PubMed Health⌕ Search

Biomedical subjects

M C Maas

Publications and source records attributed to M C Maas.

8 recordsLinked to original sources
PubMed2005

No effect of C-reactive protein on early atherosclerosis development in apolipoprotein E*3-leiden/human C-reactive protein transgenic mice.

OBJECTIVE: C-reactive protein (CRP) has been associated with risk of cardiovascular disease. It is not clear whether CRP is causally involved in the development of atherosclerosis. Mouse CRP is not expressed at high levels under normal conditions and increases in concentration only several-fold during an acute phase response. Because the dynamic range of human CRP is much larger, apolipoprotein E*3-Leiden (E3L) transgenic mice carrying the human CRP gene offer a unique model to study the role(s) of CRP in atherosclerosis development. METHODS AND RESULTS: Atherosclerosis development was studied in 15 male and 15 female E3L/CRP mice; E3L transgenic littermates were used as controls. The mice were fed a hypercholesterolemic diet to induce atherosclerosis development. Cholesterol exposure did not differ between E3L/CRP and E3L mice. Plasma CRP levels were on average 10.2+/-6.5 mg/L in male E3L/CRP mice, 0.2+/-0.1 mg/L in female E3L/CRP mice, and undetectable in E3L mice. Quantification of atherosclerosis showed that lesion area in E3L/CRP mice was not different from that in E3L mice. CONCLUSIONS: This study demonstrates that mildly elevated levels of CRP in plasma do not contribute to the development of early atherosclerosis in hypercholesterolemic E3L/CRP mice.

Animals↗
PubMed2004

GATE: a simulation toolkit for PET and SPECT.

Monte Carlo simulation is an essential tool in emission tomography that can assist in the design of new medical imaging devices, the optimization of acquisition protocols and the development or assessment of image reconstruction algorithms and correction techniques. GATE, the Geant4 Application for Tomographic Emission, encapsulates the Geant4 libraries to achieve a modular, versatile, scripted simulation toolkit adapted to the field of nuclear medicine. In particular, GATE allows the description of time-dependent phenomena such as source or detector movement, and source decay kinetics. This feature makes it possible to simulate time curves under realistic acquisition conditions and to test dynamic reconstruction algorithms. This paper gives a detailed description of the design and development of GATE by the OpenGATE collaboration, whose continuing objective is to improve, document and validate GATE by simulating commercially available imaging systems for PET and SPECT. Large effort is also invested in the ability and the flexibility to model novel detection systems or systems still under design. A public release of GATE licensed under the GNU Lesser General Public License can be downloaded at http:/www-lphe.epfl.ch/GATE/. Two benchmarks developed for PET and SPECT to test the installation of GATE and to serve as a tutorial for the users are presented. Extensive validation of the GATE simulation platform has been started, comparing simulations and measurements on commercially available acquisition systems. References to those results are listed. The future prospects towards the gridification of GATE and its extension to other domains such as dosimetry are also discussed.

Computer Simulation↗
PubMed1996

Dental microwear and microstructure in early oligocene primates from the Fayum, Egypt: implications for diet.

Textbook descriptions usually portray the Fayum anthropoideans as frugivores, with Parapithecus grangeri including a folivorous component in its diet and Apidium a component of hard-object feeding. Recent work with modern mammals has shown that analyses of both dental microwear and dental microstructure may yield insights into diet and tooth use. The purpose of this study was to combine these two techniques to gain a better perspective on the paleobiology of the Fayum higher primates. Dental microwear analyses involved the use of high resolution epoxy casts of Aegyptopithecus, Parapithecus, and Apidium housed in the Duke University Primate Center. Scanning electron micrographs were taken at x500, and all microwear features in each micrograph were digitized. For microstructure analyses, molar teeth were sectioned in a variety of planes, lightly etched, and photographed in the SEM. Results of the dental microwear analyses indicate that the three Fayum anthropoideans all clustered with modern primate frugivores but that there were also significant differences between Aegyptopithecus and the other two Fayum genera. By contrast, dental microstructure analyses showed important differences between Apidium and the other two genera. The reason for these differences probably lies in a combination of body size and dietary differences, with Aegyptopithecus occasionally feeding on hard objects and Apidium maximizing wear resistance through a unique emphasis of radial (rather than decussating) enamel.

Animals↗
PubMed1994

Enamel microstructure in Lemuridae (Mammalia, Primates): assessment of variability.

This study describes the molar enamel microstructure of seven lemurid primates: Hapalemur griseus, Varecia variegata, Lemur catta, Lemur macaco, Lemur fulvus rufus, Lemur fulvus fulvus, and Lemur fulvus albifrons. Contrary to earlier accounts, which reported little or no prism decussation in lemurid enamel, both Lemur and Varecia molars contain a prominent inner layer of decussating prisms (Hunter-Schreger bands), in addition to an outer radial prism layer, and a thin, nonprismatic enamel surface layer. In contrast, Hapalemur enamel consists entirely of radial and, near the surface, nonprismatic enamel. In addition, for all species, prism packing patterns differ according to depth from the tooth surface, and for all species but Varecia (which also has the thinnest enamel of any lemurid), average prism area increases from the enamel-dentine junction to the surface; this may be a developmental solution to the problem of accommodating a larger outer surface area with enamel deposited from a fixed number of cells. Finally, contradicting some previous reports, Pattern 1 prisms predominate only in the most superficial prismatic enamel. In the deeper enamel, prism cross-sections include both closed (Pattern 1) and arc-shaped (Pattern 2 or, most commonly, Pattern 3). This sequence of depth-related pattern change is repeated in all taxa. It should also be emphasized that all taxa can exhibit all three prism patterns in their mature enamel. The high degree of quantitative and qualitative variation in prism size, shape, and packing suggests that these features should be used cautiously in phylogenetic studies. Hapalemur is distinguished from the other lemurids by unique, medially constricted or rectangular prism cross-sections at an intermediate depth and the absence of prism decussation, but, without further assessment of character polarity, these differences do not clarify lemurid phylogenetic relations. Some characters of enamel microstructure may represent synapomorphies of Lemuridae, or of clades within Lemuridae, but homoplasty is likely to be common. Homoplasy of enamel characters may reflect functional constraints.

Ameloblasts↗
PubMed1994

A scanning electron-microscopic study of in vitro abrasion of mammalian tooth enamel under compressive loads.

Microscopic tooth-wear (microwear) patterns can be an important tool for assessing modes and rates of abrasive tooth wear, but their analysis and interpretation is complicated by the fact that microwear is influenced by many factors. Three of these factors were here tested under conditions of compressive loading: (1) species differences in enamel structure, (2) abrasive particle size and (3) magnitude of force. Teeth of four species (Homo sapiens, Lemur fulvus, Ovis aries and Crocodylus rhombifer) were abraded in vitro using three sizes of abrasive silicon-carbide grit (average diameters 73, 23 and 14 microns), at two loads (50 and 100 kg). Microwear features were assessed by scanning electron microscopy of lightly etched enamel surfaces and epoxy replicas. Microwear pits (length:width < 4:1) were the predominant feature type. Factorial analysis of variance of rank-transformed, feature-area measurements demonstrated that, under conditions of compressive loading, the size of abrasive particles was the primary determinant of microwear size. These results contrast with previous experimental tests of abrasion by predominantly shearing loads, where feature size was influenced by interaction among experimental factors, including the microscopic orientation of enamel crystallites. Although magnitude of compressive force was not a factor in microwear size variation, it may be a critical factor in explaining the presence or absence of microwear on tooth surfaces. The relatively small compressive bite force generated during typical chewing may not consistently produce abrasive pitting. These experiments demonstrate that, as the same abrasive regime can produce both large and small pits, the mechanism by which wear features are formed (i.e. compression or adhesion) cannot be determined from the size of features alone. Nevertheless, the dependence of pit size on abrasive particle size demonstrates that metrical variation in wear features can elucidate important attributes of an animal's diet.

Alligators and Crocodiles↗
PubMed1993

Enamel microstructure and molar wear in the greater galago, Otolemur crassicaudatus (Mammalia, Primates).

This study describes the molar enamel microstructure of the greater galago, based on SEM study of four individuals. Galago molar enamel consists primarily of radially oriented Pattern 1 prisms. However, the most superficial enamel is characterized by regions of poorly developed prisms or nonprismatic enamel, and Pattern 3 prisms can be found at depths intermediate and deep to the enamel surface. Orientations of prism long axes relative to wear surfaces differ among functionally distinct regions (cuspal facets, Phase I/II facets, and crushing basins). Consequently, orientations of enamel crystallites relative to these surfaces also differ. Because crystallites are the structural unit involved in enamel abrasion, these differences in orientation may have important effects on molar wear patterns. Crystallite orientations differ most between cuspal facets and Phase I/II facet surfaces. Cuspal facets are characterized by near surface-parallel interprismatic and surface-oblique prismatic crystallites. Previous experimental studies suggest that this arrangement is most resistant to wear when surface-normal (compressive) loads predominate. In contrast, prismatic and interprismatic crystallites intercept Phase I/II facet surfaces obliquely, an arrangement expected to resist abrasion when surface-parallel (shearing) loads predominate. Superficial enamel is preserved at most basin surfaces, indicating that these regions are subject to comparatively little abrasive wear. These results support the hypothesis that galago occlusal enamel is organized so as to resist abrasion of different functional regions, a property that may prove important in maintaining functional efficiency. However, this largely reflects constraints of occlusal topography on a microstructure typical of many mammals and thus does not appear to represent a structural innovation.

Aging↗
PubMed1991

Enamel structure and microwear: an experimental study of the response of enamel to shearing force.

The anisotropic fracturing and differential wear properties of enamel microstructure represent factors that can obscure the predictive relationship between dental microwear and diet. To assess the impact of enamel structure on microwear, this in vitro experimental study examines the relative contributions to wear of three factors: 1) species differences in microstructure, 2) direction of shearing force relative to enamel prisms and crystallites, and 3) size of abrasive particles. Teeth of Lemur, Ovis, Homo, and Crocodylus, representing, respectively, the structural categories of prismatic patterns 1, 2, and 3 and nonprismatic enamel, were abraded by shearing forces (forces having a component directed parallel to abraded surfaces) and examined by scanning electron microscopy. Striation width increased with particle size for nonprismatic, but not for prismatic, specimens. Direction of shear relative to prism and crystallite orientation had a significant influence on striation width in only some prismatic enamels. The different responses of prismatic and nonprismatic enamels to abrasion reflect the influence of structure, but at the level of organization of crystallites rather than prisms per se. Such interactions explain in part the inability of striation width to discriminate among animals with different dietary habits. Heteroscedasticity and deviations from normality also may confound parametric analyses of microwear variables. Variation in crystallite orientation in prismatic enamels may contribute to optimal dental function through the property of differential wear in functionally distinct regions of teeth.

Alligators and Crocodiles↗