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Biomedical subjects

C Sandri

Publications and source records attributed to C Sandri.

At least 37 records · Page 2Linked to original sources
PubMed1980

Cell types and synaptic organization of the medullary electromotor nucleus in a constant frequency weakly electric fish, Sternarchus albifrons.

The medullary electromotor nucleus (EMN) of Sternarchus albifrons was studied at the light and electron microscopic levels. The EMN consists of a dense meshwork of myelinated axons and glial elements with interposed large neurons; it is provided with an abundant supply of capillaries. Two types of essentially adrendritic nerve cells were distinguished on the basis of size: giant neurons (approx. 70 micrometers in diameter) and large neurons (approx. 30 micrometers in diameter). Their population ratio is 1:4. Only giant cells are labelled following the injection of retrograde tracer into the spinal cord; they are therefore identified with the so-called "relay cells" of other gymnotids. Tracer experiments further suggest that the descending axons of these relay cells give off collateral branches throughout the elongated spinal electromotor nucleus. In contrast, the large cells remain unlabelled and therefore lack spinal projections; they most likely correspond to "pacemaker cells." The perikaryal surface, including axon hillock and proximal part of initial segment of both types of EMN cells, is contacted by clusters of synaptic terminals and astrocytic processes. Two main varieties of synaptic terminals occur: (1) large endings and (2) ordinary end feet with standard size (S-type) and variable size (Sv-type) clear, spherical vesicles. The junction between large endings and EMN cells is characterized by the combination of gap junctions and surrounding intermediate junctions whose freeze-fracture characteristics were morphometrically analyzed. The large endings were formed by nodes of Ranvier as well as by fiber terminations, and synchronization within the EMN may be achieved by presynaptic fibers. Some of the contacts occur directly on the initial segment, which could allow activity to bypass the soma. It is concluded that the elctromotor system of Sternarchus is comprised of a rapid conduction pathway where medullary pacemaker and relay cells as well as spinal electromotor neurons are coupled by synapses with gap junctions. In contrast to the spinal electromotor neurons, the medullary EMN cells receive synapses with morphological characteristics of chemical transmission, and the S-type and SV-type terminals may possibly correspond to Gray's Type I and Type II synapses, respectively. These synapses may be involved in modulation of the electric organ discharge frequency.

Animals↗
PubMed1980

Bacilliform inclusions in cells of the proximal pars distalis in the pituitary of four species of Tilapia (Teleostei).

Thin sections and replicas of freeze-etched pituitaries from six species of the teleostean family of Cichlidae were studied by electron microscopy. Four species belonging to the genus Tilapia exhibit rod-like structures, i.e., "bacilliform inclusions" (BI), about 1-2 micrometer in length in cells of the proximal pars distalis. The BI are found either isolated or fused in small groups. They are enclosed in an envelope similar to that of secretory granules. Both the BI and the secretory granules give a positive PAS-reaction.

Animals↗
PubMed1980

The influence of high pressure freezing on mammalian nerve tissue.

Vitrification of biological specimens in liquid nitrogen can be achieved under high pressure (2,100 bars). This procedure obviates the use of aldehyde fixation and cryoprotection (glycerol). The present work demonstrates its applicability to the freeze-etching of mammalian brain tissue. Freeze-fracture repicas from rat cerebellar cortex and subfornical organ prepared by this method are compared to conventionally processed material using aldehyde fixation, glycerination and freezing with Freon. The formation of large ice crystals is prevented in tissue blocks up to 0.5 mm thick; deep etching is markedly enhanced. Cytoplasmic microstructures such as mitochondrial cristae, microtubules and microfilaments, are readily observable against a finely granulated cytosol matrix. An additional advantage is the combined application with freeze-substitution.

Animals↗
PubMed1979

Increase of large intramembranous particles in the presynaptic active zone after administration of 4-aminopyridine.

The effects of 4-aminopyridine (4-AP, 1 mg/kg i.v.) on the distribution of intramembranous particles in spinal motoneuronal synapses was examined in anesthetized (pentobarbital sodium, 50 mg/kg, i.p.) rats by freeze-etching electron microscopy. The particles were classified according to their size into small (4.2--10.5 nm in diameter) and large (10.5--18.9 nm in diameter). The active zone of the presynaptic membrane was identified on the basis of ultrastructural criteria. The large intramembranous particles in both protoplasmic (PF) and external fracture faces (EF) were aggregated more densely in the active zone as compared with its surroundings both in the control and 4-AP-treated group (P less than 0.001). The density of large particles in the PF rose from 300/sq.micron under the influence of 4-AP (P less than 0.001). The increased frequency of vesicle attachment sites at the presynaptic membrane (from 115/sq.micron in barbiturized controls to 150/sq.micron in 4-AP treated animals) seemed to be positively correlated with the increasing density of large particles. Based on these findings it is suggested that the large intramembranous particles of the presynaptic membrane may participate in the control of calcium fluxes which are essential to the transmitter release by exocytosis.

Aminopyridines↗
PubMed1979

Freeze-etch study of the teleostean pituitary.

The structure of membranes in the pituitary of several teleostean species was studied with the freeze-etching technique. Nuclear pore density is higher in hormone-secreting than in stellate cells, suggesting a higher rate of nucleo-cytoplasmic interactions in the secretory cells. The perinuclear endoplasmic reticulum (ER) in the MAS-secreting cells has a large number of fenestrae, whereas in the ER whorls of the prolactin-secreting cells the fenestration occurs only rarely. Three different types of membrane specializations are described, presumably related to exo-endocytotic events at the level of the plasmalemma.

Animals↗
PubMed1979

Ultrastructural effects of 4-aminopyridine on the presynaptic membrane in the rat spinal cord.

The effects of 4-aminopyridine (4-AP, 1 mg/kg, i.v.) on the ultrastructure of synapses in the ventral horn of the spinal cord were examined in rats anesthetized with 50 and 100 mg/kg, i.p., pentobarbital sodium and alpha-chloralose respectively. The presynaptic membrane of the 4-AP-treated animals, in both S- and F-type synapses, was wrinkled and gave frequently rise to omega-shaped profiles. However, it was not prominently lifted in comparison with the controls. In both extracellular (EF) and protoplasmic fracture faces (PF) of freeze-etched replicas, the frequency of the presynaptic membrane modulations (PMM) per unit area rose significantly under the influence of 4-AP. The ratio of open (crater-like) to total PMM in EF increased almost twice under the experimental condition. No differential 4-AP effects were seen between pentobarbital- and chloralose-treated animals. The morphological evidence is consistent with electrophysiological data showing that 4-AP facilitates the transmitter release at synaptic endings.

Aminopyrine↗
PubMed1979

Ultrastructural observations on intercellular contacts of nigral dendrites.

Possible morphological correlates of dendritic release sites for dopamine have been investigated in the rat substantia nigra. Electronmicroscopic and freeze etching studies show that nigral dendrites fail to form dendrodendritic contacts but on the contrary are always separated by glial elements, which in turn are connected by multiple gap junctions. No dendro-axonic synapses are present but large specialized symmetrical junctions could provide a possible morphological specialization for a dopamine transfer from presynaptic dendrites to axon endings.

Animals↗
PubMed1979

Three types of membrane modulations during transmitter release in rat spinal cord synapses.

Protoplasmic fracture faces of vesicle attachment sites (VSA) in the presynaptic active zone of rat spinal motoneurons were classified morphologically on the basis of particle-to-membrane relationships. Intramembranous particles are absent in type 1, located around the edges of type 2 and covering the center of type 3. Application of 4-aminopyridine (4-AP), which is known to enhance transmitter release, affected strikingly the total number of VAS (P less than 0.001) but not the percentage of the three types. It is postulated that the particle-free VAS (types 1 and 2) sharing 80--85% of VAS within the active zone, are related to exocytosis. The predominance of type 3 outside the active zone and its similarity to particle-loaded indentations possibly corresponding to coated vesicle formation, suggested that type 3 may represent endocytosis.

Aminopyridines↗
PubMed1978

Comparison of active zones in spinal cord of anesthetized and unanesthetized rats: a high voltage electron microscope study.

High voltage electronmicroscopy was applied to the study of presynaptic nerve terminals and specifically the arrangement and staining properties of synaptic vesicles in and around the active zone. The zinc iodideosmium tetroxide technique was used for the impregnation of neuropil derived from the spinal cord of anesthetized (40 mg/kg pentobarbital sodium) and unanesthetized rats. Under optimal conditions of section thickness and tilt angles stereoscopic views of a paracrystalline lattice arrangement of synaptic vesicles could be ascertained. The ZIO affinity was significantly higher in synaptic vesicles of the anesthetized as compared with the unanesthetized preparations.

Animals↗
PubMed1978

Junctional complexes and variations in gap junctions between spinal cord ependymal cells of a teleost. Sternarchus albifrons (Gymnotoidei).

Junctions between ependymal cells lining the central canal in the spinal cord of the weakly electric teleost, Sternarchus albifrons, were investigated by means of thin-section and freeze-fracture electron microscopy. Junctional complexes representing the 'terminal bar' consist of a zonula occludens, followed by intermediate junctions and desmosomes. This pattern is common in many types of epithelia but uncommon in ependyma. Three types of gap junctions were found below the level of the terminal bar and intermixed with desmosomes: (1) gap junctions at least partially enclosed by a single strand of apparent tight junction, (2) ordinary gap junctions, partially surrounded by a particle-free halo and (3) segmented gap junctions, in which linear arrays several particles wide are separated by fairly regular linear particle-free regions. Some of the observations may have implications for the mechanism of formation of gap junctions.

Animals↗
PubMed1978

Intramembranous particles at the nodes of Ranvier of the cat spinal cord: a morphometric study.

Size and distribution of intramembranous particles at nodes of Ranvier of the cat spinal cord were investigated by the freeze-etching technique and compared with those at the internodal axon. The particles are larger (up to 20 nm) at the nodal than at the internodal segment (up to 13 nm), and these large particles are more densely packed in the nodal (400 per sq micrometer) than in the internodal E (external) face (4 per sq micrometer). The nodal E face reveals a much denser overall population (1200--1300 per sq micrometer) of particles than the internodal E face (100--200 per sq micrometer), while at the P (protoplasmic) face the particle density is similar in nodal and internodal segments (1200--1600 per sq micrometer). It is suggested that the large nodal particles may be related to the mechanism of nerve excitation.

Animals↗
PubMed1977

Membrane associated particles of the presynaptic active zone in rat spinal cord. A morphometric analysis.

The distribution of membrane-associated particles in the presynaptic membrane was investigated in the spinal cord of unanesthetized and anesthetized rats by freeze etching electron microscopy. Both 'external' face (EF) and 'protoplasmic' face (PF) were examined. Particles were classified according to size in two categories: small particles (diameter 5.0-8.7 nm) and large particles (diameter 8.7-13.7 nm). The presynaptic region was subdivided into an active and a surrounding zone, depending on specific ultrastructural criteria. The density of large particles in the PF was found to be significantly higher in the active as compared with the surrounding zone in both unanesthetized and anesthetized rats. Thus, the presence of large particles represents an important feature of the active zone. Considerably more large particles were found in the waking than in the barbiturized state. This difference is paralleled by a vast increase of vesicle attachment sites in the presynaptic membrane of unanesthetized animals reported by Streit et al. and confirmed in the present study. It is suggested that the two phenomena could be interrelated and that the large particles may represent calcium channels and thus provide the morphological substrate for the mechanism of excitation-secretion coupling.

Animals↗
PubMed1977

Neuritic growth cone and ependymal gap junctions in the feline subfornical organ during early development.

Intercellular contacts in the subfornical organ (SFO) of kittens 3, 16, and 29 days old were studied in thin sections and by the freeze-etch method. Gap junctions appeared between growing nerve processes and target cells. The junctions were interspersed between immature synapses lacking mitochondria as well as full pre- and postsynaptic membrane specializations. Gap junctions were seen on filopodia as well as on more mature processes. The morphology of these junctions was typical of those described earlier but they were of small size (0.2-0.3 micron). Gap junctions of peculiar form were also seen between ependymal elements in the SFO at 16 days. These were of large size (0.5-0.8 micron) and were often of segmented character. This segmentation consisted of bands 3-4 particles in width with a center-to-center spacing of 90 nm with particle free corridors between corresponding to the width of about two rows of particles. The margin of the group might be circumscribed by a row of particles. Although gap junctions of large size were seen between ependymal cells in thin section, features corresponding to the particle free corridors have not been observed to date.

Aging↗
PubMed1977

The Ranvier nodes in the neurogenic electric organ of the knifefish Sternarchus: a freeze-etching study on the distribution of membrane-associated particles.

The two types of Ranvier nodes (type I with narrow gap, type II with giant gap) and internodes in nerve fibers composing the Sternarchus electric organ have been studied by means of freeze-etching electron microscopy. Numerical analysis of the distribution of membrane-associated particles revealed the following features: (1) the P-faces of both types of nodes and of the internodal axon bear a similarly high density of particles (1000-1200 particles/sq. micron on the average). (2) particle density is differential in E-faces: the histogram for type I nodes has a wider range of particle concentrations (114-1522 particles/sq. micron) than that for type II nodes (45-576 particles/sq. micron) whose density values are in the same range as those of the internodal axon. At least some type I nodes (narrow gaps) generate spikes and probably have a low resistivity; these nodes may be those with high particle density on E-faces. The low particle density on E-faces of type II nodes may be associated with high resistivity and absence of excitability. Similarly, the low particle density in internodes may reflect inexcitability. There is evidence that the transition from one nodal type to the next is gradual: as the gap width of type I nodes increases, there is an occurrence of surface elaborations and the density of E-face particles tends to drop towards the range of type II nodes.

Animals↗
PubMed1976

The fine structure of the perineural endothelium.

Fine strands of motor nerves were examined with the electron microscope using thin section as well as freeze-etching techniques. The specimens were taken from frog cutaneous pectoris nerve, rat sciatic nerve, mouse and shrew phrenic nerves and from human skin nerves. The perineural sheath (Henle, Ranvier, Key and Retzius) consists of one to several concentric laminae of endothelial cells; it encases nerve fascicles and eventually individual nerve fibers and terminals. The endothelial cells are extremely thin and fitted togeether smoothly by overlap and dove-tailing of their border zones. The cell contacts are formed by continuous zonulae occludentes, often reinforced by maculae adhaerentes, and in depth they comprise 3-15 strands with an average of 5-6 strands per junction. The membranes of endothelial cells are studded with attachment sites and stomata of plasmalemmal vesicles suggesting a high level of pinocytotic activity. This phenomenon is by no means restricted to the external laminae of the endothelial sheath. Each endothelial lamina is vested with basement membranes on both (epineural and endoneural) sides, and the spaces between laminae contain a few collagen fibers and fibroblasts. Occasionally, punctate tight junctions are seen between laminae. Cytological evidence supports the hypothesis that the perineural endothelium provides a relatively tight and highly selective barrier separating the peripheral nerves from surrounding tissue and its extracellular fluid spaces. This effect is achieved on the one hand by the sealing of pericellular spaces and on the other hand by a membrane controlled transcellular transport mechanism (pinocytosis), both of which are enhanced by their serial arrangement.

Animals↗