Electron microscopy of stomatogastric ganglion in the lobster Homarus americanus

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

Dense-core vesicles and non-synaptic exocytosis in the central body of the crayfish brain

Summary The central body in the median protocerebrum of the brain of the crayfish Cherax destructor is a distinctive area of dense neuropile, the nerve fibres of which contain three main types of vesicles: electronlucent vesicles (diameter 35 nm), dense-core vesicles (diameter 64 nm), and large structured dense-core vesicles (diameter 98 nm, maximum 170 nm). Different vesicle types were found together in the same neurons. Electronlucent vesicles were seen at presynaptic sites and rarely observed in the state of exocytosis. Exocytosis of densecore and structured dense-core vesicles was a regular feature on non-synaptic release sites either close to, or at some distance from pre- and subsynaptic sites. Non-synaptic exocytotic sites are more often observed than chemical synapses. Different forms of exocytosis seen at non-synaptic sites included the release of single densecore vesicles, packets of dense-core vesicles, and rows of dense-core vesicles lined up along cell membranes and around fibre invaginations. Swelling and the enhanced electron density of extracellular non-synaptic spaces may mark the positions of prior exocytotic events. In vitro treatment of the brain with tannic acid buffer solution followed by conventional double fixation resulted in the augmentation of non-synaptic exocytosis. Electron microscopy of proctolin- and serotonin-immunoreactive nerve fibres shows them to contain dense-core and electron-lucent vesicles and to be surrounded by many unlabelled profiles similarly laden with dense-core vesicles and electron-lucent vesicles, indicating the presence of other, not yet identified, neuroactive compounds.     

Catecholaminergic innervation of muscles in the hindgut of crustaceans

Summary The crustacean species Pacifastacus leniusculus and Gammarus pulex were investigated by electron microscopy in a search for possible neuromuscular junctions in the hindgut, which has a rich supply of catecholaminergic fibres. True neuromuscular synapses were found in both species between nerve terminals containing dense-core vesicles (80–110 nm in diam.) and muscle fibres. We suggest that the dense-core vesicle terminals contain a catecholamine, and this is supported by ultrahistochemical tests for monoamines. Two types of junctions are found: one in which the nerve terminal is embedded in the muscle cell (both species) and one in which protrusions from the muscle cell meet nerve terminals (Pacifastacus). Gammarus pulex, which has only circular muscles in the hindgut, has only catecholaminergic innervation, whereas Pacifastacus leniusculus has circular and longitudinal muscles both with at least two types of innervation.The investigation was supported by grants from the Swedish Natural Science Research Council (B 2760-009), the Hungarian Academy of Sciences, the Royal Swedish Academy of Sciences, and the Magnus Bergvall Foundation. We are also indebted to Mrs. Lena Sandell for her skilful technical assistance     

Ultrastructure of sympathetic ganglion cells and granule-containing cells in the paracervical (Frankenhäuser) ganglion of the newborn rat

Summary A study was made of the ultrastructure of the paracervical (Frankenhäuser) ganglion of the newborn rat, using immersion fixation by glutaraldehyde (2.5%) followed by OsO₄ (1%), or KMnO₄ (3%) fixation. The cells containing dense—core vesicles were divided into three groups: (1) primitive sympathetic cells, (2) cells containing some dense-core vesicles 700–1100 Å in size and structurally resembling sympathetic neurons, called principal neurons, and (3) cells containing many dense-core vesicles with a larger, darker dense core, 800–2000 Å in diameter, called granule-containing cells. Using glutaraldehyde-osmium fixation, the principal neurons were further divided into dark and light cells on the basis of electron opacity of the cytoplasmic matrix. The granule-containing cells were believed to correspond to the small, intensely fluorescent cells (SIF-cells) previously described using the formaldehyde-induced fluorescence technique. On the basis of the amount of granules, the granulecontaining cells were classified as mature or maturing SIF-cells and as more primitive SIF-cells, and developing sympathicoblasts. The development of synapses in autonomic ganglia was discussed.Grant: The Finnish Medical Foundation.     

Morphology and location of dense-core vesicles in the stomatogastric ganglion of the lobster,Panulirus interruptus

The appearance and distribution of dense-core vesicles in the stomatogastric ganglion of the spiny lobster, Panulirus interruptus, were examined using transmission electron microscopy. Following five fixation techniques, three types of dense-core vesicles were identified on the basis of size and morphology. Type-I vesicles are found in a distinct neuronal fiber system that appears to be involved in chemical transmission within the ganglion. Type-II vesicles occur in nerve processes in the ganglion, in major nerve trunks and in the perineural sheath of the nerves and ganglion. Type-III vesicles are present in all neuronal somata of the ganglion. The distinct morphology and location of the three types of vesicles suggest that their functional roles differ. Furthermore, the histochemical, biochemical and physiological data available for the stomatogastric ganglion indicate that Type-I vesicles may store dopamine.     

Immunocytochemical and electron-microscopic study of the elasmobranch nucleus sacci vasculosi

Summary The elasmobranch nucleus sacci vasculosi was studied by means of electron microscopy (in the dogfish) and immunocytochemistry (in the dogfish and the skate) by using antibodies against tyrosine hydroxylase, alpha-melanocyte-stimulating hormone, somatostatin, serotonin, and substance P. Ultrastructural study of the dogfish nucleus sacci vasculosi shows the presence of medium-sized cells that possess numerous mitochondria but that have no dense-core vesicles in the cytoplasm or in cell processes. Fibres of the conspicuous tractus sacci vasculosi have a beaded appearance and form conventional synapses with dendrites and cell perikarya of the nucleus sacci vasculosi. The perikarya of this hypothalamic nucleus were not immunoreactive to any of the antibodies tested, and fibres immunopositive to tyrosine hydroxylase, alpha-melanocyte-stimulating hormone, somatostatin, serotonin, and substance P were scarce within this nucleus, in both the dogfish and the skate. Dorsal to the nucleus sacci vasculosi, there are numerous positive neuronal processes in addition to many small neurons that show immunoreactivity to alpha-melanocyte-stimulating hormone, somatostatin and tyrosine hydroxylase. Two types of neuron occur in this dorsal region, displaying dense-core vesicles of either 100–160 nm or 60–100 nm diameter in their cytoplasm; they were identified as peptide-containing and monoamine-containing neurons, respectively. The neuropil of this region has a significantly different ultrastructure from that of the nucleus sacci vasculosi, with many processes containing dense-core vesicles. This group of neurons, located dorsal to the nucleus sacci vasculosi and showing (a) immunoreactivity to neuropeptides or to monoamine-synthesizing enzyme, and (b) cytoplasm with dense-core vesicles, was considered not to be a part of the nucleus sacci vasculosi but rather part of the nucleus tuberculi posterioris. These results support the non-peptidergic and non-aminergic character of the nucleus sacci vasculosi.     

Vesicle and granule content of sympathetic ganglion cells during limb regeneration of the newt Triturus

Summary Fine structural observations were made on the vesicle and granule content of ganglion cells in the posterior subclavian ganglion and peripheral nerve fibers of the upper forelimb of the newt Triturus. The populations of vesicles and granules in normal ganglion cells and nerve fibers were compared with those observed after limb transection. In normal neurons, clear vesicles range in size from 250 to 1000 Å in diameter, but are most frequently 400–500 Å. Vesicles with dense contents (granules) also vary greatly in size, but most are 450–550 Å in diameter and correspond to dense-core vesicles. Large granules that contain acid phosphatase activity are thought to be lysosomes. During limb regeneration, in both the ganglion cells and peripheral nerves, the ratio of dense vesicles to clear vesicles increases. There is a large increase in number of dense granules with a diameter over 800 Å, particularly in the peripheral regenerating fibers. This study shows that regenerating neurons differ from normal in their content of vesicular structures, especially large, membrane-bounded granules.This work was supported by grants from the National Science Foundation (GB 7912) and from the National Cancer Institute (TICA-5055), National Institutes of Health, United States Public Health Service.     

Endocrine-like cells in the lungs of the newt,Triturus alpestris Laur.

Summary Endocrine-like cells (ELC) scattered in the basal part of the ciliated epithelium were identified in the lungs of the newt, Triturus alpestris. These ELC have a clear cytoplasm containing large amounts of secretory vesicles (69–180 nm in diameter), especially in their basal parts, but do not display formaldehyde-induced fluorescence. The ELC may be associated with nerve fibres.     

Lymphoid cells in the Harderian gland of birds

Summary Numerous secretory parvocellular perikarya were found in the preoptic region of the domestic fowl (Gallus gallus). The dense-core secretory vesicles belong to two categories: vesicles with a diameter of (i)80–90 nm and (ii) 110–140nm. Scattered magnocellular elements display larger dense-core granules. The parvocellular neurons form unit-like clusters, showing also zones of direct apposition of neuronal membranes. The surrounding neuropil is rich in synaptic structures, formed by at least three types of axon terminals, distinguishable on the basis of vesicular morphology. These observations confirm the findings in other avian species. The hypothetical function of this system of peptidergic neurons in the rostral hypothalamus of birds is discussed.     

Ultrastructural Correlates of Enhanced Norepinephrine and Neuropeptide Y Cotransmission in the Spontaneously Hypertensive Rat Brain

The spontaneously hypertensive rat (SHR) replicates many clinically relevant features of human essential hypertension and also exhibits behavioral symptoms of attention-deficit/hyperactivity disorder and dementia. The SHR phenotype is highly complex and cannot be explained by a single genetic or physiological mechanism. Nevertheless, numerous studies including our own work have revealed striking differences in central catecholaminergic transmission in SHR such as increased vesicular catecholamine content in the ventral brainstem. Here, we used immunolabeling followed by confocal microscopy and electron microscopy to quantify vesicle sizes and populations across three catecholaminergic brain areas—nucleus tractus solitarius and rostral ventrolateral medulla, both key regions for cardiovascular control, and the locus coeruleus. We also studied colocalization of neuropeptide Y (NPY) in norepinephrine and epinephrine-containing neurons as NPY is a common cotransmitter with central and peripheral catecholamines. We found significantly increased expression and coexpression of NPY in norepinephrine and epinephrine-positive neurons of locus coeruleus in SHR compared with Wistar rats. Ultrastructural analysis revealed immunolabeled vesicles of 150 to 650 nm in diameter (means ranging from 250 to 300 nm), which is much larger than previously reported. In locus coeruleus and rostral ventrolateral medulla, but not in nucleus tractus solitarius, of SHR, noradrenergic and adrenergic vesicles were significantly larger and showed increased NPY colocalization when compared with Wistar rats. Our morphological evidence underpins the hypothesis of hyperactivity of the noradrenergic and adrenergic system and increased norepinephrine and epinephrine and NPY cotransmission in specific brain areas in SHR. It further strengthens the argument for a prohypertensive role of C1 neurons in the rostral ventrolateral medulla as a potential causative factor for essential hypertension.     

Electron microscopic autoradiography of serotonin uptake in the ganglia of the fresh-water mussel (Anodonta cygneak L.).

After in vitro incubation, the uptake of labelled serotonin was investigated by electron microscopic autoradiography in the ganglia of fresh-water mussel (Anodonta cygnea L.) The labelled serotonin was primarily taken up by the axons of the neurons. The silver grains could always be localized over axons containing eccentric dense-core vesicles with a diameter of 100--200 nm. The results suggest (a) the possibility of the electron microscopic identification of serotonin-containing neurons, and (b) the direct role of the eccentric dense-core vesicles in the storage of serotonin.     

Electron-microscopic investigations of vasoactive intestinal peptide (VIP)-like immunoreactive terminal formations in the lateral septum of the pigeon

Vasoactive intestinal peptide (VIP)-like immunoreactive terminal fields were examined in the lateral septum of the pigeon by means of immunocytochemistry. According to light-microscopic observations, these projections originated from VIP-like immunoreactive cerebrospinal fluid (CSF)-contacting neurons, which are located in the ependymal layer of the lateral septum and form a part of the lateral septal organ. The processes of these cells gave rise to dense terminal-like structures in the lateral septum. Pre-embedding immuno-electron microscopy revealed that VIP-like immunoreactive axon terminals had synaptoid contacts with perikarya of small VIP-immunonegative neurons of the lateral septum, which were characterized by an invaginated nucleus, numerous mitochondria, a well-developed Golgi apparatus, endoplasmic reticulum and a small number of dense-core vesicles (about 100 nm in diameter). VIP-like immunoreactive axons were also seen in contact with immunonegative dendrites in the lateral septum. In both axosomatic and axodendritic connections, VIP-like immunoreactive presynaptic terminals contained large dense-core vesicles, clusters of small vesicles and mitochondria. These findings suggest that VIP-immunoreactive neurons of the lateral septal organ project to small, presumably peptidergic nerve cells of the lateral septum and that the VIP-like neuropeptide serves as a neuromodulator (-transmitter) in this area.     

Adrenergic neurons in the spinal cord of the pike (Esox lucius) and their relation to the caudal neurosecretory system

Summary The lower spinal cord including the caudal neurosecretory system of the pike (Esox lucius) was investigated by means of light and electron microscopy and also with the fluorescence histochemical method of Falck and Hillarp for the visualization of monoamines. A system of perikarya displaying a specific green fluorescence of remarkably high intensity is disclosed in the basal part of the ventrolateral and lateral ependymal lining of the central canal. The area corresponding to the upper half of the urophysis has most cells; their number decreases caudally and cranially. A considerable number of their beaded neurites reach the neurosecretory neurons by different routes but are only occasionally present in the actual neurohemal region. An intensely fluorescent dendritic process is sometimes observed terminating with a bulbous enlargement at the ependymal surface in the central canal. Besides small, electron lucid vesicles in the terminal parts of the axons, the neurons contain numerous large dense-core vesicles which can apparently take up and store 5-hydroxydopa (5-OH-dopa) and 5-hydroxydopamine (5-OH-DA). These neurons are thought to be adrenergic and to contain a primary catecholamine, possibly noradrenaline.The varicosities of the adrenergic terminals are repeatedly observed contiguous to some of the neurosecretory axons, the membrane distance at places of contacts generally ranging from 150–200 Å. Another type of nerve terminals that contain only small empty vesicles, also after pretreatment with 5-OH-dopa or 5-OH-DA, are frequent among the neurosecretory neurons. These axons establish synaptic contacts with membrane thickenings on most of the neurosecretory neurons. Thus it seems that the neurosecretory neurons are innervated by neurons morphologically similar to cholinergic neurons and that part of them receive an adrenergic innervation, which supports the view hat the caudal neurosecretory cells do not constitute a functionally homogeneous population.Supported by the Deutsche Forschungsgemeinschaft and the Joachim-Jungius Gesellschaft zur Förderung der Wissenschaften, Hamburg.Supported by the Swedish Natural Research Council (No. 99-35). This work was in part carried out within a research organization sponsored by the Swedish Medical Research Council (Projects No. B70-14X-56-06 and B70-14X-712-05).Supported by the Deutsche Forschungsgemeinschaft and USPHS Research Grant TW 00295-02.     

Somatostatin-like immunoreactivity in human sympathetic ganglia

Summary The localization of somatostatin-like immunore-activity (SOM-LI) was examined in human lumbar sympathetic ganglia using the peroxidase-antiperoxidase method. Few of the principal neurons showed immunolabelling for somatostatin and sparse networks of nerve terminals were unevenly associated with ganglion cells. Using light microscopy, the most intense SOM-LI was seen in the perinuclear zone of the neurons. Electron-microscopically, the staining was localized on the membranes of the Golgi apparatuses. In the nerve terminals, SOM-LI was seen inside the small vesicles (40–60 nm diameter). All neurons with SOM-LI were also found to be tyrosine-hydroxylase immunoreactive when excamined with a double-staining technique. These results provide evidence that somatostatin and noradrenaline co-exist in human sympathetic neurons.     

Licht- und elektronenmikroskopische Untersuchungen des Saccus vasculosus und des Nervus und Tractus sacci vasculosi

Zusammenfassung Der Saccus vasculosus von Anguilla anguilla, Cyprinus carpio und Amiurus nebulosus wurde lichtmikroskopisch mit der AChE-Reaktion und dem fluoreszenzhistochemischen Monoaminnachweis, sowie elektronenmikroskopisch untersucht.Lichtmikroskopisch weisen die Liquorkontaktneurone und ihre Fortsätze eine starke AChE-Aktivität auf, während die Krönchenzellen inaktiv sind. Die AChE-positiven Fortsätze der Nervenzellen bilden Bündel, die in den Nervus sacci vasculosi eintreten und im Tractus sacci vasculosi weiterziehen. Diese AChE-positive Bahn kann nach Kreuzung zur Gegenseite bis in das Neuropil des Thalamus ventralis verfolgt werden. Die Liquorkontaktneurone des Saccus vasculosus, der Nervus und Tractus sacci vasculosi, sowie der Nucleus sacci vasculosi weisen keine Monoaminfluoreszenz auf.Auf den Perikaryen der Krönchenzellen kommen Synapsen vor, deren praesynaptisches Cytoplasma außer synaptischen Bläschen und Mitochondrien 800–1000 Å große granulierte Vesikel aufweist. Die Perikaryen der Liquorkontaktneurone enthalten neben den üblichen Cytoplasmabestandteilen dense core Vesikel, deren Durchmesser 700–900 Å beträgt. Axone, in denen granulierte Vesikel (Durchmesser 800 oder 1300 Å) vorkommen, bilden mit diesen Perikaryen Synapsen. Im basalen Teil des Saccusepithels findet man granulierte Bläschen (Durchmesser 800 oder 1400 Å) enthaltende Nervenfasern unterschiedlichen Durchmessers, ferner Synapsen. Der Nervus sacci vasculosi enthält klein- und großkalibrige, marklose Nervenfasern und vereinzelte Synapsen, während der Tractus sacci vasculosi aus vorwiegend kleinkalibrigen, marklosen Fasern besteht.

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Light and electron microscopic studies of the vascular sac and of the nervus and tractus sacci vasculosi

Summary The vascular sac of Anguilla anguilla, Cyprinus carpio and Amiurus nebulosus has been studied by light microscopy using AChE reaction and the fluorescence histochemical method for the demonstration of monoamines, and by electron microscopy.As demonstrated light microscopically, the cerebrospinal fluid (CSF) contacting neurons and their processes exert a strong AChE activity, while the coronet cells are inactive. The AChE-positive processes of the neurons form bundles that enter the nervus sacci vasculosi and pass on in the tractus sacci vasculosi. After crossing to the opposite side, this AChE-positive bundle can be traced into the neuropil of the ventral thalamus. Neither the CSF contacting neurons of the vascular sac, nor the nervus, tractus and nucleus sacci vasculosi show any monoamine fluorescence.As demonstrated electron microscopically, there are synapses on the perikarya of the coronet cells, their presynaptic cytoplasm being characterized by mitochondria, synaptic and granulated vesicles (diameter about 800 to 1000 Å). The perikarya of the CSF contacting neurons contain dense-core vesicles (diameter about 700 to 900 Å) besides of the usual cytoplasmic components. Axons displaying granulated vesicles with a diameter of 800 Å or 1300 Å, form synapses on these perikarya. In the basal part of the saccus epithelium, there are nerve fibres of different calibres containing dense-core vesicles (diameter about 800 Å or 1400 Å) and forming synapses. The nervus sacci vasculosi is characterized by thin and thick, unmyelinated nerve fibres, and rare synapses, while the tractus sacci vasculosi is composed of mainly small, unmyelinated fibres.

     

The structure of the coeliac ganglion of a teleost fish Myoxocephalus scorpius

Summary In order to compare the structure of a teleost sympathetic ganglion with those of other vertebrates, light, fluorescence histochemical and electron microscopy were carried out on the coeliac ganglion of the scorpion fish, Myoxocephalus scorpius. In common with studies on other vertebrates, fluorescence histochemistry distinguished two cell types: a) principal neurones which exhibited low levels of specific catecholamine fluorescence and comprise the majority of neurones in the ganglia, and b) smaller intensely fluorescent cells, some of which had processes tens of micrometers long.With the electron microscope, the principal cells were seen to make axodendritic and axosomatic synapses with axons containing mainly 30 nm agranular vesicles at the synaptic site while in other vertebrates usually only one or other synaptic association is present.Both the somata and the processes of intensely fluorescent cells contain 300–600 nm diameter vesicles many of which have electron dense cores. These cells are also innervated by axons containing 30 nm agranular vesicles.     

Catecholaminergic innervation of the rat adrenal cortex

Summary The zona glomerulosa of the rat adrenal gland is innervated by catecholaminergic nerves. Using histofluorescence techniques, we observed catecholaminergic plexuses surrounding adrenal capsular and subcapsular blood vessels. Individual varicose nerve fibers that branched off these plexuses were distributed among adrenal glomerulosa cells. This innervation was permanently eliminated after neonatal sympathectomy with guanethidine or 6-hydroxydopamine, but was not affected by ligation of the splanchnic nerve or extirpation of the suprarenal ganglion. At the ultrastructural level, axonal varicosities were commonly observed in close proximity to glomerulosa cells and blood vessels. Nerve fibers and varicosities were found to contain small (30–60 nm) clear vesicles as well as large (60–110 nm) and small (30–60 nm) dense-cored vesicles. In tissue fixed for the dichromate reaction with or without pretreatment with the false transmitter 5-hydroxydopamine, many nerve terminals contained numerous small dense-cored vesicles which are thought to contain catecholamines. These results establish the anatomical substrate for the catecholaminergic innervation of the rat adrenal cortex.     

Electron microscopic analysis of tyrosine hydroxylase,dopamine-β-hydroxylase and phenylethanolamine-N-methyltransferase immunoreactive innervation of the hypothalamic paraventricular nucleus in the rat

Summary The catecholaminergic innervation of the hypothalamic paraventricular nucleus (PVN) of the rat was studred by preembedding immunocytochemical methods utilizing specific antibodies which were generated against catecholamine synthesizing enzymes. Phenylethanolamine-N-methyltransferase (PNMT)-immunoreactive terminals contained 80–120 nm dense core granules and 30–50 nm clear synaptic vesicles. The labeled boutons terminated on cell bodies and dendrites of both parvo- and magnocellular neurons of PVN via asymmetric synapses. The parvocellular subnuclei received a more intense adrenergic innervation than did the magnocellular regions of the nucleus. Dopamine--hydroxylase (DBH)-immunopositive axons were most numerous in the periventricular zone and the medial paryocellular subnucleus of PVN. Labeled terminal boutens contained 70–100 nm dense granules and clusters of spherical, electron lucent vesicles. Dendrites, perikarya and spinous structures of paraventricular neurons were observed to be the postsynaptic targets of DBH axon terminals. These asymmetric synapses frequently exhibited subsynaptic dense bodies. Paraventricular neurons did not demonstrate either PNMT or DBH immunoreactivity. The fibers present within the nucleus which contained these enzymes are considered to represent extrinsic afferent connections to neurons of the PVN.Tyrosine hydroxylase (TH)-immunoreactivity was found both in neurons and neuronal processes within the PVN In TH-cells, the immunolabel was associated with rough endoplasmic reticulum, free ribosomes and 70–120 nm dense granules. Occasionally, nematosome-like bodies and cilia were observed in the TH-perikarya. Unlabeled axons established en passant and bouton terminaux type synapses with these TH-immunopositive cells. TH-immunoreactive axons terminated on cell bodies as well as somatic and dendritic spines of paraventricular parvocellular neurons. TH-containing axons were observed to deeply invaginate into both dendrites and perikarya of magnocellular neurons.These observations provide ultrastructural evidence for the participation of central catecholaminergic neuronal systems in the regulation of the different neuronal and neuroendocrine functions which have been related to hypothalamic paraventricular neurons.Supported by NIH Grant NS 19266 to W.K. Paull     

Morphology and central synaptic connections of the efferent neurons innervating the crayfish hindgut

Summary The distribution, morphology and synaptic connections of the hindgut efferent neurons in the last (sixth) abdominal ganglion of the crayfish, Orconectes limosus, have been investigated using light and electron microscopy in conjunction with retrograde cobalt/nickel and HRP labeling through the intestinal nerve. The hindgut efferent neurons occur singly and in clusters, and are unipolar. Their axonal projections are uniform and consist of a thick primary neurite with typical lateral projections and limited arborization of varicose fibers in the ganglionic neuropil. They also send lower order axon processes to the ganglionic neural sheath, where they arborize profusely, forming a network of varicose fibers. The majority of the efferent neurons project to the anterior part of the hindgut. HRP-labeled axon profiles are found in both pre- and postsynaptic position in the neuropil of the ganglion. HRP-labeled axon profiles also establish pre- and postsynaptic contacts in the intestinal nerve root. All hindgut efferent terminals contain similar synaptic vesicle populations: ovoid agranular vesicles (50–60 nm) and a few large granular vesicles (100–200 nm). It is suggested that the hindgut efferent neurons in the last abdominal ganglion are involved in: (1) innervation of the hindgut; (2) central integrative processes; (3) en route synaptic modification of efferent and afferent signals in the intestinal nerve; (4) neurohumoral modulation of peripheral physiological processes.Fellow of the Alexander von Humboldt Stiftung     

Two types of neuromuscular junctions in the pharyngeal retractor muscle ofHelix lucorum

The pharyngeal retractor muscle of the snailHelix lucorum is innervated by a pair of nerves containing axons of two types, for which there are two corresponding types of myoneural junctions with the muscle cells. The junctions of type I correspond to the thick axons. The terminals of these axons, which contain numerous spherical transparent vesicles (41±5 nm) and fewer vesicles of the dense-core type (67±3 nm), make contact mainly with noncontracting sarcoplasmic projections of the muscle cells. Junctions of type II correspond to thin axons, containing many granules. The terminals of these axons make contact with contractile parts of the muscle cells and they contain a heterogeneous population of vesicles: small spherical clear vesicles (44±2 nm), granules with fine-grained contents (135±5 nm), and a few spherical dense-core vesicles. The distance between the muscle cells is usually great — over 50 nm, but in the region of the sarcoplasmic processes the surface membranes come together to form a gap which in some areas does not exceed 10 nm.N. K. Kol'tsov Institute of Developmental Biology, Academy of Sciences of the USSR, Kiev. Translated from Neirofiziologiya, Vol. 9, No. 5, pp. 539–542, September–October, 1977.