Showing papers by "University of Leeds published in 1988"

Curvature‐compensated convective transport: SMART, A new boundedness‐ preserving transport algorithm

Abstract: The paper describes a new approach to approximating the convection term found in typical steady-state transport equations. A polynomial-based discretization scheme is constructed around a technique called ‘curvature compensation’; the resultant curvature-compensated convective transport approximation is essentially third-order accurate in regions of the solution domain where the concept of order is meaningful. In addition, in linear scalar transport problems it preserves the boundedness of solutions. Sharp changes in gradient in the dependent variable are handled particularly well. But above all, the scheme, when used in conjunction with an ADI pentadiagonal solver, is easy to implement with relatively low computational cost, representing an effective algorithm for the simulation of multi-dimensional fluid flows. Two linear test problems, for the case of transport by pure convection, are employed in order to assess the merit of the method.

Degree of Pyritization of Iron as a Paleoenvironmental Indicator of Bottom-Water Oxygenation

Abstract: A new method is presented for recognizing the degree of bottom-water oxygenation in organic carbon-bearing rock. The method uses the parameter Degree of Pyritization (DOP), which is the ratio of pyritic Fe to pyritic Fe + acid-soluble Fe. Sediments with ages ranging from Devonian to Cretaceous have been classified on paleoecological and sedimentological criteria as having been deposited in aerobic conditions (deposited in fully oxygenated bottom waters); restricted marine conditions (deposited in waters with low oxygen concentrations); or inhospitable bottom conditions (where little or no oxygen is present, and H2S may be continually or intermittently present). Aerobic (DOP < 0.42) can be clearly separated from restricted samples (0.46 < DOP < 0.80), but the latter have some overlap with the inhospitable bottom category (0.55 < DOP < 0.93). A boundary at DOP 0.75 separates more than 90 percent of this data. DOP values discriminate between paleoenvironments more effectively than organic carbon contents or the ratio of organic carbon to pyritic sulfur.

Why animals have different muscle fibre types

Abstract: Animals have different muscle fibre types: slow fibres with a low maximum velocity of shortening (Vmax) and fast fibres with a high Vmax. An advantage conferred by the use of different fibre types during locomotion has been proposed solely on the basis of their in vitro properties. Isolated muscle experiments show that force generation, mechanical power production and efficiency are all functions of V/Vmax, where V is the velocity of muscle shortening. But it is not known whether animals actually use the different fibres at shortening velocities that are optimal for mechanical power production and efficiency. Here we compare the V of muscle fibres during locomotion with their Vmax. This comparison shows that during slow locomotion, the slow fibres shorten at a velocity that gives peak mechanical power and efficiency and the fast fibres shorten at their optimal velocity when powering maximal movements. Our results also show that maximal movements are impossible without fast fibres because the slow ones cannot shorten rapidly enough.

Why are mammalian tendons so thick

Abstract: The maximum stresses to which a wide range of mammalian limb tendons could be subjected in life were estimated by considering the relative cross‐sectional areas of each tendon and of the fibres of its muscle. These cross‐sectional areas were derived from mass and length measurements on tendons and muscles assuming published values for the respective densities. The majority of the stresses are low. The distribution has a broad peak with maximum frequency at a stress of about 13 MPa, whereas the fracture stress for tendon in tension is about 100 MPa. Thus, the majority of tendons are far thicker than is necessary for adequate strength. Much higher stresses are found among those tendons which act as springs to store energy during locomotion. The acceptability of low safety factors in these tendons has been explained previously (Alexander, 1981). A new theory explains the thickness of the majority of tendons. The muscle with its tendon is considered as a combined system which delivers mechanical energy: the thickness of the tendon is optimized by minimizing the combined mass. A thinner tendon would stretch more. To take up this stretch, the muscle would require longer muscle fibres, which would increase the combined mass. The predicted maximum stress in a tendon of optimum thickness is about 10 MPa, which is within the main peak of the observed stress distribution. Individual variations from this value are to be expected and can be understood in terms of the functions of the various muscles.

On the complexity of computing the volume of a polyhedron

Abstract: We show that computing the volume of a polyhedron given either as a list of facets or as a list of vertices is as hard as computing the permanent of a matrix.

The prediction of local recurrence in rectal adenocarcinoma by histopathological examination.

Abstract: Local recurrence of rectal adenocarcinoma is mainly due to failure to remove all the tumour. A method is described for the routine detection of involvement of the circumferential (lateral) resection margin. Current definitions of the length of the rectum are inadequate for the assessment of the risk of local recurrence as the rectum frequently extends higher than 15 cm. Use of the term recto-sigmoid should be replaced clinically by sigmoidoscopic measurement of the height of a tumour and pathologically by its anatomical relationship to the level of peritoneal reflection, i.e. lower or upper segment of the rectum or the sigmoid colon. Tumours above the peritoneal reflection (upper segment) are at risk of circumferential resection margin involvement due to their retroperitoneal component. The amount of tissue excised varies considerably from surgeon to surgeon. Meticulous attention to the clearance of the tumour at the circumferential resection margin is essential if local recurrence rates are to be reduced. A trial of postoperative radiotherapy should be instigated based on the pathologist's identification of patients at high risk of local recurrence.

Modulation of the GABAA receptor by depressant barbiturates and pregnane steroids.

Abstract: 1. The modulation of the gamma-aminobutyric acidA (GABAA) receptor by reduced metabolites of progesterone and deoxycorticosterone has been compared with that produced by depressant barbiturates in: (a) voltage-clamp recordings from bovine enzymatically isolated chromaffin cells in cell culture, and (b) an assay of the specific binding of [3H]-muscimol to a preparation of porcine brain membranes. 2. The progesterone metabolites 5 alpha- and 5 beta-pregnan-3 alpha-ol-20-one (greater than or equal to 30 nM) reversibly and dose-dependently enhanced the amplitude of membrane currents elicited by locally applied GABA (100 microM), and over the concentration range 30 nM-100 microM stimulated the binding of [3H]-muscimol. In contrast, 5 alpha- and 5 beta-pregnan-3 beta-ol-20-one (30 nM-100 microM) had little effect in either assay, indicating a marked stereoselectivity of steroid action. 3. Scatchard analysis of the ligand binding data suggested an apparent increase in the number, rather than the affinity, of detectable [3H]-muscimol binding sites as the principle action of the active steroid isomers. 4. GABA-evoked currents were also potentiated by androsterone (1 microM) and the deoxycorticosterone metabolite 5 alpha-pregnane-3 alpha,21-diol-20-one (100 nM). 5. Secobarbitone (10-100 microM), pentobarbitone (10-300 microM) and phenobarbitone (100-500 microM) reversibly and dose-dependently potentiated the amplitude of GABA-evoked currents in the absence of any change in their reversal potential. 6. At relatively high concentrations (greater than or equal to 30 microM) secobarbitone and pentobarbitone directly elicited a membrane current. It is concluded that such currents result from GABAA receptor-channel activation since they share a common reversal potential with GABA-evoked responses (approximately 0 mV), are reversibly antagonized by bicuculline (3 microM), and potentiated by either diazepam (1 microM) or 5 beta-pregnan-3 alpha-ol-20-one (500 nM). 7. Secobarbitone (1 microM-1 mM) dose-dependently enhanced the binding of [3H]-muscimol. In common with the active steroids, an increase in the apparent number of binding sites was responsible for this effect. 8. A saturating concentration (1 mM) of secobarbitone in the ligand binding assay did not suppress the degree of enhancement of control binding produced by 5 beta-pregnan-3 alpha-ol-20-one (30 nM-100 microM). Similarly the steroid, at a concentration of 100 microM, did not influence the enhancement of [3H]-muscimol binding by secobarbitone (1 microM-1 mM). In all combinations of concentrations tested, the effects of secobarbitone and 5#-pregnan-3a-ol-20-one on [3H]-muscimol binding were additive. 9. In conjunction with previously published observations, the present data indicate close similarities in the GABA-mimetic and potentiating actions of barbiturates and steroids. However, the results obtained with combinations of steroids and barbiturates in the ligand binding assay appear inconsistent with the two classes of compound interacting with a common site to modulate the GABAA receptor activity.

Deep fluid circulation in fault zones

Abstract: Massive fluid circulation in retrogressive ductile shear zones is a well-established but poorly understood phenomenon. In some cases, surface-derived fluids, which must initially have been at hydrostatic pressures, can be shown to have entered shear zones in which fluids would normally be expected to be at lithostatic pressure. In these circumstances, thermal convection is an unlikely driving force for fluid movement. Underthrusting of a surficial fluid reservoir beneath shear zones is a viable mechanism in some cases, but not for metasomatic shear zones in the Pyrenees where insufficient underthrusting has occurred. Seismic pumping provides an alternative mechanism to explain the paradox of fluid access into ductile shear zones; a kinematic analysis of a hypothetical fault zone shows that stress and dilatancy cycles will be out of phase above and below the frictional/quasiplastic transition. If this effect is sufficiently large, hydraulic gradients may force fluid downward across the transition immediately after earthquake rupture through highly permeable microcrack networks. Between earthquake cycles, plastic creep in mylonites will overprint microcrack networks, increase fluid pressure, and promote slow upward movement of fluid at low permeability. For smaller shear zones, such as those in the Pyrenees, seismic pumping could occur down a shallow decollement with subsequent upward fluid flow through shear zones in the hanging wall of the decollement.

Observer variation in the assessment of dysplasia in ulcerative colitis

Abstract: Six histopathologists allocated 100 sections from patients with long-standing ulcerative colitis into four diagnostic categories, regular hyperplasia, reactive atypia, low-grade and high-grade dysplasia. Their allocations were analysed using kappa statistics, including Fleiss's multiple kappa for groups of observers, and agreement on specific diagnoses was explored by constructing a conditional probability matrix. The nature of their disagreements was investigated using coefficients for systematic and haphazard errors. Over the four diagnostic categories there was a wide range of pairwise agreement from a low of 49% up to 72% and kappa values were only ‘fair’ or ‘moderate’. As expected, agreement over the two categories ‘dysplasia’vs‘no dysplasia’ was better, ranging from 68% to 84%, and for ‘atypia present’ (reactive atypia, low- and high-grade dysplasia) vs‘no atypia’ two pairings achieved over 90% and 11 pairings over 80% agreement. In view of its clinical importance, conditional agreement on high-grade dysplasia was examined. Given that a first observer allocates a case as high-grade dysplasia, pairwise agreement on this diagnosis ranged from 100% down to as low as 33%. However, most of these disagreements fell into the low-grade dysplasia category so that closer follow-up and further biopsies would still have been indicated. It is a truism that the basis for safe management is careful co-operation between clinicians and pathologists who have all the relevant facts and who know and trust one another's judgement. Thus, several aspects of the ideal diagnostic process cannot be evaluated in inter-observer studies and the element of artificiality should be borne in mind when applying the findings to diagnostic practice. Nevertheless, the low level of agreement on the diagnosis of high-grade dysplasia achieved by certain pairings of specialist pathologists is a disturbing outcome of this study. Inaccuracies should be minimized by a concensus approach and we therefore recommend referral of putative cases of dysplasia to interested pathologists for further opinions. We would also advocate that pathologists faced with appearances which are indefinite between reactive atypia and dysplasia, would do better to describe them in terms of ‘atypia, significance uncertain’, so that closer surveillance is undertaken, rather than force them into more precise diagnostic categories which may be incorrect.

An introduction to Hankel operators

Abstract: Introduction 1. Compact operators on a hilbert space 2. Hardy spaces 3. Basic properties of hankel operators 4. Hankel operators on the half plane 5. Linear systems and H 6. Hankel-norm approximation 7. Special classes of hankel operator Appendix Exercises Bibliography Index.

Ectoenzymes of the kidney microvillar membrane. Differential solubilization by detergents can predict a glycosyl-phosphatidylinositol membrane anchor.

Abstract: The pattern of solubilization of nine kidney microvillar ectoenzymes by a range of detergents distinguished two classes of membrane proteins: those released from the membrane by bacterial phosphatidylinositol-specific phospholipase C and those not so released. The latter group of transmembrane proteins were solubilized efficiently (greater than 80%) by all the detergents examined. In contrast, proteins released by phosphatidylinositol-specific phospholipase C were solubilized effectively only by octyl glucoside, 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulphonate and sodium deoxycholate. Octyl glucoside solubilized the amphipathic forms of the ectoenzymes examined, suggesting that this may be a useful detergent in the purification of glycosyl-phosphatidylinositol-anchored ectoenzymes.

A spatial thresholding method for image segmentation

Abstract: Several model-based algorithms for threshold selection are presented, concentrating on the two-population univariate case in which an image contains an object and background. It is shown how the main ideas behind two important nonspatial thresholding algorithms follow from classical discriminant analysis. Novel thresholding algorithms that make use of available local/spatial information are then given. It is found that an algorithm using alternating mean thresholding and median filtering provides an acceptable method when the image is relatively highly contaminated, and seems to depend less on initial values than other procedures. The methods are also applicable to multispectral k-population images. >

Lymphocytic gastritis ― relationship to Campylobacter pylori infection

Abstract: The existence of a distinctive form of chronic gastritis characterized by marked infiltration of the surface and pitlining epithelium by mature T lymphocytes has been confirmed. Seventeen cases were identified amongst 382 patients with active chronic gastritis (4·5 per cent). The cases with lymphocytic gastritis had significantly higher counts of intra-epithelial lymphocytes than sex and age-matched controls drawn from a series of patients with the usual form of active Type B chronic gastritis. Furthermore, the lymphocytic gastritis group exhibited unusual endoscopic findings, namely erosions and ‘raised lesions’, in the body of the stomach. Such appearances have been previously described as ‘varioliform’ gastritis. Only seven of the patients (41 per cent) had C.pylori-positive biopsies, compared with over 90 per cent in the generality of active chronic gastritis, but all but two of the eleven cases tested had serological evidence of C. pylori infection. While the pathogenesis of this condition is unclear, the histological resemblance to coeliac disease suggests that lymphocytic gastritis might also represent an abnormal response to a local antigen to which the patient has become sensitized. It is possible that this antigen is C. pylori.

Recent developments in Carboniferous geology: a critical review with implications for the British Isles and N.W. Europe

Abstract: Significant increases in our understanding of Carboniferous geological and geographical processes, including plate tectonics, palaeomagnetism, climatology and sea level changes have occurred in recent years. Further advances will increasingly depend on the accurate determination of radiometric ages for the boundaries of the major Carboniferous stratigraphie subdivisions. The recent 39 Ar/ 40 Ar dating of sanidines from European Silesian tonsteins holds out great hopes that structural, igneous and metamorphic events dated by radiometric methods can be better correlated with stratigraphie events defined by goniatite zonation. Palaeomagnetic and tectonic studies in the European Hercynides have established that the Upper Palaeozoic geological evolution of the British Isles took place to the north (present coordinates) of an active micro-plate collision zone along the Galician-Brittany-Massif Central line. Lithospheric stretching of the British/Irish Hercynian ‘foreland’ in the Lower Carboniferous was followed by a belt of north-migrating crustal shortening which disrupted the thermal sag phase of extensional subsidence in northern Britain from Westphalian C times onwards. Backstripped subsidence curves for north British Carboniferous basins indicate that subsidence may have occurred in response to lithospheric thinning of up to 50%. The proposal that there was crustal extension and limited seafloor spreading between Greenland and Scotto-Scandinavia along the Rockall/Faroes line during Carboniferous times is discussed and it is suggested that strike-slip tectonics, known to have been active in Maritime Canada may have played a more important role. Radiometric studies of detrital zircons reveal that the nature of the sourcelands for the huge amounts of Carboniferous detritus in the northern British Isles changed little during the course of the period. They were dominated by outcrops of post-Archaean sediments, minor Archaean basement and abundant Caledonian granitoids with little evidence for Proterozoic crustal growth in the hinterlands. A combination of Mid-Carboniferous climatic change, to a more humid regime, and granite/gneiss terrane unroofing, substantially explains the flushing-out of huge amounts of feldspathic detritus in the Namurian. This Carboniferous climatic change itself must have been influenced by the growth of the E-W Hercynian mountain chain and the accompanying fusion of Gondwanaland with Pangea. Some palaeomagnetic evidence also exists for latitudinal shift at this time. Perhaps the most important influence was the early Namurian expansion of the great Gondwanan ice centre. Waxing and waning of this on a Milankovich time scale dominated Silesian sea level changes and facies evolution. Many late Dinantian and Silesian ‘minor’ sedimentary cycles are probably of glacio-eustatic origins, but there seems little evidence that supposed Dinantian and Namurian mesothemic cycles have such an origin. There are, in fact, increasing doubts as to the actual existence of these particular cycles.

British bird species distributions and the energy theory

Abstract: The steep decline in the number of species from Tropic to Pole may possibly be explained by 'the weather', that is, the latitudinal decline in the input of solar energy. This explanation, often obvious to the lay person, is dismissed or even ignored by most ecologists1–3. Only J. H. Brown and colleagues have proposed a direct 'species–energy' relationship4–6, an excellent scientific theory which implies simple tests that would refute the theory if it failed them. One such is the change in species numbers according to the seasonal fluctuation in the distribution of available energy: we will show that the varying distribution of small insectivorous birds in Britain in summer and winter corresponds well with the predictions of the species–energy theory; it also provides support for the theoretical model which explains it6,7. We find no support for the alternative 'historical' and 'habitat' theories.

Use of a pharmacologic indicator to compare compliance with tablets prescribed to be taken once, twice, or three times daily.

Abstract: By use of an interview, return tablet count, and a pharmacologic indicator (low-dose phenobarbital), we compared compliance with tablets prescribed to be taken once, twice, or three times daily. One hundred seventy-nine patients with type II diabetes were randomly allocated to take one 2 mg phenobarbital tablet once, twice, or three times daily for 28 days. Phenobarbital level/dose ratios indicated that compliance was similar with once- and twice-daily regimens, and both were better than thrice-daily dosing. Mean return tablet counts suggested that compliance was best with the once-daily regimen; both twice- and thrice-daily regimens were similarly inferior. This difference between the techniques may be explained by the inadequacies of the residual tablet count, which identified only 13% of cases identified by phenobarbital. We conclude that compliance with the once-daily regimen was best, but that compliance with a twice-daily regimen was very similar, and both were superior to dosing three times a day.

Sedimentology, palaeoecology and palaeoenvironmental evolution of the 1985 Lhasa to Golmud Geotraverse

Abstract: Vertical and horizontal measurements of almost 30 km of sections were made along the Geotraverse route at 113 localities ranging in age from Ordovician to Tertiary. Over 280 palaeocurrent measurements were taken and 200 thin sections were studied. Ordovician strata occur only in the Kunlun Terrane, w here thick metamorphosed sequences of elastics and carbonates occur. These are tentatively interpreted as platform margin and slope deposits. During Carboniferous times in the Kunlun Terrane transgressive late-Dinantian marine limestones with tropical to subtropical Eurasian reefoidal faunas overlie fluviatile redbeds derived from an unroofed orogenic belt. The Lhasa Terrane contains shelf basin elastics with low -diversity faunas succeeded by thick late Carboniferous/early Permian glacio-marine mixtites deposited by iceberg meltout. Permian carbonate ramp and shelf facies with reefoidal developments occur over both the Lhasa and Qiangtang Terranes, with coal-bearing elastics and fluviatile redbeds also occurring in the latter.

Chemical model for the origin of minor limestone-shale cycles by anaerobic methane oxidation

Abstract: Chemical and isotopic studies of Jurassic carbonate concretions indicate an origin by anaerobic methane oxidation. Concretion growth occurred within the top 1 m of sediment in a thin zone where methane was consumed to stimulate a late, rejuvenated phase of sulfate reduction. Similar chemical and isotopic characteristics are shown by diagenetic carbonate in several limestone-shale sequences, and an origin by anaerobic methane oxidation is proposed for these sequences also. The evolution of isolated concretions into diagenetic limestones depends on the extent of, and depth variations in, carbonate supersaturation arising from anaerobic methane oxidation and the persistence of a reduced sedimentation rate or a depositional hiatus. Diagenetic limestones formed in this way can often be recognized by their association with a later pyrite phase that is more abundant and has more positive /sup 34/S values than the adjacent shales.

Immunoregulatory cells in human decidua: morphology, immunohistochemistry and function.

Abstract: Suggestions that local intrauterine materno-fetal immune interactions may be important for normal pregnancy have led to investigation of immunoregulatory function by decidualised endometrium. Human decidua is complex and cell types may be difficult to distinguish at the light microscope level. Immunohistochemical techniques have allowed antigenic identification of cells within decidua and abundant leucocytes are present throughout pregnancy. Macrophages (CD14+) are abundant in decidua basalis and decidua parietalis throughout pregnancy and may be closely associated with extravillous trophoblast. Expression of class II MHC antigens and CD11c may suggest an immunological role but their content of lysosomal enzymes could indicate phagocytic functions. Antigen-presenting capacity has been noted in early human decidua and may be due to macrophages. Decidual macrophages have also been attributed with immunosuppressive function due to secretion of prostaglandin E2. Decidual granulated lymphocytes are abundant in the first trimester and correspond to the so-called endometrial stromal granulocytes. They express CD2, CD7, CD38 and NKH1 but are negative for classical T cell and NK cell markers and they do not express the IL2 receptor. Semipurified populations show low levels of cytotoxicity in a standard NK assay. Thus, immunohistochemical techniques have allowed characterisation of potentially immunocompetent cells in human decidua. However, their roles both in vitro and in vivo remain to be established with certainty.