Microscopic studies, limitations

Transmission electron microscopy (TEM) can resolve features down to about 1 nm and allows the use of electron diffraction to characterize the structure. Since electrons must pass through the sample however, the technique is limited to thin films. One cryoelectron microscopic study of fatty-acid Langmuir films on vitrified water [13] showed faceted crystals. The application of TEM to Langmuir-Blodgett films is discussed in Chapter XV. 

Osbome-Mendel rats administered 0, 1, 3, or 6 ppm endrin in feed for 10 weeks, and then 0, 2, 6, or 12 ppm endrin for an additional 106 weeks, had incidences of malignancies that were similar to control animals (Deichmann et al. 1970). The authors concluded that endrin was not carcinogenic or tumorigenic. Not all tissues were examined microscopically, however, limiting the conclusions that can be drawn from this study. 

Iron-chromium alloys, free from carbon, may be prepared from chromite by the alumino-thermic method. From a study of the cooling-and freezing-point curves it has been suggested that a compound, Cr Fe, exists, but this is questioned by Janecke, who studied the iron-chromium system by means of fusion curves and by the microscopic study of polished sections of various alloys between the limits 10 Fe 90 Cr and 90 Fe 10 Cr, and came to the conclusion that the system consists of a single eutectic which can form mixed crystals with either component. The eutectic contains 75 per cent, of chromium and melts at 1320° C. The addition of chromium to iron increases the readiness of attack by hydrochloric and sulphuric acids, but towards concentrated nitric acid the alloys are rendered passive. They remain bright in air and in water. The presence of carbon increases the resistance to acids and renders them very hard if carbon-free, they are softer than cast iron. All the alloys up to 80 per cent, chromium are magnetic. Molybdenum, titanium, vanadium, and tungsten improve the mechanical properties and increase the resistance to acids. 

Administration of clldlnlum for 1 yr at 5.0, 25, and 50 mg/kg In the diet of rats did not result In drug-related toxicity. Blood counts, clinical-chemistry measures, and results of gross and microscopic studies remained within normal limits (28). 

Sestak [31] has warned of some of the limitations of the techniques used and of interpretations based on microscopic studies. Only a small portion of the sample is observed and there is a need to ensure that the observed behaviour is representative. The possible destabilizing effects of conditions in the electron microscope have already been mentioned. The effects of temperature changes and possible further... 

Let us first consider the structure of myosin. The results of electron microscopic studies of skeletal-muscle myosin show it to be a two-headed structure linked to a long stalk (Figure 34.2). As we saw in Chapter 33, limited proteolysis can be a powerful tool in probing the activity of large proteins. The treatment of myosin with trypsin and papain results in the formation of four fragments two SI fragments an S2 fragment, also called heavy... 

A decrease in erythrocyte 6-aminolevulinate dehydratase activity was observed in rats exposed to 200 but not 50 ppm tetrachloroethylene for 4 weeks (Soni et al. 1990). It is not clear if exposure was intermittent or continuous. Rats exposed to 230 or 470 ppm tetrachloroethylene for up to 160 days had splenic congestion and increased hemosiderin deposits (Carpenter 1937). Study limitations include the use of sick animals (parasites, pneumonia), nonstandard study protocols, rats of undefined strain, and inadequate controls. A transient increase in reticulocytes was observed in mice exposed to tetrachloroethylene at 135 and 270 ppm during the first few weeks of an 11.5-week study (Seidel et al. 1992). Microscopic examination of bone marrow revealed no effect on pluripotent stem cells and only a small reduction in erythroid committed cells. Because of a lack of statistical analysis, NOAELs and LOAELs were not clearly identified in the Seidel et al. (1992) study. 

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