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J Pawley

Publications and source records attributed to J Pawley.

6 recordsLinked to original sources
PubMed1989

Observations of colloidal gold labelled platelet microtubules: high voltage electron microscopy and low voltage-high resolution scanning electron microscopy.

18 nm colloidal gold-antitubulin and 4 nm colloidal gold-antitubulin were used to label microtubules in adherent, fully spread platelets. Both sizes of marker effectively labelled microtubules in the partially extracted platelets. However only the 4 nm gold penetrated the dense microfilament matrix of the inner filamentous zone so that portions of microtubules within this cytoskeletal zone could be tracked. The gold marker could be visualized well with 1 MeV high voltage transmission EM and with 5 kV or greater secondary imaging or 20 kV backscattered imaging of carbon only coated samples. 1 kV secondary imaging permitted high resolution imaging of the surface of tubules and the microfilaments with their respective associated material. Individual gold-antibody complexes were difficult to identify by shape alone due to the tendency of the antibody coats to blend together when in very close approximation and due to the presence of other molecules or molecular aggregates similar in size to the gold-antibody labels. Microtubules were seen to wind in and out of the inner and outer filamentous zones as they encircled the granulomere. Some tubules were seen to "dead end" at the peripheral web. Numerous smaller microtubule loops were present principally in the outer filamentous zone and tubules could be followed as they went from the outer filamentous zone through the inner filamentous zone and into the granulomere.

Blood Platelets↗
PubMed1989

High-resolution electron microscopic evidence for the filamentous structure of the cyst wall in Giardia muris and Giardia duodenalis.

High-resolution morphological studies of the cyst wall of Giardia spp. were performed using low-voltage scanning electron microscopy (LVSEM) and transmission electron microscopy (TEM). The cyst wall was composed of membranous and filamentous layers. The membranous layer consisted of an inner and an outer cyst membrane separated by a thin layer of cytoplasm. The filamentous layer contained individual filaments that ranged from 7 to 20 nm in diameter when measured by LVSEM, formed a dense meshwork with branches or interconnections, and were occasionally arranged on the surface in whorled patterns. Cysts of Giardia muris from mice, Giardia duodenalis from dogs, pigs, voles, beavers, muskrats, and humans, and Giardia psittaci from a bird (parakeet), possessed an essentially identical wall composed of filaments. Inducement of excystation in viable Giardia cysts produced a dramatic increase in the interfilament spacing over an entire cyst, but none was observed in heat-killed or chemically fixed control cysts. These results demonstrated that the cyst wall of Giardia spp. was composed of a complex arrangement of filaments, presumably formed during the process of encystment.

Animals↗
PubMed1987

Structure of the cytoplasmic filament system in freeze-dried whole mounts viewed by HVEM.

Freeze-drying (FD) was used as an alternative to critical point-drying (CPD) for the preparation of whole mounts to study the cytoplasmic filament system in mammalian cultured cells by high voltage electron microscopy (HVEM). Rapid quenching methods such as plunging a grid into liquid propane cooled by LN2 or collision with a clean surface of a copper block cooled to LHe temperature were used to avoid ice crystal formation. For freeze-drying, a special apparatus was built that allowed the specimen to be kept at 145 K for 2-3 days at a vacuum of about 2 X 10(-7) Torr followed by gradual stepwise warming to room temperature. Purified skeletal muscle actin served as a test object for which the structure was known through independent techniques. PtK-1 cells, grown on gold grids and fixed in buffered glutaraldehyde, were used to study the structure of the cytoplasmic filament system. Under conditions that should avoid formation of ice crystals, the structure of actin fibres was as expected on the basis of previous studies, i.e. a tangle of independent uniform filaments about 7 nm thick, Using similar conditions, the cytoplasmic filaments of PtK-1 cells were equally distinct and evenly thick along their length and were usually associated with particles about 10 nm thick. On the other hand, when the sublimation time at the low temperature was cut short, both purified actin and the cytoplasmic filament system formed a network of tapering filaments devoid of particles, similar to that which has been described as a 'microtrabecular lattice'. We therefore conclude that when this structure is seen in FD preparations, it is probably the result of distortion produced by faulty FD procedure.

Actins↗
PubMed1984

An HVEM for high resolution low-dose studies of biomacromolecules.

A 1 MeV EM-7 TEM has been modified to investigate the possibility that low-dose studies of biomacromolecular crystals might be more effectively carried out in an HVEM. The modifications include a TV viewing system, a deflection-type minimum exposure system, a side-entry, -150 degrees C transfer cold-stage and improvement of the working resolution to 0.2 nm. The advantages of HVEM for this type of study include a possible reduction in radiation damage rate, greater depth of focus on tilted specimens, broader instrumental contrast transfer function and a reduction in dynamical effects on thicker samples. Disadvantages and possible solutions to them are also discussed. Preliminary results on radiation damage in catalase at 400, 600 and 800 kV and images of catalase and squaric acid are included.

Catalase↗
PubMed1984

Low voltage scanning electron microscopy.

The scanning electron microscope (SEM) is usually operated with a beam voltage, V0, in the range of 10-30 kV, even though many early workers had suggested the use of lower voltages to increase topographic contrast and to reduce specimen charging and beam damage. The chief reason for this contradiction is poor instrumental performance when V0 = 1-3 kV, The problems include low source brightness, greater defocusing due to chromatic aberration greater sensitivity to stray fields, and difficulty in collecting the secondary electron signal. Responding to the needs of the semiconductor industry, which uses low V0 to reduce beam damage, considerable efforts have been made to overcome these problems. The resulting equipment has greatly improved performance at low kV and substantially removes the practical deterrents to operation in this mode. This paper reviews the advantages of low voltage operation, recent progress in instrumentation and describes a prototype instrument designed and built for optimum performance at 1 kV. Other limitations to high resolution topographic imaging such as surface contamination, the de-localized nature of the inelastic scattering event and radiation damage are also discussed.

Animals↗