鲫鱼尾部神经分泌系统显微和亚显微结构的季节性变化

鲫鱼尾部神经分泌系统的神经分泌细胞和它的轴突中可观察到各种不同电子密度的颗粒。在性腺各个不同的发育阶段,该系统的分泌物具有累积、充满、释放和恢复这样一种周期性变化,由此说明鲫鱼的尾部神经分泌系统和它的生殖有关。     

Immunocytochemical Localization of Synaptic Proteins at Vesicular Organelles in PC12 Cells

The distribution of the three synaptic vesicle proteins SV2, synaptophysin and synaptotagmin, and of SNAP-25, a component of the docking and fusion complex, was investigated in PC12 cells by immunocytochemistry. Colloidal gold particle-bound secondary antibodies and a preembedding protocol were applied. Granules were labeled for SV2 and synaptotagmin but not for synaptophysin. Electron-lucent vesicles were labeled most intensively for synaptophysin but also for SV2 and to a lesser extent for synaptotagmin. The t-SNARE SNAP-25 was found at the plasma membrane but also at the surface of granules. Labeling of Golgi vesicles was observed for all antigens investigated. Also components of the endosomal pathway such as multivesicular bodies and multilamellar bodies were occasionally marked. The results suggest that the three membrane-integral synaptic vesicle proteins can have a differential distribution between electron-lucent vesicles (of which PC12 cells may possess more than one type) and granules. The membrane compartment of granules appears not to be an immediate precursor of that of electron-lucent vesicles.     

Structure of anterior dorsal ventricular ridge in snakes.

Anterior dorsal ventricular ridge (ADVR) is a major subcortical, telencephalic nucleus in snakes. Its structure was studied in Nissl, Golgi, and electron microscopic preparations in several species of snakes. Neurons in ADVR form a homogeneous population. They have large nuclei, scattered cisternae of rough endoplasmic reticulum in their cytoplasm, and bear dendrites from all portions of their somata. The dendrites have a moderate covering of pedunculated spines. Clusters of two to five cells with touching somata can be seen in Nissl, Golgi, and electron microscopic preparations. The area of apposition may contain a series of specialized junctions which resemble gap junctions. Three populations of axons can be identified in rapid Golgi preparations of snake ADVR. Type 1 axons course from the lateral forebrain bundle and bear small varicosities about 1 mu long. Type 2 axons arise from ADVR neurons and bear large varicosities about 5 mu long. The origin of the very thin type 3 axons is not known; they bear small varicosities about 1 mu long. The majority of axon terminals in ADVR are small (1 mu to 2 mu long), contain round synaptic vesicles, and form asymmetric active zones. This type of axon terminates on dendritic spines and shafts and on somata. A small percentage of terminals are large, 5 mu in length, contain round synaptic vesicles, and form asymmetric active zones. This type of axon terminates only on dendritic spines. A small percentage of terminals are small, contain pleomorphic synaptic vesicles, and form symmetric active zones. This type of axon terminates on dendritic shafts and on somata.     

Synaptin/synaptophysin, p65 and SV2: their presence in adrenal chromaffin granules and sympathetic large dense core vesicles.

The subcellular distribution of three proteins of synaptic vesicles (synaptin/synaptophysin, p65 and SV2) was determined in bovine adrenal medulla and sympathetic nerve axons. In adrenals most p65 and SV2 is confined to chromaffin granules. Part of synaptin/synaptophysin is apparently also present in these organelles, but a considerable portion is found in a light vesicle which does not contain significant concentrations of typical markers of chromaffin granules (cytochrome b-561, dopamine beta-hydroxylase or the amine carrier). An analogous finding was obtained for sympathetic axons. The large dense core vesicles contain most p65 and also SV2 but only a smaller portion of synaptin/synaptophysin. A lighter vesicle containing this latter antigen and some SV2 has also been found. These results establish that in adrenal medulla and sympathetic axons three typical antigens of synaptic vesicles are not restricted to light vesicles. Apparently, a varying part of these antigens is found in chromaffin granules and large dense core vesicles. On the other hand, the light vesicles do not contain significant concentrations of functional antigens of chromaffin granules. Thus, the biogenesis of small presynaptic vesicles which contain all three antigens as well as functional components like the amine carrier is likely to involve considerable membrane sorting.     

Sorting during transport to the surface of PC12 cells: divergence of synaptic vesicle and secretory granule proteins

PC12 cells, a cell line derived from a rat pheochromocytoma, have both regulated and constitutive secretory pathways. Regulated secretion occurs via large dense core granules, which are related to chromaffin granules and are abundant in these cells. In addition, PC12 cells also contain small electron-lucent vesicles, whose numbers increase in response to nerve growth factor and which may be related to cholinergic synaptic vesicles. These could characterize a second regulated secretory pathway. We have investigated the trafficking of protein markers for both these organelles. We have purified and characterized the large dense core granules from these cells using sequential velocity and equilibrium gradients. We demonstrate the copurification of the major PC12 soluble regulated secretory protein (secretogranin II) with this organelle. As a marker for the synaptic vesicle-like organelles in this system, we have used the integral membrane glycoprotein p38 or synaptophysin. We show that the p38-enriched fraction of PC12 cells comigrates with rat brain synaptic vesicles on an equilibrium gradient. We also demonstrate that p38 purifies away from the dense core granules; less than 5% of this protein is found in our dense granule fraction. Finally we show that p38 does not pass through the dense granule fraction in pulse-chase experiments. These results rule out the possibility of p38 reaching the small clear vesicles via mature dense granules and imply that these cells may have two independently derived regulated pathways.     

Compartmentalization of monoaminergic synaptic vesicles in the storage and release of neurotransmitter

Monoaminergic nerves are characterized by the presence of a population of small synaptic vesicles (40-60 nm in diameter) containing a few large vesicles (80-90 nm in diameter). Thus, although both types of vesicles contain monoamines, the small vesicles must be considered as the organoid responsible for the storage and release of the neurotransmitter, whereas the large ones possibly are involved in the modulation of the process. The small vesicles are electron-lucent or have an osmiophilic electron-dense core that is always linked to the vesicle membrane. Considering morphological and histochemical evidence under different experimental conditions, we proposed the existence of two compartments in the small vesicles: the core and the matrix, corresponding respectively to the electron-dense core and the electron-lucent space between the core and the vesicle membrane in osmium tetroxide fixations. The sizes of both compartments are inversely related, i.e., the smaller the core, the larger the matrix and vice versa. The core even disappears, giving way to a small electron-lucent vesicle made exclusively by the matrix. Thus, the matrix is a constant component of the vesicle, whereas the core is a transient one. Each compartment has a different pool of amine: a loosely bound, easily releasable pool in the matrix and a tightly bound, more resistant pool in the core. These two pools subserve, respectively, a tonic or phasic release of the neurotransmitter, correlated with a tonic or phasic stimulation of the receptor. The core may be considered as a storage or reserve pool. Experimental evidence from our laboratory supports the concept that different mechanisms are operative in both compartments in the release of the neurotransmitter. For instance, a Ca2(+)-independent release would be primarily concerned with the neurotransmitter contained in the matrix, and a Ca2(+)-dependent efflux would be primarily related with the neurotransmitter stored in the core. However, it still must be established that a simple relationship exists between each kind of stimulus and each vesicle compartment, rather than both compartments being integrated in a dynamic functional unit.     

Light and electron microscopic studies on the pineal tract of rainbow trout, Salmo gairdneri.

The pineal tract of rainbow trout from the pineal end vesicle to the posterior commissure was studied by light and electron microscopy. Five types of nerve fibres (photoreceptor basal process, ganglion cell dendrite, electron-lucent fibre and synaptic vesicles, myelinated and unmyelinated axons) and two modes of synapses (photoreceptor basal process ganglion cell dendrite and axon terminal with synaptic vesicles-photoreceptor basal process synapses) are distinguishable in the proximal region of end vesicle. The two distinct synaptic associations with the photoreceptor basal process suggest two different (excitatory and inhibitory) control of pineal sensory activity. At the distal portion of stalk about two thousand nerve fibres converge into dorsal and ventral bundles. Posterior to the habenular commissure several small branches run out laterally from the ventral bundles to the basal margin of the ependyma, but not into the habenular commissure. The dorsal bundle passes through the dorsal side of the subcommissural organ and runs ventral to the posterior commissure. The pineal tract is composed of unmyelinated axons, electron-lucent nerve fibres and myelinated axons. The number of fibres increases throughout the stalk and reaches the maximum number at the opening of pineal lumen to IIIrd ventricle, however, the number of fibres then decreases through the subcommissural organ and posterior commissure. This increase and decrease of nerve fibres suggest the continuous participation of axonal fibres of pineal nerve cells and the ramification or branching of pineal tract, respectively.     

Synaptic patterns in the dorsal lateral geniculate nucleus of the monkey

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

Ultrastructural and immunocytochemical studies of neuromuscular junctions in oviduct of Locusta migratoria

The ultrastructure of nerve endings in the oviduct visceral muscles of Locusta migratoria was studied by electron microscopy and by immunogold labeling for two kinds of neuromodulators, the pentapeptide proctolin and FMRFamide-related peptides. Nerve endings contained electron-lucent round vesicles and two kinds of granules (round and avoid), and formed two types of synapses or release sites with the muscle. The morphologically distinct nerve endings were classified into three different categories based on the composition of synaptic vesicles and granules. Type-I nerve endings were dominated by electron-lucent round vesicles and contained only a few round electron-dense granules. Type-II nerve endings contained mostly electron-dense round granules and electron-lucent round vesicles. A few electron-dense ovoid granules were also present. Electron-dense ovoid granules dominated the type-III nerve endings, which usually contained less electron-lucent vesicles than either type-I or II nerve endings. Both proctolin and FMRFamide-like immunoreactivity was associated with electron-dense round granules. However, FMRFamide-like immunoreactivity was only found in the type-II nerve endings, while proctolin immunoreactivity was found within type-I nerve endings as well as in some type-II nerve endings. Immunological results therefore allow us to further divide type-II nerve endings into type-IIa (immunonegative for proctolin) and type-IIb (immunopositive for proctolin). Type-III nerve endings show no immunolabeling to either proctolin or FMRFamide.     

Membrane Composition of Adrenergic Large and Small Dense Core Vesicles and of Synaptic Vesicles: Consequences for Their Biogenesis

The membrane proteins of adrenergic large dense core vesicles, in particular those of chromaffin granules, have been characterized in detail. With the exception of the nucleotide carrier all major peptides have been cloned. There has been a controversy whether these vesicles contain antigens like synaptophysin, synaptotagmin and VAMP or synaptobrevin found in high concentration in synaptic vesicles. One can now conclude that large dense core vesicles also contain these peptides although in lower concentrations. The biosynthesis of large dense core vesicles is analogous to that of other peptide secreting vesicles of the regulated pathway. One cannot yet definitely define the biosynthesis of small dense core vesicles which apparently have a very similar membrane composition to that of large dense core vesicles. They may form directly from large dense core vesicles when their membranes have been retrieved after exocytosis. These membranes may become sorted in an endosomal compartment where peptides may be deleted or added. Such an addition could be derived from synaptophysin-rich vesicles present in adrenergic axons. However small dense core vesicle peptides may also be transported axonally independent of large dense core vesicles. For proving one of these possibilities some crucial experiments have been suggested.     

Myoneural junctions and neurosecretory endings on spermathecal muscle fibers of two weevils,Sitophilus granarius and Hypera postica

The fine structure of muscle fibers connecting the two arms of the spermatheca and their innervating axons was studied with the electron microscope. The muscle fibers appear to be a sub-set of skeletal and not visceral muscles. Neurosecretory axons with electron dense granules are adjacent to the muscle fibers in young females O-day post-eclosion but not in the ovipositing adult. The typical nerves form synaptic junctions with muscle fibers at all ages but the nerves are divided into two types based on the morphology of the synaptic vesicles they contain, either spherical or flattened.     

The relationship between synaptic vesicles,Golgi apparatus,and smooth endoplasmic reticulum: a developmental study using the zinc iodide-osmium technique

Summary Routine electron microscopy and a zinc iodide-osmium tetroxide technique (ZIO), recently found to be specific for synaptic vesicles, were used to study the origin of synaptic vesicles during postnatal development in the lumbosacral enlargement of the albino rat. In immature nervous tissue, a large number of vesicles, indistinguishable from synaptic vesicles (S vesicles), were found in the Golgi apparatus and in different portions of the axon where they were often intermingled with elements of the smooth endoplasmic reticulum (SER). Ten to twenty percent of these S vesicles within the Golgi apparatus as well as the majority of these vesicles in all parts of the axon were positive to ZIO. Much of the SER in axons was also positive. The number of vesicles and elements of the SER showed some decrease in the non-terminal portion of axons on day 21 and even more of a decrease in adult neurons. These data suggest that synaptic vesicles are produced in the Golgi apparatus and SER in immature neurons. The decrease in S vesicles and SER in adult neurons suggests a drop in synaptic vesicle production after synaptogenesis has ended. In addition, the material that has been studied shows that ZIO staining is not limited to synaptic vesicles during development since oligodendroglia and endothelial cells are also stained during this period.     

Neuromuscular junctions in the buccal mass of Aplysia: fine structure and electrophysiology of excitatory transmission.

The lower extrinsic protractor muscle in the buccal mass of Aplysia consists of bundles of muscle fibers 4--12 mu in diameter, containing thick and thin filaments that are not arranged in a transversely striated pattern. Individual fibers come close to one another and form specialized junctional regions. Electrophysiological evidence indicates that the muscle fibers form an electrical cyncytium. Muscle bundles are innervated by more than one excitatory axon at a number of points along their length. The presynaptic terminals contain spherical electron-lucent vesicles and a few larger electron-dense vesicles. There are no obvious structural postsynaptic specializations. Graded contraction can result from summation of excitatory junctional potentials in separate axons or from summation and facilitation of junctional potentials from a single axon. The buildup of facilitation during a train of stimuli results from the linear summation of facilitation remaining from preceding impulses.     

Neuromuscular junctions in the buccal mass of Aplysia: Fine structure and electrophysiology of excitatory transmission

The lower extrinsic protractor muscle in the buccal mass of Aplysia consists of bundles of muscle fibers 4–12 m̈ in diameter, containing thick and thin filaments that are not arranged in a transversely striated pattern. Individual fibers come close to one another and form specialized junctional regions. Electrophysiological evidence indicates that the muscle fibers form an electrical syncytium. Muscle bundles are innervated by more than one excitatory axon at a number of points along their length. The presynaptic terminals contain spherical electron-lucent vesicles and a few larger electron-dense vesicles. There are no obvious structural postsynaptic specializations. Graded contraction can result from summation of excitatory junctional potentials in separate axons or from summation and facilitation of junctional potentials from a single axon. The buildup of facilitation during a train of stimuli results from the linear summation of facilitation remaining from preceding impulses.     

Neurotransmitter release

Axon terminals release more than one physiologically active substance. Synaptic messengers may be stored in two different types of vesicles. Small electron-lucent vesicles mainly store classical low molecular weight transmitter substances and the larger electron-dense granules store and release proteins and peptides. Release of the two types of substances underlies different physiological control. Release of messenger molecules from axon terminals is triggered by influx of Ca2+ through voltage sensitive Ca2+ channels and a rise in cytosolic Ca2+ concentrations. Neither the immediate Ca2+ target(s) nor the molecular species involved in synaptic vesicle docking, fusion and retrieval are known. It is, however, likely that steps involved in the molecular cascade of transmitter release include liberation of vesicles from their association with the cytonet and phosphorylation by protein kinase C of proteins which have the ability to alter between membrane bound and cytoplasmic forms and thus facilitate or initiate the molecular interaction between synaptic vesicles and the plasma membrane.     

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.     

Ultrastructure of the hypothalamic neurosecretory system of the dog

Summary The hypothalamic neurosecretory system of normal dogs was studied by light and electron microscopy after perfusion-fixation. In the supraoptic nucleus most neurons are loaded with elementary neurosecretory granules having a content of low electron density. Neurons with less neurosecretory material and signs of enhanced synthetic activity, as recognized by the changes in the endoplasmic reticulum, were also observed.The vesiculated neurons ofJewell were studied under the electron microscope and various stages of development were described. It was postulated that they originate by a localized process of cytoplasmic cytolysis which ends in the formation of a large aqueous intracellular cavity limited by a plasma membrane. The possible significance of these vesiculated neurones is discussed. Some few myelinated neurosecretory axons are found in the supraoptic nucleus.The neurons of the paraventricular nucleus are smaller and contain less neurosecretory material. This is abundant and very pale in the axons. The median eminence consists of an inner zone, mainly occupied by the neurosecretory axons of the hypothalamic-neurohypophysial tracts, and an outer zone in which some neurosecretory axons end on the capillary of the portal system. This outer zone contains numerous axons with the axoplasm rich in neurofilaments and some containing granulated and non-granulated synaptic vesicles. Some neurons with granulated vesicles were observed in this region. The adrenergic nature of these neurons and axons is postulated.The infundibular process of the neurohypophysis shows small axons with discrete amounts of elementary granules and vesicles of synaptic type at the endings. Some enlarged axons having, in addition, large polymorphic bodies are observed and related to the Herring bodies.The size and morphology of the granules are analyzed along the entire hypothalamic-neurohypophysial system. The changes in diameter and electron density are related to the maturation of the granules and the possible significance of such evolution.Supported by grants from the Consejo Nacional de Investigaciones Cientificas y Técnicas and by the Air Force Office of Scientific Research (AF-AFOSR 963-66).     

A Survey of the Cytology and Synaptic Organization of the Insular Trigeminal—Cuneatus Lateralis Nucleus in the Rat,Including an Identification of Spinal Afferent Inputs

The cytology and synaptic organization of the insular trigeminal—cuneatus lateralis (iV-Cul) nucleus was examined in the rat. In addition, the ultrastructural morphology and synaptic connectivity of anterogradely labeled spinal afferent axons terminating in iV-Cul were examined following injection of horseradish peroxidase (HRP) into the cervical spinal cord. The uniformity of the ultrastructural features of iV-Cul neurons supports the presence of a homogeneous neuronal population. The most prominent feature of the iV-Cul neuropil is the presence of numerous interdigitating astrocytic processes, which extensively isolate neuronal somata and processes. iV-Cul contains a heterogeneous population of axonal endings that can be separated into three categories, depending upon whether they contain predominantly spherical-shaped agranular synaptic vesicles (R endings), predominantly pleomorphic-shaped agranular synaptic vesicles (P endings), or a heterogeneous population of dense-core vesicles (DC endings). The R endings represent the majority of axonal endings in iV-Cul and establish asymmetrical axodendritic and axospinous synaptic contacts, primarily along the distal portions of the dendritic tree. P endings establish symmetrical axosomatic, axodendritic, and axospinous synaptic contacts and exhibit a more generalized distribution along the somadendritic tree. DC terminals establish asymmetrical axodendritic synaptic contacts with distal dendritic processes and are the least frequently observed endings in the iV-Cul neuropil. Numerous synaptic glomeruli, exhibiting a single large central R bouton that establishes multiple axodendritic or axospinous synapses, characterize the iV-Cul neuropil. Axoaxonic synapses are conspicuously absent from the iV-Cul neuropil and glomeruli. The anterograde HRP labeling of spinal afferent axons that terminate in iV-Cul indicates that the terminals along these fibers are of the R type and that they are engaged predominantly in synaptic glomeruli. The results of this study indicate that the synaptic organization of iV-Cul is distinctly different from that of neighboring somatosensory nuclei, and supports the recent suggestion that this nucleus should be considered a separate precerebellar spinal relay nucleus in the lateral medulla.     

A survey of the cytology and synaptic organization of the insular trigeminal-cuneatus lateralis nucleus in the rat, including an identification of spinal afferent inputs

The cytology and synaptic organization of the insular trigeminal-cuneatus lateralis (iV-Cul) nucleus was examined in the rat. In addition, the ultrastructural morphology and synaptic connectivity of anterogradely labeled spinal afferent axons terminating in iV-Cul were examined following injection of horseradish peroxidase (HRP) into the cervical spinal cord. The uniformity of the ultrastructural features of iV-Cul neurons supports the presence of a homogeneous neuronal population. The most prominent feature of the iV-Cul neuropil is the presence of numerous interdigitating astrocytic processes, which extensively isolate neuronal somata and processes. iV-Cul contains a heterogeneous population of axonal endings that can be separated into three categories, depending upon whether they contain predominantly spherical-shaped agranular synaptic vesicles (R endings), predominantly pleomorphic-shaped agranular synaptic vesicles (P endings), or a heterogeneous population of dense-core vesicles (DC endings). The R endings represent the majority of axonal endings in iV-Cul and establish asymmetrical axodendritic and axospinous synaptic contacts, primarily along the distal portions of the dendritic tree. P endings establish symmetrical axosomatic, axodendritic, and axospinous synaptic contacts and exhibit a more generalized distribution along the somadendritic tree. DC terminals establish asymmetrical axodendritic synaptic contacts with distal dendritic processes and are the least frequently observed endings in the iV-Cul neuropil. Numerous synaptic glomeruli, exhibiting a single large central R bouton that establishes multiple axodendritic or axospinous synapses, characterize the iV-Cul neuropil. Axoaxonic synapses are conspicuously absent from the iV-Cul neuropil and glomeruli. The anterograde HRP labeling of spinal afferent axons that terminate in iV-Cul indicates that the terminals along these fibers are of the R type and that they are engaged predominantly in synaptic glomeruli. The results of this study indicate that the synaptic organization of iV-Cul is distinctly different from that of neighboring somatosensory nuclei, and supports the recent suggestion that this nucleus should be considered a separate precerebellar spinal relay nucleus in the lateral medulla.     

Fine structural characteristics and synaptic connections of trigeminocerebellar projection neurons in rat trigeminal nucleus oralis

In order to classify the presynaptic terminals contacting trigeminocerebellar projection neurons (TCPNs) in rat trigeminal nucleus oralis (Vo), electron-microscopic examination of sequential thin sections made from TCPNs located in the border zone (BZ) of Vo, labeled by the retrograde transport of horseradish peroxidase, was undertaken. The use of BZ TCPNs, labeled in Golgi-like fashion so that many of their dendrites and axons were visible, allowed for the determination of the distribution of each bouton type along the soma and dendrites, as well as for the characterization of the morphology and synaptic relations of the labeled axon and its terminals. Three types of axon terminals contacting labeled BZ TCPNs have been recognized, depending upon whether they contain primarily spherical-shaped, agranular synaptic vesicles (S endings); predominantly flattened, agranular synaptic vesicles (F endings); or a population of pleomorphic-shaped, agranular synaptic vesicles (P endings). The S endings represent the majority of axon terminals contacting labeled BZ TCPNs and establish asymmetrical axosomatic and axodendritic synaptic contacts. Many S endings are situated in one of two types of synaptic glomeruli. One type of glomerulus has a large S ending at its core, whereas the other contains a small S ending. Large-S-ending glomeruli include only labeled distal dendrites of BZ TCPNs; small-S-ending glomeruli contain either a labeled soma, proximal dendrite, or distal dendritic shaft. The remaining S endings are extraglomerular, synapsing on distal dendrites. P endings are less frequently encountered and establish intermediate axosomatic and axodendritic synapses. These endings exhibit a generalized distribution along the entire somatodendritic tree. F endings make symmetrical axodendritic synapses with distal dendrites, are only found in glomeruli containing small S endings, and are the least frequently observed ending contacting labeled BZ TCPNs. The majority of axonal endings synapsing on labeled BZ TCPNs are located along distal dendrites, with only a relatively few synapsing terminals situated on proximal dendrites and somata. The axons of labeled BZ TCPNs arise from the cell body and generally give rise to a single short collateral near their points of origin. This collateral remains unbranched and generates several boutons within BZ, while the parent axon acquires a myelin sheath and, without branching further, travels dorsolaterally toward the inferior cerebellar peduncle. The collateral boutons resemble extraglomerular S endings. They contain agranular, spherical-shaped synaptic vesicles and make asymmetrical axodendritic synapses with small-diameter unlabeled dendritic shafts in the BZ neuropil.