Some aspects of the structural organization of the arthropod ganglia

Summary This paper deals with the fine structure of the abdominal ganglia of several species of arthropods belonging to the classes Arachnida, Crustacea, Myriapoda and Insecta. The tissues were fixed in osmium tetroxide and embedded in n-butyl methacrylate or fixed in potasium permanganate and embedded in a mixture of X 133/2097 and Araldite.A comparative study was made in order to discriminate between those structural characteristics of the nervous system appearing only in determined taxonomic groups and those belonging to a fundamental plan common to the whole Phylum. This work covers the morphology of neurons, glial cells, neuropilic nerve fibers and neuronal connections.Most arthropod neurons are pear-shaped with only one prolongation and the nucleus is located in the center of the soma, enveloped by two membranes showing numerous pores. Cisternae of the ER have frequently been observed in continuity with this nuclear envelope. After osmic fixation the nuclear content appears to consist of small dense granules distributed at random in the nucleoplasm. In addition to these small perticles there are, in some species, large chromatin blocks. The use of Permanganate as fixative introduces important changes in the nuclear aspect; most of the nuclei look washed and the nuclear content acquires an homogeneous appearance.The cytoplasm of the neurons contains a complex system of internal membranes consisting of cisternae and tubuli of the ER system, lamellae of the Golgi complex and invaginations of the plasma membrane. In most species the elements of the ER system are distributed at random in the cytoplasm but in the neurons of Bothriurus bonariensis there are parallel aggregations of membranes similar to the Nissl bodies found in vertebrates.It was found in some of the species studied (Armadillidium vulgare and Lithobius Sp.) that the internal membrane system of the nerve cells is mainly represented by Golgi elements while the ER system seems to be poorly developed.Besides the membranous components, the neuronal cytoplasm contains mitochondria, multivesicular bodies and dense granules of neurosecretory material.Neuroglial cells are mainly characterized by their nuclear structure. After the action of osmium tetroxide, glial nuclei show irregular masses of chromatin inmersed in a nucleoplasm of low electron density. In permanganate fixed material these chromatin blocks appear as blank spaces.In the cytoplasm of these cells there are mitochondria, membranes pertaining to the ER system and elements of the Golgi complex but in some of the species studied gliofibrils and granules of pigment were found.Three main types of neuroglial cells have been recognized in an arthropod ganglia. These are: subcapsular glial cells, neuron satellites and nerve fiber satellites.The neuropile occupies the central region of the ganglion and consists of a great number of nerve fibers intermingled with glial processes. The neuropilic n. fibers consistently show profiles of ER membranes and tubuli pertaining to the ER system. In some of these fibers the ER reaches a high degree of development. In Armadillidium there is a special type of n. fiber containing a regular sequence of transversally oriented cisternae. Arthropod fibers sometimes contain thin parallel filaments as well as typical ER elements.Mitochondria, small vesicles and dense granules are commonly found within the neuroplasm of the neuropilic fibers. It is important to note that in arthropods, microvesicles are not restricted to the terminal region of the nerve fibers but that they may also occur all along the fibers.Arthropod neurons are enveloped by a glial insulating capsule and therefore interneuron contacts may only occur at neuropile level. These contacts are of three different morphological types: cross contacts, longitudinal contacts and end-knob contacts. At the level of longitudinal and cross contacts the neuroplasm shows no increase in the number of microvesicles or mitochondria. In the end-knob contacts, on the contrary, large numbers of microvesicles appear concentrated in the pre-synaptic fiber only, and occasionally in both fibers the pre-synaptic and the post-synaptic.It is maintained that funcional interneuron connections may result not only from contacts between fibers containing vesicles, but also between fibers in which vesicles are absent.     

Fine Structure of the Optic Tract in the Atlantic Hagfish,Myxine glutinosa

The fine structure of the the optic tract in Myxine glutinosa was studied in three animals. The nerve fibers are unmyelinated and occur either singly or in groups surrounded by glial cytoplasm. 810 and 1467 nerve fibers were counted in two tracts. In the zone where the optic tract leaves the eye cup, a cell type is described and suggested to correspond to ganglion cells. Presynaptic terminals containing agranular vesicles were seen adjacent to this cell type and to other processes in the area. The results are discussed in view of previous reports.     

Fine structure of glial cells in the central nervous system of the tapeworm Grillotia erinaceus (Cestoda: Trypanorhyncha)

The problem of glial cells existing in parasitic and free living flatworms is correlated with organization of parenchyma in platyhelmintes. In the contrary to the widespread opinion that myelin-like envelopes and glial cells do not exist in the nervous system of parasitic flatworms, it has been shown by ultrastructural researches that Amphilina foliacea (Cestoda, Amphilinidea) has well developed glial cells and myelin-like envelopes in the ganglia and main cords, which include both glial cells and intercellular components. The aim of our research was to reveal and investigate in details structural components corresponding to the concept of the glial cell in the CNS of Grillotia erinaceus (Cestoda: Trypanorhyncha). Three types of glial cells have been found. The first type is the fibroblast-like glial cells; cells locate in the cerebral ganglion, contain in cytoplasm and extract out fibrillar matrix, form desmosomes and have supporting function. The glial cells of the second type form myeline-like envelope of the giant axons and bulbar nerves in scolex and have laminar cytoplasm. These cells are numerous and exceed in number the neurons bodies into the nerve. The glial cells of the third type form multilayer envelopes in the main nerve cords; extra cellular fibers and gap-junctions take place between the layers. There are contacts between the glial cells of the third type and excretory epithelium but specialized contacts with neurons have been not found. The existing of glial cells in free living and parasitic flatworms is discussed.     

Ultrastructure of the synganglion in the larval tickAmblyomma americanum (Ixodoidea: Ixodidae)

The synganglion in the larvalAmblyomma americanum consists of a ganglionic mass pierced by the oesophagus. The nervous tissue consisting of an outer cortex and an inner neuropile is surrounded by an external neurilemma. The cortex comprises perineurium glial cells and neurosecretory and non-neurosecretory neuronal cell bodies. The neuropile consists of nerve fibres ensheathed by glial cells. The entire ganglionic mass is enclosed within a sinus of the circulatory system. No investigations using electron microscopy appear to have been made on the synganglion in the tick larval stage.     

Ultrastructural study of the Central Nervous System of Endemic Geocentrophora(Prorhynchida,Platyhelminthes) from Lake Baikal

The ultrastructure of the central nervous system of four species of the Geocentrophoragroup (Prorhynchida, Lecithoepitheliata) has been studied with Geocentrophora waginias a model organism. This is the first ultrastructural study of the nervous system in flatworms of the endemic Geocentrophoragroup from Lake Baikal. The neurons are characterized by invaginations of the plasma membrane with extracellular material (ECM) extending deep into the neuronal cytoplasm. Two types of neuron are distinguished on the basis of the character of the cytoplasm and the content of neuronal vesicles. Several more vesicle types are observed in the nerve processes in the neuropile. The following kinds of neuronal release sites are distinguished: single synapses, shared synapses, nonsynaptic release sites characterized by omega profiles and neuromuscular contacts. Special features of the synapses in the species of Geocentrophoraare pronounced local thickenings of the presynaptic membrane connected to paramembraneous densities. The ECM-filled invaginations and the local presynaptic thickenings are features that distinguish the neurons of Geocentrophoraspp. from previously described turbellarian neurons.     

Ultrastructure of the tentacle nerve plexus and putative neural pathways in sea anemones

Abstract. Neurons of sea anemone tentacles receive stimuli via sensory cells and process and transmit information via a plexus of nerve fibers. The nerve plexus is best revealed by scanning electron microscopy of epidermal peels of the tentacles. The nerve plexus lies above the epidermal muscular layer where it appears as numerous parallel longitudinal and short interconnected nerve fibers in Calliactis parasitica . Bipolar and multipolar neurons are present and neurites form interneuronal and neuromuscular synaptic contacts. Transmission electron microscopy of cross sections of tentacles of small animals, both C. parasitica and Aiptasia pallida , reveals bundles of 50–100 nerve fibers lying above groups of longitudinal muscle fibers separated by intrusions of mesoglea. Smaller groups of 10–50 slender nerve fibers are oriented at right angles to the circular muscle formed by the bases of the digestive cells. The unmyelinated nerve fibers lack any glial wrapping, although some bundles of epidermal fibers are partially enveloped by cytoplasmic extensions of the muscle cells; small gastrodermal nerve bundles lie between digestive epithelial cells above their basal myonemes. A hypothetical model for sensory input and motor output in the epidermal and gastrodermal nerve plexuses of sea anemones is proposed.     

Embryology of the thoracic and abdominal ganglia of the large milkweed bug, Oncopeltus fasciatus (Dallas), (Hemiptera, Lygaeidae)

In this study, the condensation of the three thoracic and 11 abdominal segmental ganglia to form a prothoracic and central nerve mass during embryogenesis is described. During katatrepsis, many changes occur in the organization of these ganglia; this study suggests that some of these changes are caused by mechanical forces acting on the ventral nerve cord at this time. The ventral nerve cord begins its anterior migration and coalescence ten hours after katatrepsis and is completed 63 hours later. The central ganglion is made up of the meso- and metathoracic ganglia and seven abdominal ganglia. Intrasegmental median cord nuclei are shown to form glial elements in the median sagittal plane of the neuropile and in the longitudinal connectives. Intersegmental median cord neuroblasts migrate into the posterior gangliomeres but, apparently, degenerate soon after katatrepsis. Lateral cord cells bordering on the neuropile form a glial investment that surrounds this fiber tract region. Peripheral lateral cord cells are shown to form the cells of the outer ganglionic sheath, the perineurium.     

The subgenus Persicargas (Ixodoidea: Argasidae: Argas): A. (P.) arboreus central nervous system anatomy and histology

The anatomy and histology of the adult Argas (Persicargas) arboreus central nervous system are described and compared with these properties in other ticks. The single, integrated, central nerve mass (CNM) is formed by a fused supra-esophageal part (protocerebrum, cheliceral ganglia, palpal ganglia, and stomodeal pons) and a subesophageal part (4 pairs of pedal ganglia and the complex opisthosomatic ganglion). Single peripheral nerves (pharyngeal and recurrent) and paired peripheral nerves (compound protocerebral, cheliceral, palpal, pedal and opisthosomatic) extend from the CNM to body organs and appendages. Optic nerves, described in other Argas species, are not found in A. (P.) arboreus. Histologically, the CNM is enclosed by a thin-walled periganglionic blood sinus and invested by a collagenous neural lamella followed by a perineurial layer composed of glial cells and containing fine reticular spaces, a cortical layer of association, motor and neurosecretory cell bodies and glial cells, and inner neuropile regions of fiber tracts forming 5 horizontal levels of connectives and commissures.     

An electron microscopic study on the innervation of the gill filaments of a lamprey,Lampetra japonica

This paper reports observations on the innervation of gill filaments of the lamprey, Lampetra japonica. Nerve fibers run on each side of the afferent filament artery (AFA nerve) and in the connective tissue compartment along the efferent filament artery (EFA nerve). The AFA nerve supplies vasomotor fibers to the afferent filament artery and arteriovenous anastomoses and special visceral motor fibers to branchial muscle fibers (musculus compressor branchialis circularis). Nerve endings of the vasomotor fibers contain large, cored vesicles (60–180 nm in diameter) with a variable number of small, clear vesicles (30–70 μm in diameter), whereas those of the visceral motor fibers have many small, clear vesicles with few large, cored vesicles. The EFA nerve supplies vasomotor fibers to the efferent filament artery. Their endings, containing mixtures of predominantly large, cored vesicles and small, clear vesicles make close synaptic contacts with reticular cells. The latter in turn are connected with each other or with smooth muscle cells in the wall of the efferent filament artery by nexuses. No nerves are found in the axial plate between the afferent and efferent filament arteries nor in the secondary lamellae of individual gill filaments. No afferent nerve supply to the gill filament has been found.     

THE ELECTRON MICROSCOPIC LOCALIZATION OF CHOLINESTERASE ACTIVITY IN THE CENTRAL NERVOUS SYSTEM OF AN INSECT, PERIPLANET A AMERICANAL.

The distribution of esterase activity in the last abdominal ganglion, the connectives and the cereal nerves of the cockroach Periplaneta americana has been investigated cytochemically. Activity of an unspecific eserine-insensitive esterase (or esterases) has been found in glial elements in these regions of the nerve cord. In addition, sites of cholinesterase (eserine-sensitive) activity have been found in association with (a) the glial sheaths of the axons in the cereal nerves and connectives, (b) the glial folds encapsulating the neuron perikarya in the ganglion, and (c) in localized areas along the membranes of axon branches within the neuropile, often flanked by focal clusters of synaptic vesicles. These results are discussed with particular reference to the previously reported insensitivity of the insect nerve cord to applied acetylcholine, and to the probable existence of a cholinergic synaptic mechanism in the central nervous system of this insect.     

The fine structure of the central synaptic contacts on an identified crustacean motoneuron

Summary Small nerve terminals in the neuropile of the brain of the crab Scylla serrata make close contact with the secondary, tertiary and higher order central branches of the reflex eye-withdrawal motoneurons. Most contacts have the characteristics of chemically transmitting synapses in that the presynaptic terminals contain agranular vesicles of 25 to 50 nm in diameter and are separated from the motoneuron by a synaptic cleft of about 16 nm. Some terminals contain synaptic ribbons, others contain a mixture of larger (50 to 80 nm) agranular and also dense cored vesicles. In addition large blunt-ended contacts unaccompanied by vesicles, occur between neurons in the neuropile and the motoneuron. It is suggested that the absence of synaptic contacts over the large primary branches of the motoneuron could explain previous physiological findings that little or no resistance changes can be detected in this part of the neuron during excitation or inhibition.We thank Mrs. Joan Goodrum for the preparation of Fig. 1.     

The fine structure of neuroglia in the central nervous system of nereid polychaetes

Summary The principal supportive elements of the nereid central nervous system are non-neuronal cells that are referred to as supportive glia. Supportive glial cells form a conspicuous cortex in the nerve cord. The inner region of this cortex consists of closely packed processes and cell bodies of fibrous supportive glial cells that are arranged in concentric layers around the perimeter of the neuropile. The fibrous appearance of the glial cells results from dense bundles of cytoplasmic filaments. Many fibrous glial processes penetrate the neuropile and ramify among the neuronal elements. Larger, irregularly shaped cells are the chief supportive glial elements of the peripheral region of the cortex where they line the stromal sheath (neural lamella) and invest the neuronal perikarya with extensive concentric systems of lamellate processes. These glial cells usually possess a relatively undifferentiated cytoplasm with scattered glycogen granules, but occasionally have a well developed Golgi apparatus, endoplasmic reticulum and densely packed particulate glycogen. The supportive glia exhibits numerous desmosomes as well as 5-layered (tight) and 7-layered (gap) junctions. Interspersed among the supportive glial cells are non-neuronal cells referred to as granulocytes. These cells have abundant large, granular inclusions, electron lucent vesicles, plasmalemmal infoldings and microtubules. The granulocytes may be derived from undifferentiated glial cells or may represent coelomocytes that have invaded the nervous tissue.Supported by USPHS Grants No. NIH 5P01 NS-07512, NIH 2T01 GM-00102, and NB-00840.The author acknowledges the excellent technical assistance of Sarah Wurzelmann and Stanley Brown, and thanks Dr. Berta Scharrer for many stimulating discussions.     

Ultrastructure of the circumoral nerve ring and the radial nerve cords in holothurians (Echinodermata)

The circumoral nerve ring and the radial nerve cords (RNCs) of Eupentacta fraudatrix and Pseudocnus lubricus (Holothuroidea) were examined as an example of holothurian nervous tissue. The RNC is composed of outer ectoneural and inner hyponeural layers, which are interconnected with one another via short neural bridges. The circumoral nerve ring is purely ectoneural. Both ectoneural and hyponeural components are epithelial tubes with a thick neuroepithelium at one side. A thin ciliated non-neuronal epithelium complements the neuroepithelium to form a tube, thereby enclosing the epineural and hyponeural canals. The whole of the ectoneural and hyponeural subsystems is separated from the surrounding tissue by a continuous basal lamina. The nerve ring and the ectoneural and hyponeural parts of the radial nerves are all neuroepithelia composed of supporting cells and neurons. Supporting cells are interpreted as being glial cells. Based on ultrastructural characters, three types of neurons can be distinguished: (1) putative primary sensory neurons, whose cilium protrudes into the epineural or hyponeural canal; (2) non-ciliated neurons with swollen rough endoplasmic reticulum cisternae; (3) monociliated neurons that are embedded in the trunk of nerve fibers. Different types of synapses occur in the neuropile area. They meet all morphological criteria of classical chemical synapses. Vacuolated cells occur in the neuroepithelium of E. fraudatrix, but are absent in P. lubricus; their function is unknown. The cells of the non-neuronal epithelia that overlie the ectoneural and hyponeural canals are hypothesized to belong to the same cell type as the supporting cells of the neuroepithelium.     

On the Fine Structure of the Nervous System of Tubiluchus philippinensis (Tubiluchidae,Priapulida)

At the mouth tube/introvert border a circumenteric intraepithelial nerve ring occupies a circular ridge protruding into the body cavity. The ring has a centrally located neuropile nearly free of perikarya and two zones of different perikarya above and below the neuropile. Presumably non-neuronal perikarya have an oval nucleus, large heterochromatin clumps and marked filament bundles. Such elements resemble tanycytic glial cells. Two types of presumably neuronal perikarya contain small cytoplasmic granules, similar to those in nerve fibre profiles. One of these neurons has a pale nucleus with a prominent nucleolus, the other a rather inconspicuous nucleus similar to that of the tanycytic cells. The neuronal processes of the fibre ring differ in diameter and contain clear and dense core vesicles, small granules (high or medium electron density) or granules with a dense periphery and a light centre. Sometimes neighbouring processes seem interconnected by electrical synapses. Images suggesting chemical synapses are rare. A large intraepithelial nerve lies in the wall of the introvert and ventral body wall close to the musculature, possibly innervated by this nerve. Frontal of the anus lies an intraepithelial ganglion demonstrating again a central neuropile. two neuronal types and tanycytic elements with filament bundles. Comparative aspects of the characters of the Tubiluchus nervous system are also discussed.     

A serotonergic system in the brain of the Antarctic fish, Trematomus bernacchii

The immunohistochemical distribution of serotonin-containing nerve fibres and cells has been described in the brain of the Antarctic fish, Trematomus bernacchii. The largest serotonergic system was associated with the diencephalic and rhombencephalic ventricles. In particular, serotonin-positive cells have been found in the lateral recess and neuropile zone of the diencephalic ventricle, where we have identified the serotonergic portion of the paraventricular organ. Numerous serotonin cells were localized in the dorsal nucleus of the raphe, the dorsal tegmental nucleus and the central gray. Two large cell groups, arranged in a pair of well-defined columns and connecting the central gray with the dorsal reticular formation, were immunostained in the region of the trigeminal nuclei. In addition, few positive cells have been found in the preoptic area and the cerebellar valvula, and few serotonergic nerve fibres, probably belonging to the lateral lemniscus, have been identified. The distribution of serotonin elements in the brain of T. bernacchii has been compared with that described in other fish, where it showed some modifications in the immunoreactive pattern. Finally, the lack of a serotonergic system at the level of the reticular superior formation has been reported; however, it was not possible to rule out a phylogenetic or environmental explanation.     

Distribution and ultrastructure of neurosecretory cells in the cerebral ganglion of the earthworm

Neurosecretory (Nsy) cells within the cerebral ganglion of Lumbricus terrestris were classified ultrastructurally. The Nsy cells within the subesophageal ganglion, nerve cord ganglion, and the peripheral nervous system were also examined. A comparative survey of Nsy cells of four other species of oligochaetes, Eisenia feotida, octolasion cyaneum, Dendrobeona subrubicunda, and Allolophora longa, was also carried out. Seven cell types (A1, A2, A3, A4, A5, C, and SEF), distinguished by special cytological and ultrastructural features, were found within the cerebral ganglion. Distribution of these cells inside and outside the cerebral ganglion was studied in detail by light and electron microscopy. The nerve terminals of each cell type were followed into the neuropile region. Exocytosis from cell bodies appears to be the main release mechanism for the Nsy granules, whereas small Nsy vesicles are released through synapses in the neuropile. Peripheral fibers of some cell types (A1, A2, and A3) extend through the capsule to the pericapsular epithelium. It is possible that Nsy cells secrete hormones from their cell bodies and peripheral processes and that their centrally directed axons release modulators/transmitters within the neuropile.     

Neuronal pathways from the dorsal acelli of the house cricket,Acheta domesticus

Each ocellar nerve in the house cricket Acheta domesticus contains giant nerve fibers of 10-15 μ diameter, characterized in Golgi Cox preparations by a single row of short collaterals which runs along nearly the entire length of a fiber. Numerous long collaterals are given off by thin fibers in the ocellar nerve; medium-size fibers give off relatively few collaterals. The lateral ocellar tracts extend posteriorly through the dorsal protocerebrum, crossing the protocerebral bridge dorsally. The smaller median ocellar tract runs more ventrally through the pars intercerebralis; posterior to the bridge its fibers turn out toward the lateral nerves. Golgi and cobalt preparations reveal branching of giant and mediu_-size ocellar fibers posterior to the bridge at two levels, forming bilateral regions of ocellar neuropile. No ocellar processes appear to be given off to the corpora pedunculata, centra! body, nervi corporis cardiaci, antenna! lobes, or circumesophageal connectives; it is uncertain whether ocellar collaterals extend into the protocerebral bridge or optic lobes. Cell bodies of giant and medium-sized fibers are located in the pars intercerebralis.     

A crossed projection from the optic tectum to craniocervical premotor areas in the brainstem reticular formation. An anterograde and retrograde tracing study in the mallard (Anas platyrhynchos L.)

The optic tectum in birds receives visual information from the contralateral retina. This information is passed through to other brain areas via the deep layers of the optic tectum. In the present study the crossed tectobulbar pathway is described in detail. This pathway forms the connection between the optic tectum and the premotor area of craniocervical muscles in the contralateral paramedian reticular formation. It originates predominantly from neurons in the ventromedial part of stratum griseum centrale and to a lesser extent from stratum album centrale. The fibers leave the tectum as a horizontal fiber bundle, and cross the midline through the caudal radix oculomotorius and rostral nucleus oculomotorius. On the contralateral side fibers turn to ventral and descend caudally in the contralateral paramedian reticular formation to the level of the obex. Labeled terminals are found in the ipsilateral medial mesencephalic reticular formation lateral to the radix and motor nucleus of the oculomotor nerve, and in the contralateral paramedian reticular formation, along the descending tract. Neurons in the medial mesencephalic reticular formation in turn project to the paramedian reticular formation. Through the crossed tectobulbar pathway visual information can influence the activity of craniocervical muscles via reticular premotor neurons.     

Neurobiology of the basal platyhelminth <Emphasis Type="Italic">Macrostomum lignano</Emphasis>: map and digital 3D model of the juvenile brain neuropile

We have analyzed brain structure in Macrostomum lignano, a representative of the basal platyhelminth taxon Macrostomida. Using confocal microscopy and digital 3D modeling software on specimens labeled with general markers for neurons (tyrTub), muscles (phalloidin), and nuclei (Sytox), an atlas and digital model of the juvenile Macrostomum brain was generated. The brain forms a ganglion with a central neuropile surrounded by a cortex of neuronal cell bodies. The neuropile contains a stereotypical array of compact axon bundles, as well as branched terminal axons and dendrites. Muscle fibers penetrate the flatworm brain horizontally and vertically at invariant positions. Beside the invariant pattern of neurite bundles, these “cerebral muscles” represent a convenient system of landmarks that help define discrete compartments in the juvenile brain. Commissural axon bundles define a dorsal and ventro-medial neuropile compartment, respectively. Longitudinal axons that enter the neuropile through an invariant set of anterior and posterior nerve roots define a ventro-basal and a central medial compartment in the neuropile. Flanking these “fibrous” compartments are neuropile domains that lack thick axon bundles and are composed of short collaterals and terminal arborizations of neurites. Two populations of neurons, visualized by antibodies against FMRFamide and serotonin, respectively, were mapped relative to compartment boundaries. This study will aid in the documentation and interpretation of patterns of gene expression, as well as functional studies, in the developing Macrostomum brain.     

Ultrastructural characteristics and innervation of the pineal organ of the antarctic seal Leptonychotes weddelli

The electron microscopy of the epiphysis cerebri of the antarctic seal Leptonychotes weddelli revealed a highly organized organ. The general cytological characteristics of the pinealocytes and the glial cells are described. The capillary blood vessels are the nonfenestrated type. The organ is richly innervated by mainly unmyelinated nerve fibers. Most of the axons end in the inner part of the organ, around vessels, some of them in relation with pinealocytes. The significance of the findings is discussed.