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J B Pawley

Publications and source records attributed to J B Pawley.

At least 19 recordsLinked to original sources
PubMed1993

Cryo-crinkling: what happens to carbon films on copper grids at low temperature.

A study of the surface flatness of carbon films on copper grids used for cryo-electron microscopy has been carried out using a Hitachi S-900 low-voltage SEM. Dramatic changes in flatness were observed after cooling from room temperature to -170 degrees C. The changes were similar both for carbon films that had been floated from a mica surface and for those initially deposited on the surface of plastic films. Results demonstrate that films prepared on copper grids that appear flat at room temperature become extensively, but reversibly, puckered at -170 degrees C. The linear thermal expansion coefficient (alpha) for copper is 16.2 x 10(-6)/degrees C and the puckering can be explained by assuming that the coefficient for amorphous carbon is substantially less. Measurements on grids made of titanium, molybdenum and tungsten (coefficients 8.5, 5 and 4.5 x 10(-6)/degrees C, respectively) showed significantly less puckering.

Carbon↗
PubMed1993

Scanning electron microscopy of high-pressure-frozen sea urchin embryos.

High-pressure-freezing permits direct cryo-fixation of sea urchin embryos having a defined developmental state without the formation of large ice crystals. We have investigated preparation protocols for observing high-pressure-frozen and freeze-fractured samples in the scanning electron microscope. High-pressure-freezing was superior to other freezing protocols, because the whole bulk sample was reasonably well frozen and the overall three-dimensional shape of the embryos was well preserved. The samples were either dehydrated by freeze-substitution and critical-point-drying, or imaged in the partially hydrated state, using a cold stage in the SEM. During freeze-substitution the samples were stabilized by fixatives. The disadvantage of this method was that shrinking and extraction effects, caused by the removal of the water, could not be avoided. These disadvantages were avoided when the sample was imaged in the frozen-hydrated state using a cold-stage in the SEM. This would be the method of choice for morphometric studies. Frozen-hydrated samples, however, were very beam sensitive and many structures remained covered by the ice and were not visible. Frozen-hydrated samples were partially freeze-dried to make visible additional structures that had been covered by ice. However, this method also caused drying artifacts when too much water was removed.

Animals↗
PubMed1992

High-resolution scanning electron microscopy.

The spatial resolution of the scanning electron microscope is limited by at least three factors: the diameter of the electron probe, the size and shape of the beam/specimen interaction volume with the solid for the mode of imaging employed and the Poisson statistics of the detected signal. Any practical consideration of the high-resolution performance of the SEM must therefore also involve a knowledge of the contrast available from the signal producing the image and the radiation sensitivity of the specimen. With state-of-the-art electron optics, resolutions of the order of 1 nm are now possible. The optimum conditions for achieving such performance with the minimum radiation damage to the specimen correspond to beam energies in the range 1-3 keV. Progress beyond this level may be restricted by the delocalization of SE production and ultimate limits to electron-optical performance.

Electrons↗
PubMed1992

High-resolution scanning electron microscopy of frozen-hydrated cells.

Cryo-fixed yeast Paramecia and sea urchin embryos were investigated with an in-lens type field-emission SEM using a cold stage. The goal was to further develop and investigate the processing of frozen samples for the low-temperature scanning electron microscope (LTSEM). Uncoated frozen-hydrated samples were imaged with the low-voltage backscattered electron signal (BSE). Resolution and contrast were sufficient to visualize cross-fractured membranes, nuclear pores and small vesicles in the cytoplasm. It is assumed that the resolution of this approach is limited by the extraction depth of the BSE which depends upon the accelerating voltage of the primary beam (V0). In this study, the lowest possible V0 was 2.6 kV because below this value the sensitivity of the BSE detector is insufficient. It is concluded that the resolution of the uncoated specimen could be improved if equipment were available for high-resolution BSE imaging at 0.5-2 kV. Higher resolution was obtained with platinum cryo-coated samples, on which intramembranous particles were easily imaged. These images even show the ring-like appearance of the hexagonally arranged intramembranous particles known from high-resolution replica studies. On fully hydrated samples at high magnification, the observation time for a particular area is limited by mass loss caused by electron irradiation. Other potential sources of artefacts are the deposition of water vapour contamination and shrinkage caused by the sublimation of ice. Imaging of partially dehydrated (partially freeze-dried) samples, e.g. high-pressure frozen Paramecium and sea urchin embryos, will probably become the main application in cell biology. In spite of possible shrinkage problems, this approach has a number of advantages compared with any other electron microscopy preparation method: no chemical fixation is necessary, eliminating this source of artefacts; due to partial removal of the water additional structures in the cytoplasm can be investigated; and finally, the mass loss due to electron beam irradiation is greatly reduced compared to fully frozen-hydrated specimens.

Animals↗
PubMed1991

Early results using high-resolution, low-voltage, low-temperature SEM.

Recent advances in the design of the scanning electron microscope (SEM) column, such as the coupling of a field-emission gun to a low-aberration immersion lens and the availability of a high-stability cryo-transfer stage, make low-temperature, low-voltage SEM (LTLVSEM) possible at very high resolution. We have used this combination to obtain results with uncoated biological specimens. The trichocyst from a Paramecium was used as a test specimen to observe the shrinkage of this structure as the temperature is raised from 170 K to room temperature following freeze-drying. High-magnification stereo images were obtained of trichocysts that had been prepared by freezing, freeze-substitution and critical-point drying and which were subsequently viewed by LTLVSEM to reduce beam damage and contamination.

Animals↗
PubMed1991

Backscattered electron imaging for high resolution surface scanning electron microscopy with a new type YAG-detector.

Double Layer Coating for backscattered electron imaging is a coating and imaging method especially suitable for high resolution scanning electron microscopy (SEM) of large biological samples. Since the backscattered electron (BSE) signal from thin metal coating layers is quite low, field emission SEM's and very sensitive BSE-detectors are required for this method. In this study an improved BSE-detector of the YAG type was used with an in-lens type field emission SEM. Two samples were investigated in order to demonstrate and to improve the potential of this new approach: (1) cryo-prepared cultured kidney cells were shadowed by electron beam evaporation with platinum-carbon (unidirectionally at a fixed angle of 45 degrees) and then coated with an additional 10 nm carbon layer; and (2) cryo-prepared trichocyst matrixes (paracrystalline structures contained in secretory granules, the trichocysts, found in Paramecium) were coated by ion beam sputtering with about 1 nm of platinum. This sample was rotated and tumbled during coating in order to obtain as uniform a metal layer as possible and then an additional 10 nm carbon layer was evaporated over the metal. When these samples were viewed at a primary beam accelerating voltage (Vo) of 10 kV or higher, contrast was good on the unidirectionally coated cell culture samples. However, trichocyst matrixes with the thinner and more uniform coating showed very poor contrast because most of the BSE detected represented beam-specimen interactions from the bulk of the sample and not in the thin platinum layer. The situation was improved by using low Vo (4 kV). Under these conditions the penetration depth of the electrons is reduced and a greater proportion of the BSE electrons are scattered by the platinum layer. The results were compared with freeze-fracture and deep-etch transmission electron microscope studies of the trichocyst matrixes from the literature: Almost similar resolution is achieved on the biological structures but a better impression of the three dimensional arrangement of the whole trichocyst matrix is obtained with the SEM. The globular particles form disc-like structures that are connected with each other by thin fibers.

Animals↗
PubMed1990

Optimizing parameters for correlative immunogold localization by video-enhanced light microscopy, high-voltage transmission electron microscopy, and field emission scanning electron microscopy.

Correlative video-enhanced light microscopy, high-voltage transmission electron microscopy, and low-voltage high resolution scanning electron microscopy were used to examine the binding of colloidal gold-labeled fibrinogen to platelet surfaces. Optimal conditions for the detection of large (18 nm) and small (3 nm) gold particles are described.

Blood Platelets↗
PubMed1990

High-resolution immunogold localization of Giardia cyst wall antigens using field emission SEM with secondary and backscatter electron imaging.

We describe here the ultrastructural localization of Giardia cyst antigens in the filaments associated with the outer portion of intact cysts and on developing cyst wall filaments in encysting trophozoites. Post-embedding immunogold labeling of thin sections of intact Giardia cysts with polyclonal and monoclonal antibodies specific for cyst wall antigens (major protein bands of approximately 29, 75, 88, and 102 KD on Western blots) showed strong labeling of the filamentous cyst wall, whereas no labeling was seen on the membranous portion. High-resolution field emission scanning electron microscopy (FESEM) of Giardia cysts revealed that the cyst wall-specific polyclonal rabbit antisera and monoclonal mouse antibody produced gold labeling of 20-nm filaments in the cyst wall as detected with secondary electron imaging (SEI) and backscatter electron imaging (BEI) at 10 kV, despite coating of the cells with platinum by ion sputtering. FESEM studies of encysting Giardia trophozoites demonstrated that immunostaining with antibodies to cyst wall antigens produced colloidal gold labeling of developing cyst wall filaments on the cell surface; however, the intervening membrane domains were unlabeled. Substitution of normal serum for cyst wall-specific antibodies, or preabsorption of specific antibodies with Giardia cysts, eliminated immunolabeling of the filaments.

Animals↗
PubMed1989

The case for low voltage high resolution scanning electron microscopy of biological samples.

Dried biological samples are low in scattering power, non-conducting and sensitive to radiation damage. These facts complicate the choice of the optimum beam voltage Vo at which they should be observed in the scanning electron microscope (SEM) because they add as variables the type and thickness of the coating material and degradation/contamination of the specimen by the beam. Heretofore, high resolution SEM could only be carried out at relatively high Vo (20-30kV) because available equipment could not produce small beam diameters at low Vo. Modern instruments can produce beam diameters of about 3nm at 1.5kV. As normal preparative procedures (fixation, critical point drying, coating) are unlikely to preserve reliable structure below this level, it is now possible to investigate the possible advantages associated with low Vo operation such as a reduction in charging and radiation damage and improved topographic contrast. The conclusion recommended by this paper is that the term resolution needs careful definition. The size of the smallest features visible in a micrograph is a function of many variables. Although probably the most important is specimen preparation, a number of others (probe size, beam penetration range, contamination, coating thickness needed to provide contrast and avoid charging etc) are functions of Vo. Of these variables at least probe size and possibly contamination become more favorable at higher Vo while the remainder favor low Vo. As a result the optimum will occur at a Vo where the best balance of these factors occurs for a particular sample. When using the Hitachi S-900, we have found that the optimum seems to be at 1.5-2.5kV for topologically diverse samples, but may extend to 5kV on samples on which very small structural details have been preserved and which are relatively stable to radiation damage.

Animals↗
PubMed1988

Wave of free calcium at fertilization in the sea urchin egg visualized with fura-2.

A wave front of increased free calcium traversing the egg at fertilization is demonstrated in the sea urchin Lytechinus pictus. The use of the fluorescent calcium chelator fura-2 in combination with low-light-level TV microscopy and image processing allows the visualization of the Ca2+ wave front with high spatial and temporal resolution. Such a wave is demonstrated as increased fluorescence after an excitation of 340-nm wavelength and as the reciprocal image in form of a reduced fluorescence when excited at 380 nm. The band-like appearance of the wave resembles the Ca2+ wave described for larger eggs of other species. In a dispermic egg the high resolution of the system used allows us to recognize two waves of Ca2+ originating from the respective points of sperm entry.

Animals↗
PubMed1988

Freeze-fracture of 3T3 cells for high-resolution scanning electron microscopy.

Triton-extracted, freeze-fractured 3T3 cells have been examined in the Hitachi S-900 field-emission SEM, after light platinum coating, at low beam voltage to evaluate the performance of the microscope under these conditions. For unstained material fixed in glutaraldehyde alone, high-resolution images can be obtained, at accelerating voltages of 1.5-5kV, after rotary deposition of platinum to an average thickness of 1.5-3 nm. Comparisons are made between these results and those of studies by TEM of deep-etch replicas of similar material previously published.

Cell Nucleus↗
PubMed1980

Preparation of experimental animal tissue for SEM.

Methods for preparing experimental animal tissues for the scanning electron microscope (SEM) have evolved from so many sources, with such lack of standardization that interpretation of the results is difficult, and inter-investigation comparisons usually impossible. To distinguish the surface changes inherent in any protocol for the preparation of bulk tissue blocks from those produced by experimentation or disease, the preparative procedures must become standardized at least to the extent that preparation for light microscopy (LM) and transmission electron microscopy (TEM) is standardized. Rationales for the selection of a particular preparative procedure which will result in minimal alteration from the living tissue are discussed. The methods for handling the specimen, stabilization, dehydration and drying, rendering the surface conductive, and exposing the surface of interest are described for a wide variety of experimental animals. The use of SEM to yield maximum morphological information in the secondary electron mode is also described, as are the methods used for evaluating the results in terms of minimal distortion due to preparative procedures.

Animals↗
PubMed1978

A chamber attached to the SEM for fracturing and coating frozen biological samples.

A chamber for introducing, fracturing and coating frozen biological samples has been developed as an attachment to the sepcimen chamber of a scanning electron microscope. Together with a eucentric-tilt cold-stage, this chamber constitutes a complete system for viewing fractured biological surfaces of the type normally only seen by replica techniques. An air-lock on the chamber accepts a transfer module to allow insertion of the frozen sample without frost build-up. Fracturing is carried out with a precisely adjustable cooled knife under a 10--100X binocular microscope. The sample can tilt and rotate while being coated with carbon or metals evaporated from rechargeable sources introduced through the air-lock. Cooling in the chamber is provided by a cylindrical copper tank filled with liquid nitrogen. The chamber has its own LN2 trapped high vacuum system. After preparation the sample can be placed directly into the SEM through an isolation valve. The cold-stage utilizes a Joule-Thomson refrigerator. The sample can be kept below 103 K at all times though there are provisions for heating it in the fracturing and cold-stage positions. A system of controls, sensors and interlocks simplifies the operation of the system.

Animals↗
PubMed1977

Using simultaneous three colour X-ray mapping and digital-scan-stop for rapid elemental characterization of coal combustion by-products.

A system is described for rapid, simultaneous three colour elemental mapping with a scanning electron microscope (SEM) and an energy dispersive X-ray analyser. The technique, which use a SEM scanning at TV rate, minimizes the disadvantages of long scan times such as inefficient use of linear amplifer and observable dead-time shadowing. The system also employs a digital scan-stop assembly utilizing a light pen to rapidly and reproducibly direct the beam to an object of choice for spot-mode analysis. Application of the system to analysis of fine particulates with emphasis on fly ash derived from coal-fired electric power plants is discussed. Chemical heterogeneity of fine particles in standard reference material fly ash and in phagocytized fly ash within pulmonary macrophages is demonstrated. This system combines the morphologic capability of the SEM with X-ray multielement mapping to provide a needed tool for particulate source identification.

Air Pollutants↗