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R R Shivers

Publications and source records attributed to R R Shivers.

63 records · Page 4Linked to original sources
PubMed1976

Trans-glial channel-facilitated translocation of tracer protein across ventral nerve root sheaths of crayfish.

Trans-glial channels, which traverse the multilamellate glial sheath of crayfish nerves, are easily recognized in freeze-fracture preparations. Their structure and position in the glial layers of the sheath strongly supports the suggestion that they serve to facilitate rapid movement of molecules and fluids from outside the sheath to the surface of axons contained within. Segments of ventral ganglion nerve roots, which were ligated at their free ends, were immersed in crayfish Ringer solution containing 10 mg/ml horseradish peroxidase (HRP). Electron microscopic examination of the nerve sheath 30 sec after exposure to peroxidase showed that the protein had passed across the sheath and was present near the axon surface. Reaction product was present in trans-glial channels as well as in extracellular clefts and adaxonal tubular lattices thereby supporting the notion that these structures constitute a specialized conduit traversing the sheath. Often, 'fronts' of reaction product were observed across the sheath from its exterior to the interior reflecting a gradual accumulation of protein in extracellular clefts toward the axon. After 5 min in HRP-Ringer, protein appeared in all channels, extracellular clefts, and tubular lattices. With increased length of exposure of ligated nerve segments to HRP-Ringer, reaction product was found in vesicles in glial cytoplasm adjacent to axons. Results from this study suggest that trans-glial channels constitute an efficient system for rapid solute movement across nerve sheaths and may represent a mechanism whereby ions and nutrients are made available to nerves isolated in an avascular sheath.

Animals↗
PubMed1976

Trans-glial channels in ventral nerve roots of crayfish.

The sheath around the roots of the sixth abdominal ganglion in the ventral nerve cord of the crayfish consists of concentric layers of thin glial processes alternating with wide clefts containing filamentous connective tissue. Regions of each glial lamella are perforated by single, short, tubular channels: the trans-glial channels. In thin plastic sections examined in the electron microscope, the channels appear as slits that are 240 A wide and 450-550 A long which traverse glial lamellae less than 1,500 A thick. Branched tubular channels cross glial sheets that are thicker than 1,500 A. The thickest glial wrap is adaxonal; it closely encapsulates individual axons and its cell membrane is separated from the axolemma by a collagen-free space of only 150 A. The adaxonal glial cytoplasm contains unique, three-dimensional networks of interconnected tubules. Separate tubular lattices occur along these thicker processes. In replicas of freeze-fractured sheaths, the outer half of the plasma membrane belonging to the thin glial sheets exhibits many volcano-like protrusions which represent cross fractures through the necks of trans-glial channels. Corresponding depressions on the inner half of these membranes are sites where the plasma membrane invaginates to form the channels. Although some channels are randomly dispersed, others are lineraly positioned in restricted areas across successive glial layers. The number of channels is far more readily appreciated in replicas than in thin sections. The average frequency of channels is 16 per mu2 (range 8 to 33) in normal roots and does not differ significantly from the average of 13 per mu2 in proximal stumps of roots fixed three to four weeks after the roots were cut. The channels are not precisely aligned from one glial layer to the next but do appear to coincide approximately with the adaxonal tubular lattice. The combination of trans-glial channels and adaxonal tubular lattices may provide a complex conduit that could facilitate a rapid, passive flow of electrolytes and nutrients across the nerve sheath to the axonal surface. Horseradish peroxidase solutions bathing the ventral roots enter the trans-glial channels, extracellular clefts and finally the tubular lattices. This distribution supports the proposed role of the channels in a rapid extracellular passage of solutes. The channel profiles have a range of forms consistent with the supposition that they are not static but continually reforming. There are indications that, proximal to the cut, the areas of glial plasma membrane with channel profiles contain more junctional complexes between regenerating cells than between glial cells of normal sheaths. The channel profiles and aggregates of particles belonging to junctions are closely associated when they occupy the same region of the membrane.

Animals↗
PubMed1975

The mandibular organ of the lobster, Homarus americanus.

The lobster mandibular organ is well vascularized and its polygonal cells are arranged loosely around blood vessels and blood sinuses. Numerous mitochondria and microbodies (peroxisomes) give the acidophilic cytoplasm a finely granular appearance, but there is no evidence of secretory granules. The abundant endoplasmic reticulum is almost entirely agranular and occurs in two morphologically distinct forms: tubular and cisternal. The tubular reticulum is randomly distributed and may represent the site of synthesis and transport of the mandibular organ product. The cisternal reticulum is frequently associated with microbodies. Both forms of endoplasmic reticulum proliferate during mid to late premolt. Mandibular organ ultrastructure closely resembles that of cells known to synthesize steroids or lipids, which suggests that this organ may have a similar function. There is no functional evidence of involvement in molt control in Homarus, but ultrastructural and other evidence suggests an analogy with insect corpus allatum.

Adaptation, Physiological↗
PubMedDate not supplied

Immortalized mouse brain endothelial cells are ultrastructurally similar to endothelial cells and respond to astrocyte-conditioned medium.

Studies of brain microvessel endothelial cell physiology and blood-brain barrier properties are often hampered by the requirement of repeatedly producing and characterizing primary endothelial cell cultures. The use of viral oncogenes to produce several immortalized brain microvessel cell lines has been reported. The resulting cell lines express many properties of the blood-brain barrier phenotype but do not completely mimic primary endothelial cells in culture. As immortalized brain microvessel endothelial cell lines have not yet been produced from mice, we transformed mouse brain endothelial cells with the adenovirus E1A gene using a retroviral vector (DOL). Eight of 11 clones produced exhibited an endothelial-like cobblestone morphology and were characterized as endothelial with a panel of antibodies, lectins, and ultrastructural criteria. These cells are endothelial in origin and share ultrastructural features with primary cultures of endothelial cells. Examination of freeze fracture and transmission electron micrographs show adherens junctions exist between the transformed cells, and culture in astrocyte-conditioned medium induces the formation of gap junctions. This is one indication that responses to astrocyte-derived factors are retained by the transformed cell lines.

Animals↗
PubMedDate not supplied

Opening of the blood-brain barrier in Anolis carolinensis. A high voltage electron microscope protein tracer study.

The tight junctions between endothelial cells of capillaries in the forebrain of Anolis carolinensis are a common component of the structural basis for the blood-brain barrier in this reptile. The complexity of these junctions, which is apparent in platinum replicas of freeze-fractured brain capillaries, is unchanged by treatments designed to render the blood-brain barrier of these lizards leaky to horseradish peroxidase. An alternative route for extravasation of horseradish peroxidase, following injection of chameleons with 2.7 mg of D-glucose to render their brain capillaries leaky, is a system of transient cytoplasmic vesicles and vesiculo-tubular channels whose lumina may be open to the luminal or abluminal surface (or both) of the capillary endothelial cell. High voltage electron microscopy (HVEM) of 0.25 and 0.5 micron thick plastic sections of experimental brain capillary endothelium confirmed the existence of vesiculo-tubular conduits. These channels display a sigmoid morphology and are situated in the cytoplasm at angles oblique to the luminal and abluminal surfaces of the endothelium. Occasionally, the channels spanned the entire endothelial wall of the capillary, and in such cases, appeared to connect the lumen with the brain extracellular compartment. HVEM images (including stereo pairs) of the vesiculo-tubular channels show them to have a scalloped, irregular profile consistent with their proposed formation by fusion of pinocytotic vesicles. Also, HVEM examinations of experimental capillaries from peroxidase-treated lizards reveal massive quantities of dense reaction product in cytoplasmic vesicles and vesiculo-tubular membrane compartments of the endothelium, and the complex pleomorphism exhibited by these structures. Observations made in the present study suggest that as a consequence of severe hyperglycaemia, transendothelial channels form in the brain capillaries by fusion of pinocytotic vesicles generated by accelerated pinocytosis at the luminal surface of the endothelium, and subsequently serve as open routes for massive floods of tracer into the central nervous system.

Animals↗