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  • 1
    ISSN: 1573-7381
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary Frog, snake and rat neuromuscular junctions were prepared for electron microscopy by the quick-freeze, deep-etch, rotary replication procedure. The postsynaptic membrane was exposed by treating muscles with 1 mg/ml collagenase to remove the basal lamina. Present on the apices of the postsynaptic folds are regular arrays of 8–9 nm protrusions. These are not seen in the depths of the folds nor elsewhere on the muscle surface, thus they presumably represent the heads of cholinergic receptor molecules. These protrusions tend to be arranged in parallel rows two-abreast. Their high concentration (10 000/μm2) and their orderly arrangement is basically similar to the receptors seen inTorpedo postsynaptic membrane. Their distribution did not appear to change after denervation. Efforts were made to expose possible anchoring structures of these receptors, by treating muscles with 0.1% Saponin immediately before and/or during fixation in 1% formaldehyde, or by homogenizing muscles after brief formaldehyde fixation. This washed most soluble protein out of the cytoplasm and exposed a submembraneous meshwork just beneath the postsynaptic membrane. This meshwork appears to connect the membrane to underlying bundles of intermediate filaments which course through the postsynaptic processes that border each fold. This meshwork is presumably equivalent to the postsynaptic ‘density’ seen in thin sections. Its three-dimensional structure suggests that it could anchor receptor molecules to underlying cytoskeletal elements and thus immobilize receptors in the plane of the postsynaptic membrane.
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  • 2
    ISSN: 1573-7381
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary Two types of autonomic nerve cell in the cloacal region of lamprey,Lampetra japonica have been studied by electron microscopy. Large ganglion cells (LGC) were unipolar and individually invested with a satellite cell sheath. The LGC-satellite cell complex measured 24 μm × 38 μm on average. Granular endoplasmic reticulum and cored vesicles (80–140 nm in diameter) were scattered in the perikaryon. Two kinds of peculiar cytoplasmic filament were seen in LGC: one type was about 20 nm in diameter with periodic dense material on the surface and the other had a diameter of about 8 nm and showed an undulating profile. Nerve endings containing abundant small clear vesicles (30–50 nm in diameter) and a few larger cored vesicles (50–100 nm in diameter) were found in synaptic contact with LGC. Small ganglion cells (SGC) were also unipolar and covered incompletely by a satellite cell sheath. The SGC-satellite cell complex measured 6 and 12 μm on average. The SGC was packed with organelles and the perikaryon appeared more electron dense than that of LGC. SGC perikaryonal cytoplasm contained dispersed granular endoplasmic reticulum and numerous large cored vesicles (55–220 nm in diameter). Nerve endings containing numerous large cored vesicles (70–170 nm in diameter) and variable numbers of small clear vesicles (30–50 nm in diameter) were seen on the surface of SGC.
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  • 3
    ISSN: 1573-7381
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary X-ray diffraction was used to probe the effects of triethyl tin (TET) on the periodicity and amount of membrane disorientation in the lamellar myelin from respiring optic and sciatic nervesin vitro as well as from nerves of rats treatedin vivo through their drinking water. The diffraction patterns show thatin vitro TET at concentrations of 4–100 μm affects C.N.S. but not P.N.S. myelin structure. A planar, concentric membrane array with a 200 Å period is detected in the C.N.S.; this ordered, swollen myelin contrasts with the vacuolar and vesicular structure seen in thin-sections in TET-induced oedema. No effects of short-termin vivo treatment with TET are observed in either the C.N.S. or P.N.S. The finding that carbonic anhydrase (CA) inhibitors have no effect on the TET-induced structural changes indicates that the swelling we observe is not related to a CA-dependent process. In comparison, the TET effect is prevented by replacing the mobile ions with isotonic sucrose. We conclude that TET-induced swelling in C.N.S. myelin arises from an increase in ion transport followed by obligatory fluid movement. Further, the ordered, swollen structure we detect may be an intermediate state that exists transientlyin vivo in TET intoxication and that precedes the gross swelling and vacuolization usually observed.
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  • 4
    ISSN: 1573-7381
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary Foetal retina was removed from donor rats at 15 days of gestation and transplanted to the occipital cortex of neonatal host rats. The purpose of this procedure was to examine the development of retinal neurons and photoreceptors, and document synaptic patterns during maturation of the transplanted retina in an environment lacking a normal target for optic axons. Host animals were sacrificed at 5, 10, 15, 20 and 30 days and samples of cortex containing the transplant were subjected to a light and electron microscopic analysis. During early stages of development, (5 days) the retina assumes a radial orientation with the scleral (outer) surface located centrally and the vitreal (inner) surface occupying the periphery. Numerous mitotic figures are found at the centre of the transplant and columns of primitive neuroblasts appear to radiate out from this zone. By 10 to 15 days after transplantation the retinal tissue contains numerous small rosettes each of which displays a histotypic organization with recognizable layers of sensory cells and their centrally-projecting processes, an outer limiting membrane, made up of a network of zonulae adherentes, and a rudimentary outer and inner plexiform layer which delineate the cells of the inner nuclear layer. Ultrastructural analysis of such rosettes confirmed the presence of typical bipolar, amacrine, horizontal and ganglion cells, but revealed that while the plexiform layers were occupied by numerous processes from these neurons, few if any, of these exhibited synaptic vesicles. By 20 to 30 days following transplantation sensory cells have completely differentiated, giving rise to prominent inner and outer segments which display typical cilia, centrioles and basal bodies, together with numerous stacked lamellae of photoreceptors which were contorted, presumably due to growth in an abnormal site. It should be further emphasized that these structures developed in the absence of pigment cells. Synaptic development ensues during this period to form characteristic dyads within the outer and inner plexiform layers. Additionally, clusters of amacrine to amacrine contacts occurred in the inner plexiform layer and were found to be increased relative to other types of junctions. In general, synaptogenesis takes place in the outer and inner plexiform layers and all categories of retinal synapses are established, but the process was found to be significantly delayed in comparison to normal retina at the same stage of development. Quantitative analysis revealed a reduced number of presumptive ganglion cells in proportion to the other categories of neurons. Optic fibres remained small and failed to myelinate. It is suggested that lack of an appropriate target for optic axons induced this alteration and may be indirectly related to the delay in the onset of synaptic development.
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  • 5
    ISSN: 1573-7381
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary In acute electrophysiological experiments on anaesthetized cats, single identified hair follicle afferent fibres were injected with horseradish peroxidase (HRP). The HRP was injected from an intra-axonal microelectrode in the lumbosacral spinal cord. One to six hours after injection the animals were perfused and the tissue prepared for light and electron microscopy (EM). Axon collateral arborizations containing HRP reaction product were identified in thick sections under the light microscope and the same tissue then cut on the ultramicrotome for EM study. The terminal branches of the collaterals kept their myelin sheaths until they were 0.45–l.0 μm in diameter, just before they formed synaptic boutons. Synaptic boutons (1.0–4.0 μm in diameter) were usually of theen passant variety and made contact with dendrites. The contacts were asymmetrical (Type I) and contained round, clear synaptic vesicles of 35–60 nm diameter. Both the non-myelinated portion of the terminal axon and the synaptic boutons received axo-axonic contacts. These axo-axonic boutons contained clear (agranular) vesicles irregular in profile.
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  • 6
    ISSN: 1573-7381
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary A light and electron microscopic study of the developing corpus callosum was carried out in foetal and neonatal rats in order to determine the mode of growth of the earliest callosal axons across the midline and to investigate the potential role played by non-neuronal cells during the formation of the tract. The axons of the corpus callosum first cross the midline between the 18th and 19th days of gestation by traversing the anterodorsal aspect of the pre-existing hippocampal commissure. Prior to the appearance of the callosal axons at the midline, there is an aggregation of astrocyte processes anterior and dorsal to the hippocampal commissure. Careful examination of these processes in different planes of section shows that they are not organized in any obvious way that would provide a clearly defined path for the growing axons; nor are there any preferentially oriented extracellular spaces at the midline. No specialized membrane contacts could be seen between non-neuronal cell processes and the early callosal axons. Thus, there is no overt morphological evidence for an active role of non-neuronal cells in axon guidance in the initial formation of the corpus callosum. The development of the corpus callosum is accompanied by the formation of a temporary cavum septi pellucidi, which is always closed to the subarachnoid space. The cavum persists during the first postnatal week, after which time it becomes populated by cells of the lateral septal nuclei. Macrophages are present within the cavum and may play a role in its formation. Macrophages are also found within parts of the corpus callosum. No obvious degeneration of axons is seen in the corpus callosum during its early development.
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  • 7
    ISSN: 1573-7381
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary A new culture technique is described for dissociated ganglia of embryos ofPeriplaneta americana. Neurons obtained by the mechanical dissociation of ganglia from 23–26 day old embryos were grown for seven days in a combination of Schneider's Drosophila Medium and Eagle's Basal Medium and then transferred to a combination of Leibovitz's L-15 Medium and Yunker's Modified Grace's Medium supplemented with ecdysone. Neurons cultured in this way survived for periods in excess of one month. The cells had a typically neuronal ultrastructure and produced axonal processes that associated to form fibre bundles. Within these processes neurosecretory granules, dense-core vesicles and clear vesicles were found. In some instances these vesicles aggregated at points of contact between fibres giving the appearance of presynaptic endings. These cultures would appear to be suitable for pharmacological studies of the insect nervous system.
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  • 8
    ISSN: 1573-7381
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary Axon plasma membranes (axolemma) were studied by freeze-fracture electron microscopy at stages prior to and during myelination in the optic nerves of neonatal rats. In unensheathed axons, intramembranous particles associated with the internal (P) and external (E) leaflets of the axolemma increased in number before reaching a plateau (approximately 600/μm2 in both leaflets) at about 9 days postnatally. In newly myelinated fibres, by contrast, the distribution of particles was asymmetrical; fewer particles (approximately 200/μm2) were found on the E-face and greater numbers (approximately 1400/μm2) were present on the P-face, distributions similar to those observed in mature myelinated fibres. Node-like aggregations of particles were not found in unensheathed pre-myelinated axons nor were they present in axons presumed to be ensheathed by glial cytoplasm but not yet myelinated, although nodal specializations could be easily identified in fibres with only a few turns of compact myelin. These observations show first that there is a redistribution of particles in the P- and E-faces of the internodal axolemma coincident with the onset of myelination and secondly, that nodal specializations (represented by the increased densities of E-face particles) appear after ensheathment but before the formation of compact myelin in fibres of the rat optic nerve.
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  • 9
    ISSN: 1573-7381
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary Samples of cerebral cortex (parietal and occipital) and thalamic nuclei (ventrobasal, posterolateral, dorsal lateral geniculate) from normal, adult, aldehyde perfusion fixed mice and rats were examined by electron microscopy for the presence of free postsynaptic-like densities (FPSDs). FPSDs are plaques of intracellular paramembranous electron-dense material, ultrastructurally indistinguishable from postsynaptic densities, but not aligned with a presynaptic specialization. In a systematic survey of the neuropil around 6000 neuronal perikarya, 250 FPSDs were encountered. Almost all of these were within dendritic spines and shafts and about 90% of them were apposed by a neuronal perikaryon, the remainder by a dendritic shaft. Inevery case a subsurface cistern (SSC) was present in the cell body or dendrite apposed to the FPSD, and was flattened along the extent of the FPSD. In none of the material were the FPSDs associated, even remotely, with degenerating elements, suggesting that they are not formed by degeneration of presynaptic boutons. The incidence of FPSD-SSC complexes was higher in thalamus than in cerebral cortex which, together with previous observations indicating their absence from normal cerebellar cortex, suggests significant regional variations in distribution. It is suggested that FPSDs might represent synaptic precursors perhaps induced to form as a response to loss (possibly age-dependent loss) of synaptic contacts on a neuron and that the SSCs are somehow involved in maintaining the FPSDs and/or preparing them for innervation by adjacent axon terminals to form new synaptic contacts.
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  • 10
    ISSN: 1573-7381
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary This report gives a detailed description of the fine structure of the olfactory mucosa in man. Using a special biopsy instrument and technique, fresh biopsies of olfactory epithelium were taken under local anaesthesia from eight normal volunteers. Transmission electron microscopy reveals that human olfactory epithelium has four major cell types: ciliated olfactory receptors, supporting cells, basal cells and microvillar cells. The ciliated olfactory receptors, as in other mammals, are bipolar neurons; the dendrite tip, modified to form the olfactory vesicle, bears 10–30 cilia that lack dynein arms. The supporting cells, markedly different from the goblet cells of respiratory epithelium, are not specialized for mucus secretion. Instead they are equipped to contribute materials to, and remove materials from, the surface mucus. The basal cells are stem cells that serve to replace epithelial cells and receptors lost during normal turnover or injury. In addition to ciliated olfactory neurons, supporting cells and basal cells, the human olfactory mucosa contains a distinct fourth cell type, the microvillar cell, of unknown function. The apical pole of the cell sends a tuft of short microvilli into the nasal cavity; its basal pole gives rise to a slender cytoplasmic process that resembles an axon. If microvillar cells prove to be sensory cells, the current concept of the human olfactory epithelium will have to be revised to include two morphologically distinct classes of receptors.
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