Neuroglandular synapses in the pharynx of the sea anemone Aiptasia pallida (Cnidaria, Anthozoa)

Scanning and transmission electron microscopy of the pharynx of the sea anemone Aiptasia pallida revealed a heavily ciliated epidermis and two types of gland cells not known previously to be innervated. By tracing serial cross sections of the pharynx, we located and characterized two types of neuroglandular synapses (i.e., those having clear vesicles and those with dense-cored vesicles). The diameters of the vesicles at each synapse were averaged; clear vesicles ranged from 70 to 103 nm in diameter and were observed at synapses to both mucous and zymogenic gland cells. Dense-cored vesicles ranged from 53 to 85 nm in diameter and were observed at synapses to two mucous gland cells. One mucous gland cell had three neuroglandular synapses, one with clear vesicles and two with dense-cored vesicles. The occurrence of either clear or dense-cored vesicles at neuroglandular synapses suggests that at least two types of neurotransmitter substances control the secretion of mucus in the sea anemone pharynx. To date, only clear vesicles have been observed at a neurozymogenic gland cell synapse in the pharynx. No evidence of immunoreactivity to phenylethanolamine-N-methyl transferase was observed at neuroglandular synapses, suggesting that adrenaline is not a transmitter in the pharynx of A. pallida.     

Acetylcholinesterase activity at the synapses of presynaptic boutons with presumed alpha-motoneurons in chicken ventral horn. Light- and electron-microscopic studies

Acetylcholinesterase (AChE) activity at the synapses of presynaptic boutons on presumed alpha-motoneurons in the chicken ventral horn was studied histochemically at the light- and electron-microscope levels. At the light-microscope level, many dot-like AChE-active sites were observed on the soma and dendrites of presumed alpha-motoneurons. On electron microscopy, reaction products for AChE activity were observed mainly in the synaptic clefts of the four kinds of presynaptic boutons: (1) S type boutons, (2) boutons containing small, spherical, dense cored vesicles (diameter range, 60-105 nm) and spherical, clear vesicles, (3) boutons containing medium-sized, spherical, dense cored vesicles (65-115 nm) and spherical, clear vesicles, and (4) boutons containing large, spherical, dense cored vesicles (80-130 nm) and spherical, clear vesicles. In the light of previous physiological and biochemical studies, the present results suggest the possibility that each of these presynaptic boutons which are AChE-active in their synaptic clefts may contain acetylcholine, substance P, or enkephalins which acts as a neurotransmitter or modulator.     

Neural pathways and innervation of cnidocytes in tentacles of sea anemones

Our previously published studies are here reviewed detailing neuro-cnidocyte synapses, demonstrating putative neurotransmitter substances, and identifying complex neural pathways in sea anemones. Synapses were traced to their contacts on nematocytes and spirocytes by transmission electron microscopy of serial thin sections of tentacles. In five animals, cells containing microbasic p-mastigophores had synapses with clear vesicles, whereas cells containing basitrichous isorhizas had synapses with dense-cored vesicles, providing preliminary evidence for a selectivity of neurotransmitter types for different nematocysts. Either clear or dense-cored synaptic vesicles were also present at neuro-spirocyte contacts. Antho-RFamide immunoreactivity occurred in some anthozoan synaptic vesicles and immunogold labeling of serotonin was found at a neuro-spirocyte synapse. Neural pathways included direct innervation of spirocytes by sensory cells, sequential neuro-neuro-spirocyte and neuro-neuro-nematocyte synapses and reciprocal synapses involving axons of both sensory cells and ganglion cells. Such synaptic patterns resemble neuro-effector pathways found in higher animals and lay to rest the independent effector hypothesis for cnidocyte discharge in tentacles of sea anemones.     

Electron microscope investigation of synapses,synapse-like structures,and possible sites of release of neurosecretory material in the buccal ganglia of Helix pomatia (Mollusca)

Summary In the buccal ganglia of Helix pomatia synapses and sites of possible release of neurosecretory material were investigated electron microscopically. There is one chemical synapse and one electrotonic synapse in the neuropile of the ganglion. No synapses could be detected in the buccal nerves, cerebro-buccal connectives, or in the buccal commissure. The synaptic cleft of the chemical synapse is about 25 nm wide and contains electron-dense material whereas the cleft of the electrotonic synapse is only 5 nm wide. The presynaptic fibre of the chemical synapse contains clear vesicles and dense core vesicles. The release sites of neurosecretory material are found at the initial segment of the axons, at perikarya of neurones, and at the perineurium of the ganglion. If the terminals are located at the plasmalemma of a nerve cell, these release sites are called synapse-like structures according to Roubos and Moorer-van Delft (1979). The synapse-like structures show all structural elements of synapses, except the 25 nm cleft containing dense material; the cleft is only 15–20 nm wide here like the normal cleft between neurones and glial cells or between two fibres. If the secretory material is released at the periphery through the perineurium the terminal is called synaptoid according to Scharrer (1970). In all cases, i.e. synapses, synapse-like structures, and synaptoids, clear vesicles were found in the axon terminal. This finding provides further evidence that clear vesicles always accompany the release of substances from axon endings.     

An Ultrastructural Account of Otoplanid Turbellaria Neuroanatomy I. The cerebral ganglion and the peripheral nerve net

The otoplanid nervous system investigated in Otoplana truncaspina Lanfranchi, 1969 and Parotoplanella heterorhabditica Lanfranchi, 1969 consits of: (a) an ellipsoidal cerebral ganglion located between the gut and the cephalic intestine and invested by a fibrillar collagen-like capsule 0.3 μm thick; (b) anterior extracapsular ganglion cell clusters; (c) a peripheral nerve plexus locally thickened at the level of the epithelial sensory and glandular areas, with extensive synaptic connections. At least two neuron types can be identified within the ganglion: (a) an inner layer close to the central neuropile of the 1st type of neurons, showing a vesicular cytoplasm rich in RER and Golgi complexes processing both round, clear, 25–45 nm in diameter, and dense cored vesicles, 50–80 nm in diameter; (b) an outer layer of the 2nd type of neurons, adjoining the capsule and filled with uniformly dense vesicles, 60–90 nm in diameter. Synaptic endings in the neuropile are provided with clear vesicles and dense cored vesicles or uniformly dense vesicles. The presynaptic side has paramembranous projections channelling the vesicles to the active zone; omega-like profiles are also observed. Thin banded muscle fibres run within the brain. A comparison is drawn with the other turbellarian neuron types described in the literature, to suggest their possible function. The functional implications of the synaptic ultrastructure are discussed.     

Different nematocytes have different synapses in the sea anemone Aiptasia pallida (Cnidaria,Anthozoa)

Sea anemones feed by discharging nematocysts into their prey, but the pathway for control of nematocyst discharge is unknown. The purpose of this study was to investigate the ultrastructural evidence of neuro-nematocyte synapses and to determine the types of synaptic vesicles present at different kinds of nematocyst-containing cells. The tip and middle of tentacles from small specimens of Aiptasia pallida were prepared for electron microscopy and serial micrographs were examined. We found clear vesicles in synapses on mastigophore-containing nematocytes and dense-cored vesicles in synapses on basitrich-containing nematocytes and on one cnidoblast with a developing nematocyst. In addition, we found reciprocal neuro-neuronal and sequential neuro-neuro-nematocyte synapses in which dense-cored vesicles were present. It was concluded that : (1) neuro-nematocyte synapses are present in sea anemones, (2) different kinds of synaptic vesicles are present at cells containing different types of nematocysts, (3) synapses are present on cnidoblasts before the developing nematocyst can be identified and these synapses may have a trophic influence on nematocyst differentiation, and (4) both reciprocal and sequential synapses are present at the nematocyte, suggesting a complex pathway for neural control of nematocyst discharge. J. Morphol. 238:53–62, 1998. © 1998 Wiley-Liss, Inc.     

An ultrastructural study of the polyp and strobila of Atorella japonica (Cnidaria,Coronatae) with respect to muscles and nerves

Atorella japonica were observed by TEM to examine the nerve plexus in the capitulum of the polyp and the cross-striated muscle cells of the strobila. The nerve plexus included a number of neuromuscular junctions and many interneural synapses. Neuromuscular junctions contained two types of synaptic vesicle: clear and small (ca 75 nm diam.), and dense cored and large (ca 120 nm diam.). The first type of vesicle always appeared near the presynaptic membrane and the second type was distributed behind the former. In interneural synapses, two types of vesicle which were similar to neuromuscular synaptic vesicles were recognized. They were distributed in a pattern similar to that of the neuromuscular synaptic vesicles, but these vesicles were found on both sides of the two synaptic membranes.     

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

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

Catecholamine-rich cells and varicosities in bovine splenic nerve,vesicle contents and evidence for exocytosis

The bovine splenic nerve trunk contins mast cells, ganglion cells, small intensely flurescent (SIF) cells, and varicosities which exhibit a brilliant fluorescence characteristic for noradrenaline (NA) and dopamine (DA) after formaldehyde exposure. All these catecholamine-rich structure could contribute particles to isolated nerve vesicle fractions. Mast cells are recognized ultrastructurally by their large (300–800nm) dense granules. SIF cells may be represented by cells and processes containing dense cored vesicles (120–140 nm) which are larger than the typical vesicles in axons and terminals. Terminal-like areas with typical large dense cored vesicles (LDV, 75 nm) and small dense cored vesicles (SDV, 45–55 nm) probably correspond to the fluorescent varicosities. The LDV constitute about 40% of all vesicle in terminal-like areas and terminals. Their staining properties indicate the presence of protein, phospholipids, and ATP. Tyramine depletes NA without loss of matrix density. The LDV can fuse with the terminal membrane, and released material outside omega profiles is interpreted to depict exocytosis. Large and small vesicles are easily distinguished from the very large mast cell granules and the moderately dense Schwann cell vesicles. Neither appear to contaminate the LDV fractions but the latter may contain a small population of SIF cell vesicles. Golgi vesicles from the Schwann cells mainly occur in the lighter zones of the gradient.     

Presynaptic dense bars at neuromuscular synapses of the lobster,Homarus americanus

Summary The threedimensional ultrastructure of presynaptic dense bars was examined by serial section electron microscopy in the excitatory neuromuscular synapses of the accessory flexor muscle in the limbs of larval, juvenile, and adult lobsters. The cross-sectional profile of the dense bar resembles an asymmetric hourglass, the part contacting the presynaptic membrane being larger than that projecting into the terminal. The bar has a height of 55–65 nm and varies in length from 75–600 nm. In its dimensions it resembles the dense projections in the synapses of the CNS of insects and vertebrates. The usual location of these dense bars is at well defined synapses, though a few are found at extrasynaptic sites either in the axon or terminal. In the latter case the bars are close to synapse-bearing regions, particularly in the larval terminals, suggesting that the extrasynaptic bars denote early events in synapse formation. In all cases the bars are intimately associated with electron lucent, synaptic vesicles located on either side, in the indentation of its hourglass-shaped cross sectional profile. The vesicles occur along the length of the bar and contact the presynaptic membrane. Consequently the dense bar may serve to align the vesicles at the presynaptic membrane prior to exocytosis. A similar role has been suggested for the presynaptic dense bodies at the neuromuscular junction of the frog, where synaptic vesicles form a row on either side of this structure.Supported by Muscular Dystrophy Association of Canada and NSERCC. Generous use of laboratory facilities at Woods Hole was provided by the late Fred Lang     

Axons containing neuropeptide Y innervate arginine vasopressin-containing neurons in the rat paraventricular nucleus

Summary Synaptic connections between neurons immunoreactive for arginine vasopressin (AVP) and axon terminals immunoreactive for neuropeptide Y (NPY) were found in the magnocellular part of the paraventricular nucleus (PVN) in the rat hypothalamus. In pre-embedding double immunolabeling, NPY axon terminals labeled with diamin-obenzidine (DAB) reaction product established synaptic junctions on the perikarya and neuronal processes of AVP neurons labeled with silver-gold particles. Ultrastructural morphology of the neurons was more suitably preserved by a combination of pre- and post-embedding procedures. The presynaptic NPY terminals contained many small clear vesicles and a few cored vesicles, and DAB chromogen (immunoreaction product) was located on the surface of the vesicular profiles and on the core. The postsynaptic AVP neurons possessed many large secretory granules labeled with gold particles. At the synaptic junctions, small clear vesicles were accumulated at the presynaptic membrane, and the postsynaptic membrane was coated with a dense accumulation of fine electron dense particles. The perikarya also received synapses made by immuno-negative axon terminals containing many small clear vesicles and a few cored vesicles. These terminals were found more frequently than those containing NPY.     

Axons containing neuropeptide Y innervate arginine vasopressin-containing neurons in the rat paraventricular nucleus. Dual electron microscopic immunolabeling

Synaptic connections between neurons immunoreactive for arginine vasopressin (AVP) and axon terminals immunoreactive for neuropeptide Y (NPY) were found in the magnocellular part of the paraventricular nucleus (PVN) in the rat hypothalamus. In pre-embedding double immunolabeling, NPY axon terminals labeled with diaminobenzidine (DAB) reaction product established synaptic junctions on the perikarya and neuronal processes of AVP neurons labeled with silver-gold particles. Ultrastructural morphology of the neurons was more suitably preserved by a combination of pre- and post-embedding procedures. The presynaptic NPY terminals contained many small clear vesicles and a few cored vesicles, and DAB chromogen (immunoreaction product) was located on the surface of the vesicular profiles and on the core. The postsynaptic AVP neurons possessed many large secretory granules labeled with gold particles. At the synaptic junctions, small clear vesicles were accumulated at the presynaptic membrane, and the postsynaptic membrane was coated with a dense accumulation of fine electron dense particles. The perikarya also received synapses made by immuno-negative axon terminals containing many small clear vesicles and a few cored vesicles. These terminals were found more frequently than those containing NPY.     

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

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

Large dense-core vesicles at the frog neuromuscular junction: characteristics and exocytosis

The population of large dense-core vesicles (LDCVs) in motor nerve terminals of the frog cutaneous pectoris muscle was analysed after various experimental protocols leading to large acetylcholine release. Three types of LDCVs classified according to their size and the core density were detected. Vesicles, 100–150 nm in diameter, with a large and very dense core (type 1) or with an irregular and diffuse dense core (type 2) were present in similar proportions (45 and 50% respectively) in controls. Smaller vesicles, 50–80 nm in diameter, with a very dense core (type 3) were rare, representing around 5% of the cored vesicles. The relative proportion of type 1 and type 2 LDCVs was not modified after prolonged treatment with 25 mM K+. In contrast, the proportion of type 2 LDCVs significantly increased whereas that of type 1 LDCVs decreased after two or three series of 20 Hz electrical stimuli applied to the nerve at 5 s intervals. These changes suggest that type 2 LDCVs are newly recycled LDCVs in the process of reloading. Images of fusion of LDCVs with the axolemma in regions facing Schwann cell digitations were observed both in K+- and in electrically stimulated preparations. They indicate that exocytosis of LDCVs at the frog neuromuscular junction takes place preferentially away from the active zones. The presence of a clathrin-like coat on large pockets still containing a core and of both type 1 and type 2 LDCVs in the vicinity of coated pockets strongly suggests that LDCVs might undergo a combined process of exo–endocytosis at the same site.     

Interneuronal synapses in the local ganglia of the cat urinary bladder.

The interneuronal synapses of the urinary bladder in the cat were studied by electron microscopy. The great majority of the fibres containing vesicles are found within the ganglia occurring in the trigonum area. Morphologically differentiated synaptic contacts could be observed on the surface of the local neurons and between the different nerve processes. The presynaptic terminals can be divided into three types based on a combination of synaptic vesicles. Type I terminals, presumably cholinergic synaptic terminals, contain only small clear vesicles of 40-50 nm in diameter. Type II terminals, presumably adrenergic terminals, are characterized by small granulated vesicles of 40-60 nm in diameter. Type III terminals, probably of local origin, contain a variable number of large granulated vesicles of 80-140 nm in diameter. Occasionally, a single nerve fibre contacted several (two or four) other nerve processes forming a typical synapse. In other cases, on one nerve cell soma or on other nerve processes there are two or three different-type nerve terminals establishing synapses. It might be inferred from these observations that convergence and divergence can occur in the local ganglia and that cholinergic and adrenergic synaptic terminals can modulate the ganglionic activity. However, a local circuit also can play an important role in coordinating the function of the bladder.     

Neurobiology of the scallop. II. structure of the parietovisceral ganglion lateral lobes in relation to afferent projections from the mantle eyes

The lateral lobes of the scallop parietovisceral ganglion have been examined morphologically with respect to their functional role as optic lobes. The gross morphology of the lateral lobe and projections of optic nerve fibers within it were investigated by 1) supravital methylene blue staining, and 2) autoradiography using tritiated proline injected intraocularly for incorporation and transport by the optic fibers. Ultrastruc‐turally, the lateral lobe was examined using standard electron microscopic techniques. The lateral lobe is composed of a cortical rind of cells, 8–15 μm in diameter at the ventral surface and 15–20 μm in diameter at the ventral surface, surrounding a central neuropil. The neuropil contains three distinct regions: 1) the glomerular neuropil, a series of densely staining spherical subunits associated with the eyes and pallial nerves, 2) the subcellular neuropil, a synaptic region adjacent to the ventral cell layer also having a visual function, and 3) the subglomerular neuropil, the remaining, rather unspecialized neuropil of the lateral lobe. Synaptic profiles with symmetrical membrane thickenings, a 32 nm synaptic cleft, and three types of vesicles are seen throughout the neuropil, although the density of synapses is greater in the glomerular region. Clear, dense core and neurosecretory vesicles are seen individually or as mixed populations in the presynaptic terminals. Autoradiographic experiments have revealed that optic fibers enter the lateral lobe and project directly to the subcellular neuropil where they synapse with cells located on the ventral surface of the lateral lobe cells. These cells in turn form the dense glomerular structures previously identified as visual association centers and send efferent fibers into the pallial nerves. The projection of optic fibers to the ventral surface of the lobe is consistent with previous electrophysiological recordings of visual activity at this site.     

A freeze-fracture study of afferent and efferent synapses of hair cells in the sensory epithelium of the organ of Corti in the guinea pig

Summary Afferent and efferent synapses of hair cells in the organ of Corti of the guinea pig have been examined in freeze-fracture replicas.Afferent synapse In the inner hair cells, intramembranous particles 10 nm in diameter are aggregated on the ridge on the P-face of the presynaptic membrane directly beneath the synaptic rod. In the outer hair cells, in which the synaptic rod is located in the presynaptic cytoplasm underneath the presynaptic membrane, small aggregations of intramembranous particles 10 nm in diameter can be found on the P-face of the presynaptic membrane corresponding to the site of the presynaptic dense projection. Intramembranous particles 10 nm in diameter are also densely aggregated on the P-face of the postsynaptic membrane of the outer hair cells.Efferent synapse of the outer hair cells Large intramembranous particles 13 nm in diameter are distributed in clusters composed of four to ten particles on the P-face of the presynaptic membrane. In the P-face of the postsynaptic membrane, disc-like aggregations of intramembranous particles 9 nm in diameter are found. The subsynaptic cistern covers the cytoplasmic surface of the postsynaptic membrane of the efferent synapse; it may cover more than one postsynaptic membrane when several efferent synapses are in close proximity to one another.     

Ultrastructure of neurons and synapses in the tentacle gastrodermis of the sea anemone Calliactis parasitica

Little is known about gastrodermal neurons and synapses in the tentacles of sea anemones. Using transmission electron microscopy of serial thin sections of Calliactis parasitica, we have identified both a sensory cell and a ganglion cell with granular vesicles originating from the Golgi complex and have identified four types of synapses in the tentacular gastrodermal nerve plexus. The sensory cell has a recessed apical cilium with a basal body and a perpendicularly oriented centriole, below which are several strands of striated rootlets surrounded by mitochondria. The ganglion cell lacks a cilium and resembles a bipolar neuron, with oppositely directed processes lying parallel to the basally located circular smooth muscle. Both one-way and two-way interneuronal synapses are present with 60- to 90-nm granular vesicles of various densities aligned at the paired electron-dense membranes and fine cross filaments in the intervening 13-nm cleft. Two types of neuroeffector synapses have been located. Dense granular vesicles are present at neuromuscular synapses, whereas less dense vesicles are present at neuroglandular synapses. Most of the synaptic vesicles range from 60 to 120 nm in diameter. Two types of nerve cells and a variety of synaptic loci provide morphological substrates for the spontaneous SS2 conduction pulses in the tentacular gastrodermis of C. parasitica. J Morphol 231:217–223, 1997. © 1997 Wiley-Liss, Inc.     

The variety of synaptic vesicles in the interpeduncular nucleus (ITP) of the frog.

The electron microscope has revealed a large variety of synaptic vesicles in the interpeduncular nucleus (ITP) of the frog "Rana esculenta". They vary in shape, size and electron density. There are two types of synapses which show only translucent spherical vesicles: in one type the vesicles are 40 nm, in the other type they are 70 nm in diameter. In other types of synapses the translucent vesicles may be mixed with those with dense core. Large granules, 160 nm in diameter, already reported in the ITP (KEMALI 1977a), are also shown as well as tiny flat mixed with large flat dense core vesicles of dumb-bell shape. Two types of axo-axonic synapses are illustrated while no crest synapses have been demonstrated. The results suggest that the afferents to the ITP might be more numerous than those reported in the literature or that--as in the case of the habenular afferents which consist of cholinergic and peptergic fibres--each projecting nucleus to the ITP has different types of fibres with more than one type of transmitter. Furthermore, due to the vesicles sizes, we may consider the ITP as a site in the vertebrate central nervous system where conventional neurotransmitter structures coexist with probable neurohumoral elements.