The organization of the cerebral ganglion in the shore crab, Carcinus maenas. II. The relation of intracerebral blood vessels to other brain elements

Adult neurogenesis: ultrastructure of a neurogenic niche and neurovascular relationships

The first-generation precursors producing adult-born neurons in the crayfish (Procambarus clarkii) brain reside in a specialized niche located on the ventral surface of the brain. In the present work, we have explored the organization and ultrastructure of this neurogenic niche, using light-level, confocal and electron microscopic approaches. Our goals were to define characteristics of the niche microenvironment, examine the morphological relationships between the niche and the vasculature and observe specializations at the boundary between the vascular cavity located centrally in the niche. Our results show that the niche is almost fully encapsulated by blood vessels, and that cells in the vasculature come into contact with the niche. This analysis also characterizes the ultrastructure of the cell types in the niche. The Type I niche cells are by far the most numerous, and are the only cell type present superficially in the most ventral cell layers of the niche. More dorsally, Type I cells are intermingled with Types II, III and IV cells, which are observed far less frequently. Type I cells have microvilli on their apical cell surfaces facing the vascular cavity, as well as junctional complexes between adjacent cells, suggesting a role in regulating transport from the blood into the niche cells. These studies demonstrate a close relationship between the neurogenic niche and vascular system in P. clarkii. Furthermore, the specializations of niche cells contacting the vascular cavity are also typical of the interface between the blood/cerebrospinal fluid (CSF)-brain barriers of vertebrates, including cells of the subventricular zone (SVZ) producing new olfactory interneurons in mammals. These data indicate that tissues involved in producing adult-born neurons in the crayfish brain use strategies that may reflect fundamental mechanisms preserved in an evolutionarily broad range of species, as proposed previously. The studies described here extend our understanding of neurovascular relationships in the brain of P. clarkii by characterizing the organization and ultrastructure of the neurogenic niche and associated vascular tissues.

Novel organization and development of copepod myelin. ii. nonglial origin

Nerve-impulse conduction is greatly speeded by myelin sheaths in vertebrates, oligochaete annelids, penaeid and caridean shrimp, and calanoid copepods. In the first three invertebrate cases, myelin arises from glial cells, as it does in vertebrates. The contribution of the glial cells to the layered structure of the myelin is clear: their nuclei are either embedded in the layers or reside in contiguous cytoplasmic compartments, and their cell membranes are seen to be continuous with those of the myelin layers. However, with calanoids, the association with glial cells presumed necessary to generate the myelin has never been satisfactorily identified. We have conducted a systematic examination of thin sections through different parts of the copepod nervous system to identify the structural organization of copepod myelin and the likely mechanism for its formation. We find that myelination appears to commence by laying down and compacting a cisternal tongue against the inside of the axolemma. This is followed by the successive layering and compaction of additional tongues to create a stack of tongues. The margins of the tongues then expand to encircle the interior of a neurite, meeting and fusing to form complete concentric myelin. No sign of glial involvement could be detected at any stage. Unlike glially derived myelin, the extracellular tracer lanthanum did not penetrate between the myelin layers in copepods, further evidence against a glial source. We believe this to be the first demonstration of a nonglial origin for myelin in any species.

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.

Invertebrate neurophylogeny: suggested terms and definitions for a neuroanatomical glossary

BACKGROUND

Invertebrate nervous systems are highly disparate between different taxa. This is reflected in the terminology used to describe them, which is very rich and often confusing. Even very general terms such as 'brain', 'nerve', and 'eye' have been used in various ways in the different animal groups, but no consensus on the exact meaning exists. This impedes our understanding of the architecture of the invertebrate nervous system in general and of evolutionary transformations of nervous system characters between different taxa.

RESULTS

We provide a glossary of invertebrate neuroanatomical terms with a precise and consistent terminology, taxon-independent and free of homology assumptions. This terminology is intended to form a basis for new morphological descriptions. A total of 47 terms are defined. Each entry consists of a definition, discouraged terms, and a background/comment section.

CONCLUSIONS

The use of our revised neuroanatomical terminology in any new descriptions of the anatomy of invertebrate nervous systems will improve the comparability of this organ system and its substructures between the various taxa, and finally even lead to better and more robust homology hypotheses.

Adult neurogenesis in the crayfish brain: proliferation, migration, and possible origin of precursor cells

The birth of new neurons and their incorporation into functional circuits in the adult brain is a characteristic of many vertebrate and invertebrate organisms, including decapod crustaceans. Precursor cells maintaining life-long proliferation in the brains of crayfish (Procambarus clarkii, Cherax destructor) and clawed lobsters (Homarus americanus) reside within a specialized niche on the ventral surface of the brain; their daughters migrate to two proliferation zones along a stream formed by processes of the niche precursors. Here they divide again, finally producing interneurons in the olfactory pathway. The present studies in P. clarkii explore (1) differential proliferative activity among the niche precursor cells with growth and aging, (2) morphological characteristics of cells in the niche and migratory streams, and (3) aspects of the cell cycle in this lineage. Morphologically symmetrical divisions of neuronal precursor cells were observed in the niche near where the migratory streams emerge, as well as in the streams and proliferation zones. The nuclei of migrating cells elongate and undergo shape changes consistent with nucleokinetic movement. LIS1, a highly conserved dynein-binding protein, is expressed in cells in the migratory stream and neurogenic niche, implicating this protein in the translocation of crustacean brain neuronal precursor cells. Symmetrical divisions of the niche precursors and migration of both daughters raised the question of how the niche precursor pool is replenished. We present here preliminary evidence for an association between vascular cells and the niche precursors, which may relate to the life-long growth and maintenance of the crustacean neurogenic niche.

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.

Hormonal and synaptic influences of serotonin on adult neurogenesis

New neurons are incorporated into the adult brains of a variety of organisms, from humans and higher vertebrates, to non-vertebrates such as crustaceans. In virtually all of these systems serotonergic pathways appear to provide important regulatory influences over the machinery producing the new neurons. We have developed an in vitro preparation where adult neurogenesis can be maintained under highly controlled conditions, and are using this to test the influence of hormones on the production of neurons in the crustacean (Homarus americanus) brain. Serotonin levels have been manipulated in this in vitro preparation, and the resulting effects on the rate of neurogenesis have been documented. In addition we have compared in vitro influences of serotonin with results acquired from in vivo exposure of whole animals to serotonin. These experiments suggest that there are multiple mechanisms and pathways by which serotonin may regulate neurogenesis in the crustacean brain: (1) serotonin is effective in regulating neurogenesis at levels as low as 10(-10)M, suggesting that circulating serotonin may have hormonal influences on neuronal precursor cells residing in a vascular niche or the proliferation zones; (2) contrasting effects of serotonin on neurogenesis (up- vs. down-regulation) at high concentrations (10(-4)M), dependent upon whether eyestalk tissue is present or absent, indicate that serotonin elicits the release of substances from the sinus glands that are capable of suppressing neurogenesis; (3) previously demonstrated (Beltz, B.S., Benton, J.L., Sullivan, J.M., 2001. Transient uptake of serotonin by newborn olfactory projection neurons. Proc. Natl. Acad. Sci. USA 98, 12730-12735) serotonergic fibers from the dorsal giant neuron project directly into the proliferation zone in Cluster 10, suggest synaptic or local influences on neurogenesis in the proliferation zones where the final cell divisions and neuronal differentiation occur. Serotonin therefore regulates neurogenesis by multiple pathways, and the specific mode of influence is concentration-dependent.

Brain architecture in the terrestrial hermit crab Coenobita clypeatus (Anomura, Coenobitidae), a crustacean with a good aerial sense of smell

Background

During the evolutionary radiation of Crustacea, several lineages in this taxon convergently succeeded in meeting the physiological challenges connected to establishing a fully terrestrial life style. These physiological adaptations include the need for sensory organs of terrestrial species to function in air rather than in water. Previous behavioral and neuroethological studies have provided solid evidence that the land hermit crabs (Coenobitidae, Anomura) are a group of crustaceans that have evolved a good sense of aerial olfaction during the conquest of land. We wanted to study the central olfactory processing areas in the brains of these organisms and to that end analyzed the brain of Coenobita clypeatus (Herbst, 1791; Anomura, Coenobitidae), a fully terrestrial tropical hermit crab, by immunohistochemistry against synaptic proteins, serotonin, FMRFamide-related peptides, and glutamine synthetase.

Results

The primary olfactory centers in this species dominate the brain and are composed of many elongate olfactory glomeruli. The secondary olfactory centers that receive an input from olfactory projection neurons are almost equally large as the olfactory lobes and are organized into parallel neuropil lamellae. The architecture of the optic neuropils and those areas associated with antenna two suggest that C. clypeatus has visual and mechanosensory skills that are comparable to those of marine Crustacea.

Conclusion

In parallel to previous behavioral findings of a good sense of aerial olfaction in C. clypeatus, our results indicate that in fact their central olfactory pathway is most prominent, indicating that olfaction is a major sensory modality that these brains process. Interestingly, the secondary olfactory neuropils of insects, the mushroom bodies, also display a layered structure (vertical and medial lobes), superficially similar to the lamellae in the secondary olfactory centers of C. clypeatus. More detailed analyses with additional markers will be necessary to explore the question if these similarities have evolved convergently with the establishment of superb aerial olfactory abilities or if this design goes back to a shared principle in the common ancestor of Crustacea and Hexapoda.

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.

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.

Regionally specific distribution of the binding of anti-glutamine synthetase and anti-S100 antibodies and of Datura stramonium lectin in glial domains of the optic lobe of the giant prawn

We previously characterized some crustacean glial cells by markers such as 2',3'-cyclic nucleotide 3'-phosphodiesterase and glial fibrillary acidic protein. Here we use antibodies against glutamine synthetase full-length molecule (anti-GS/FL), a GS C-terminal peptide (anti-GS/20aa-C), and brain S100 (anti-S100), as well as the binding of the insect glia and rat astrocytic marker Datura stramonium lectin (DSL), in the optic lobe of the prawn Macrobrachium rosenbergii. All markers label the lamina ganglionaris cartridge region (lighter: anti-GS/FL; heavier: DSL). In addition, anti-GS/FL labels superficial somata of external and internal medullas and internal chiasm cells. Both anti-GS/20aa-C and anti-S100 label heavily the glial sheaths of the lamina ganglionaris. In addition, anti-S100 binds to the perineurial glia of medullary parenchymal vessels. Western blot analyses show that both anti-GS/FL and anti-GS/20aa-C bind mostly to a band of 50-55 kDa, compatible with a long isoform of vertebrate GS, and accessorily to a possible dimer and, in the case of anti-GS/20aa-C, to an ill-defined band of intermediate mass. Binding of anti-S100 is selective for a single band of about 68 kDa but shows no protein in the weight range of the canonical S100 protein superfamily. DSL reveals two bands of about 75 and about 120 kDa, thus within the range of maximal recognition for rat astrocytes. Our results suggest that phenotype protein markers of the optic lobe glia share antigenic determinants with S100 and (a long form of) GS and that, similarly to vertebrate and insect glia, crustacean glia protein and N-glycan residue markers display regional heterogeneity.

Magnetic resonance imaging at 9.4 T as a tool for studying neural anatomy in non-vertebrates

This report describes magnetic resonance imaging (MRI) methods we have developed at 9.4 T for observing internal organs and the nervous system of an invertebrate organism, the crayfish, Cherax destructor. We have compared results acquired using two different pulse sequences, and have tested manganese (Mn(2+)) as an agent to enhance contrast of neural tissues in this organism. These techniques serve as a foundation for further development of functional MRI and neural tract-tracing methods in non-vertebrate systems.

The decapod crustacean circulatory system: a case that is neither open nor closed

Historically, the decapod crustacean circulatory system has been classed as open. However, recent work on the blue crab, Callinectes sapidus, suggests the circulatory system may be more complex than previously described. Corrosion casting techniques were refined and used to map the circulatory system of a variety of crab species (order: Decapoda; family: Cancridae) to determine if the complexity observed in the blue crab was present in other species. Seven arteries arose from the single chambered heart. The anterior aorta, the paired anterolateral arteries, and the paired hepatic arteries exited from the anterior aspect of the heart. The small-diameter posterior aorta exited posteriorly from the heart. Exiting from the ventral surface of the heart, the sternal artery branched to supply the legs and mouthparts of the crab. These arteries were more complex than previously described, with arterioles perfusing all areas of the body. The arterioles split into fine capillary-like vessels. Most of these capillaries were blind ending. However, in several areas (antennal gland, supraesophageal ganglion) complete capillary beds were present. After passing through the capillary-like vessels, blood drained into a series of sinuses. However, rather than being arbitrary spaces as previously described, scanning electron micrographs showed the sinuses to be distinct units. Most of the sinuses formed a series of flattened membrane-bound lacunae. This complexity may qualify the decapod crustacean circulatory system as one that is "partially closed" rather than open.

Neurons in the third abdominal ganglion of the early postnatal crayfish: a quantitative and ultrastructural study

Ultrastructural data on the third abdominal ganglion of the crayfish was heretofore only available for adult individuals. The fine structure of neurons in the adult that are involved in the escape response has been described in detail, but no similar data existed for the postnatal individual. An increase in the number of neurons in the third abdominal ganglion during postnatal stages had been reported, which suggested that several changes in the features of neurons may occur. Here we describe the general anatomy and ultrastructure of the early postnatal third abdominal ganglion, with emphasis on neurons, and we compare their characteristics to those of the adult. Abdominal ganglia of 56 crayfish of 0, 8, 10, 18, 25, 50, 110, and 150 postnatal days were processed under cacodylate buffered aldehyde fixatives, osmicated, embedded in plastic, sectioned, and examined by light and electron microscopy. The anatomy of postnatal ganglia is homologous to the anatomy of the adult ganglia except that the perineurium is not developed in postnatals. The area of neurons within the postnatal ganglion shows no stratification, but neurons are grouped in nuclei according to their size. Neurons constitute a homogeneous population in different stages of maturity, as revealed particularly by the ultrastructure of the nucleolus. Postnatal development is evident in the perineurium, which may provide structural support to the ganglion.

A vertebrate-like blood--brain barrier, with intraganglionic blood channels and occluding junctions, in the scorpion

India ink and ionic lanthanum injections have revealed that the central nervous system (CNS) of the scorpion possesses a highly vascularized cephalothoracic ganglionic mass. It, together with other abdominal ganglia which form a ventral nerve cord, are all ensheathed by an outer layer of modified glial, or perineurial, cells. These cells resemble those which line the blood channels permeating the CNS, in exhibiting both inverted gap and tight junctions. Although the latter show close or fused membrane appositions, lanthanum appears to penetrate past a number, but not all, of them. Freeze-fracturing reveals that these junctions are composed of E-face particles aligned into a network of rows, or ridges, which are frequently discontinuous, especially near the periphery of the perineurium. This produces a somewhat 'leaky' system but occlusion to tracers occurs ultimately, for in the CNS none can be found beyond the perineurium. The existence of this perineurial blood-brain barrier is also demonstrable electrophysiologically where cations such as Mg2+ are unable to penetrate beyond the perineurial layer although they can, it seems, leak in via the blood vascular system. Relative differences in tightness between the perineurium and the cells lining the blood channels may be attributed to differences in the relative number of discontinuous ridges. This is borne out by the observation that the peripheral nervous system has a highly attenuated perineurium with many fewer junctions, and some of these nerves tend to be leaky with respect to tracer penetration. In fixed material the junctional ridges may fracture on to the E-face or partly on both the EF and PF, while in unfixed tissue they are usually found on the PF. In both cases they exhibit complementary grooves that are coincident with the ridges across membrane transitions; in such cases the cell membranes are fused with concomitant obliteration of the intercellular space. These tight junctions, often closely associated with EF gap junctional particle aggregates which may be very loosely clustered, appear to form the basis of the observed blood-brain barrier in the scorpion CNS.

Organization of crustacean neuropil. I. Patterns of synaptic connections in lobster stomatogastric ganglion

The stomatogastric ganglion of the lobster consists of about thiry neurons, mainly large monopolar cells, which have been well characterized physiologically. This paper presents an anatomical description of this ganglion, emphasizing synaptic connections in the neuropil. The neuron cell bodies are located on the dorsal surface of the ganglion. They send processes into the underlying neuropil mass. The neuropil is differentiated into two regions: a core of coarse neuropil consists of large heavily ensheathed processes; a surrounding region of fine-textured synaptic neuropil consists of smaller unsheather processes. Synapses are found only in synaptic neuropil, not in the core of coarse neuropil. Synaptic contacts, about one million in the entire neuropil, are easily recognized by a set of criteria including presynaptic vesicles and pre- and postsynaptic membrane specializations. Most synaptic contacts invole at least three neural processes, usually one pre- and two postsynaptic elements. Synapses are clustered onto irregular swellings or varicosities on neural processes. These varicosities make both pre- and postsynaptic contacts. Three differenty types of presynaptic profile are recognized. Pyloric dilator, ventricular dilator and lateral posterior gastric neurons belong to type A with clear irregular synaptic vesicles. Lateral pyloric, pyloric, anterior median and dorsal gastric neurons belong to type B with larger clear round vesicles. Many unidentified fibres, presumably stomatogastric nerve afferents, blong to type C with both small clear irregular vesicles and also large dense-core vesicles. The synaptic vesicle types are tentatively correlated with neurotransmitter: type A with acetylcholine, type B with an unknown transmitter, possibly glutamate, and type C with dopamine. The distribution of synaptic contacts on the processes of identified neurons reconstructed from serial section is presented in the following paper.

The brain of the horseshoe crab (Limulus polyphemus). I. Neuroglia

The brain of the horseshoe crab, Limulus polyphemus, harbors three populations of neuroglial cells, whose distribution and cellular details are best appreciated by a combination of silver impregnation, scanning, and transmission electron microscopy. Stellate astrocytes envelop neurons as satellite cells, permeate the neuropile, and secrete a framework of sustentacular trabeculae throughout the brain. Velate astrocytes are restricted to Kenyon cells, i.e. small association neurons, of which they harbor up to 150 per neuroglial cell. Vascular neuroglia is composed of glycogen and mitochondria-laden, interlocked cells that form an open meshwork in the hemocoelic spaces of the brain. Aside from supportive functions of neuroglia, the vascular neuroglial cells in particular seem to subserve the role of a metabolic reserve cell for the central nervous system.