PubMed Health⌕ Search

Biomedical subjects

A T Marshall

Publications and source records attributed to A T Marshall.

At least 19 recordsLinked to original sources
PubMed2003

Calcium associated with a fibrillar organic matrix in the scleractinian coral Galaxea fascicularis.

Field emission scanning electron microscopy of frozen-hydrated preparations of the scleractinian coral Galaxea fascicularis revealed organic fibrils which have a diameter of 26 nm and are located between calicoblastic ectodermal cells and the underlying CaCO(3) skeleton. Small (37 nm in diameter) nodular structures observed upon this fibrillar organic material possibly correspond to localised Ca-rich regions detected throughout the calcifying interfacial region of freeze-substituted preparations by X-ray microanalysis. We propose that these Ca-rich regions associated with the organic material are nascent crystals of CaCO(3). Significant amounts of S were also detected throughout the calcifying interfacial region, further verifying the likely presence of organic material. However, the bulk of this S is unlikely to be derived from mucocytes within the calicoblastic ectoderm. It is suggested that in the scleractinian coral G. fascicularis, nodular crystals of CaCO(3) establish upon a fibrillar, S-containing, organic matrix within small but distinct extracellular pockets formed between calicoblastic ectodermal cells and skeleton. This arrangement conforms with the criteria necessary for biomineralisation and with the long-held theory that organic matrices may act as templates for crystal formation and growth in biological mineralising systems.

Animals↗
PubMed2002

Changes in surface features during desiccation of the anhydrobiotic plant parasitic nematode Ditylenchus dipsaci.

The anhydrobiotic nematode Ditylenchus dipsaci is a fast-dehydration strategist, itself generating the slow rate of water loss necessary for survival. A permeability slump occurs during the initial phases of desiccation. This may be produced by changes in the nematode's cuticle. Two scanning electron microscopic techniques were used to follow changes in surface structures during desiccation. Freeze substitution and critical-point drying produced artifacts that obscured changes produced by the desiccation of the nematode. Low-temperature field emission scanning electron microscopy (FESEM) was successful in following changes that reflected those observed by light microscopy (LM). Significant changes in diameter, the lateral alae, and the cuticular annulations were demonstrated using this technique. Two types of annulations were observed: the major annulations, which extended to meet the margins of the lateral alae, and the minor annulations, which did not. With desiccation the prominence of the annulations increased, their spacing decreased, and the minor annulations extended closer to the margins of the lateral alae. These observations are consistent with the permeability slump resulting from a decrease in the width of the annulation groove and an increase in its depth. However, this requires confirmation using techniques that can follow annulation changes in individual nematodes.

Adaptation, Physiological↗
PubMed2002

Low temperature FESEM of the calcifying interface of a scleractinian coral.

The ultrastructural nature of the calcifying interface in the scleractinian coral Galaxea fascicularis has been investigated using high-resolution, low temperature field emission scanning electron microscopy (FESEM). This technique permitted structural analyses of soft tissue and skeleton in G. fascicularis in a frozen-hydrated state, without the need for chemical fixation or decalcification. Structural comparisons are made between frozen-hydrated polyps and polyps that have undergone conventional fixation and decalcification. Vesicles expelled by the calicoblastic ectodermal cells into sub-skeletal spaces and previously suggested to play a role in calcification were commonly observed in fixed samples but were distinctly absent in frozen-hydrated preparations. We propose that these vesicles are fixation artefacts. Two distinct types of vesicles (380 and 70 nm in diameter, respectively), were predominant throughout the calicoblastic ectodermal cells of frozen-hydrated preparations, but these were never seen to be entering, or to be contained within, sub-skeletal spaces, nor did they contain any crystalline material. In frozen-hydrated preparations, membranous sheets were seen to surround and isolate portions of aboral mesogloea and to form junctional complexes with calicoblastic cells. The calicoblastic ectoderm was closely associated with the underlying skeleton, with sub-skeletal spaces significantly smaller (P<0.0001) in frozen-hydrated polyps compared to fixed polyps. A network of organic filaments (26 nm in diameter) extended from the apical membranes of calicoblastic cells into these small sub-skeletal cavities. A thin sheath was also frequently observed adjacent to the apical membrane of calicoblastic cells.

Animals↗
PubMed1998

Quantitative elemental X-ray imaging of frozen-hydrated biological samples.

It is shown that quantitative X-ray imaging of planed, frozen-hydrated, biological bulk samples that have not been etched is possible. X-ray imaging represents a better alternative to static beam (selected area) analysis of fractured frozen-hydrated samples. This procedure avoids the undesirable necessity of etching planed frozen-hydrated samples to provide an interpretable electron image. Qualitative oxygen and carbon X-ray images, which can be acquired in a short time, can be used for distinguishing morphological features and remove the requirement for electron images. In test samples of frozen-hydrated albumin, containing salts, analyses by X-ray images compared well with static beam (selected area) analyses from the same samples. An example of an analysis of frozen-hydrated insect Malpighian tubules is given in which the response to ouabain treatment was analysed. In this example X-ray imaging showed that ouabain resulted in a significant increase in cytoplasmic and luminal Na and a significant decrease in cytoplasmic and luminal K. X-ray imaging also showed that there was a significant increase in cellular water content. The presence of a potassium gradient in soybean root nodules was also demonstrated. The use of standard deviation images for processing low count images increases analytical precision but results in underestimates of the true concentrations.

Animals↗
PubMed1994

Light element X-ray microanalysis in biology.

It is shown that both qualitative and quantitative light element X-ray microanalysis of biological samples is feasible. These analyses were carried out using ultrathin window (UTW) detectors. Quantitative analysis yields a total element analysis with H estimated by difference or "guesstimated". Comparison with calculated concentrations, or concentrations obtained by chemical analysis, shows that X-ray microanalysis of sections, by the peak to continuum ratio model, give sufficiently accurate results for biological purposes. The measurement of O concentrations to yield water content is carried out using x-ray imaging techniques, so that the distribution of heavier elements can be spatially related to water and dry mass distribution. Similarly light element and heavy/light element ratios are readily visualised by X-ray imaging. These ratios can indicate the subcellular distribution of different molecular species e.g., nitrogenous compounds such as urates. It is possible to derive quantitative images of water distribution in both sections and bulk samples. Comparisons of the same sample type both as frozen sections and frozen bulk samples show that the water estimates obtained by the two different analytical methods are similar. Oxygen analysis of C films at different specimen temperatures unequivocally reveals the temperature at which ice deposition on the specimen commences. This establishes safe conditions for reducing mass loss in model samples and freeze-dried sections to minimal levels and for avoiding artefactual oxygen analyses of both frozen-hydrated and freeze-dried sections.

Animals↗
PubMed1994

Morphometric analysis of renal proximal tubules in cadmium-treated rats.

The morphological changes in renal proximal tubules of Sprague Dawley rats given acute or chronic exposure to cadmium were analysed. Cadmium chloride was administered either by five subcutaneous injections of cadmium at 2 mg kg-1 body weight or in drinking water at 100 micrograms ml-1 for 39 weeks. The mean cadmium concentration in the kidneys of these rats was 45 and 102 micrograms g-1 wet weight respectively. The rats were anaesthetized and the kidneys were fixed by perfusion and processed for electron microscopy. Proximal tubule profiles were larger in the acute exposure rats. The brush border in both groups of treated rats was shorter and there were focal areas of loss of microvilli. The surface density of microvillus membrane per unit cell volume was reduced by 25% and 19% for chronic and acute dosed rats respectively. There were few significant changes in organelles detected by morphometric analysis of the entire kidney tissue, however there was a reduction in volume density of lysosomes following chronic exposure and individual necrotic cells and distorted nuclei were observed. The morphological changes observed in chronic and acute dosed rats were consistent with a primary site of toxic insult on the apical plasma membrane. There appeared to be no evidence for change in fluid and electrolyte homeostasis in the epithelium. The results suggest that the cadmium may produce a selective deficit of transport mechanisms for macromolecules.

Animals↗
PubMed1994

X-ray microanalysis of renal proximal tubules in cadmium-treated rats.

Cadmium chloride was administered to Sprague-Dawley rats in drinking water at 100 ppm for 39 weeks. The mean cadmium concentration in kidneys from 5 rats at the end of this period was 102 +/- 17 micrograms g-1 wet weight (mean +/- SE) compared with 0.22 +/- 0.15 microgram g-1 wet weight in a similar group of control rats. Right kidneys were rapidly frozen in propane and small pieces of cortex were prepared for X-ray microanalysis by freeze-substitution in ether/acrolein for 21 days. Analysis of freeze-substituted sections showed that Cd concentrations rose to 3.9 mmol kg-1 of resin embedded tissue (approximately wet weight) in cytoplasm and 6.5 mmol kg-1 in nuclei. At the same time S concentrations rose by 31 percent in nuclei and 23 percent in the cytoplasm. Cd was also present in lysosomes. Cytoplasmic and nuclear ionic (Na, K, Cl, Ca, Mg) concentrations did not change in spite of a considerable decrease in microvillus membrane surface area. The concentration of P also did not change suggesting that nucleoside phosphate levels remained stable. The distribution of Cd and S supports current concepts of cadmium toxicity. Although total kidney Cd was at the critical concentration and extensive damage to microvilli had occurred, there appeared to be no effect on ionic permeability and cell electrolyte balance and, by inference, NaCl reabsorption.

Animals↗
PubMed1993

Evaluation of several common standards for the X-ray microanalysis of thin biological specimens.

The question of the best type of standard to use for X-ray microanalysis of thin biological specimens remains unanswered. Standards embedded in an organic matrix have the advantage that they resemble biological specimens, but their composition is generally not known exactly. We compared several standards and, surprisingly, inorganic binary salts sprayed onto a supporting film were the most suitable: they corresponded closely with several other methods using organic matrices; they were easily produced; and their composition is known. Glutaraldehyde-urea aminoplastic resin thin sections and thin films containing dissolved salts were problematic. The composition of the polymer appears to be variable, and the thin films did not correspond with any other standard tested. Chelex100 bio-standard beads and flakes loaded with accurately determined concentrations of ions, embedded in epoxy resin and thin sectioned, tended to correspond to the results obtained with the binary salts. However, the results from some bio-standards were inexplicably aberrant. An epoxy resin standard was used for bromine, and was found to agree closely with the binary standards.

Electron Probe Microanalysis↗
PubMed1993

Use of ultra-thin window detectors for biological microanalysis.

Films and bulk samples of Nylon, gelatin, Makrofol, epoxy resin, aminoplastic resin and sodium acetate have been used as models of biological samples. It is shown that the use of ultrathin window (UTW) detectors in scanning transmission and scanning electron microsopes permits the quantitative analysis of light elements, yielding a total element analysis with hydrogen estimated by difference or "guesstimated". Comparison with known concentrations of concentrations obtained by chemical analysis shows that X-ray microanalysis of selections by the peak to continuum ratio model and bulk samples by the phi(pz) model gives sufficiently accurate results for biological purposes. It is also shown that sections may be analysed by the standardless ratio model. The application of UTW detectors to total element analysis by quantitative elemental imaging is demonstrated of bulk biological samples. which have been freeze-substituted, embedded in epoxy resin and surface polished. The possibility of imaging the oxygen content of frozen-hydrated bulk tissue samples which have been surface polished is also demonstrated. This may lead to the imaging of water distribution in frozen-hydrated bulk samples of biological tissues. UTW detectors are also useful for detecting mass loss in organic samples by monitoring the decrease in oxygen counts and for detecting contamination by monitoring the increase in carbon counts. It is also shown that changes in carbon counts are good indicators of folds in sections.

Animals↗
PubMed1991

X-ray microanalytical investigation of the response of chicken proximal tubule cells to infection with avian infectious bronchitis virus.

The technique for X-ray microanalysis of frozen-hydrated bulk specimens was used to determine the intracellular and luminal fluid electrolyte concentrations in the proximal tubules of kidneys from chickens infected with infectious bronchitis virus. Eight days post-infection with this virus there were significant changes in the electrolyte composition when compared with values from normal control chickens. The intracellular sodium decreased from 43 to 36 mmol/l, the chloride fell from 41 to 31 mmol/l and the potassium went from 125 to 115 mmol/l. Sodium counts in the luminal fluid rose from .73 to 1.03 cps. These disturbances in electrolyte composition are consistent with alterations in sodium reabsorption in the proximal tubule due to decreased transport of sodium into the cells across the microvillus membrane. It appears that the Na-K-ATPase pump is unaffected. The results demonstrate the value of X-ray microanalysis methods for the study of electrolyte transport in pathologically affected cells and provide further information for the definition of viral-host cell interactions in the pathogenesis of viral disease. As a check on methodology two normal rat kidneys were analysed in the same way. Intracellular sodium and potassium concentrations were 22 and 138 mmol/l respectively.

Animals↗
PubMed1990

Reversal and inhibition of cholera toxin-induced secretion in isolated rabbit ileum.

1. Cholera toxin (1 microgram/ml) abolished net fluid absorption by everted sacs of rabbit ileum, leading to net fluid secretion. This action occurred via the toxin-catalysed ADP ribosylation of the stimulatory GTP-binding protein Gs which is linked to adenylate cyclase. Nicotinamide (10 mM), a reaction product of ADP ribosylation, reversed cholera toxin-induced secretion, restoring absorption. Lower concentrations of nicotinamide induced partial reversal. 2. Nicotinamide (1 mM) blocked the secretory action of cholera toxin applied to ileal sacs. This inhibitory action was more effective in the presence of methionine (1 mM). 3. Other inhibitors of ADP ribosylation, benzamide and adenine, blocked the secretory action of cholera toxin. Hypoxanthine, an analogue and metabolite of adenine, was similarly effective. 4. Nicotinamide was not, however, effective in blocking or reversing the secretory action of theophylline (10 mM) or of heat-stable E. coli enterotoxin STa. This indicates that nicotinamide has a highly specific action against ADP ribosylating toxins. 5. It is proposed that nicotinamide reverses the secretory action of cholera toxin by reversing ADP ribosylation, simply by the law of mass action. This counters the established idea that the effects of cholera and other ADP-ribosylating toxins are irreversible under physiological conditions.

Adenine↗
PubMed1988

Progress in quantitative X-ray microanalysis of frozen-hydrated bulk biological samples.

The analysis of bulk frozen-hydrated biological samples has developed now to a level where practical application of the technique is possible. Provided the sample is carefully coated with a conductive metal, the development of a space charge capable of causing a significant distortion of the electron diffusion volume does not seem to occur, and analytical resolution can be conveniently held to approximately 2 micron (both depth and lateral resolution). Two valid quantitative methods are available, and two methods of determining dry weight fractions are also available. An area where further research could lead to improvement in analysis of frozen-hydrated bulk samples is in the investigation of fracturing methods. If fracture planes that were flat and reproducible could be easily obtained, some of the difficulties of analysing frozen-hydrated bulk samples would be considerably reduced.

Animals↗