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Biomedical subjects

M Chew

Publications and source records attributed to M Chew.

At least 37 records · Page 2Linked to original sources
PubMed1998

Myosin crossbridge configurations in equilibrium states of vertebrate skeletal muscle. Heads swing axially or turn upside-down between resting and rigor.

The positions and orientations of the myosin heads in relaxed, active, rigor and S1-labelled fish muscle are being determined by analysis both of electron micrographs and of low-angle X-ray diffraction patterns. The X-ray analysis of resting muscle makes use of the head shape defined from the study of S1 crystals, with variable head configurational parameters being used on each of the three different 3-fold symmetric 14.3 nm-spaced 'crowns' of myosin heads within the 42.9 nm axial repeat of the myosin filaments. Diffraction patterns were stripped using CCP13 fibre diffraction software. Searches and optimisation were carried out using simulated annealing and local refinement procedures to give a 'best fit' relaxed structure with a crystallographic R-factor of about 4%. It had heads oriented all the same way up (i.e. with similar rotations around their own long axes) on the myosin filament, but with a small range of axial tilts. Head configuration in rigor fish muscle is being determined by X-ray diffraction and electron microscopy of normal rigor muscle and of skinned muscle soaked with extrinsic myosin S1. Computed 3-D reconstructions of acto-S1 using X-ray amplitudes and phases from electron microscopy are informative and help to analyse the X-ray diffraction data that extend axially to about 1 nm resolution. An ambiguity is the axial direction of the observed resting myosin head array relative to the known polarity of the actin filaments. One polarity would give little axial displacement (2-3 nm) between opposite ends of the resting and rigor heads, and in this case the heads would need to rotate around their own long axes by about 115 degrees to make a rigor attachment. The other (preferred) filament polarity would provide considerable axial swinging (14-15 nm) between the two states. We are attempting to define the absolute polarity of the resting muscle myosin head array using electron microscopy and image processing either of cryo-sections or of replicas from shadowed, freeze-fractured, rapidly frozen fish muscle fibres.

Actins↗
PubMed1997

Quantification of radiation-induced regional lung injury with perfusion imaging.

PURPOSE: To better understand the dose and time dependence of radiation therapy (RT)-induced regional lung dysfunction as assessed by changes in regional lung perfusion. METHODS AND MATERIALS: Patients who were to receive RT for tumors in and around the thorax, wherein portions of healthy lung would be incidentally irradiated, were prospectively studied. Regional function was assessed pre- and post-RT with single photon emission computed tomography (SPECT) lung perfusion scans, obtained following the intravenous administration of approximately 4 mCi of technetium-99m macroaggregated albumin. Pre-RT computed tomography (CT) scans were used to calculate the three-dimensional (3D) dose distribution, reflecting tissue density inhomogeneity corrections. Each SPECT scan was correlated with the pre-RT CT scan, and the 3D dose distribution. Changes in regional lung perfusion were correlated with regional RT dose, at various time intervals following radiation. RESULTS: The data from 20 patients (7 breast cancer, 5 lymphoma, 1 esophagus, 1 sarcoma, and 6 lung cancer) have been analyzed. Patients with gross intrathoracic lung cancers causing obstruction of regional pulmonary arteries were not included. For most patients, there is a statistically significant dose-dependent reduction in regional blood flow at all time points following radiation. While a time dependence is suggested in the high dose range, the limited amount of data prevents meaningful statistical evaluation. CONCLUSIONS: Radiation therapy-induced regional lung dysfunction occurs in a dose-dependent manner and develops within 3-6 months following radiation. In contrast to classical "sigmoid" dose-response curves, described mainly for changes following whole lung irradiation, these data suggest a more gradual relationship between regional dysfunction and RT dose. Retraction of irradiated lung with secondary movement of unirradiated lung into the "3D-defined irradiated volume" may have introduced inaccuracies into this analysis. Additional studies are currently underway to assess this possibility and better refine this dose-response curve. Studies are underway to determine if changes in assessments of whole lung function, such as pulmonary function tests, can be predicted by summing the regional changes observed.

Dose-Response Relationship, Radiation↗
PubMed1997

Temporalis muscle transfer in the correction of lagophthalmos due to leprosy.

In the correction of lagophthalmos due to leprosy neuritis temporalis muscle transfer (TMT) is used to provide a motor to assist in lid closure. This study of TMT in 51 eyes was carried out to assess the effectiveness of TMT in achieving lid closure and corneal protection. The average lid gap preoperatively on light closure was 7.3 mm which was reduced to 3.2 mm on final follow-up. The average lid gap pre-operatively on tight closure was 5.3 mm which was reduced to 0.4 mm at final follow-up. It is possible to train patients with partial or total anesthesia of the cornea in a visual THINK-BLINK reflex. The common complications encountered were ectropion in 6 eyes (12%) and ptosis in 3 eyes (6%).

Adolescent↗
PubMed1992

Biochemical fractionation and characterization of proteins from Golgi-enriched membranes.

Fractions enriched in Golgi membranes were prepared from rat liver by sucrose gradient ultracentrifugation. These enriched membranes were further subfractionated on the basis of their solubilities in EGTA, 150 mM sodium carbonate, pH 11.5, sodium deoxycholate, Triton X-100, or sodium dodecyl sulfate. This led to isolation of peripheral, luminal, and integral membrane proteins of the Golgi-enriched membranes. Luminal and membrane proteins were further purified by wheat germ agglutinin and concanavalin A lectin affinity chromatographies. Some proteins from these lectin columns were resolved by preparative gel electrophoresis and microsequenced. Subsequently, antibodies were produced for two proteins by immunization of either mice or rabbits. Immunofluorescence microscopy suggests that these proteins are confined to Golgi apparatus-like structures. The protocol described is well suited for the study of organelle structure and function.

Amino Acid Sequence↗
PubMed1991

Population genetic analysis of host seeking and resting behaviors in the malaria vector, Anopheles balabacensis (Diptera: Culicidae).

During the intermonsoon period from mid-September to mid-October 1986, wild-caught Anopheles balabacensis Baisas females were marked and released in a host-choice experiment. Association between capture and recapture of marked mosquitoes from human and bovid hosts and blood meal host identification of recaptured females were determined on a daily basis. Although the mark-recapture and blood meal data indicated behavioral heterogeneity between buffalo and human biters, restriction endonuclease fragment length polymorphism analysis revealed no differences in repeat sequence profiles. Doubly-marked recaptures strongly indicated a "learning" component involved in a separate host preference experiment. In a "habitat loyalty" experiment conducted in January 1987, females of An. balabacensis preferentially returned to the resting sites (indoor surfaces and exit traps) where they were first caught. Of nine isozyme loci found to be polymorphic, the genotypic frequencies of Esterase-3 and Isocitrate dehydrogenase-3 were different in "faithfully" endophilic and exophilic subpopulations. Genetic heterozygosity, as determined by polyacrylamide gel electrophoresis, was greater in exophilic than endophilic population components. These results confirm that genetic and learning components can significantly influence house resting and host seeking behavior and may contribute to local epidemiological patterns of malaria transmission observed in Sabah, Malaysia.

Animals↗
PubMed1988

Government relations. Communication--the most powerful tool.

Your vote is only the beginning of the support your legislator needs; he or she also needs to know your stand on issues. Use the tools of communication as issues come before your representatives to tell them how to best represent you.

Communication↗
PubMed1988

Enzyme polymorphism of the malaria vector, An. balabacensis (Diptera: Culicidae) revisited--why sample natural populations?

Two separate observations from recent electrophoretic studies of the systematics and population genetics of laboratory-reared populations which had a long history of colonization in various laboratories, were found to be inconsistent with the present study which used wild-caught populations from East Malaysia. Reanalysis of the two data sets generally indicated a low amount of genetic variation in laboratory colonies. The latter is characterized by higher frequency of monomorphic loci, low average heterozygosity values and, in one extreme case, no variability at two loci. However, natural populations of An. balabacensis and An. leucosphyrus showed more protein variability by the use of polyacrylamide gel electrophoresis. Since laboratory-maintained mosquitoes are genetically and phenotypically different from those in the field, results of laboratory studies on the systematics and population genetics of Anopheles species complexes may be biased.

Animals↗
PubMed1987

Thinning of fetal pulmonary arterial wall and postnatal remodelling: ultrastructural studies on the respiratory unit arteries of the pig.

Adaptation to extra-uterine life and postnatal remodelling of intra-acinar arteries was followed in 34 Large White pigs, from birth to adult life, applying morphometry to light and electronmicroscopic studies. After birth, percentage wall thickness decreased rapidly due to a reduction in overlap of adjacent smooth muscle cells and an increase in smooth muscle cell surface area/volume ratio, (p less than 0.01 at 12 h), without a reduction in the volume density of smooth muscle cells. Smooth muscle cells appeared immature at birth and synthetic rather than contractile organelles predominated. Between 3 weeks and 6 months myofilament volume density doubled (p less than 0.0001). At all ages, pericytes, intermediate and smooth muscle cells showed similar volume densities of contractile and synthetic organelles. Thus, the high fetal pulmonary vascular resistance appeared to be due to the shape and arrangement of smooth muscle and other contractile cells within the vessel wall, rather than an excessive contractility of these cells. After birth rapid remodelling of arterial wall structure achieved a reduction in wall thickness by 30 min, continuing during the first week of life. After 3 weeks, remodelling involved an increase in wall thickness, connective tissue deposition with more collagen than elastin (p less than 0.0001), and smooth muscle cell differentiation.

Animals↗
PubMed1977

Comparative evaluation of 99mTc GH, 99mTcO4, and 99mTc DTPA as brain imaging agents.

The brain imaging properties of 99mTc glucoheptonate, 99mTc pertechnetate, and 99mTc DPTA are compared. Results demonstrate that optimum images are obtained at 90, 180, and 180 min., for 99mTc GH, 99mTc DTPA, and 99mTc perterchnetate, respectively. The former two images are not affected by prior bone imaging with 99mTc pyrophosphate, while 99mTc pertechnetate images are adversely affected. 99mTc glucoheptonate appears to be the superior agent for brain imaging, followed by 99mTc DTPA and 99mTc pertechnetate.

Brain Diseases↗
PubMedDate not supplied

Three-Dimensional Reconstruction of a Collagen IV Analogue in the Dogfish Egg Case Wall.

The wall of the egg case of the dogfish (Scyliorhinus canicula) contains an analogue of collagen Types IV and VIII organised into a regular 3-dimensional network. It presumably provides both a protective and a filtering role for the eggs contained within it. Electron micrographs of longitudinal and transverse sections, including systematically tilted sections, have been used both to define, for the first time, the space group symmetry of the lattice and to carry out 3-D reconstruction of the unit cell contents. This cell was found to be tetragonal, space group I422, with a = b = 11 ± 1 nm, and c = 74 ± 4 nm. Consistent with this, projection symmetries were c2mm for the [1,0,0] view, p2mm for the [1,1,0] view, and p4mm for the projection down the c axis (the [0,0,1] view), and all observed reflections in the computed Fourier transform obeyed the rule h + k + l = 2n (n integer) for body-centred lattices. The 3-D reconstruction, the first electron micrograph 3-D reconstruction of a collagen-containing material, is interpreted in terms of variations of previous molecular models for this structure. Type IV collagen is a constituent of the basal lamina, where it forms a network with both structural and filtering properties. The dogfish egg case structure may throw light on the (less regular) collagen IV structure of the basal lamina.

Journal Article↗