Dynamics of ultrastructural transformations of paired biological membranes in the nervous system

The paper describes ultrastructural mechanisms of formation of nanoscopic pores and large syncytial perforations of paired membranes located between neurons, between neuron and glia, in multiglial envelopes, as well as in the endoplasmic reticulum and Golgi apparatus. The mechanisms of pores and perforation formation are similar in all cases. Adjacent membranes fuse or form either continuous or punctate tight junctions. The pores and their extensions—perforations—are formed only on the basis of the tight junctions; the intermembrane cleft acquires a varicose, bead-like shape. The pore between the “beads” is formed as a result of transformation of the ellipsoid shape of the bead into the spherical one. Upon autoamputation of the varicose structures, they are transformed into residual bodies in the lumen of perforations.     

Glio-neuronal and glio-glial syncytial cytoplasmic connections in peripheral nerve trunks of the crayfish Astacus leptodactylus

The paper considers various aspects of glial sheaths of neuritis in the crayfish peripheral nerve trunks and roots. There are revealed dotted glio-neurite tight junctions and a varicose deformation of the intercellular glio-neurite cleft. Rupture of membranes in the area of contact leads to formation of the glio-neurite pore (less than 10 nm) that is enlarged and forms wide (up to 240 nm) syncytial perforations. At the edge of perforation, either remnants of tight junctions are present or damaged membranes that fuse and are rounding. The lumen of perforations always contains residual membranous bodies in the form of vesicles. Their deviation from the median line can indicate a mutual translocation of substances of the glio- and neuroplasm. In the adjacent layers of the multilayer glial sheath there is noted a similar phenomenon of formation of the glio-glial syncytial connection terminating by fusion of neighbor glial layers, which is terminated by fusion of neighbor glial layers into the single lamina. The process begins from the varicose deformation of interglial clefts, which appears as a result of massive formation of dotted and expanded tight membranous contacts. As a result of transformation of ellipsoid varicose deformations into the spherical ones, syncytial pores (less than 10 nm) between them are formed, which are enlarged and break the paired gliolemmas into fragments. As a result, the adjacent glial layers are united. Since this process in intact animals occurs on the background of undamaged nerve structures, a suggestion is put forward about its reversibility and the functional nature.     

Glio-neuronal and glio-glial syncytial cytoplasmic connections in peripheral nerve trunks of the crayfish <Emphasis Type="Italic">Astacus leptodactylus</Emphasis>

The paper considers various aspects of glial sheaths of neurites in the crayfish peripheral nerve trunks and roots. There are revealed dotted glio-neuritic tight junctions and a varicose deformation of the intercellular glio-neuritic cleft. Rupture of membranes in the area of contact leads to formation of the glio-neuritic pore (less than 10 nm) that is enlarged and forms wide (up to 240 nm) syncytial perforations. At the edge of perforation, either remnants of tight junctions are present or damaged membranes that fuse and become rounded. The lumen of perforations always contains residual membranous bodies in the form of vesicles. Their deviation from the median line can indicate a mutual translocation of substances of the glio- and neuroplasm. In the adjacent layers of the multilayer glial sheath there is noted a similar phenomenon of formation of the glio-glial syncytial connection terminating by fusion of neighbor glial layers, which is terminated by fusion of neighbor glial layers into the single lamina. The process begins from the varicose deformation of interglial clefts, which appears as a result of massive formation of dotted and expanded tight membranous contacts. As a result of transformation of ellipsoid varicose deformations into the spherical ones, syncytial pores (less than 10 nm) between them are formed, which are enlarged and break the paired gliolemmae into fragments. As a result, the adjacent glial layers are united. Since this process in intact animals occurs on the background of undamaged nerve structures, a suggestion is put forward about its reversibility and the functional nature.     

Syncytial perforations of human neuronal embryo membranes

An electron microscopy study of the anlage of cerebral cortex of human embryo has been carried with the aim of determining the presence of syncytial interneuronal connections in embryogenesis. It has been determined that, in part of the neurons, the glial embryo is absent and their external cell membranes are directly attached to each other by forming elongated or dotted tight junctions. Sometimes these junctions are perforated and, on their basis, the true syncytial interneuronal connections are formed. Natural structural properties of these connections are the following: formation of the base of tight membrane contacts, obligatory rounding of perforation edges, and the presence of residual particles in the form of spherical vesicles in the lumen of perforations. Results obtained allowed us to conclude that, in the anlage of cerebral cortex of embryos obtained during surgical abortion of pregnancy, apart from the formation of synaptic contacts, or until their formation, there is the possibility of syncytial interneuronal connections appearing. This should be considered during the transplantation of the developing brain.     

Syncytial perforations of human embryo membranes

An electron microscopy study of the anlage of cerebral cortex of human embryo has been carried with the aim of determining the presence of syncytial interneuronal connections in embryogenesis. It has been determined that, in part of the neurons, the glial embryo is absent and their external cell membranes are directly attached to each other by forming elongated or dotted tight junctions. Sometimes these junctions are perforated and, on their basis, the true syncytial interneuronal connections are formed. Natural structural properties of these connections are the following: formation of the base of tight membrane contacts, obligatory rounding of perforation edges, and the presence of residual particles in the form of spherical vesicles in the lumen of perforations. Results obtained allowed us to conclude that, in the anlage of cerebral cortex of embryos obtained during surgical abortion of pregnancy, apart from the formation of synaptic contacts, or until their formation, there is the possibility of syncytial interneuronal connections appearing. This should be considered during the transplantation of the developing brain.     

Cytoplasmic syncytial interneuronal connection in molluscs

The absolute criteria developed by the authors have been presented; they allow revealing cytoplasmic syncytial connections between processes of nerve cells in vivo and in vitro at the light microscopy level by using classical methods and time lapse videoshooting in the phase contrast. With aid of electron microscopy, metastable membrane contacts and their perforations, cytoplasmic syncytial interneuronal pores, and fusion of nerve processes are demonstrated. In the culture of isolated molluscan neurons, the process of formation of syncytial connections between processes of the same neuron or of different neurons is reproduced. Processes of one neuron, which have syncytial connection with another neuron, are shown to remain viable after death of its neuronal soma. The cytoplasmic varicosities formed on processes of one neuron are able to overcome the place of syncytial contact with processes of another neuron and to move to the body of the latter. A hypothesis is put forward that the cytoplasmic syncytial connection between nerve processes is formed under the conditions of the absence of their glial sheaths.     

Ultrastructural adaptations for viviparity in the female reproductive system of gyrodactylid monogeneans

The female reproductive system of viviparous monogeneans (Gyrodactylus and Macrogyrodactylus) has been examined using fluorescence microscopy and transmission electron microscopy. The female system is tubular, made up of a thin-walled proximal seminal receptacle/ootype and a distal uterus, separated by a complex cellular region. Both parts have a continuous syncytial cytoplasmic lining. Maturing oocytes in the seminal receptacle/ootype are in intimate contact with the receptacle lining. The uterus cytoplasmic lining completely surrounds the developing embryo, and is continuous with anterior and posterior cell bodies which fluoresce strongly when stained with bisBenzimide. This lining is most extensive around small embryos, when it contains specialised organelles including star-shaped configurations of electron-dense membranes and multilamellate bodies. Pits in the uterus wall bridged by membranous structures connect the cytoplasmic lining to parenchyma or digestive cells. The cytoplasmic lining regresses as the embryo develops, but remains continuous and in intimate contact with the embryonic tegument (at least until the near-term embryo begins independent movement). Numerous ribosomes, membranes and mitochondria in the uterine cytoplasmic layer indicate a high metabolic rate, and exo/endocytotic vesicles in the F1 tegument suggest transfer of materials occurs between parent and embryo. Putative vitelline cells in the posterior of the body contain abundant RNA, ribosomes and membrane-bound secretory bodies, and are filled with an electron-lucent secretion. However, there are no ducts associated with these cells, and their function remains unknown. The cytoplasmic lining of both the seminal receptacle/ootype and the uterus appears to regulate oocyte/embryo nutrition. Similar syncytial layers occur in rotifers, but are unlike the nutritive epithelia of most other viviparous organisms.     

Autotypical septal contacts of cerebellar glial cells as protective reaction under toxic action of glutamate and nitric oxide

Structural transformations of the frog cerebellar glial cells (astrocytes) in the presence of high concentrations of glutamate and nitric oxide (NO) were studied by electron microscopy method. It was found that under these conditions mimicking a natural stroke, glial processes storing sufficient amounts of glycogen grains could protect the neurons of the molecular layer. Under these conditions glial processes transformed (stretched out) into strands, virtually deprived of cytoplasm. The distance between two membranes of such strands dropped to 25–30 nm, and cross-bridges appeared inside. These structures are called “autotypic septal contact” (ASC). A modified glial process with ASC could wrap injured synapse or synaptic elements (buttons, dendritic spines) and form a tight capsule. Glial processes with ASC can also intrude between two neurons of the cerebellum granular layer, preventing their pathological fusion. The results suggest that ASC can have a protecting role in the toxic action of glutamate and NO-generating agents.     

呼吸道合胞病毒诱导细胞凋亡与自噬

呼吸道合胞病毒感染与细胞凋亡、自噬的关系错综复杂。研究发现呼吸道合胞病毒感染细胞后,既能产生促细胞凋亡作用,也能产生抗细胞凋亡作用,还能诱导细胞发生自噬。研究这些过程机理,能帮助我们更好地认识呼吸道合胞病毒感染发病机制,为预防和治疗呼吸道合胞病毒感染提供一些新的方向。     

Intimate relationship between interstitial cells of Cajal and enteric nerves in the guinea-pig small intestine

Recent studies have suggested that enteric inhibitory neurotransmission is mediated via interstitial cells of Cajal in some gastrointestinal tissues. This study describes the physical relationships between enteric neurons and interstitial cells of Cajal in the deep muscular plexus (IC-DMP) of the guinea-pig small intestine. c-Kit and vimentin were colocalized in the cell bodies and fine cellular processes of interstitial cells of the deep muscular plexus. Anti-vimentin antibodies were subsequently used to examine the relationships of interstitial cells with inhibitory motor neurons (as identified by nitric oxide synthase-like immunoreactivity) and excitatory motor neurons (using substance P-like immunoreactivity). Neurons with nitric oxide synthase- and substance P-like immunoreactivities were closely associated with the cell bodies of interstitial cells and ramified along their processes for distances greater than 300 7m. With transmission electron microscopy, we noted close relationships between interstitial cells and the nitric oxide synthase- and substance P-like immunoreactive axonal varicosities. Varicosities of nitric oxide synthase and substance P neurons were found as close as 20 and 25 nm from interstitial cells, respectively. Specialized junctions with increased electron density of pre- and postsynaptic membranes were observed at close contact points between nitric oxide synthase- and substance P-like immunoreactive neurons and interstitial cells. Close structural relationships (approximately 25 nm) were also occasionally observed between either nitric oxide synthase- and substance P-like immunoreactive varicosities and smooth muscle cells of the outer circular muscle layer. The data suggest that interstitial cells in the deep muscle plexus are heavily innervated by excitatory and inhibitory enteric motor neurons. Thus, these interstitial cells may provide an important, but probably not exclusive, pathway for nerve-muscle communication in the small intestine.     

Neuropeptides in insect mushroom bodies

Owing to their experimental amenability, insect nervous systems continue to be in the foreground of investigations into information processing in – ostensibly – simple neuronal networks. Among the cerebral neuropil regions that hold a particular fascination for neurobiologists are the paired mushroom bodies, which, despite their function in other behavioral contexts, are most renowned for their role in learning and memory. The quest to understand the processes that underlie these capacities has been furthered by research focusing on unraveling neuroanatomical connections of the mushroom bodies and identifying key players that characterize the molecular machinery of mushroom body neurons. However, on a cellular level, communication between intrinsic and extrinsic mushroom body neurons still remains elusive. The present account aims to provide an overview on the repertoire of neuropeptides expressed in and utilized by mushroom body neurons. Existing data for a number of insect representatives is compiled and some open gaps in the record are filled by presenting additional original data.     

Some peculiarities of the ultrafine structure of the neurons of the cestode Pelichnibothrium speciosum (Monticelli, 1889) (Cestoda: Tetraphyllidea)]

In the scolex ganglia of the cestode Pelichnibothrium speciosum uni - and multipolar neurons can be found. Their neuroplasm is rich in free ribosomes. The ergastoplasmas membranes are frequently arranged in compact finger print-like structures. The nerve cells processes form tight junctions which cannot be interpreted as synaptic contacts.     

An Ultrastructural Study of Cell-to-Cell Membrane Interaction at Early Stages of Postnatal Ontogenesis of Cortical Brain Neurons in White Rats

In early postnatal ontogenesis of cerebral cortex (visual area) of the white rat, a wide distribution of different types of membrane contacts have been found between developing nervous cells and their processes. The following types of contacts were observed: 1. Penetration of thin filopodia into specialized invaginations having all the features of coated vesicles; 2. Contacts of filopodia with thickened surface membrane; 3. Contacts of opposit filopodia; 4. Contacts of membranes with reciprocal invaginations alternating with filopodia or surface blebs.
These types of membrane interaction were regularly distributed along the surface of cells and their processes, and were situated in close approximation to typical tight junctions and other adhesional complexes. As a rule, filopodia were components of axon branches, and almost all invaginations were situated on plasma membranes of cell bodies or on dendrites, although sometimes there were invaginations on axon profiles and filopodia on dendrites.
It is suggested that the distribution and structural specialization of these membrane contacts reflect their participation in the process of programmed cell-to-cell recognition that precedes the formation of synaptic contact. Other reports and the current data reveal the special morphogenetic role of membrane communication in the formation and stability of integrative cell systems.     

Intercellular junctions between human odontoblasts. A freeze-fracture study after demineralization

Intercellular junctions in the odontoblastic layer have been studied with a freeze-fracture technique. Children's tooth germs were fixed, sliced and demineralized. Samples of the pulpodentinal border were routinely prepared for freeze-fracture. Three kinds of intercellular junctions were detected between human odontoblast cell bodies: gap junctions, desmosomes and tight junctions. Numerous gap junctions are responsible for intercellular communication at different levels of the cell bodies. Focal tight junctions, parallel to the axis of the cell, and desmosomes are sites of cell-to-cell adhesion between lateral plasma membranes. At the distal end of the cell bodies, junctional complexes consist of zonular tight junctions and gap junctions. These zonular tight junctions, never before described between odontoblasts, contribute to the pseudo-epithelial organization of the odontoblastic layer. They constitute a predentin-pulp barrier, the permeability of which must be studied to establish their role in relation to dentin formation.     

Development of catecholaminergic neurons in the human medulla oblongata

Distribution and maturation of catecholaminergic (CA) neurons have been studied by tyrosine hydroxylase immunohistochemistry in the medulla oblongata of human fetuses aged 14.5-25 weeks of gestation. Already at 14.5 weeks, CA neurons were observed in two longitudinally oriented cell clusters, one located ventrolaterally in the area of the lateral reticular and ambiguous nuclei, the other one dorsomedially forming 4 groups related to the dorsal vagal nucleus, the commissural nucleus of the vagus, the nucleus of the tractus solitarius and the area postrema. CA neurons in the area postrema were often found close to blood vessels. Scattered intermediate CA neurons were seen between these two larger clusters. CA neurons still appeared immature exhibiting bipolar morphology with only one or two short stout processes, which hardly branched. At 21 weeks, CA neurons occupied essentially the same location, but had a more mature morphology. Though still bipolar in shape, they had thinner and much longer processes which frequently branched. Both in the ventrolateral and the dorsomedial cell clusters, these processes were frequently lying close to blood vessels. At 25 weeks, CA cells had matured into multipolar neurons with long thin processes forming fine fiber networks in the ventrolateral medulla as well as around and within the dorsal vagal and solitarius nuclei. Only at this stage, a distinct CA fiber tract was seen located in the region of the tractus solitarius. Our results indicate that CA neurons in the human medulla, which are presumably involved in the control of ventilation and blood pressure, though generated rather early during development, mature relatively late.     

Identification of two cardioacceleratory neurons in the isopod crustacean, Ligia exotica and their effects on cardiac ganglion cells

We identified two pairs of cardioacceleratory (CA1, CA2) neurons in the central nervous system of the isopod Ligiaexotica and examined their effects on the cardiac ganglion (CG). CA1 neurons had cell bodies in the 2nd thoracic ganglion and had arborizations in the subesophageal ganglion and the 1st and 2nd thoracic ganglia. CA2 neurons had cell bodies in the 3rd thoracic ganglion and had arborizations in the 2nd, 3rd and 4th thoracic ganglia. They sent axons to the heart through the ipsilateral 3rd roots of the ganglia where their cell bodies were located. Repetitive stimulation of the CA1 axon rapidly increased the burst frequency of the CG, and that of CA2 rather slowly. The increased burst rate caused by the CA1 stimulation was significantly higher than that caused by CA2. Overall depolarization of a quiescent CG cell produced by the CA1 stimulation was significantly larger in amplitude than that produced by CA2. Facilitation was obviously seen in the excitatory post-synaptic potentials evoked by the CA1 stimulation. These results show that the synaptic properties of CA1 and CA2 neurons are different, suggesting that they have different functional roles in heart regulation. Accepted: 19 July 1997     

The fine structure of the nerve cord of Myxicola infundibulum (annelida,polychaeta)

Summary Ultrastructural observations of the giant axon of Myxicola infundibulum reveal that the axoplasm contains neurofilaments, a few neurotubules and mitochondria. Finger-like projections issuing from the glial cells of the sheath encircle the giant axon at various angles. The space between the axolemma and sheath is 125 Å. Branches of the giant axon are also surrounded by a glial sheath as they course through the neuropil. Some branches of the giant axon seem to fuse with certain neurons, creating a syncytial arrangement between the giant axon and these neurons.Many small nerve fibers course longitudinally in the neuropil of the nerve cord. Most of these axons are separated from each other by a space of 200 Å without intervening glial processes. Synapses in the neuropil have both clear 600 Å vesicles and larger dense core vesicles suggesting chemical transmission. Some, but not all, of the synaptic areas show thickened membranes and dense material in the synaptic cleft.This study was supported in part by PHS NS-07740 to R.L.P., J.A.B. is a NDEA Predoctoral Fellow in the Department of Physiology.     

Thylakoid membrane perforations and connectivity enable intracellular traffic in cyanobacteria

Cyanobacteria, the progenitors of plant and algal chloroplasts, enabled aerobic life on earth by introducing oxygenic photosynthesis. In most cyanobacteria, the photosynthetic membranes are arranged in multiple, seemingly disconnected, concentric shells. In such an arrangement, it is unclear how intracellular trafficking proceeds and how different layers of the photosynthetic membranes communicate with each other to maintain photosynthetic homeostasis. Using electron microscope tomography, we show that the photosynthetic membranes of two distantly related cyanobacterial species contain multiple perforations. These perforations, which are filled with particles of different sizes including ribosomes, glycogen granules and lipid bodies, allow for traffic throughout the cell. In addition, different layers of the photosynthetic membranes are joined together by internal bridges formed by branching and fusion of the membranes. The result is a highly connected network, similar to that of higher-plant chloroplasts, allowing water-soluble and lipid-soluble molecules to diffuse through the entire membrane network. Notably, we observed intracellular membrane-bounded vesicles, which were frequently fused to the photosynthetic membranes and may play a role in transport to these membranes.     

SUBSURFACE CISTERNS AND THEIR RELATIONSHIP TO THE NEURONAL PLASMA MEMBRANE

Subsurface cisterns (SSC's) are large, flattened, membrane-limited vesicles which are very closely apposed to the inner aspect of the plasma membranes of nerve cell bodies and the proximal parts of their processes. They occur in a variety of vertebrate and invertebrate neurons of both the peripheral and central nervous systems, but not in the surrounding supporting cells. SSC's are sheet-like in configuration, having a luminal depth which may be less than 100 A and a breadth which may be as much as several microns. They are separated from the plasmalemma by a light zone of ~50 to 80 A which sometimes contains a faint intermediate line. Flattened, agranular cisterns resembling SSC's, but structurally distinct from both typical granular endoplasmic reticulum (ER) and from Golgi membranes, also occur deep in the cytoplasm of neurons. It is suggested that membranes which are closely apposed may interact, resulting in alterations in their respective properties. The patches of neuronal plasmalemma associated with subsurface cisterns may, therefore, have special properties because of this association, resulting in a non-uniform neuronal surface. The possible significance of SSC's in relation to neuronal electrophysiology and metabolism is discussed.