Ultrastructure of different neuropil zones of the procerebrum in snails and slugs

Ultrastructure and peculiarities of interneuronal connections in various zones of neuropil of procerebral olfactory centers of the brain in snails and slugs: in the outer and inner neuropil, zone of input of afferent fibers of labial nerves, as well as zones of running of afferent and efferent fibers of tentacular nerves, were studied. A pronounced spatial morpho-functional differentiation and a complex zonal synaptoarchitectonics of procerebrums is revealed. It has been shown that the procerebrum neural elements, both intrinsic and numerous ones coming from other brain regions and chemosensory systems, contain an enormous variety of vesicles. These vesicles provide connections between neural elements in various synapses and synapse-like junctions and in the composite divergent and convergent complexes formed by them. A positive polychemical nature of granular cells, the main neural elements of procerebrums, and functional significance of symmetric junctions predominant in procerebrums is discussed.     

Cerebral parts of chemosensory systems of the snail: Structural bases of intersensory integration

Presented are data on distribution of afferent fibers from tentacular and labial nerves innervating chemosensory tentacular organs, lips, and mouth area in the cerebral ganglia of terrestrial pulmonary snails are in the article. By using Golgi impregnation and infusion of cobalt chloride and nickel, it has been shown that most terminal branches of afferent fibers of all chemosensory organs are located in several symmetric areas of both metacerebrums. A part of fibers both of tentacular and of labial nerves form peculiar tracts and terminate in the interior neuropil of procerebrums. In metacerebrums, various neurons providing connections with afferent fibers of chemosensory organs are revealed. Many of them also innervate interior neuropil and procerebrum cell bodies area. The data obtained allow considering procerebrums as higher integrative centers of chemosensory information.     

Comparative study of nerve elements and their relationships with Endocrine glands and muscle retractors in Ommatophores of snails and slugs

A comparative study of the total cytoarchitectonics and structural interrelations of receptor and nerve elements at the area of receptor cell bodies, in tentacular ganglia and their digital outgrowths as well as in the lateral wall of tentacular olfactory organs of several species of terrestrial snails and slugs from different families was carried out using Golgi and Colonnier silver nitrate impregnation technique and horseradish peroxidase labeling. The presence of a peculiar cluster of intraepithelial receptor cells has been established for the first time in sensory epithelium of ommatophores. A connection of endocrine cells of the optic gland with nerve and receptor elements of tentacular ganglia was shown. Peculiarities of innervation of muscle refractors and lateral wall of ommatophores were revealed. FMRFamide-positive nerve fibers were found to participate in innervation of muscle elements of ommatophores. Neurotensin-positive and occasional serotonin-positive nerve elements also were revealed in the tentacular organs. Electron microscopic study showed the presence of complicated synaptic complexes of either convergent or divergent type and of many symmetric synaptic structures in neuropil of tentacular ganglia. Many nervous fibers and synapses contain several types of granular and agranular vesicles, which indicates possible co-localization of several bioactive substances in the same nerve elements.Translated from Zhurnal Evolyutsionnoi Biokhimii i Fiziologii, Vol. 40, No. 6, 2004, pp. 556–568.     

Structural organization of procerebrums of terrestrial molluscs: Characteristics of neuronal pattern, plasticity, and age peculiarities

In terrestrial snails and slugs, the presence of five types of neurons has been shown: typical granular, horizontal, stellate, apical, and basal cells. A peculiarity of procerebrum neurons is a loop-like segment of the basal process. Granular cells have been established to spread both as cell columns and as single cells. The number of the columns and the number of cells in each column depend on the age and individual peculiarities of the animal. In the course of ontogenesis, there also occur changes of the number of processes-outgrowths of granular cell bodies as well as of the character of thickenings and the type of endings on their main processes. Basal neurons and single granular cells of the columns form several independent tracts running from the procerebrum cell body areas to metacerebrums. The single granular cells, horizontal cells, and a part of basal neurons are considered as associative elements providing formation in procerebrums of the single system that is a basis for the synchronized activity of the whole neuronal complex. A similarity of organization of procerebrums and of associative-integrative centers of higher animals is discussed.     

Regulating synchronous oscillations of cerebellar granule cells by different types of inhibition

Synchronous oscillations in neural populations are considered being controlled by inhibitory neurons. In the granular layer of the cerebellum, two major types of cells are excitatory granular cells (GCs) and inhibitory Golgi cells (GoCs). GC spatiotemporal dynamics, as the output of the granular layer, is highly regulated by GoCs. However, there are various types of inhibition implemented by GoCs. With inputs from mossy fibers, GCs and GoCs are reciprocally connected to exhibit different network motifs of synaptic connections. From the view of GCs, feedforward inhibition is expressed as the direct input from GoCs excited by mossy fibers, whereas feedback inhibition is from GoCs via GCs themselves. In addition, there are abundant gap junctions between GoCs showing another form of inhibition. It remains unclear how these diverse copies of inhibition regulate neural population oscillation changes. Leveraging a computational model of the granular layer network, we addressed this question to examine the emergence and modulation of network oscillation using different types of inhibition. We show that at the network level, feedback inhibition is crucial to generate neural oscillation. When short-term plasticity was equipped on GoC-GC synapses, oscillations were largely diminished. Robust oscillations can only appear with additional gap junctions. Moreover, there was a substantial level of cross-frequency coupling in oscillation dynamics. Such a coupling was adjusted and strengthened by GoCs through feedback inhibition. Taken together, our results suggest that the cooperation of distinct types of GoC inhibition plays an essential role in regulating synchronous oscillations of the GC population. With GCs as the sole output of the granular network, their oscillation dynamics could potentially enhance the computational capability of downstream neurons.     

Synaptic Contacts of Neurons of Fascia Dentata Transplants with Nonspecific Targets in Neocortex of Recipients

We carried out an electron microscopy study of possible synaptic contacts of the neurons of intracortical transplants of the rat brain fascia dentata with targets in the recipient somatosensory cortex. The axons of fascia dentata granular cell and their synaptic terminals could be easily identified in the neocortex due to their distinct morphological features (mossy fibers), although the fascia dentate cells normally do not interact with the neocortex. Thin nonmyelenized mossy fibers were found in both an intermediate zone between the transplant and brain and in the adjacent brain. Their presynaptic buds, like in situ, had large size and formed characteristic terminal, intraterminal, and en passant multiple synaptic contacts and desmosome-like junctions. The aberrant nerve fibers used perykaryons, dendrites of varying diameter, and dendrite spikes of the somatosensory cortex pyramidal neurons as postsynaptic targets in the neocortex. In addition to vacant spaces that appeared in the brain as a result of transplantation, the ingrowing axons induced the formation of additional contact sites: deep invaginations of the plasmalemma of perykaryons, somatic spikes, terminal branchings of dendrites, and dendritic outgrowths of complex branched shape. These aberrant contacts were characterized by the presence of polyribosomes, endoplasmic reticulum cisternae, and mitochondria in the postsynaptic loci. Osmiophility and extension of desmosome-like junctions were also enhanced in such synapses. Thus, it was shown that mossy fibers ingrowing in the recipient neocortex were capable of forming cell-to-cell contacts with signs of functional synapses to atypical cell targets.     

Synaptic contacts of neurons of fascia dentata transplants with nonspecific targets in neocortex of recipients

We carried out an electron microscopy study of possible synaptic contacts of the neurons of intracortical transplants of the rat brain fascia dentata with targets in the recipient somatosensory cortex. The axons of fascia dentata granular cell and their synaptic terminals could be easily identified in the neocortex due to their distinct morphological features (mossy fibers), although the fascia dentate cells normally do not interact with the neocortex. Thin nonmyelenized mossy fibers were found in both an intermediate zone between the transplant and brain and in the adjacent brain. Their presynaptic buds, like in situ, had large size and formed characteristic terminal, intraterminal, and en passant multiple synaptic contacts and desmosome-like junctions. The aberrant nerve fibers used perykaryons, dendrites of varying diameter, and dendrite spikes of the somatosensory cortex pyramidal neurons as postsynaptic targets in the neocortex. In addition to vacant spaces that appeared in the brain as a result of transplantation, the ingrowing axons induced the formation of additional contact sites: deep invaginations of the plasmalemma of perykaryons, somatic spikes, terminal branchings of dendrites, and dendritic outgrowths of complex branched shape. These aberrant contacts were characterized by the presence of polyribosomes, endoplasmic reticulum cisternae, and mitochondria in the postsynaptic loci. Osmiophility and extension of desmosome-like junctions were also enhanced in such synapses. Thus, it was shown that mossy fibers ingrowing in the recipient neocortex were capable of forming cell-to-cell contacts with signs of functional synapses to atypical cell targets.     

Ultrastructure of celiac plexus nodes in the dog

A normal structure of the celiac plexus nodes has been studied in 12 mature dogs. As demonstrate the results of the investigation, gangliocytes of the celiac plexus are characterized with a well developed granular cytoplasmic reticulum and a large number of Golgi complexes. In perikaryon of the gangliocytes, an essential number of mitochondria, microtubules, free ribosomes and polysomes, lysosomes, multivesicular bodies, agranular and granular vesicles and neurofilaments are found. The gangliocyte has one nucleus which occupies about 1/3 of the whole area of the cell. The nucleus is rich in chromatin. The nucleolus makes about 1/5 of the whole area of the nucleus and is intensively rich in heterochromatin. In the celiac plexus nodes amyelinated neural fibers predominate. Dendrites in the gangliocytes differ from axons by a higher electron density of their matrix and contain the same organells that does the perikaryon of the gangliocyte. Rather complex glyoneuronal interrelations are observed in the canine celiac plexus nodes. Synapses are revealed in all ganglionar zones. The axodendritic synaptic contacts predominate over the axosomatic ones.     

Inductive role of mossy fibers of hippocampus in the development of dendritic spines in aberrant synaptogenesis at neurotransplantation

The dentate fascia of the hippocampal formation isolated from 20-day-old Wistar rat fetuses was subjected to heterotopic transplantation into the somatosensory area of the neocortex of adult rats of the same strain. Five months after surgery, neurotransplantates, together with neighboring area of the neocortex, were studied using light and electron microscopy. We carried out a detailed study of the ultrastructure of the ectopic synaptic endings formed by the axons of granular neurons of the dentate fascia (mossy fibers) with neurons of the neocortex unusual for them in a normal state. Ultrastructural analysis revealed that most ectopic synaptic endings produce its determinant morphological features: giant sizes of presynaptic knobs, active zones with branched dendritic spines, and adherens junctions with the surface of dendrites. The data indicate that the mossy fibers growing from neurotransplantates induce structural and chemical reorganization of dendrites of the neocortex using transmembrane adherens junctions, such as puncta adherentia junctions. This results in the differentiation of active zones and development of dendritic spines typical for giant synaptic endings that are invaginated into presynaptic endings. Thus, the ability of neurons of the dentate fascia to form aberrant synaptic connections at transplantation results from the inductive synaptogenic properties of mossy fibers.     

Transient structures of the human fetal brain: subplate, thalamic reticular complex, ganglionic eminence

Morphological features of the subplate, the thalamic reticular complex and the ganglionic eminence, which represent three major transient structures of the human fetal forebrain, are summarized with special reference to their functional roles. The subplate harboring various neuronal types is an outstandingly wide zone subjacent to the cortical plate in the human fetal brain. Within the subplate various cortical afferents establish synaptic contacts for a prolonged period before entering the cortical plate. Therefore, the subplate is regarded as a "waiting compartment" which is required for the formation of mature cortical connections. Next to the thalamic reticular nucleus, within the fibers of internal capsule, the perireticular nucleus is located which has been established as a distinct entity during development. Its various neuronal types express a number of different neuroactive substances. Perinatally, the perireticular nucleus is drastically reduced in size. It is involved in the guidance of corticofugal and thalamocortical fibers. The ganglionic eminence is a conspicuous proliferative area that persists throughout nearly the entire fetal period. In the human fetal brain it extends medially upon the dorsal thalamic nuclei which receive precursor cells from the ganglionic eminence. Postmitotic cells in the marginal zone of the ganglionic eminence serve as an intermediate target for growing axons. On the whole, all three structures establish transient neural circuitries that may be essential for the formation of adult projections. The characteristics of the three transient structures are particularly relevant for developmental neuropathology as these structures may be damaged in disorders that preferentially occur in preterm infants.     

Possible mechanisms of synapse formation during ontogenesis

Electron microscopic investigation of synaptogenesis in the sensomotor cortex and in the caudate nucleus has been performed in the prenatal ontogenesis (16-22 days) and in newborn rats. The first immature synapses are demonstrated to appear beginning on the 16th day of embryogenesis. At the end of the prenatal development and especially in newborn animals desmosome-like, asymmetric and symmetric, mixed and complex forms of the synaptic contacts are revealed. As a result of the analysis performed on the ultrastructural organization of the contacts, a hypothesis explaining mechanisms of development of various elements of the synapses has been suggested. A part of the synaptic contacts of the asymmetric and symmetric types is supposed to be genetically programmed and membrane specialization of these contacts is formed earlier than synaptic vesicles appear. Other part of the synapses undergoes certain stages of differentiation before the functionally mature contact is formed. The initial stage in the synapses formation in formation of the desmosome-like junction. The second stage is appearance of synaptic vesicles in the area of this contact. The third stage includes development of pre- and postsynaptic membranous specialization and owing to this the contact acquires either asymmetric or symmetric appearance. For the ontogenetic periods investigated establishment of complex forms of the intercellular junctions (tangent, reciprocal, etc.) is specific; this evidently demonstrates certain plastic rearrangements in the synapses during the process of development.     

Distribution and the fine structure of serotonin-containing fibers in the rat small intestine

Serotonin immunoreactive fibers were observed under the electron microscopy in all layers of the small intestine, with greatest abundance in the mucosa. Submucosal blood vessels were often surrounded by immuno positive nerves. In the inner circular muscle layer the immunoreactive serotonin positive fibers were closely associated with the smooth muscle cells. In the ganglia of the myenteric and submucous plexuses, labelled fibers surrounded the immunonegative neural cell bodies, but rarely formed conventional synaptic junctions. It is concluded that the serotoninergic system of the small intestine may influence the activity of associated structures in a diffuse non-synaptic manner.     

The catenin, p120ctn, is a common membrane-associated protein in various epithelial and non-epithelial cells and tissues

The cadherin-binding catenin p120ctn was originally identified as an Src-tyrosine kinase substrate. More recently, p120ctn has been shown in some cell types to be associated with catenin/cadherin complexes of adherens junctions. To address the question whether p120ctn is restricted to certain cell types or whether it is a general cellular component we investigated tissue distribution of p120ctn by immunohistochemistry and immunoblotting in the rat. We found p120ctn to be widely distributed in several tissues where it is mainly restricted to the plasma membrane. In various epithelia p120ctn was found in association with different adherens junctions such as the zonula adherens and puncta adherentia. In addition, p120ctn was localized along infoldings of the basal cell membrane, most prominently in renal proximal and distal tubules. pl20ctn was not restricted to epithelia. It was also found at intercalated discs of cardiomyocytes. In the nervous system, immunostaining was particularly prominent in areas rich in synapses suggesting that pl20ctn is a component of synaptic adherens junctions as well. By immunoblotting, four different isoforms of pl20ctn could be detected displaying similar electrophoretic mobilities as the isoforms 1A, 1B, 2A, and 2B reported from mice. Whereas all epithelia assayed contained at least two isoforms, testis, heart, brain, and retina contained a single 110-kDa band that corresponds to isoform 1B in mice.     

Synaptic patterns in the dorsal lateral geniculate nucleus of the monkey

Summary Synaptic junctions are found in all parts of the nucleus, being almost as densely distributed between cell laminae as within these laminae.In addition to the six classical cell laminae, two thin intercalated laminae have been found which lie on each side of lamina 1. These laminae contain small neurons embedded in a zone of small neural processes and many axo-axonal synapses occur there.Three types of axon form synapses in all cell laminae and have been called RLP, RSD and F axons. RLP axons have large terminals which contain loosely packed round synaptic vesicles, RSD axons have small terminals which contain closely packed round vesicles and F axons have terminals intermediate in size containing many flattened vesicles.RLP axons are identified as retinogeniculate fibers. Their terminals are confined to the cell laminae, where they form filamentous contacts upon large dendrites and asymmetrical regular synaptic contacts (with a thin postsynaptic opacity) upon large dendrites and F axons. RSD axons terminate within the cellular laminae and also between them. They form asymmetrical regular synaptic contacts on small dendrites and on F axons. F axons, which also occur throughout the nucleus, form symmetrical regular contacts upon all portions of the geniculate neurons and with other F axons. At axo-axonal junctions the F axon is always postsynaptic.Supported by Grant R 01 NB 06662 from the USPHS and by funds of the Neurological Sciences Group of the Medical Research Council of Canada. Most of the observations were made while R. W. Guillery was a visiting professor in the Department of Physiology at the University of Montreal. We thank the Department of Physiology for their support and Mr. K. Watkins, Mrs. E. Langer and Mrs. B. Yelk for their skillful technical assistance.     

The biochemical anatomy of cortical inhibitory synapses

Classical electron microscopic studies of the mammalian brain revealed two major classes of synapses, distinguished by the presence of a large postsynaptic density (PSD) exclusively at type 1, excitatory synapses. Biochemical studies of the PSD have established the paradigm of the synapse as a complex signal-processing machine that controls synaptic plasticity. We report here the results of a proteomic analysis of type 2, inhibitory synaptic complexes isolated by affinity purification from the cerebral cortex. We show that these synaptic complexes contain a variety of neurotransmitter receptors, neural cell-scaffolding and adhesion molecules, but that they are entirely lacking in cell signaling proteins. This fundamental distinction between the functions of type 1 and type 2 synapses in the nervous system has far reaching implications for models of synaptic plasticity, rapid adaptations in neural circuits, and homeostatic mechanisms controlling the balance of excitation and inhibition in the mature brain.     

The catenin/cadherin adhesion system is localized in synaptic junctions bordering transmitter release zones

Molecular mechanisms linking pre- and postsynaptic membranes at the interneuronal synapses are little known. We tested the cadherin adhesion system for its localization in synapses of mouse and chick brains. We found that two classes of cadherin-associated proteins, alpha N- and beta-catenin, are broadly distributed in adult brains, colocalizing with a synaptic marker, synaptophysin. At the ultrastructural level, these proteins were localized in synaptic junctions of various types, forming a symmetrical adhesion structure. These structures sharply bordered the transmitter release sites associated with synaptic vesicles, although their segregation was less clear in certain types of synapses. N-cadherin was also localized at a similar site of synaptic junctions but in restricted brain nuclei. In developing synapses, the catenin-bearing contacts dominated their junctional structures. These findings demonstrate that interneuronal synaptic junctions comprise two subdomains, transmitter release zone and catenin-based adherens junction. The catenins localized in these junctions are likely associated with certain cadherin molecules including N-cadherin, and the cadherin/ catenin complex may play a critical role in the formation or maintenance of synaptic junctions.     

Ultrastructural Characteristics of Interneuronal Connections of the Central Nervous System of Bivalve Molluscs

Ultrastructure and peculiarities of interneuronal contacts are studied in visceral ganglia of two species of bivalve molluscs, Anadara broughtoni and Mactra sulcatoria. Gap, desmosome-like, symmetrical, and classic synaptic junctions between neuronal bodies and their main processes are described. The major part of interneuronal junctions in the ganglia of the molluscs studied are symmetrical. Complex synaptic complexes formed by specialized and non-specialized junctions are observed.     

Fine structure of the carp torus longitudinalis

Fine structure of the carp torus longitudinalis was studied by electron and light microscopy and compared with the granular layer of the carp cerebellum. There are three types of cells in the torus longitudinalis, that is, small, medium-sized and large cells. From the small cells to the medium-sized cells, there is gradual transition in size and in the amounts of cell organelles. These cell profiles and the closely packed cell pattern in a part of the torus longitudinalis are quite similar to those in the granular layer of the cerebellum. Moreover, peculiar synaptic configurations of so-called “synaptic glomeruli” are found both in the torus longitudinalis and in the granular layer of the cerebellum. In the torus longitudinalis, unmyelinated nerve fibers are seen to have bulbous swellings along their course, most of which contain a mitochondrion, as do they in the carp tectum opticum. It is speculated that the torus longitudinalis may be partly related to the granular layer of the cerebellum and partly to the tectum opticum.     

Electrical synapses in mammalian CNS: Past eras,present focus and future directions

Gap junctions provide the basis for electrical synapses between neurons. Early studies in well-defined circuits in lower vertebrates laid the foundation for understanding various properties conferred by electrical synaptic transmission. Knowledge surrounding electrical synapses in mammalian systems unfolded first with evidence indicating the presence of gap junctions between neurons in various brain regions, but with little appreciation of their functional roles. Beginning at about the turn of this century, new approaches were applied to scrutinize electrical synapses, revealing the prevalence of neuronal gap junctions, the connexin protein composition of many of those junctions, and the myriad diverse neural systems in which they occur in the mammalian CNS. Subsequent progress indicated that electrical synapses constitute key elements in synaptic circuitry, govern the collective activity of ensembles of electrically coupled neurons, and in part orchestrate the synchronized neuronal network activity and rhythmic oscillations that underlie fundamental integrative processes. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.     

Intercellular junctions in the corpora cardiaca of locusts

Summary The intercellular junctions in the corpora cardiaca of the locusts Schistocerca gregaria and Locusta migratoria were investigated by transmission electron microscopy. In the glandular lobes, complexes consisting of scalariform junctions and associated mitochondria, comparable to those previously observed in ion transporting epithelia, are formed between gland cells, and more rarely between gland cells and the neurons innervating them. Their structure and abundance are apparently unaffected by the stage of development or by the various experimental conditions employed. In the neural lobe, scalariform junctions form between glial cells and show close association with the endoplasmic reticulum. Gap junctions are present among glandular, neural and glial elements, and are formed between cells of the same type and of different types. Contacts resembling punctate tight junctions are widely distributed in the gland, but would be unlikely to form a barrier to diffusion. Septate junctions are formed exclusively between glial cells.