Neurons with dopamine-like immunoreactivity target mushroom body Kenyon cell somata in the brain of some hymenopteran insects

The mushroom bodies of the insect brain are centers for olfactory and multimodal information processing and they are involved in associative olfactory learning. They are comprised of numerous (340,000 in the bee brain), small (3–8 μm soma diameter) local interneurons, the Kenyon cells. In the brain of honeybees (Apis mellifera) of all castes (worker bees, drones and queens), wasps (Vespula germanica) and hornets (Vespa crabro) immunostaining revealed fibers with dopamine-like immunoreactivity projecting from the pedunculus and the lip neuropil of the mushroom bodies into the Kenyon cell perikaryal layer. These fibers terminate with numerous varicosities, mainly around the border between medial and lateral Kenyon cell soma groups. Visualization of immunostained terminals in the transmission electron microscope showed that they directly contact the somata of the Kenyon cells and contain presynaptic elements. The somata of the Kenyon cells are clearly non-immunoreactive. Synaptic contacts at the somata are unusual for the central nervous systems of insects and other arthropods. This finding suggests that the somata of the Kenyon cells of Hymenoptera may serve an integrative role, and not merely a supportive function.     

Non-amidated and amidated members of the C-type allatostatin (AST-C) family are differentially distributed in the stomatogastric nervous system of the American lobster, <Emphasis Type="Italic">Homarus americanus</Emphasis>

The crustacean stomatogastric nervous system (STNS) is a well-known model for investigating neuropeptidergic control of rhythmic behavior. Among the peptides known to modulate the STNS are the C-type allatostatins (AST-Cs). In the lobster, Homarus americanus, three AST-Cs are known. Two of these, pQIRYHQCYFNPISCF (AST-C I) and GNGDGRLYWRCYFNAVSCF (AST-C III), have non-amidated C-termini, while the third, SYWKQCAFNAVSCFamide (AST-C II), is C-terminally amidated. Here, antibodies were generated against one of the non-amidated peptides (AST-C I) and against the amidated isoform (AST-C II). Specificity tests show that the AST-C I antibody cross-reacts with both AST-C I and AST-C III, but not AST-C II; the AST-C II antibody does not cross-react with either non-amidated peptide. Wholemount immunohistochemistry shows that both subclasses (non-amidated and amidated) of AST-C are distributed throughout the lobster STNS. Specifically, the antibody that cross-reacts with the two non-amidated peptides labels neuropil in the CoGs and the stomatogastric ganglion (STG), axons in the superior esophageal (son) and stomatogastric (stn) nerves, and ~ 14 somata in each commissural ganglion (CoG). The AST-C II-specific antibody labels neuropil in the CoGs, STG and at the junction of the sons and stn, axons in the sons and stn, ~ 42 somata in each CoG, and two somata in the STG. Double immunolabeling shows that, except for one soma in each CoG, the non-amidated and amidated peptides are present in distinct sets of neuronal profiles. The differential distributions of the two AST-C subclasses suggest that the two peptide groups are likely to serve different modulatory roles in the lobster STNS.     

The ganglion-connective junction in the central nervous system of the leech,Hirudo medicinalis

Classical studies of the nervous system of the leech revealed that there were specific types of very large glial cells associated with various parts of the neuron. Recent microelectrode studies demonstrated that there was a low resistance to the flow charge from any one of these large glial cells to another. The present study describes a previously unreported type of glial cell, the glial cell of the fascicles. These cells, which resemble the glial cells of the connectives but are smaller, are found in the fascicles of axons that unite the connectives to the neuropil. Thus, these cells are located between the glial cells of the connectives on the one hand and the glial cells of the neuropil and packets on the other and must be taken into account in considerations of the low resistance to the transfer of charge from one glial cell to another.     

Glial cells of the crayfish and their relationships with neurons. An ultrastructural study

1. Glial cells of the crayfish abdominal ganglia have been studied by transmission electron microscopy. Special attention is paid to the interrelationships between neurons and glial cells. Covers and hemocyte-related elements have also been considered. 2. Glial cells are identified by a common ultrastructure and close relationships with neurons. Four glial classes are considered, depending on their morphology, the compartment of neurons they ensheathe and neuron-glia interface. 3. Four ultrastructural classes of neurons are proposed. They differ in geometry and ultrastructure, as well as in glial covers (complexity and evaginations into the neuron somata). The morphology and organization of glial covers is specific for the neuron type they ensheathe. Specific glial covers do not differ in glia-glia communicatory structures. 4. The morphological and metabolical compartments of neurons are separated from the extracellular matrix or blood by specific glial systems. A system of two cells is interposed between neuron somata and hemolymph or the extracellular matrix. 5. Glial processes are crossed by membraneous tubular systems, at neuron perikarya and axons. Frequent gap junctions of varying area, density and number of IMP are found in the covers of neuron somata. 6. Neuron-glia interface bears numerous communicatory structures for both ionic and macromolecular exchange. They include junctions and transient modifications of membranes. Some of them suggest active transport mechanisms. 7. Modified endocytotic mechanisms seem to be responsible for the glia-to-neuron transfer of macromolecules as well as for the neuron-to-glia transfer of lamellar bodies. 8. The neuropil is divided into glomeruli (electrical or chemical) by glial processes and the trabeculae of the extracellular dense matrix. Neuron-glia membrane appositions have been found in electrical glomeruli. In chemical glomeruli, dense cored vesicles can release their content at neuron-neuron or neuron-glia intercellular cleft, at non-synaptic loci. 9. Neurons of type II contain peripheral complex Golgi systems, associated to subsurface cisternae and neuron-glia gap junctions, suggesting a cooperation of glial cells in specific macromolecular synthesis.     

Neo-Timm and selenium stainable glial cells of the rat telencephalon

The Neo-Timm and selenium methods predominantly stain the neuropil of the rat brain and have been found to visualize zinc in synaptic vesicles. A fraction of glial cells and neuronal somata is also stained, especially when the Neo-Timm method is used. In the present study the localization and appearance of stained glial cells in the rat telencephalon are described using the two methods and the effect of metal chelating agents on the stained glial cells is examined. Neo-Timm stained glial cells were observed in both white and grey matter, with a preponderance in the major fiber tracts of the telencephalon, and were seen to contain rather large silver grains in their cytoplasm. Chelation with diethyldithiocarbamate (DEDTC) or dithizone prevented this staining. Brains from rats treated intravitally with selenium contained only occasionally stained glial cells. However, when present they showed the same characteristics as the Neo-Timm stained glial cells, including the reaction to chelation. Although both the Neo-Timm and selenium methods primarily visualize zinc in the neuropil of the rat brain, the possibility that copper could contribute to the glial cell staining cannot be ruled out. This possibility is further discussed.     

Gliogenesis in the embryonic brain of the grasshopper Schistocerca gregaria with particular focus on the protocerebrum prior to mid-embryogenesis

I investigate the pattern of gliogenesis in the brain of the grasshopper Schistocerca gregaria prior to mid-embryogenesis, with particular focus on the protocerebrum. Using the glia-specific marker Repo and the neuron-specific marker HRP, I identify three types of glia with respect to their respective positions in the brain: surface glia form the outmost cell layer ensheathing the brain; cortex glia are intermingled with neuronal somata forming the brain cortex; and neuropil glia are associated with brain neuropils. The ontogeny of each glial type has also been studied. At 24 % of embryogenesis, a few glia are observed in each hemisphere of the proto-, deuto- and tritocerebrum. In each protocerebral hemisphere, such glia form a cluster that expands rapidly during later development. Closer examination reveals proliferative glia in such clusters at ages spanning from 24 to 36 % of embryogenesis, indicating that glial proliferation may account for the expansion of the clusters. Data derived from 33–39 % of embryogenesis suggest that, in the protocerebrum, each type of glia is likely to be generated by its respective progenitor-forming clusters. Moreover, the glial cluster located at the anterior end of the brain can give rise to both surface glia and cortex glia that populate the protocerebrum via subsequent migration. Proliferation is observed for all three glial types, indicating a possible source for the glia.     

Glial fibrillary acidic protein in the brain of the caecilian Typhlonectes natans (Amphibia,Gymnophiona): an immunocytochemical study

The astroglia of adult and juvenile (metamorphosed) Typhlonectes natans (Fischer) was investigated immunocytochemically with a monoclonal antibody directed against glial fibrillary acidic protein (GFAP). The astroglia of this member of the Order Gymnophiona of the class Amphibia is mainly composed of radial glial cells. Their somata limit the ventricles. They each give rise to a thick process that extends through the periventricular gray and arborizes within the neuropil. At the subpial surface, endfeet establish the membrana gliae limitans externa. Some extraependymal radial glial cells are immunoreactive, but no mammalian-like astrocytes are visualized. In the spinal cord, perikarya of radial glia are displaced from the GFAP-immunonegative ependyma. Perivascular endfeet and processes lining blood vessels are abundantly labeled. An increase in GFAP immunoreactivity extends from the exclusive labeling of subpial endfeet in newborn, recently metamorphosed animals, to the subsequent staining of distal processes and of the entire cell in older juveniles. The midline glia of the brainstem is immunoreactive at all ages examined. Strong glial wedges separate and delineate fiber tracts. Radial glial fibers in the habenulae are particularly thick and exhibit strong GFAP immunoreactivity, even in juveniles where GFAP immunoreactivity is otherwise minimal. The pattern of GFAP immunolabeling in the caecilian T. natans is similar to that in salamanders, but not to that in frogs.     

Tracing cells throughout development: Insights into single glial cell differentiation

In the article “Predetermined embryonic glial cells form the distinct glial sheaths of the Drosophila peripheral nervous system” we combined our expertise to identify glial cells of the embryonic peripheral nervous system on a single cell resolution with the possibility to genetically label cells using Flybow. We show that all 12 embryonic peripheral glial cells (ePG) per abdominal hemisegment persist into larval (and even adult) stages and differentially contribute to the three distinct glial layers surrounding peripheral nerves. Repetitive labelings of the same cell further revealed that layer affiliation, morphological expansion, and control of proliferation are predetermined and subject to an intrinsic differentiation program. Interestingly, wrapping and subperineurial glia undergo enormous hypertrophy in response to larval growth and elongation of peripheral nerves, while perineurial glia respond to the same environmental changes with hyperplasia. Increase in cell number from embryo (12 cells per hemisegment) to third instar (up to 50 cells per hemisegment) is the result of proliferation of a single ePG that serves as transient progenitor and only contributes to the outermost perineurial glial layer.     

An ultrastructural study of peripheral neurons and associated non-neural structures in the bivalve mollusc, Spisula solidissima

The ultrastructural morphology of peripheral neurons and associated structures in the bivalve mollusc, Spisula solidissima have been studied in an effort to describe the synaptic topography and to provide anatomical correlates of previous physiological observations. The somata of the peripheral neurons are located within the perineurium at branch points of the siphonal nerves. There are many fiber-fiber synaptic contacts which are characterized by isolated sites of contact with membrane specialization and unilateral accumulation of synaptic vesicles. There are also synaptic contacts involving the somata, both axo-somatic and somato-axonic, the two being distinguishable on the basis of the polarity of vesicle accumulation. All of the observed synaptic profiles were similar in appearance regardless of the neuron loci involved. Much of the non-synaptic soma surface is covered with processes of glial cells. Likewise, in many cases, individual fibers and groups of fibers are encased with glial processes. Unique clusters of membrane bound, pigment containing glial like cells occur throughout the nervous system of Spisula. The heterogeneous appearance of the inclusions and the presence of lysosome-like bodies in the cytoplasm of these cells suggest a possible phagocytic role.     

Structure of allotransplanted ganglia and regenerated neuromuscular connections in crayfish

In adult crayfish, Procambarus clarkii, motoneurons to a denervated abdominal superficial flexor muscle regenerate long-lasting and highly specific synaptic connections as seen from recordings of excitatory postsynaptic potentials, even when they arise from the ganglion of another crayfish. To confirm the morphological origins of these physiological connections we examined the fine structure of the allotransplanted tissue that consisted of the third abdominal ganglion and the nerve to the superficial flexor muscle (the fourth ganglion and the connecting ventral nerve cord were also included). Although there is considerable degeneration, the allotransplanted ganglia display intact areas of axon tracts, neuropil, and somata. Thus in both short (6–8 weeks) and long (24–30 weeks) term transplants approximately 20 healthy somata are present and this is more than the five axons regenerated to the host muscle. The principal neurite and dendrites of these somata receive both excitatory and inhibitory synaptic inputs, and these types of synaptic contacts also occur among the dendritic profiles of the neuropil. Axon tracts in the allotransplanted ganglia and ventral nerve cord consist largely of small diameter axons; most of the large axons including the medial and lateral giant axons are lost. The transplanted ganglia have many blood vessels and blood lacunae ensuring long-term survival. The transplanted superficial flexor nerve regenerates from the ventral to the dorsal surface of the muscle where it has five axons, each consisting of many profiles rather than a single profile. This indicates sprouting of the individual axons and accounts for the enlarged size of the regenerated nerve. The regenerated axons give rise to normal-looking synaptic terminals with well-defined synaptic contacts and presynaptic dense bars or active zones. Some of these synaptic terminals lie in close proximity to degenerating terminals, suggesting that they may inhabit old sites and in this way ensure target specificity. The presence of intact somata, neuropil, and axon tracts are factors that would contribute to the spontaneous firing of the transplanted motoneurons. © 1996 John Wiley & Sons, Inc.     

Developmental regulation of glial cell phagocytic function during Drosophila embryogenesis

The proper removal of superfluous neurons through apoptosis and subsequent phagocytosis is essential for normal development of the central nervous system (CNS). During Drosophila embryogenesis, a large number of apoptotic neurons are efficiently engulfed and degraded by phagocytic glia. Here we demonstrate that glial proficiency to phagocytose relies on expression of phagocytic receptors for apoptotic cells, SIMU and DRPR. Moreover, we reveal that the phagocytic ability of embryonic glia is established as part of a developmental program responsible for glial cell fate determination and is not triggered by apoptosis per se. Explicitly, we provide evidence for a critical role of the major regulators of glial identity, gcm and repo, in controlling glial phagocytic function through regulation of SIMU and DRPR specific expression. Taken together, our study uncovers molecular mechanisms essential for establishment of embryonic glia as primary phagocytes during CNS development.     

Tonotopic gradients of Eph family proteins in the chick nucleus laminaris during synaptogenesis

Topographically precise projections are established early in neural development. One such topographically organized network is the auditory brainstem. In the chick, the auditory nerve transmits auditory information from the cochlea to nucleus magnocellularis (NM). NM in turn innervates nucleus laminaris (NL) bilaterally. These projections preserve the tonotopy established at the level of the cochlea. We have begun to examine the expression of Eph family proteins during the formation of these connections. Optical density measurements were used to describe gradients of Eph proteins along the tonotopic axis of NL in the neuropil, the somata, and the NM axons innervating NL at embryonic day 10, when synaptic connections from NM to NL are established. At E10-11, NL dorsal neuropil expresses EphA4 at a higher concentration in regions encoding high frequency sounds, decreasing in concentration monotonically toward the low frequency (caudolateral) end. In the somata, both EphA4 and ephrin-B2 are concentrated at the high frequency end of the nucleus. These tonotopic gradients disappear between E13 and E15, and expression of these molecules is completely downregulated by hatching. The E10-11 patterns run counter to an apparent gradient in dendrite density, as indicated by microtubule associated protein 2 (MAP2) immunolabeling. Finally, ephrin-B2 is also expressed in a gradient in tissue ventral to the NL neuropil. Our findings thus suggest a possible conserved mechanism for establishing topographic projections in diverse sensory systems. These results of this study provide a basis for the functional examination of the role of Eph proteins in the formation of tonotopic maps in the brainstem.     

Embryonic brain tract formation in Drosophila melanogaster

 During embryogenesis in insects, the axonscaffold of the brain is built around the embryonic foregut which separates the anlagen of the brain hemispheres. Here, we investigate this process in Drosophila and show that the major longitudinal and horizontal tracts of the embryonic brain form superficially near the interface between the foregut and embryonic brain hemispheres. We identify three types of cellular structures which might be involved in tract formation. These are rows of glial cells at the medial brain margin, cellular bridges composed of neuronal somata and the epithelial surface of the foregut itself. The close proximity to the outgrowing axons suggests that the structures at the brain-foregut interface may play a role in the morphogenesis of embryonic brain tracts in Drosophila. Received: 11 November 1996 / Accepted: 3 January 1997     

Electron microscopy of stomatogastric ganglion in the lobster Homarus americanus

The stomatogastric ganglion and two of the associated afferent and efferent nerve trunks (stomatogastric and dorsal ventricular nerves) from Homarus americanus have been examined with light and electron microscopy after glutaraldehyde-osmium tetroxide fixation. The dorsally located neuron somata, rich in ribosomes and glycogen, are encased in multi-layered glial and fibrous sheaths. The synaptic neuropil regions occur scattered throughout the central and ventral part of the ganglion, interspersed amonglarger nerve fibres of extrinsic and intrinsic origin from which the neuropil is derived. Neural processes containing masses of small clear vesicles plus larger dense-core vesicles make apparent synaptic contacts at points of increased membrane density with smaller, non-vesicle-containing or sometimes other vesicle-containing nerve fibres.     

Organization of the antennal motor system in the sphinx moth Manduca sexta

The antennae of the sphinx moth Manduca sexta are multimodal sense organs, each comprising three segments: scape, pedicel, and flagellum. Each antenna is moved by two systems of muscles, one controlling the movement of the scape and consisting of five muscles situated in the head capsule (extrinsic muscles), and the other system located within the scape (intrinsic muscles) and consisting of four muscles that move the pedicel. At least seven motoneurons innervate the extrinsic muscles, and at least five motoneurons innervate the intrinsic muscles. The dendritic fields of the antennal motoneurons overlap one another extensively and are located in the neuropil of the antennal mechanosensory and motor center. The density of motoneuronal arborizations is greatest in the lateral part of this neuropil region and decreases more medially. None of the motoneurons exhibits a contralateral projection. The cell bodies of motoneurons innervating the extrinsic muscles are distributed throughout an arching band of neuronal somata dorsal and dorsolateral to the neuropil of the antennal mechanosensory and motor center, whereas the cell bodies of motoneurons innervating the intrinsic muscles reside mainly among the neuronal somata situated dorsolateral to that neuropil. Received: 30 March 1996 / Accepted: 23 June 1996     

Synaptic connections of dopamine-immunoreactive neurons in the antennal lobes of Periplaneta americana. Colocalization with GABA-like immunoreactivity

Summary Dopamine-like immunoreactivity was demonstrated histochemically in about ten local interneurons in the antennal lobe of Periplaneta americana. The somata of these neurons are within the ventrolateral group of cell bodies. Additional immunohistochemical tests revealed that the same neurons also have a GABA-like immunoreactivity.Immunohistochemical dopamine staining (preembedding) of preparations in which the antennal receptor fibers had been caused to degenerate showed that in the glomerular neuropil these antennal fibers form output synapses on dopamine-immunoreactive neurons. The latter form output synapses on unstained neuron profiles.     

Secondary retinae of a primitive jumping spider,Yaginumanis (Arachnida,Araneida, Salticidae)

Summary Retinae of the secondary eyes of a primitive salticid spider, Yaginumanis sexdentatus (Yaginuma 1967) are described at the ultrastructural level. The structures of the anterior lateral, posterior lateral and posterior median eyes are identical. Receptor somata lie in the retinal cups. Each receptor bearing twin rhabdomeres is ensheated by (i) much-divided processes of non-pigmented glial cells whose somata lie distally in the retinal cups; and (ii) four processes of pigmented glial cells whose somata lie basally, below the receptive segments. Pigment granules in the latter are concentrated in the basal retina, and are not present at the level of the rhabdoms. The present findings support the placement of Yaginumanis in a newly erected Subfamily Spartaeinae by Wanless (1984), because of the likelihood of homology in the fine structural organisation of the secondary retinae of this genus and of the genus Portia.