The giant neurone system in ophiuroids

Summary The morphology of the circumoral nerve ring of an ophiuroid, Ophiura texturata, is described. Particular attention is given to a system of fibres which are giant by echinoderm standards, and which occur both in the ectoneural and hyponeural parts of the nerve ring. The giant fibres in the ectoneural tissue do not show the complicated pattern of distribution present in the segmental ganglia of the radial nerves. The main areas of neuropil in the ectoneural tissue are associated with small axon bundles which leave the nerve ring to innervate the gut and disc. The hyponeural tissue is exclusively motor and is involved in the innervation of the main radial and inter-radial muscles of the disc. Branches of the motor nerves are also associated with juxtaligamental tissue, the secretory products from which are thought to influence the plasticity of collagenous connective tissue. The structure of the circumoral nerve ring suggests that it serves as a functional connection between the nerve cords in adjacent radii. The ultrastructural evidence does not support the view that the circumoral nerve ring represents a central nervous system.     

NADPH-diaphorase localization in the radial nerve cords of the starfish Patiria pectinifera

The presence and localization of NADPH-diaphorase (NADPH-d) in the radial nerve cords of Patiria pectonifera was shown by electron histochemistry. NADPH-d-positive structures were found in ectoneural and hyponeural regions of the radial nerve cord. Ultrastructural localization of NADPH-d was detected in neurons, sensory cells, supporting cells, and in the nerve plexus. The highest enzymatic activity in ectoneural region of the radial nerve cord is due, presumably, to the involvement of NADPH in sensory signal processing.     

Ultrastructure of the Statocysts in the Apodous Sea Cucumber Leptosynapta inhaerens (Holothuroidea, Echinodermata)

The burrowing sea cucumber Leptosynapta inhaerens possesses five pairs of statocysts, one pair on either side of each radial nerve cord where it arises from the circumoral nerve ring. The nerve cords exhibit only ectoneural components at the level of the statocysts. A sinus-like epineural canal lies superjacent to each cord. This canal is lined by a robust monociliated neuroepithelium which lacks any special support cells. Beneath the neuroepithelium, the somata of the ectoneural neurons form a perikaryal layer whereas the axons are located within the proximal parts of the cords. Glial cells have not been found. Each statocyst is a hollow sense organ. Its central cavity is lined by a monolayer of monociliated parietal cells. Axons of these parietal cells extend towards the statocyst nerve which connects each statocyst with the ectoneural pathways of the cord. A single lithocyte floats within each central statocyst cavity. This unciliated cell contains a voluminous vacuole with the statolith and several smaller vacuoles. It is concluded that statocysts do not belong to the basic organization of the Holothuroidea but have been evolved within this group. The statement, that the statocysts of apodous sea cucumbers and that of the enigmatic Xenoturbella bocki are homologous organs, is rejected.     

The giant neurone system in ophiuroids

Summary The nervous system of Ophiura texturata contains nerve fibres and cell bodies that are an order of magnitude larger than anything previously described in the Asteroidea and Echinoidea. These large nerve cells are designated giant fibres. Giant nerve cells are present in both the ectoneural and hyponeural nervous system. The layout of these nerve cells is described and it is shown that the organization is repeated in each segmental ganglion that makes up the radial nerve cord. The circumoral nerve ring is composed, in the main, of tracts of nerve fibres joining the radial nerves, and it contains only limited areas of neuropil associated with the alimentary canal and muscles of the disc and jaws. Degeneration studies have shown that each segmental ganglion of the radial nerve cords contains a discrete population of neurones separate from adjacent ganglion and that there are not anatomically continuous giant fibres along the whole length of the nerve cord.     

Localization of S1- and S2-like immunoreactivity in the nervous system of the brittle star Amphipholis squamata (Delle Chiaje 1828).

The recent isolation and characterization of the SALMFanide neuropeptides S1 GFNSALMFamide; and S2 (SGPYSFNSGLTFamide) from the sea stars. Asterias rubens and Asterias forbesi have initiated numerous studies on their morphological localization and distribution within the phylum Echinodermata. It has been shown by immunocytochemistry and radioimmunoassay that these peptides are widely distributed in the nervous system of some asteroids, echinoids and ophiuroids. A physiological approach has also shown that S1 and S2 potentiate the luminescence of the small ophiuroid Amphipholis squamata. In the present study. S1- and S2-like immunoreactivity have been localized in A. squamata by immunocytochemistry on both wholemount preparation and histological sections. The results reveal a widespread neuronal distribution of S1-like immunoreactivity in the circumoral ring, radial nerve cord, and tube feet. S1-like immunoreactivity was found to be associated with axons and cell bodies in both the ectoneural and hyponeural components of the nervous. S2-like immunoreactivity was detected only in the ectoneural plenus of the circumoral ring and radial nerve cord.     

Immunocytochemical mapping of the novel echinoderm neuropeptide SALMFamide 1 (S1) in the starfish Asterias rubens

The recent isolation and characterization of the SALMFamide neuropeptides S1 and S2 from the starfish Asterias rubens has initiated a series of studies on their distribution. Specific antisera have been raised against S1 and used in light-microscopical immunocytochemistry. The results of this study reveal for the first time a possible hyponeural innervation of the visceral musculature of the gut and the widespread neuronal distribution of S1, (i) in axons and cell bodies of both ectoneural and hyponeral regions of the radial nerve cord and circumoral nerve ring, (ii) in the nerve ring and nerve plexus of the tube feet, (iii) in the apical muscle, (iv) in skin, and (v) extensively throughout the digestive system. These discoveries are of particular interest in terms of the possible functional roles for S1 in Asterias rubens.Part of this work has been previously reported as an abstract: (Moore et al.1990, 1991)     

Base composition, selection, and phylogenetic significance of indels in the recombination activating gene-1 in vertebrates

Background

Echinoderms and chordates belong to the same monophyletic taxon, the Deuterostomia. In spite of significant differences in body plan organization, the two phyla may share more common traits than was thought previously. Of particular interest are the common features in the organization of the central nervous system. The present study employs two polyclonal antisera raised against bovine Reissner's substance (RS), a secretory product produced by glial cells of the subcomissural organ, to study RS-like immunoreactivity in the central nervous system of sea cucumbers.

Results

In the ectoneural division of the nervous system, both antisera recognize the content of secretory vacuoles in the apical cytoplasm of the radial glia-like cells of the neuroepithelium and in the flattened glial cells of the non-neural epineural roof epithelium. The secreted immunopositive material seems to form a thin layer covering the cell apices. There is no accumulation of the immunoreactive material on the apical surface of the hyponeural neuroepithelium or the hyponeural roof epithelium. Besides labelling the supporting cells and flattened glial cells of the epineural roof epithelium, both anti-RS antisera reveal a previously unknown putative glial cell type within the neural parenchyma of the holothurian nervous system.

Conclusion

Our results show that: a) the glial cells of the holothurian tubular nervous system produce a material similar to Reissner's substance known to be synthesized by secretory glial cells in all chordates studied so far; b) the nervous system of sea cucumbers shows a previously unrealized complexity of glial organization. Our findings also provide significant clues for interpretation of the evolution of the nervous system in the Deuterostomia. It is suggested that echinoderms and chordates might have inherited the RS-producing radial glial cell type from the central nervous system of their common ancestor, i.e., the last common ancestor of all the Deuterostomia.     

Localization of the neuropeptide NGIWYamide in the holothurian nervous system and its effects on muscular contraction

NGIWYamide is a peptide recently isolated from the sea cucumber Apostichopus japonicus. It stiffens the connective tissue of the holothurian body wall. Localization of NGIWYamide was investigated by immunohistochemical staining with antiserum raised against NGIWYamide. In holothurian nervous systems NGIWYamide-like immunoreactivity (NGIWYa-LI) was observed in the hyponeural and ectoneural regions of the radial nerve cord, as well as in the circumoral nerve ring, podial nerves, tentacular nerves, the basiepithelial nerve plexus of the intestine and in cellular processes running through the body wall dermis. Labelled nerve fibres from the hyponeural part of the radial nerve running towards the circular muscle and from the podial nerve into the body wall dermis suggest that NGIWYamide controls both muscle and connective tissue. We examined the effect on muscle activity of the sea cucumber. NGIWYamide (10^-7 to 10^-4 M) caused contraction of the longitudinal body wall muscle. Tentacles showed contraction only at a higher dose (10^-4 M). NGIWYamide (10^-4 M) inhibited spontaneous contraction of the intestine.     

Ultrastructure and organization of the epineural canal and the nerve cord in sea urchins (Echinodermata,Echinoida)

Summary The radial nerve cord ofMespilia globulus has been examined as an example of echinoid nerve cords. In the radius of echinoids only the ectoneural component of the nerve cord is present which is a derivative of the ectoderm. The nerve cord runs in the interior of the body and is accompanied by the epineural canal. In echinoids, the neuroepithelium makes up the upper and side walls of the epineural canal. Each lateral branch of the nerve cord forms a sort of neural tube. It encloses a branch of the epineural canal which represents an open connection with the sea water. Thus, the epineural canal exhibits numerous openings which probably allow sea water to flow back and forth. This organization is unique in echinoderms. — The neuroepithelium exhibits the organization of an epidermis with well-developed nervous elements. Glial cells are not present. The support cells are the true epithelial cells. Their monociliated cell bodies border the lumen and, by means of cytoplasmic stems that contain a bundle of filaments, they reach up to the basal lamina. The nerve cells and their trunk of nerve fibres fill the spaces between the support cells. — Three types of nerve cells can be distinguished according to their polarity: (1) Primary sensory cells that project a cilium into the epineural canal, the axon hillock region is at the opposite pole. (2) Subluminal cells whose cilium originates in the axon hillock region. (3) Neurones that lie within the trunk of nerve fibres. They are highly stretched in the direction of the nerve cord and are also provided with a cilium. Types 2 and 3 may be homologized with the basal nerve cells of the epidermis. They are possibly multipolar. — The lateral nerve cords make contact with the ampulla and pass the ambulacral plate parallel to the channel that connects the ampulla and the tube foot. The activity of the tube foot-ampulla system is possibly controlled by means of transmitter substances that diffuse through the connective tissue layer between the nerve cord and the myoepithelia of the ampulla and the tube foot respectively.     

Novel Insights into the Echinoderm Nervous System from Histaminergic and FMRFaminergic-Like Cells in the Sea Cucumber Leptosynapta clarki

Understanding of the echinoderm nervous system is limited due to its distinct organization in comparison to other animal phyla and by the difficulty in accessing it. The transparent and accessible, apodid sea cucumber Leptosynapta clarki provides novel opportunities for detailed characterization of echinoderm neural systems. The present study used immunohistochemistry against FMRFamide and histamine to describe the neural organization in juvenile and adult sea cucumbers. Histaminergic- and FMRFaminergic-like immunoreactivity is reported in several distinct cell types throughout the body of L. clarki. FMRFamide-like immunoreactive cell bodies were found in the buccal tentacles, esophageal region and in proximity to the radial nerve cords. Sensory-like cells in the tentacles send processes toward the circumoral nerve ring, while unipolar and bipolar cells close to the radial nerve cords display extensive processes in close association with muscle and other cells of the body wall. Histamine-like immunoreactivity was identified in neuronal somatas located in the buccal tentacles, circumoral nerve ring and in papillae distributed across the body. The tentacular cells send processes into the nerve ring, while the processes of cells in the body wall papillae extend to the surface epithelium and radial nerve cords. Pharmacological application of histamine produced a strong coordinated, peristaltic response of the body wall suggesting the role of histamine in the feeding behavior. Our immunohistochemical data provide evidence for extensive connections between the hyponeural and ectoneural nervous system in the sea cucumber, challenging previously held views on a clear functional separation of the sub-components of the nervous system. Furthermore, our data indicate a potential function of histamine in coordinated, peristaltic movements; consistent with feeding patterns in this species. This study on L. clarki illustrates how using a broader range of neurotransmitter systems can provide better insight into the anatomy, function and evolution of echinoderm nervous sytems.     

The catecholaminergic nerve plexus of Holothuroidea

Catecholamines have been extensively reported to be present in most animal groups, including members of Echinodermata. In this study, we investigated the presence and distribution of catecholaminergic nerves in two members of the Holothuroidea, Holothuria glaberrima (Selenka, 1867) (Aspidochirotida, Holothuroidea) and Holothuria mexicana (Ludwig, 1875) (Aspidochirotida, Holothuroidea), by using induced fluorescence for catecholamines on tissue sections and immunohistochemistry with an antibody that recognizes tyrosine hydroxylase. The presence of a catecholaminergic nerve plexus similar in distribution and extension to those previously reported in other members of Echinodermata was observed. This plexus, composed of cells and fibers, is found in the ectoneural component of the echinoderm nervous system and is continuous with the circumoral nerve ring and the radial nerves, tentacular nerves, and esophageal plexus. In addition, fluorescent nerves in the tube feet are continuous with the catecholaminergic components of the radial nerve cords. This is the first comprehensive report on the presence and distribution of catecholamines in the nervous system of Holothuroidea. The continuity and distribution of the catecholaminergic plexus strengthen the notion that the catecholaminergic cells are interneurons, since these do not form part of the known sensory or motor circuits and the fluorescence is confined to organized nervous tissue.     

Developmental origin of the adult nervous system in a holothurian: an attempt to unravel the enigma of neurogenesis in echinoderms

In adult echinoderms, the nervous system includes the ectoneural and hyponeural subsystems. The former has been believed to develop from the ectoderm, whereas the latter is considered to be mesodermal in origin. However, this view has not been substantially supported by embryological examinations. Our study deals with the developmental origin of the nervous system in the direct-developing sea cucumber Eupentacta fraudatrix. The rudiment of the adult nervous system develops from ectodermally derived cells, which ingress into the primary body cavity from the floor of the vestibule. At the earliest stages, only the rudiment of the ectoneural nerve ring is laid down. The radial nerve cords and tentacular nerves grow out from this subcutaneous rudiment. The ectoneural cords do not develop simultaneously but make their appearance in the following order: unpaired mid-ventral cord, paired dorsal lateral cords, and ventral lateral cords. These transitional developmental stages probably recapitulate the evolution of the echinoderm body plan. The holothurian hyponeural subsystem, as other regions of the metazoan nervous system, has an ectodermal origin. It originally appears as a narrow band of tissue, which bulges out of the basal region of the ectoneural neuroepithelium. Our data combined with those of other workers strongly suggest that the adult nervous tissue in echinoderms develops separately from the superficial larval system of ciliary nerves. Therefore, our data are neither in strict accordance with Garstang's hypothesis nor do they allow to refuse it. Nevertheless, in addition to ciliary bands, other areas of neurogenetic epidermis must be taken into account.     

The giant neurone system in ophiuroids

Summary The radial nerve cords of members of the class Ophiuroidea consist of two parts, the ectoneural and the hyponeural tissues, which are separated by an acellular basal lamina. The hyponeural tissue is composed entirely of motor fibres. The cell bodies of the hyponeural neurones are arranged in ganglia, one to each segment of the arm, and each containing approximately one hundred cell bodies. Synaptic contact between the two tissues occurs across the basal lamina. Ultrastructural evidence shows that the majority of these synapses operate in the ectoneural to hyponeural direction. Three pairs of nerve bundles, each containing approximately thirty five large motor fibres arise from each ganglion and innervate the intervertebral muscles. The large motor fibres divide into a number of pre-terminal axons in the region in which the motor fibre enters the muscle block. The terminal axons run at right-angles across the muscle fibres and neuromuscular junctions are found at the points of contact between the two; each terminal axon makes contact with a large number of muscle fibres. The hyponeural axons also pass through the juxtaligamental tissue before they reach the muscle blocks and there is some evidence of synaptic contact with the juxtaligamental cells. The juxtaligamental tissue is thought to be associated with changes in the structural properties of the collagenous ligaments of the arm during arm autotomy (Wilkie 1979). Degeneration studies confirmed the layout of the hyponeural motor axons.     

Ultrastructure of the Alimentary Canal in Five-Month-Old Pentactulae of the Holothurian Eupentacta fraudatrix

Five-month-old pentactulae (juveniles) of the holothurian Eupentacta fraudatrixpossess a well-developed alimentary canal comprising an esophagus, a stomach, an intestine, and a rectum. The intestine in turn consists of five parts. The esophagus, stomach, and rectum are lined with a cuticular epithelium. The intestinal lining lacks a cuticle and is composed of mainly polyfunctional vesicular enterocytes. Granular enterocytes are less abundant; their cytoplasm contains electron-dense granules, which are probably zymogenic. The gut connective tissue consists of electron-lucent ground substance with collagen fibers and embedded coelomocytes. The gut mesothelium is composed of myoepithelial and peritoneal cells and contains the neurons of the hyponeural nerve plexus.     

The fine structure of the buccal tentacles of Holothuria forskali (Echinodermata,Holothuroidea)

Summary Ultrastructural study of the buccal tentacles of Holothuria forskali revealed that each tentacle bears numerous apical papillae. Each papilla consists of several differentiated sensory buds.The epidermis of the buds is composed of three cell types, i.e. mucus cells, ciliated cells, and glandular vesicular cells (GV cells). The GV cells have apical microvilli; they contain bundles of cross striated fibrillae associated with microtubules. Ciliated cells have a short non-motile cilium. Bud epidermal cells intimately contact an epineural nervous plate which is located slightly above the basement membrane of the epidermis. The epineural plate of each bud connects with the hyponeural nerve plexus of the tentacle. This nerve plexus consists of an axonic meshwork surrounded in places by sheath cells. The buccal tentacles have well-developed mesothelial muscles. Direct innervation of these muscles by the hyponeural nerve plexus was not seen.It is suggested that the buccal tentacles of H. forskali are sensory organs. They would recognize the organically richest areas of the sediment surface through the chemosensitive abilities of their apical buds. Tentacles presumably trap particles by wedging them between their buds and papillae.     

NGIWYamide-induced contraction of tube feet and distribution of NGIWYamide-like immunoreactivity in nerves of the starfish Asterina pectinifera

NGIWYamide, a neuropeptide recently isolated from sea cucumbers, was tested on tube feet of the starfish Asterina pectinifera. NGIWYamide (10(-6)-10(-4) M) caused contraction of isolated tube feet. NGIWYamide-like immunoreactivity (NGIWYa-LI) was investigated with an antiserum against NGIWYamide. NGIWYa-LI was found in the radial nerve cord (RNC), the marginal nerve, and the tube feet. Both ectoneural and hyponeural parts of the RNC showed NGIWYa-LI. Immunoreactive cell bodies were found in both parts of RNC. Extensive labeling in the basal region of the ectoneural part suggests that a substantial proportion of axons in this part contains NGIWYamide or a similar substance. In tube feet, NGIWYa-LI was found in the sub-epithelial nerve plexus and in the basal nerve ring. Double labeling along with 1E11, a neuron-specific monoclonal antibody developed from A. pectinifera, indicated that the structures with NGIWYa-LI are neurons. These results suggest that NGIWYamide or an NGIWYamide-like peptide exists in starfish and functions as a neurotransmitter or a neuromodulator.     

Neurobiology of Polyorchis. II. Structure of effector systems

The gross and fine morphology of the major effector systems in the anthomedusan, Polyorchis penicillatus, is described and discussed in relation to the known physiological and behavioral properties of these systems. Swimming is controlled by an anastomosing network of giant neurons within the inner nerve ring and radial nerves. Although these neurons may be coupled by gap junctions it is likely that they form a syncytium. The photosensitivity of the “giants” is attributed to reflexive membranes within the cytoplasm. Giant neurons act as both the pre- and postsynaptic cell when forming synapses with other neurons of the inner nerve ring. Neuromuscular synapses between “giants” and the striated swimming muscle are found around the margin and along the radii. Swimming muscle cells are connected laterally by gap junctions and end-to-end by desmosomes which are sometimes elaborated with extra-thick filaments. Unstriated sphincter and radial muscles, the major muscles associated with crumpling, are both greatly folded over mesogloeal ridges and have processes that cross the mesogloea to contact the ring and radial canals, respectively. Synapses or other sites that might be responsible for exciting these muscles during crumpling have not been found. The ability of the endodermal lamella and canals to propagate action potentials can be accounted for by the numerous gap junctions that are seen in these tissues. The precise location where excitation is transferred to the nervous system to initiate crumpling is not known but epithelial bridges crossing the mesogloea are likely routes. Synapses between neurons originating in the outer nerve ring and tentacle longitudinal muscle can account for the control of tentacle length. Neurons of the outer nerve ring also synapse onto velar, radial fibers and the sphincter muscle. The inner and outer nerve rings have nervous connections. The organisation of the outer nerve ring and the arrangement of nerves within the endodermal plexus is described. A diagram showing the major connections and interactions of components of the effector systems is presented.     

Nitric oxide (NO) synthase immunoreactivity in the starfish Marthasterias glacialis

The neuroendocrine system of the starfish Marthasterias glacialis was investigated immunocytochemically using antisera specific for rat neuronal, bovine aortic endothelial, and mouse macrophage, nitric oxide (NO) synthases. Immunoreactivity was detected only with the antibodies specific for the neural enzyme, in the ectoneural and hyponeural tissues of the radial nerve cords and in the basiepithelial plexus and endocrine cells of the digestive tract. The pyloric stomach showed more immunoreactive structures than the other digestive organs, with the rectal caeca showing the least activity. Immunoreactive endocrine cells were located in the cardiac and pyloric stomachs and in the pyloric caeca. Co-localization of the enzyme immunoreactivity, and the staining for NADPH-diaphorase, demonstrate the presence of NO synthase in echinoderms. These results provide further evidence that NO is a neuronal messenger of early phylogenetic origin which has been conserved throughout evolution.     

Secretory Radial Glia in the Ectoneural System of the Sea Star Asterias rubens (Echinodermata)

Previous studies of epithelial nervous systems have focused on the neuronal elements, but generally neglected the origin of neuro–glial interactions. In this study, we use a polyclonal antiserum directed against Reissner's substance to label non-neuronal bipolar cells in the ectoneural part of the radial nerve cord in the sea star Asterias rubens. Ultrastructural results show secretory activity in these bipolar cells. Immunolabelled material is released into the extracellular matrix in the hyaline layer as well as in the region of the basal end-feet. As a first step towards characterising the antigen, a specific protein band of 36 kD was demonstrated with immunoprecipitation. Cells of this type: (1) traverse the epithelium to full extent from the outer surface to the basal lamina; (2) carry a single apical cilium; (3) contain conspicuous bundles of intermediate filament; (4) produce a secretion which is, at least in part, homologous to the Reissner's substance which is produced by a primitive radial glia cell type in chordates. It is concluded that the bipolar cells in the ectoneural part of the surface epithelium of the sea star Asterias rubens are secretory radial glia, which evidently have a common origin to the radial glia which secretes Reissner's substance in chordates.