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Biomedical subjects

R C Black

Publications and source records attributed to R C Black.

At least 19 recordsLinked to original sources
PubMed1996

Translocation and exudation of tumor metabolites in crown galled plants.

Crown gall tumors are induced on susceptible plants by pathogenic strains of Agrobacterium. These neoplastic plants cells produce metabolites, called opines, which provide a source of nutrients to colonizing agrobacteria. Opine production previously has been shown to influence microbial communities in the immediate vicinity of the tumor. We have obtained evidence for opine translocation to and exudation from distal uninfected regions of the plant host. Grafted plants made from an opine-producing transgenic scion with a wild-type stock, or with a wild-type scion and an opine-producing stock accumulate opines in the wild-type portions of the plant. Moreover, opines were detected in root, stem, and leaf tissues of nontransgenic plants on which stem crown galls had been induced by pathogenic strains of Agrobacterium. These plants exuded opines from their roots as a component of their root exudates. Translocation of opines from the tumor to other parts of the plant, and their exudation from roots, indicates that these biologically active compounds are available to opine-catabolizing microbes that have not induced the tumors but are present in the rhizosphere or on portions of the plant distant from the site of the gall.

Biological Transport↗
PubMed1994

Cell-autonomous cytokinin-independent growth of tobacco cells transformed by Agrobacterium tumefaciens strains lacking the cytokinin biosynthesis gene.

Mutations at the cytokinin biosynthesis locus (tmr) of Agrobacterium tumefaciens usually result in strains that induce tumors exhibiting the rooty phenotype associated with high auxin-to-cytokinin ratios. However, tobacco (Nicotiana tabacum cv Havana 425) leaf disc explants responded to tmr- mutant strain A356 by producing rapidly growing, unorganized tumors, indicating that these lines can grow in a cytokinin-independent fashion despite the absence of a functional tmr gene. Several methods have been used to characterize the physiological and cellular basis of this phenotype. The results indicate that tmr- tumors have a physiologically distinct mechanism for cytokinin-independent growth in comparison to tumors induced by wild-type bacteria. The cytokinin-independent phenotype of the tmr- transformants appears to be cell autonomous in nature: only the transformed cells and their progeny were capable of cytokinin-independent growth. Specifically, the tmr- tumors did not accumulate cytokinin, and clonal analysis indicated the tmr- transformed cells were not capable of stimulating the growth of neighboring nontransformed cells. Finally, the cytokinin-independent phenotype of the tmr- transformants was shown to be cold sensitive, whereas the wild-type tumors exhibited a cold-resistant cytokinin-independent phenotype. Potential mechanisms for this novel form of cytokinin-independent growth, including the role of the dehydrodiconiferyl alcohol glucosides found in both tumor types, are discussed.

Agrobacterium tumefaciens↗
PubMed1993

Two processes of bone remodeling in plated intact femora: an experimental study in dogs.

Bone loss is known to occur under plates used for internal fixation. Its exact location and extent and its relationship to the plate-bone contact area was investigated in 36 adult beagles. Plates with two different contact areas were fixed unilaterally to intact femora for periods of 8 and 24 weeks. After removal, the bones were assessed radiographically, histologically, histomorphometrically, and biomechanically. Two processes that affect the bone after plating became apparent. One is a short-term process, possibly caused by the interference of the plate with the blood supply to the cortex. This is followed by necrosis, which induces porosis. The second independent process is caused by the tendency of the bone to adapt to the new state of mechanical loads that results from the load sharing by the plate. The first stage of this process is expressed through swift changes in the bone cross-sectional area under the plate by remodeling in all three envelopes: haversian, periosteal, and endosteal. This stage is followed by a gradual remodeling towards a steady state, in which the final cross-section of the bone is attained. Both processes affected the bone simultaneously during plate fixation. The process caused by the vascular interference ceases after some time, and only the process due to stress-shielding prevails for a long period.

Animals↗
PubMed1993

Relation of vertebral bone screw axial pullout strength to quantitative computed tomographic trabecular bone mineral content.

Noninvasive prediction of the maximum axial load that a spinal bone screw will be able to withstand after anterior surgical placement would be highly useful. To investigate if this is feasible, we first performed preliminary experiments to distinguish the trabecular and cortical contributions to overall stiffness; the trabecular component was found to dominate. We then used a commercial computed tomography bone mineral package to determine the mineral density of the trabecular region of 41 porcine vertebrae in terms of equivalent K2HPO4 concentration; values ranged from 104 to 343 mg/cm3. A 6.5-mm diameter cancellous bone screw was then inserted laterally in each vertebra, and the ultimate tensile strength (UTS) of the screw/bone interface was measured using a tensile testing machine. The UTS values ranged from 589 to 2,620 Newtons. A superlinear relation was found between UTS and the projected K2HPO4 concentration in the direction of the screw axis, expressed in units of mg/cm2.

Animals↗
PubMed1993

Healing of plated femoral osteotomies in dogs. A mechanical study using a new test method.

We investigated changes in whole bone flexural rigidity during healing of plated osteotomized beagle femora. Using a recently developed mechanical method, healing femora and their intact contralateral controls were tested in non-destructive bending in 24 planes at 15 degree angular increments. The elliptical distributions of flexural rigidity were used to define 4 parameters which describe the mechanical status of a healing bone relative to its control. Plates of 3 different rigidities were used in 21 beagles; 6 for 2 months and 15 for 6 months. 2 healing efficiency parameters, describing bone rigidity, indicated that plated femora may never reach the rigidity of their controls. One of the parameters, describing bone asymmetry, showed that changes in bone asymmetry occurred early in the healing process. Results at 6 months showed no differences in rigidity and asymmetry of bones plated with the different plates. This is attributed to antagonistic effects of axial and flexural rigidities of the plates on bone healing. Our results indicate that the method may be useful for bone healing research.

Animals↗
PubMed1990

Quantitative measures for fracture healing: an in-vitro biomechanical study.

A method is presented to assess the functional capabilities of plated osteotomized canine femora as load bearing structures and to quantify their healing status. In this method the osteotomized bones and their intact contralateral controls are tested in nondestructive bending, in twenty-four planes of loading at 15 degree angular increments. Flexural rigidity (EI), in each of the planes, is determined by using classical beam theory. It has been found that the EI values have the expected elliptical distribution. The 24 EI values, obtained for each bone, are curve fitted by regression analysis and the characteristics of each bone are described by the three parameters defining its ellipse. The parameters of the ellipses of the osteotomized bone and of its contralateral control are used to define four new parameters that may serve as measures for the healing efficiency. One of these serves as an indicator for the fragility of the healed bone and the other three add quantitative information on its mechanical state.

Animals↗
PubMed1988

A contoured anterior spinal fixation plate.

There is a need for a specifically designed plate for anterior fixation of the vertebral bodies, contoured to fit closely around the lateral aspect of the spine and wide enough to allow multiple options for placement of at least three screws in each vertebra. The large-diameter cancellous screws should penetrate the opposite cortex. Existing bone plates are inadequate, because they are too narrow and do not allow positioning of more than two screws in each vertebra. The Biomedical Engineering Department of the National Research Council of Canada designed and tested the plate described in this article, with particular attention to providing smooth surfaces to prevent vascular complications. Three lengths of plates have been developed and are used in the area from T11 to L5, with a specific tapered plate for the L5 area to prevent contact with the overlying iliac vessels. This device should be used for stabilization following corpectomy for tumor, decompression of burst fractures, severe disc degeneration, pseudarthrosis, and the multiply operated back.

Adult↗
PubMed1985

Current distributions in the cat cochlea: a modelling and electrophysiological study.

The current distribution of bipolar electrodes implanted into the scala tympani of the cat cochlea was investigated using a two-electrode masking technique. Two electrode masking is a non-invasive technique which requires two electrically independent electrodes and relies upon the forward masking of the electrically evoked brainstem response to a probe stimulus by that of a preceding test stimulus. The technique was described in terms of a model, which enabled an approach for estimating the scala tympani length constant to be established. Model results have shown good agreement with electrophysiological results. Application of the model confirmed the scala tympani length constant within the basal turn of the cochlea to lie between 3 and 4 mm.

Animals↗
PubMed1983

Current distributions in cochlear stimulation.

Animal experimental studies have shown length constants of 2-4 mm for bipolar and 8-16 mm for monopolar stimulations. Studies in models using saline-solution-filled tubes have allowed us to examine the radial and longitudinal current distribution for pseudobipolar stimulation and have demonstrated that current localization is the same for bipolar and pseudobipolar stimulation over a 6-10-dB operating range. With coincident pseudobipolar multiple-channel stimulation there was suppression of the current between the stimulus maxima and enhancement at the edges leading to less stimulus interaction. Experiments performed with a pseudobipolar electrode implanted into the human cochlea showed that there was significant spread of current along the ground electrode because the electrode ground impedance was significantly greater than the cochlear tissue impedances. Because this leads to less current returned at each ground electrode, the pseudobipolar array will result in less interaction for coincident stimulation.

Animals↗
PubMed1983

Intracochlear electrical stimulation of normal and deaf cats investigated using brainstem response audiometry.

Brainstem response audiometry for intracochlear electrical stimulation of normal-hearing and deafened cats was investigated. In normal cochleas the brainstem response amplitude grew slowly near threshold as a current-amplitude dependent process, identified as electrophonic in origin. This terminated in a rapidly growing charge-dependent process at approximately 20 dB above threshold, identified as direct electrical stimulation of the auditory nerve. Small levels of white noise (25-35 dB SPL) were sufficient to mask most of the electrophonic response, leaving the direct stimulation process essentially unmodified. In cochleas damaged with d.c. currents and loud sounds, only a rapidly growing charge-dependent process was observed which grew similarly to that in normal-hearing cats but occurred at lower currents. This indicates that possibly the electrical properties of the cochlea were altered in the deafening process, suggesting the inadequacy of normal animals as deaf models for electrical stimulation. Using the technique of derived brainstem responses, it was shown that direct electrical stimulus components were localized to the vicinity of the stimulus electrode with electrophonic components distributed more widely. However, at high currents there was some evidence of the stimulus spreading into the internal auditory meatus.

Animals↗
PubMed1983

Chronic electrical stimulation of the auditory nerve in cats. Physiological and histopathological results.

The ability of spiral ganglion cells to survive long-term electrical stimulation is a precondition for the success of cochlear prostheses. In this study 10 cats were implanted bilaterally with bipolar scala tympani electrodes, and stimulated for periods of up to 2029 hours using charge balanced biphasic current pulses. The status of the auditory nerve was monitored periodically by recording electrically evoked auditory brainstem responses. At the conclusion of the stimulation program, spiral ganglion cell survival was assessed for stimulated and control cochleas; comparison of the two groups showed no statistically significant difference. The results of this study indicate that long-term intracochlear electrical stimulation, using carefully controlled biphasic pulses, does not adversely affect the spiral ganglion cell population.

Animals↗
PubMed1980

Differential electrical excitation of the auditory nerve.

The multichannel cochlear prosthesis requires an electrode stimulus configuration which produces a stimulus field spatially localized to each electrode. In this paper, a three-dimensional discrete resistance model of the cochlea was developed which exhibits electrical response properties similar to those observed during electrical stimulation of the cochlea. The model results suggest that the spatial attenuation of current within the cochlea varies greatly in magnitude, depending on the stimulus configuration. In addition, the model suggests that the spatial attenuation of current in both the auditory nerve fiber endings in the organ of Corti and in the myelinated fibers within the cochlear ground paths is different from the voltage attenuation in the scalar fluids. Therefore the efficacy with which a particular stimulus configuration differentially excites local terminal auditory nerve fiber populations cannot be deduced from scalar voltage measurements which have previously been recorded in the literature. Consequently physiological experiments were performed in the cat to measure the current distributions in the terminal nerve fiber region for monopolar and bipolar stimulation of the scala tympani, and also for stimulation between the scala tympani and the scala vestibuli. The mean length constants measured in the basal turn for these stimuli were found to be 12, 3, and 7.5 mm, respectively.

Animals↗
PubMed1980

Dissolution of smooth platinum electrodes in biological fluids.

Quantitative measurements of the dissolution of smooth platinum electrodes resulting from stimulation of physiological saline with sinusoidal, square biphasic, square monophasic and direct currents have been made by spectrophotometric analysis of the stimulated solution. Results presented give an indication of the stimulus parameter region in which platinum dissolution is minimal and therefore potentially suitable as an implant electrode material.

Body Fluids↗