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Biomedical subjects

D F Parsons

Publications and source records attributed to D F Parsons.

At least 55 records · Page 3Linked to original sources
PubMed1976

Hypobaric helium atmospheres for light and electron microscopy of mammalian cells.

The development of electron microscope environmental/hydration chambers (EMC's) for the examination of living biological specimens has necessitated a search for a suitable gaseous environment that is compatible with both electron imaging and cell survival. Helium has a lower electron scattering cross section than air. The effects of hydrated helium and hydrated air atmospheres in an EMC at pressures 50-200 Torr were examined with a JEM 200 transmission electron microscope operated at 200 kV and 22 degrees C. Results showed that whereas 200 Torr of air reduced beam transmission by more than 90% under these conditions, 200 Torr of moist helium only reduced beam transmission by 30%. The effects of sub-atmospheric or hypobaric helium atmospheres on human white blood cells attached to glass coverslips were investigated by phase-contrast light microscopy in a simulated EMC or light microscope environmental chamber (LMC) at ...37 degrees C. gross morphological changes were observed in cells held at pressures of 300 Torr of moist helium for exposure times of 15 min. At pressures below 300 Torr, morphological changes occurred more rapidly as the pressure was reduced. At the lowest pressures tested (60 and 80 Torr) 50% of the cell populations died within 6-7 min. The development of morphological changes produced by hypobaric stress followed a characteristic pattern. The filopodia, or microvilli attaching the cells to the substrate, became withdrawn, with only occasional retraction fibers remaining. Blends appeared around the cell surface and increased in size as hypobaric exposures progressed. Vacuoles, or cler spherical regions, appeared within the cytoplasm and the overall shape of the cells became circular. The cells became flatter during circularization, showing an apparent 2-2-5 X increase in size as they respread on the substrate...

Cytoplasm↗
PubMed1976

Minimizing false negatives in electron microscope searches for virus or other specific features of cancer cells.

Electron microscopic searches for virus particles and other specific features of cancer cells involve exceptionally poor sampling statistics. It is probable that most searches conducted in the past were of limited value in establishing whether such viruses or special features are associated with one or more tumor types. On the other hand, in a few cases, the electron microscope has provided the first, or even the only, evidence of virus particles. A radical change in the way the electron microscope is applied to this problem is suggested whereby a high-voltage microscope is used to examine thicker sections and an image-processing arrangement is used to focus and select images and to search the images for virus particles. It is suggested that it is important to distinguish virus producers amongst oncogenic viral genome carriers since these may be in a more advanced state of tumor induction and may be capable of transmitting the virus to others.

False Negative Reactions↗
PubMed1976

Phase transition of plasma membranes of rat hepatocyte and hepatoma cells by electron diffraction.

The transition temperatures (Tc) of the plasma membranes of Reuber H-35 hepatoma and normal ACl rat hepatocytes were measured by electron diffraction under physiological conditions. Diffraction rings below Tc indicate the existence of solid lipid domains. The Tc of both membranes increase to a plateau value of 17.5 degrees during the storage period of 4 days at 4 degrees, the rate of increase being slower for normal liver membranes. The extrapolations to zero storage suggest an innate difference of Tc for these two membranes. Storage in oxygen-reduced media slows down the initial increase in Tc in normal liver membranes, while the depletion of divalent cations accelerates the increase. Differences in lipid composition are partly responsible for this behavior.

Animals↗
PubMed1975

Direct observation of domains in wet lipid bilayers.

Domain structure and phase separation in hydrated lipid bilayers have been imaged directly by selected reflection dark-field electron microscopy. Domains in multicomponent bilayers are much smaller than those in single component bilayers, in agreement with results obtained by selected area electron diffraction.

Cholesterol↗
PubMed1975

Electron diffraction study of hydrated phospholipid single bilayers. Effects of temperature hydration and surface pressure of the "precursor" monolayer.

The molecular packing and phase transition of hydrated dipalmitoylphosphatidylcholine single bilayers are studied by electron diffraction, using an electron microscope equipped with a hydration stage. The phase transition and area per molecule are measured as functions of temperature, hydration and the surface pressure of the monolayer from which the bilayer is formed. The transition temperature of a bilayer agrees with calorimetric measurements on bulk lipid/water mixtures. The molecular packing of a bilayer corresponds to that of the precursor monolyer at a surface pressure of 47 dyne/cm.

Binding Sites↗
PubMed1975

The effects of hypobaric atmospheres on living cells in layers of thin media and implications for electron microscopy.

At present, the only possibility for improved resolution of intracellular movement appears to be electron microscopy using a hydration chamber (EMC) at reduced gas pressures with thinned medium around the cells. The environmental constraints of examining cells under such conditions were examined in the absence of ionizing radiation by using an EMC-analogue fitted to a light microscope. White blood cells and baby hamster kidney cells were examined. A 50% survival pressure value, for hypobaric atmospheres, was determined for these cells. Morphological changes of the hypobarically-exposed cells are described.

Air Pressure↗
PubMed1974

Structure of wet specimens in electron microscopy. Improved environmental chambers make it possible to examine wet specimens easily.

Several recent technological advances have increased the practicality and usefulness of the technique of electron microscopy of wet objects. (i) There have been gains in the effective penetration of high-voltage microscopes, scanning transmission microscopes, and high-voltage scanning microscopes. The extra effective penetration gives more scope for obtaining good images through film windows, gas, and liquid layers. (ii) Improved methods of obtaining contrast are available (especially dark field and inelastic filtering) that often make it possible to obtain sufficient contrast with wet unstained objects. (iii) Improved environmental chamber design makes it possible to insert and examine wet specimens as easily as dry specimens. The ultimate achievable resolution for wet objects in an environmental chamber will gradually become clear experimentally. Resolution is mainly a function of gas path, liquid and wet specimen thickness, specimen stage stability, acceleration voltage, and image mode (fixed or scanning beam) (13). Much depends on the development of the technique for controlling the thickness of extraneous water film around wet objects or the technique for depositing wet objects onto dry, hydrophobic support films. Although some loss of resolution due to water or gas scattering will always occur, an effective gain is anticipated in preserving the shape of individual molecules and preventing the partial collapse that usually occurs on drying or negative staining. The most basic question for biological electron microscopy is probably whether any living functions of cells can be observed so that the capabilities of the phase contrast and interference light microscopes can be extended. Investigators are now rapidly approaching a final answer to this question. The two limiting factors are (i) maintaining cell motility in spread cells immersed in thin layers of media and (ii) reducing beam radiation damage to an acceptable level. The use of sensitive emulsions and image intensifiers can bring the observation dose below that required to stop cell motility. Use of a timed, pulsed deflector system enables sufficiently short exposures to be obtained to eliminate blurring due to Brownian motion. Environmental chambers have enhanced the possibilities of electron diffraction analysis of minute crystals and ordered biological structures. High-resolution electron diffraction patterns (especially kinematic) of protein crystals can only be obtained in a wet environment. Hence, it may now be possible to obtain undistorted images of protein molecules. Moreover, by subjecting diffraction patterns to image-iterative techniques (56), it will be possible to phase the electron diffraction patterns to give a calculated image with a higher resolution than that which can be produced by electron microscope objective lenses. Environmental chambers offer exciting prospects for the determination of water structure and water and ice nucleation (atmospheric science). Nucleation data near the molecular level have been badly needed for some time. The application of environmental chambers in industrial chemistry, for example, in studies of polymerization, catalysis, and corrosion, are awaiting exploration. They offer an unusual approach to measurements of reaction kinetics through images that should be both sensitive and rapid.

Animals↗
PubMed1974

Electron diffraction of wet biological membranes.

Electron diffraction patterns were obtained for the first time from single wet phospholipid bilayers and from wet human erythrocyte membranes by using a temperature-controlled electron microscope hydration stage. Selective area diffraction showed the existence of semicrystalline domains. A structural transition was observed at the transition temperature of the wet dipalmitoyl lecithin bilayer.

Cell Membrane↗
PubMed1974

Electron diffraction of wet phospholipid bilayers.

The structure of fully hydrated dipalmitoyl lecithin single bilayers, and monolayers deposited on Formvar substrates are studied by electron diffraction, using a hydration stage fitted to an electron microscope. Selective area diffraction patterns of these films indicate that there are domains consisting of mosaics of crystallites of hexagonally packed lipid chains. The size of these domains are typically several mum in diameter. The diffraction intensity agrees with that calculated from the electron scattering factor of the hydrocarbon chains of the lipid molecule.

Crystallography↗
PubMed1972

Electron diffraction of wet proteins: catalase.

Electron diffraction patterns having 3500 reflections out to 2 angstromns were obtained from wet microcrystals of catalase. No diffraction was obtained if the water vapor pressure was set below 90 percent of the equilibrium value.

Catalase↗