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Biomedical subjects

J H Willison

Publications and source records attributed to J H Willison.

At least 19 recordsLinked to original sources
PubMed1985

Ultrastructure of Saccharomyces cerevisiae strain AG1-7 and its responses to changes in environment.

Asynchronous populations of the budding yeast Saccharomyces cerevisiae strain AG1-7 were examined by freeze-fracture electron microscopy for ultrastructural changes occurring in response to changes in the environment, specifically the following: temperature (23 or 37 degrees C); cell density (exponential, early stationary, and stationary phases); various periods of nitrogen starvation at low cell density, and return of nitrogen-starved cells to nitrogen-replete medium. This information has been gathered in preparation for ultrastructural examination of comparable responses of temperature-sensitive cell-cycle mutants. The plasma membrane was found to be particularly responsive to changes in environment. A high proportion (75%) of cells in exponential phase populations at 37 degrees C displayed paracrystalline arrays of plasma membrane particles, whereas this proportion was much lower (20%) at 23 degrees C in the same medium; plasma membrane grooves were longer at 37 than at 23 degrees C. In budded cells, the mother cell displayed paracrystalline arrays more frequently than the bud. Entry of cells into stationary phase, either through permitting population growth or by limiting nitrogen supply, resulted in increases in numbers of paracrystalline arrays and grooves. Groove depth also increased. The paracrystalline-array and groove-density responses were independent, both during entry into stationary phase and during the subsequent lag phase. Unusual groove forms appeared during stationary phase in high cell density populations, but not in low cell density nitrogen-starved populations. "Aggregate" and "geometric" tonoplast forms, previously described in strain A364A when grown under some of the conditions used here, were not found in AG1-7 under any of the conditions used here. It was demonstrated that particle-free patches can arise rapidly on the tonoplast of AG1-7 in response to temperature change from 37 to 23 degrees C. During stationary phase, spherosomes (lipid droplets) increased in size, particularly in response to nitrogen depletion. After 72 h of nitrogen starvation, about 10% of cell volume consisted of spherosomes. Changes in vacuolar content and mitochondrial form were also noted during entry into stationary phase.

Cell Count↗
PubMed1985

An initial test of a method for the estimation of real mean particle size from shadowed samples.

During shadowing, a "cap" of metal develops on small particles. This cap increases apparent particle with (measured normal to the shadowing direction) by an extent which cannot be predetermined. The extent of this increase in particle size (here defined as the "cap," X) is estimated in the present method by using opposite (180 degrees sample rotation) bidirectional shadowing. It is argued that the bidirectional cap is the sum of the two unidirectional caps, and therefore that X = 2A - (B + C), where A is the mean bidirectionally shadowed particle size, and B and C are the two mean unidirectionally shadowed particle sizes. As a validation of the method, the mean diameter of air-dried ferritin was estimated and the results appear to confirm the hypothesis (mean diameter by present method, 10.7 +/- 0.2 nm; mean diameter by previous methods, 10.89 nm).

Carbon↗
PubMed1983

Bidirectional shadowing in freeze-etching.

Bidirectional shadowing in freeze-etching may be achieved by firing an electron-beam shadowing source, rotating the specimen stage through a desired angle, and re-firing the shadowing source. It is demonstrated that portrait shadow-casting, which permits information to be drawn from much of the specimen region lying within primary shadows, can be readily achieved using a 90 degree specimen rotation. With 180 degree specimen rotation, particle-size analysis is feasible. Particle-height analysis is demonstrated using membrane-associated particles as an example. Data from suitable sets of micrographs can also be used for the estimation of particle-width exaggeration due to the accumulation of the shadowing-metal cap. Fibre-width analysis, using the linear regression method, is demonstrated by a study of native cellulosic microfibrils. Mean microfibril widths were found to be 5.5-7.0 nm.

Cell Membrane↗
PubMed1978

Cell wall structure and deposition in Glaucocystis.

Events leading to cell wall formation in the ellipsoidal unicellular alga Glaucocystis are described. The wall is deposited in three phases: (a) a thin nonfibrillar layer, (b) cellulosic microfibrils arranged in helically crossed polylamellate fashion, and (c) matrix substances. At poles of cells, microfibrils do not terminate but pass around three equilaterally arranged points, resulting in microfibril continuity between the twelve helically wound wall layers. These findings were demonstrated in walls of both mother cells and freeze-fractured growing cells, and models of the wall structure are presented. Cellular extension results in spreading apart, and in rupture, of microfibrils. On freeze-fractured plasma membranes, there were 35 nm X 550 nm structures associated with the ends of microfibrils. These are interpreted as representing microfibril-synthesizing centers (terminal complexes) in transit upon the membrane. These terminal complexes are localized in a zone, or zones. The plasma membrane is subtended by flattened sacs, termed shields, which become cross-linked to the plasma membrane after completion of wall deposition. During wall deposition, microtubules lie beneath the shields, and polarized filaments lie between shields and plasma membrane. The significance of these findings in relation to understanding the process of cellulose deposition is discussed, and comparisons are made with the alga Oocystis.

Cell Division↗
PubMed1978

Altered nuclear pore diameters in G1-arrested cells of the yeast Saccharomyces cerevisiae.

Nuclear pores in cells of the yeast Saccharomyces cerevisiae were examined by using the freeze-fracture technique. Nuclear pore diameters in actively growing cells appear to be exclusively of the normal diameter (75 to 115 nm), whereas some pore diameters in abnormally small G1-arrested cells produced by nitrogen starvation are unusually wide (120 to 160 nm). There may be a correlation between nuclear pore size and nuclear envelope size, the larger pores tending to occur in the smaller envelopes. The finding suggests that nuclear pore diameter may not function in regulating the flow of informational molecules from nucleus to cytoplasm, but may be implicated in regulating the flow of substrates into the nucleus.

Cell Cycle↗
PubMed1977

"Large" and "small" nuclear pore complexes; the influence of glutaraldehyde.

Aliquots of lymphocyte cell suspensions were pretreated according to the following three schedules before freeze fracturing: (a) prefixed with 2% glutaraldehyde before infiltration with 25% glycerol in medium RPMI-1640; (b) frozen in medium RPMI-1640 without additional pretreatment; and (c) frozen after pretreatment with 25% glycerol in medium RPMI-1640. The diameters of the fractured nuclear pore complexes of cells prefixed with glutaraldehyde were normally distributed within the range 70-120 nm (median 90 nm). The nuclear envelopes of 66-75% of cells processed through schedules b and c, which omitted glutaraldehyde fixation, had 70-120 nm diameter pores, while the remainder had pores with diameters in the range 120-175 nm. The large pores were structurally similar to the smaller pores except for their dimensions. These results indicate that glutaraldehyde gives rise to shrinkage of the larger pores to the minimum, smaller, diameter. Apparent orifices of at least 30 nm diameter were sometimes observed at the centres of these large pore complexes. We propose that the variation in pore diameters may indicate opening and closure of this orifice, and that the widely reported "central granule" of the nuclear pore complex corresponds with the orifice in a closed configuration.

Aldehydes↗
PubMed1977

The attachment of bacterial spinae.

Spinae are attached to protease-sensitive structural proteins in the external surface of the outer membrane. Agents and (or) treatments affecting ionic, hydrophobic, or hydorgen bonds are ineffective in releasing spinae from bacteria. As judged by thin-sectioning and freeze-fracturing techniques, the outer membrane is not modified at the attachment site to a detectable extent, and the other surface layers are not involved. The attachment of spinae is thus differentiated from that of flagella.

Binding Sites↗
PubMed1976

Cellulose biosynthesis in Acetobacter xylinum: visualization of the site of synthesis and direct measurement of the in vivo process.

In vivo synthesis of cellulose by Acetobacter xylinum was monitored by darkfield light microscopy. Cellulose is synthesized in the form of a ribbon projecting from the pole of the bacterial rod. The ribbon elongates at a rate of 2 mum min-1. The ribbon consists of approximately 46 microfibrils which average 1.6 X 5.8 nm in cross section. The observed microfibrillar elongation rate corresponds to 470 amol of glucose/cell per hr assimilated into cellulose. Electron microscopy of the process using negative staining, sectioning, and freeze-etching indicated the presence of approximately 50 individual synthetic sites organized in a row along the longitudinal axis of the bacterial rod and in close association with the outer envelope. The process of cellulose synthesis in Acetobacter is compared with that in eukaryotic plant cells.

Cellulose↗
PubMed1976

Arrangement of morphological subunits in bacterial spinae.

The filament, that is helically arranged to form the bacterial spina, is composed of morphological subunits (oligomers) about 5.6 nm in width and 11 nm in length. The oligomers are asymmetrical in that the inner surface is grooved. Image analysis of negative-stained spinae ribbons indicates that the oligomers are paired, possibly beaded structures, the arrangement of which is easily distorted during preparation. In intact spinae, the oligomer orientation may be normal to the filament axis, but in collapsed freeze-etched spinae, the oligomers are inclined at a constant angle of about 72 degrees to the filament axis.

Bacterial Proteins↗
PubMed1975

The relationship of plastic deformation in freeze-etching to the orientation of a protein particle.

Freeze-fracturing of face-centered, cubically packed, intracellular crystals of fraction 1 protein result in two distinct fracture responses of the constituent particles, according to their orientation relative to the plane of fracture. If a square-packed plane is revealed, fracture occurs either between particles or the particles plastically deform. Conversely, if a hexagonally-packed plane is revealed, fracture occurs either betwen particles or cleanly at internal planes of the particles, without any plastic deformation. It is proposed that this information may be of value in determining the arrangement of the axes of the covalently bound polypeptide protomer chains which constitute the oligomer.

Chloroplasts↗