Synaptoarchitectonics of the ganglia of the celiac plexus in white rats

In 50 intact white rats at the age of 6, 15, 23 and 30 months synapsoarchitectonics of the celiac plexus nodes was studied by an electron microscopy method. Peculiarities in synapsoarchitectonics are stipulated by pericaryon processes in neurons, some of them have no contacts with the axonal terminals, while others have contacts with the axonal terminals. The former include small (about 0.5 mkm) drop-like and large (up to 1.5 mkm) polymorphous processes within the limits of perisomatic membrane, as well as processes penetrating the neuronal capsule. All of them contain, in different combinations, vesicles, ribosomas, fibrillae, and the largest processes--small cisterns of granular cytoplasmatic network and single mitochondria. The processes of the first group are considered as original stages for the development of the second group processes. The latter are represented by different in size (about 1.0--2.0 mkm) in form (digital, cone-, pin-, goblet-shaped, cylindrical, branching) and in content formations. There is, as a rule, one contact on the processes of an uncomplicated form, while on the branching processes there can be up to three and more contacting axonal terminals. Peculiar features in the composition of the processes taken as a whole (specific forms, absence of dendritic tubes, sometimes numerous contacts with axonal terminals in spite of small size) distinguish them from newly forming dendritic processes and these formations are considered as independent specialized receptor apparatus in the pericaryon of neurons of the celiac plexus.     

Axonal patterns of disc-shaped cells in the central nucleus of the cat inferior colliculus

The axonal patterns of disc-shaped cells (Dsc) and their distribution within the central nucleus (Cn) of the inferior colliculus was studied in young cats with the rapid Golgi method. Dsc were subdivided in three main cell varieties according to their axonal branching pattern. The first type possesses local axonal collaterals inside the lamina of origin but also gives collaterals and probably terminals to adjacent lamina. The second variety is characterized by a dense axonal plexus with a restricted zone of arborization. The third axonal pattern is distributed in a radiate fashion. These results demonstrate that Dsc contribute to the intrinsic axonal system of the Cn to a larger degree than previously supposed. Axon terminals of Dsc probably establish axo-dendritic contacts with medium sized Stc which probably establish contacts with neighbouring Dsc. This suggests that reciprocal connections between Dsc and Stc could exist in the Cn.     

Interneurons of the motor region of the neocortex]

Interneurons of motor area in the brain cortex have been studied in cats and monkeys. The greatest attention has been paid to pyramidal interneurons, among which six cell types have been described according to their axonal composition. Unlike stellate interneurons, all types of pyramidal interneurons possess less developed axonal collaterals. Interneuronal contacts are situated on dendrites or cell bodies of middle and large long-axonal pyramids. Functional role of cortical interneurons seems to be different. Some of them are of inhibitory nature (basket cells and, perhaps, other types of long-axonal stellate neurons), others are exciting elements. The latter include short-axonal stellate neurons and, perhaps, pyramidal interneurons. While comparing the cortex in cats and monkeys, it is evident that the neocortex in monkeys, especially its lower layers, is rich in pyramidal interneurons.     

FGF22 signaling regulates synapse formation during post‐injury remodeling of the spinal cord

The remodeling of axonal circuits after injury requires the formation of new synaptic contacts to enable functional recovery. Which molecular signals initiate such axonal and synaptic reorganisation in the adult central nervous system is currently unknown. Here, we identify FGF22 as a key regulator of circuit remodeling in the injured spinal cord. We show that FGF22 is produced by spinal relay neurons, while its main receptors FGFR1 and FGFR2 are expressed by cortical projection neurons. FGF22 deficiency or the targeted deletion of FGFR1 and FGFR2 in the hindlimb motor cortex limits the formation of new synapses between corticospinal collaterals and relay neurons, delays their molecular maturation, and impedes functional recovery in a mouse model of spinal cord injury. These results establish FGF22 as a synaptogenic mediator in the adult nervous system and a crucial regulator of synapse formation and maturation during post‐injury remodeling in the spinal cord.     

Development of neurons in the abdominal ganglion of Aplysia californica. I. Axosomatic synaptic contacts.

The development of mariculture techniques for the raising of Aplysia californica in the laboratory from fertilized egg to reproductively mature adult permits the study of the developmental program whereby individual identified neurons in the abdominal ganglion acquire their specific adult properties. In this paper, we describe one of the early steps of this developmental program: the outgrowth of axonal processes by neurons of the abdominal ganglion. Axonal outgrowth is correlated with and may be triggered by the transient appearance of morphologically identifiable axosomatic contacts between the as yet undifferentiated cell body of specific neurons and an axon terminal from an incoming nerve fiber from the pleuroabdominal connective. The evidence that transient axosomatic contacts may signal neuronal differentiation is the following: (1) Axosomatic contacts have not been observed in the abdominal ganglion of adult animals, whereas they are commonly observed during the early stages of development. (2) Cells that receive axosomatic contacts are undifferentiated morphologically and do not as yet have axons. By contrast, cells with axons do not have soma contacts. (3) Individual cells that can be identified from animal to animal in the same and succeeding developmental stages receive axosomatic contacts on similar topographic postions of the cell body at one point in development. Axon outgrowth then occurs at the site of contact. Later in development, with further axon extension, these cells no longer have synaptic contacts on the cell body or axon.     

Morphological characteristics of various segments of local connections in the rat somatosensory cortex

Pyramidal, aspinous, sparsely-spinous bipolar and multipolar neurons of the rat sensomotor cerebral cortex, impregnated after Golgi method, have been studied at an electron microscopical level. The ultrastructural characteristics of the pyramidal neurons differs from that of the nonpyramidal cells. Distribution of various synaptic contacts on the cellular surface and cortical postsynaptic targets of the axonal arborizations of the neurons are revealed. On the body of the pyramidal cells only symmetrical synapses exist, on large dendritic trunks symmetrical synapses prevail, on the spines and the terminal dendritic branches assymetrical synapses mainly predominate. Axonal collateralies of the pyramidal cells form asymmetrical synapses on the spines, small and middle dendrites. There are more axo-somatic synapses on the bodies of the nonpyramidal neurons than on the pyramidal cells, among them both symmetrical and asymmetrical types of the synapses occur. On the trunks and small dendrites of the nonpyramidal cells both types of synaptic contacts are revealed. In the distal direction of the dendrites the number of the asymmetrical synapses becomes predominating. Axons of the bipolar cells form asymmetrical synapses on the spines, small and middle dendrites. Axons of the multipolar cells form symmetrical synapses on the dendrites and the dendritic trunks of the nondifferentiated cells. Differences in the distribution character of the synaptic inlets and various postsynaptic targets of the axonal systems in the cells assume various functional role of the identified neurons.     

Ultrastructure of the synapses of the initial axonal segment of the pyramidal neuron

Ultrastructure of the proximal part of the axon in the neurons, identified according to a number of morphological signs as pyramidal, has been studied in the layer III of the cat cerebral hemisphere sensomotor cortex. In sections, tangential to the cortical surface, in the initial axonal segment, a submembranous osmophilic layer and fasciculi of microtubules are revealed. On the initial segment spines are found, they contain cysterns resembling by their structure the spine system of the dendritic spines. Axonal terminals revealed along the axonal distribution are in contact both with the axonal trunk and with the spines. Regarding the initial segment, they are presynaptic, contain oval synaptic vesicles and form symmetric axo-axonal synapses only. In transversal sections axonal terminals are detected, arranging on the surface of the initial segment mostly as single ones, in longitudinal sections they are seen as clusters. Analysing the author's data and those from the literature, a conclusion is made that in intact animals the synaptic contacts at the initial segment of the axon are the only form of axo-axonal synapses in the neocortex.     

Ultrastructural characteristics of layer III pyramidal neurons in the cat primary auditory cortex (Al)

Electron microscope studies were made of retrogradely horseradish peroxidase-labeled pyramidal neurons forming transcallosal projections in layer III of the cat primary auditory cortex (Al). These showed a significant proportion of the somatic membrane to be covered with processes of astroglia, while synapses occupy 20% of the synaptic surface on average. Between 4 and 10 axosomatic synapses were identified on the profiles of callosal cell somata. All these were formed by axonal terminals containing small, flattened synaptic vesicles and had symmetrical contacts. Average length of these synaptic contacts equaled 1.6 µm. Numerous anterogradely horseradish peroxidase-labeled axonal terminals of callosal fibers were found in cortical area Al in amongst retrogradely HP-labeled neurons. The ultrastructural pattern of these is described.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 22, No. 4, pp. 520–526, July–August, 1990.     

Peptidergic and monoaminergic innervation of the gonads in the holothurian Cucumaria japonica

In the holothurian gonad structure of the peptidergic and monoaminergic systems has been described. Axons of their cells form tissue and hemal terminals. Epithelial cells and smooth myocytes of the gonadal wall get direct innervation, having contacts with the axonal terminals that are separated by the cleft 50-75 nm in the diameter. It is possible that neuropeptides and biogenic monoamines are transported to the germ and follicular cells of the germinative gonadal zone via hemolymph of the hemal sinus.     

Infundibular neurons of the human hypothalamus simultaneously reactive with antisera against endorphins,ACTH, MSH and β-LPH

In man, discrete neurons of the infundibular (arcuate) nucleus contain compounds that can be stained with anti-endorphin (alpha and beta), anti-ACTH, anti-MSH (alpha and beta) and anti-beta-LPH immune sera (I.S.). In the fetus, certain neurons stain with anti-beta-endorphin or anti((17--39)ACTH starting from the 11th week of fetal life. At the ultrastructural level, these neurons contain elementary granules that are immunoreactive with anti-beta-endorphin. In the adult, neurons immunoreactive with anti-beta-endorphin are found in the infundibular nucleus. Their axonal fibers terminate around blood vessels in the neurovascular zone and in the pituitary stalk, or establish contacts with non-immunoreactive perikarya of the infundibular nucleus. These neurons can be stained with anti(17--39)ACTH and anti-beta-endorphin I.S. The most reactive are also stained moderately with anti-alpha-MSH, anti-beta-MSH, anti-beta-LPH, anti-alpha-endorphin, or anti(1--24)ACTH I.S. These results indicate that, in man, compound(s) identical with or immunologically related to endorphins, beta-LPH, ACTH and MSH are secreted by certain hypothalamic neurons. These agents probably originate from a common precursor molecula similar to the so-called pro-opiocortin.     

Neuronal extracellular proteases facilitate cell migration, axonal growth, and pathfinding

The release of extracellular proteases by the axonal growth cone has been proposed to facilitate its movement by digesting cell-cell and cell-matrix contacts in the path of the advancing growth cone. The serine protease plasminogen activator (PA) has been shown to be secreted and focally concentrated at axonal growth cones of cultured mammalian neurons. Thus, PAs are well-placed to play an active role in growth cone movement and axonal pathfinding in development and regeneration. We discuss recent findings that suggest that the biological action of these proteases is more complex than originally thought. Received: 15 May 1997 / Accepted: 4 June 1997     

On the fine structure of the cerebral ganglion of Sagitta (Chaetognatha)

Summary In this study the fine structure of the cerebral ganglion of Sagitta setosa (Chaetognatha) is investigated. The ganglion is flat and superficially positioned dorsally, below the basal lamina of the cephalic epidermis. It is surrounded by a specifically differentiated sheath. This sheath is made up of cells, which are interpreted as representing glial cells, and can be divided into an outer and an inner zone. The outer zone is composed of flat sheath cells with pale nuclei and few organelles. The inner zone consists of densely packed, extremely thin lamellar cellular processes. These attenuated lamellae, which still contain cytoplasm, resemble the myelin sheath of vertebrate axons. The intercellular space between the lamellae contains electron-dense material. In the sheath specialized intercellular contacts occur. The inner zone of this sheath extends at definite points into the centre of the ganglion and separates a zone of perikarya from the neuropil, as well as the single perikarya from each other. The perikarya are relatively uniform and do not form a cortex, but are concentrated mainly in lateral parts of the cerebral ganglion. Within the neuropil are axonal endings which have synaptic contacts with several postsynaptic elements. These anatomical findings are discussed with respect to their functional significance.     

PlexinA1 signaling directs the segregation of proprioceptive sensory axons in the developing spinal cord

As different classes of sensory neurons project into the CNS, their axons segregate and establish distinct trajectories and target zones. One striking instance of axonal segregation is the projection of sensory neurons into the spinal cord, where proprioceptive axons avoid the superficial dorsal horn-the target zone of many cutaneous afferent fibers. PlexinA1 is a proprioceptive sensory axon-specific receptor for sema6C and sema6D, which are expressed in a dynamic pattern in the dorsal horn. The loss of plexinA1 signaling causes the shafts of proprioceptive axons to invade the superficial dorsal horn, disrupting the organization of cutaneous afferents. This disruptive influence appears to involve the intermediary action of oligodendrocytes, which accompany displaced proprioceptive axon shafts into the dorsal horn. Our findings reveal a dedicated program of axonal shaft positioning in the mammalian CNS and establish a role for plexinA1-mediated axonal exclusion in organizing the projection pattern of spinal sensory afferents.     

In vivo time-lapse imaging and serial section electron microscopy reveal developmental synaptic rearrangements

Dendrites, axons, and synapses are dynamic during circuit development; however, changes in microcircuit connections as branches stabilize have not been directly demonstrated. By combining in?vivo time-lapse imaging of Xenopus tectal neurons with electron microscope reconstructions of imaged neurons, we report the distribution and ultrastructure of synapses on individual vertebrate neurons and relate these synaptic properties to dynamics in dendritic and axonal arbor structure over hours or?days of imaging. Dynamic dendrites have a high density of immature synapses, whereas stable dendrites have sparser, mature synapses. Axons initiate contacts from multisynapse boutons on stable branches. Connections are refined by decreasing convergence from multiple inputs to postsynaptic dendrites and by decreasing divergence from multisynapse boutons to postsynaptic sites. Visual deprivation or NMDAR antagonists decreased synapse maturation and elimination, suggesting that coactive input activity promotes microcircuit development by concurrently regulating synapse elimination and maturation of remaining contacts.     

Role of laminin and integrin interactions in growth cone guidance

Laminin is well known to promote neuronal adhesion and axonal growth, but recent experiments suggest laminin has a wider role in guiding axons, both in development and regeneration. In vitro experiments demonstrate that laminin can alter the rate and direction of axonal growth, even when growth cone contact with laminin is transient. Investigations focused on a single neuronal type, such as retinal ganglion cells (RGCs), strongly implicate laminin as an important guidance molecule in development and suggest the involvement of integrins. Integrins are receptors for laminin, and neurons express multiple types of integrins that bind laminin. Morphologically, integrins cluster in point contacts, specialized regions of the growth cone that may coordinately regulate adhesion and motility. Recent evidence suggests that the structure and regulation of point contacts may differ from that of their nonneuronal counterparts, focal contacts. In part, this may be because the interaction of the cytoplasmic domain of integrin with the cytoskeleton is different in point contacts and focal contacts. Mutational studies where the cytoplasmic domain is truncated or altered are leading to a better understanding of the role of the α and β subunit in regulating integrin clustering and binding to the cytoskeleton. In addition, whereas integrins may regulate motility through direct physical linkages to the growth cone cytoskeleton, an equally important role is their ability to elicit signaling, both through protein tyrosine phosphorylation and modulating calcium levels. Through such mechanisms integrins likely regulate the dynamic attachment and detachment of the growth cone as it moves on laminin substrates.     

Ultrastructural changes in the neurons and interneuronal synapses of the hypothalamus in the progeny of morphine-dependent rats

The electron microscopic method has revealed early maturation of the protein-synthesizing apparatus of some cells, on the one hand, and underdevelopment of neuroblast volumes, appearance of artificial contacts, axonal degeneration and destruction in large trunks, on the other hand. Compensatory-restorative processes in the cell on the 14th and 21st days of the postnatal development result in a complete restoration of morphological organization of neurons and interneuronal connections in the hypothalamus of offspring of the morphine-dependent rats.     

Ultrastructure of the serotoninergic system of the motor area of the cerebral cortex

Owing to the microscopical investigation, using selective neurotoxin 5,7-dihydroxytryptamine, it has been possible to reveal the serotoninergic system and targets of its innervation in the rat cerebral cortex motor area. The serotoninergic axonal varicosities and synaptic boutons are present in all layers of the neocortex. Their large amount is revealed in the I and II layers. The terminals form contacts with dendrites of small size, sometimes they terminate on the head of the spines, as well as on bodies of neurons in different layers. According to their position and ultrastructural organization these neurons are, perhaps, pyramidal, that is glutamatergic, and those less in their size--refer to interneurons and can be GABAergic ones. Basing on own data and those of the literature, concerning the existence of nonsynaptic link for transmission of serotoninergic effects, a conclusion is made that a coordinating functioning of the synaptic and non-synaptic intercellular integrative mechanisms ensure a wide range of functions of the serotoninergic system in the cerebral cortex.     

Synaptic organization of the cat parietal association cortex (area 5b)

An electron microscopy study was made of synaptic organization in the cat association cortex, area 5b. A total of 1635 axonal terminals were discovered over 6215 µm² (240 electronic imagings of slices of different association cortex layers); i.e., an average of 263±16 terminals per 1000 µm² expanse. It was found that 75.5% of axon terminals contained synaptic vesicles and formed either one- or two-sided contact with postsynaptic structures; 24.5% of axonal terminals contained synaptic vesicles but formed no distinct synaptic contacts with nearby neurons; 84.9% of terminals contained round-shaped or slightly oval synaptic vesicles; 7.8% had both rounded and elongated shapes, and vesicles were very elongated in the remaining 7.3%. Of the axonal terminals having synaptic contacts, axo(dendritic)-spinal terminals accounted for 46.6%, and axodendritic and axosomatic endings amounted to 50.0% and 3.4% respectively (in all 77% of axosomatic terminals contained elongated vesicles and maintained symmetrical contact, while 23% had round-shaped vesicles and formed asymmetrical contact). Calculations show that for each 1 mm³ an average of 258 million axonal terminals are found forming synaptic contacts in the cat association cortex as well as 84 million terminals containing synaptic vesicles but not forming contact.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 21, No. 2, pp. 174–185, March–April, 1989.