characteristic of the OL in decapods. The LAN is bi-lobed and stratified as it is in decapods. However, the LAN of stomatopods differs from that of decapods in being relatively large and containing extensive serotonergic innervation. The median antennular neuropil of stomatopods has sparse serotonergic innervation, and it is more diffusely organized compared to decapods. No accessory lobes were found in N. oerstedii. Thus, the stomatopod antennular flagella have the same two, highly organized parallel pathways common to decapods-the OL pathway and the LAN pathway.     

Neuronal differentiation and long-term survival of newly generated cells in the olfactory midbrain of the adult spiny lobster, Panulirus argus

The fate of continuously generated cells in the soma clusters of the olfactory midbrain of adult spiny lobsters, Panulirus argus, was investigated by in vivo pulse-chase experiments with the proliferation marker 5-bromo-2'-deoxyuridine (BrdU) combined with immunostainings for neuropeptides of mature neurons. A BrdU injection after a survival time (ST) of 14 h labeled about 100 nuclei in the lateral soma clusters (LC), comprised of projection neurons, and about 30 nuclei in the medial soma clusters (MC), comprised of local interneurons. The BrdU-positive nuclei were confined to small regions at the inside of these clusters, which also contain nuclei in different phases of mitosis and thus represent proliferative zones. After STs of 2 weeks or 3 months, the number of BrdU-positive nuclei was doubled, indicating a mitosis of all originally labeled cells. Dependent on ST, the BrdU-positive nuclei were translocated from the proliferative zones towards the outside of the clusters, where somata of mature neurons reside. Immunostainings with antibodies to the neuropeptides FMRFamide and substance P, both of which label a large portion of somata in the MC and a pair of giant neurons projecting into the LC, revealed that in both clusters the proliferative zones are surrounded by, but are themselves devoid of, labeling. In the MC, some BrdU-positive somata were double-labeled by the FMRFamide antibody after an ST of 3 months, and by the substance P antibody after STs of 6 and 11/14 months, but not after 3 months. In the LC, BrdU-positive somata after an ST of 3 months partially and after 6 and 11/14 months widely overlapped with the arborizations of the giant neurons, indicating the establishment of synaptic input. The experiments show that cells generated in proliferative zones in the LC and MC of adult spiny lobsters after a final mitosis differentiate into neurons within months, survive for at least 1 year, and are integrated into the circuitry of the olfactory midbrain. A new hypothesis about the mechanism of adult neurogenesis in the central olfactory pathway of decapod crustaceans is developed, linking it to neurogenesis during embryonic and larval development.     

Olfactory Enrichment Influences Adult Neurogenesis Modulating GAD67 and Plasticity-Related Molecules Expression in Newborn Cells of the Olfactory Bulb

The olfactory bulb (OB) is a highly plastic region of the adult mammalian brain characterized by continuous integration of inhibitory interneurons of the granule (GC) and periglomerular cell (PGC) types. Adult-generated OB interneurons are selected to survive in an experience-dependent way but the mechanisms that mediate the effects of experience on OB neurogenesis are unknown. Here we focus on the new-generated PGC population which is composed by multiple subtypes. Using paradigms of olfactory enrichment and/or deprivation combined to BrdU injections and quantitative confocal immunohistochemical analyses, we studied the effects of olfactory experience on adult-generated PGCs at different survival time and compared PGC to GC modulation. We show that olfactory enrichment similarly influences PGCs and GCs, increasing survival of newborn cells and transiently modulating GAD67 and plasticity-related molecules expression. However, PGC maturation appears to be delayed compared to GCs, reflecting a different temporal dynamic of adult generated olfactory interneuron integration. Moreover, olfactory enrichment or deprivation do not selectively modulate the survival of specific PGC phenotypes, supporting the idea that the integration rate of distinct PGC subtypes is independent from olfactory experience.     

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.     

Tachykinin-related neuropeptide in the crayfish olfactory midbrain

Immunoreactivity indicative of tachykinin-related peptide (TRP) was detected in the olfactory midbrain of the crayfish Pacifastacus leniusculus when using an antiserum to the insect neuropeptide locustatachykinin I (LomTK-I). A monoclonal antibody to the mammalian tachykinin substance P was shown in double-labeling experiments to label structures in the crayfish brain identical to those labeled with the LomTK antiserum. Within the midbrain LomTK-like immunoreactive (LomTK-LI) material was observed in a limited population of neuronal somata and their varicose processes. A single pair of large interneurons gave rise to numerous varicose LomTK-LI processes innervating a cluster of cell bodies (cluster 10) as well as the olfactory neuropils. The latter neuropil was also innervated by a population of LomTK-LI globuli cells with cell bodies in cluster 9. Radioimmunoassay (RIA), utilizing the LomTK antiserum, and reverse-phase high-performance liquid chromatography (HPLC) were used to partially characterize the immunoreactive material in extract of the portion of the midbrain that houses the olfactory (OL) and accessory (AL) lobes and cell clusters 9 and 10 on the one hand, and in extract of the remaining parts of the brain on the other. Approximately the same amounts of LomTK-LI material were observed for the two extracts. RIA showed that the immunoreactive material of both extracts diluted roughly in parallel to synthetic LomTK-I and HPLC analysis of the extracts revealed immunoreactive material in both tissues which eluted with retention times in the range of synthetic LomTK-I and LomTK-II. These results suggest that TRPs similar to LomTKs are present in the olfactory midbrain of Pacifastacus. The distribution of immunolabeled neuronal structures suggests that in the crayfish, peptide(s) closely related to insect TRPs may act as a neuroactive substance released from nerve fibers in olfactory neuropil areas and at certain neuronal cell bodies.     

The organization of the olfactory lobes in Euphausiacea and Mysidacea (Crustacea,Malacostraca)

Summary The structural organization of the olfactory lobes in representatives of euphausiid and mysid crustaceans was investigated and compared, also with these structures described in other crustaceans and in insects. In the investigated euphasiid and mysid species, the olfactory-globular tract and the position of cell clusters associated with the olfactory lobes show a similar arrangement. This arrangement is in agreement with that described in decapod crustaceans. The olfactory lobe neuropil in representatives of both taxa shows glomerular arrangement. These glomeruli are partly enclosed in a glial wrapping, and they represent the only site where synaptic contacts are established within the olfactory lobes. This glomerular arrangement appears similar to that described in the antennal lobe of insects, but differs from the columnar arrangement described in decapod crustaceans. Furthermore, about 15–20 FMRFamide-like immunoreactive globuli cells were labeled and they occupy a similar position in the investigated species. Neurites from these cells terminate only in the olfactory lobe glomeruli, and they are therefore regarded as intrinsic interneurons. The absence of serotonin-like immunoreactivity in the olfactory lobes is a feature only ascribed to the euphausiids and mysids. A specific neuropil area is present in male mysids, and it occupy a position forward of the olfactory lobe. The male-specific neuropil in mysids and the macro-glomerular in insects complex are interpreted as analogous structures.     

TRIM32-dependent transcription in adult neural progenitor cells regulates neuronal differentiation

In the adult mammalian brain, neural stem cells in the subventricular zone continuously generate new neurons for the olfactory bulb. Cell fate commitment in these adult neural stem cells is regulated by cell fate-determining proteins. Here, we show that the cell fate-determinant TRIM32 is upregulated during differentiation of adult neural stem cells into olfactory bulb neurons. We further demonstrate that TRIM32 is necessary for the correct induction of neuronal differentiation in these cells. In the absence of TRIM32, neuroblasts differentiate slower and show gene expression profiles that are characteristic of immature cells. Interestingly, TRIM32 deficiency induces more neural progenitor cell proliferation and less cell death. Both effects accumulate in an overproduction of adult-generated olfactory bulb neurons of TRIM32 knockout mice. These results highlight the function of the cell fate-determinant TRIM32 for a balanced activity of the adult neurogenesis process.     

Immunolocalisation of crustacean-SIFamide in the median brain and eyestalk neuropils of the marbled crayfish

Crustacean-SIFamide (GYRKPPFNGSIFamide) is a novel neuropeptide that was recently isolated from crayfish nervous tissue. We mapped the localisation of this peptide in the median brain and eyestalk neuropils of the marbled crayfish (Marmorkrebs), a parthenogenetic crustacean. Our experiments showed that crustacean-SIFamide is strongly expressed in all major compartments of the crayfish brain, including all three optic neuropils, the lateral protocerebrum with the hemiellipsoid body, and the medial protocerebrum with the central complex. These findings imply a role of this peptide in visual processing already at the level of the lamina but also at the level of the deeper relay stations. Immunolabelling is particularly strong in the accessory lobes and the deutocerebral olfactory lobes that receive a chemosensory input from the first antennae. Most cells of the olfactory globular tract, a projection neuron pathway that links deuto- and protocerebrum, are labelled. This pathway plays a central role in conveying tactile and olfactory stimuli to the lateral protocerebrum, where this input converges with optic information. Weak labelling is also present in the tritocerebrum that is associated with the mechanosensory second antennae. Taken together, we suggest an important role of crustacean-SIFamidergic neurons in processing high-order, multimodal input in the crayfish brain.     

Embryonic and postembryonic neurogenesis in the ventral nerve cord of the freshwater crayfish Cherax destructor.

Previous studies of neurogenic activity in the thoracic neuromeres of indirect developing crustaceans indicated that the temporal patterns of neurogenesis can be correlated with the appearance of the thoracic appendages during larval and metamorphic development. To test further the idea that the temporal patterns of neurogenesis in crustaceans are related to their life histories, we examined neurogenesis in the ventral nerve cord of a direct developing crustacean, the freshwater crayfish Cherax destructor, whose life history contains neither larval stages nor metamorphoses. Neurogenesis was examined using the in vivo incorporation of bromodeoxyuridine into DNA. During late embryonic development the thoracic neuromeres of the crayfish contain arrays of mitotically active neuroblasts similar to those previously described in the spider crab and lobster. The arrays in the crayfish abdomen are, however, greatly reduced compared with those of the thorax. On hatching, both the thoracic and abdominal appendages of C. destructor are capable of movement. The pleopods, however, do not beat rhythmically until the second postembryonic stage whereas the pereiopods are not used in coordinated walking movements until the third stage. An examination of the time course of neurogenesis in the ventral nerve cord revealed that neurogenic activity in each neuromere ceases during or before the moult to the developmental stage in which its segmental appendage is first used in coordinated movements. These findings indicate that the patterns of neurogenesis in crustaceans are indeed related to the maturation of the segmental appendages and, in particular, to the maturation of motor behaviours.     

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.     

Adult neurogenesis in the central olfactory pathway of dendrobranchiate and caridean shrimps: New insights into the evolution of the deutocerebral proliferative system in reptant decapods

Persistent neurogenesis in the central olfactory pathway characterizes many reptant decapods such as lobsters, crayfish and crabs. In these animals, the deutocerebral proliferative system generates new neurons which integrate into the neuronal network of the first order processing neuropil of the olfactory system, the deutocerebral chemosensory lobes (also called olfactory lobes). However, differences concerning the phenotype and the mechanisms that drive adult neurogenesis were reported in crayfish versus spiny lobsters. While numerous studies have focussed on these mechanisms and regulation of adult neurogenesis, investigations about the phylogenetic distribution are missing. To contribute an evolutionary perspective on adult neurogenesis in decapods, we investigated two representatives of basally diverging lineages, the dendrobranchiate Penaeus vannamei and the caridean Crangon crangon using the thymidine analogue Bromodeoxyuridine (BrdU) as marker for the S phase of cycling cells. Compared to reptant decapods, our results suggest a simpler mechanism of neurogenesis in the adult brain of dendrobranchiate and caridean shrimps. Observed differences in the rate of proliferation and spatial dimensions are suggested to correlate with the complexity of the olfactory system. We assume that a more complex and mitotically more active proliferative system in reptant decapods evolved with the emergence of another processing neuropil, the accessory lobes. © 2018 Wiley Periodicals, Inc. Develop Neurobiol, 2018