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Abstract: Crack growth behaviour in two types of commercially available silicon carbide was examined from 600 to 850 C in ambient atmospheres containing oxygen, water vapour, and sulphur dioxide. The double-torsion specimen was used in the incremental displacement rate mode to yield (K 1,V) relations. The direct-bonded material exhibited unstable crack propagation and arrest behaviour which was not measureably affected by temperature variations or the corrosive environments. The hot-pressed material exhibited subcritical crack growth similar to the three regions of the classical (K 1,V) diagram. Oxidation of the silicon carbide is suggested to be the mechanism of stress corrosion operating in these environments.

A new member of the thrombospondin gene family, designated thrombospondin-4, has been identified in the Xenopus laevis genome. The predicted amino acid sequence indicates that the protein is similar to the other members of this gene family in the structure of the type 3 repeats and the COOH-terminal domain. Thrombospondin-4 contains four type 2 repeats and lacks the type 1 repeats that are found in thrombospondin-1 and 2. The amino-terminal domain of thrombospondin-4 has no significant homology with the other members of the thrombospondin gene family or with other proteins in the database. RNAse protection analysis establishes that the initial expression of Xenopus thrombospondin-4 is observed during neurulation. Levels of mRNA expression increase twofold during tailbud stages but decrease by the feeding tadpole stage. The size of the thrombospondin-4 message is 3.3 Kb and 3.4 Kb in the frog and human, respectively. Northern blot analysis of human tissues reveals high levels of thrombospondin-4 expression in heart and skeletal muscle, low levels in brain, lung and pancreas and undetectable levels in the placenta, liver and kidney. These data establish the existence of a new member of the thrombospondin gene family that may participate in the genesis and function of cardiac and skeletal muscle.

This preliminary study examined the effects of immunization with Bacteroides macacae. the monkey equivalent of the human species of B. gingivalis on ligature-induced periodontitis. During a 12 week immunization period, 8 out of the 12 Macacae fasicularis monkeys were immunized weekly with B. macacae washed cells and 4 were sham-immunized with saline. At the same time, all were scaled and pumiced weekly to establish gingival health. Following this period, the mandibular first molars were ligated in 8 out of the 12 monkeys to induce periodontitis. The immunized, ligated experimental group, the ligated, sham-immunized control group, and the immunized, non-ligated control group were then followed for a 6 months ligation period while plaque was allowed to accumulate. Gingival indices, attachment levels, pocket depths, plaque indices, radiographs, serum and crevicular fluid antibodies and subgingival bacteria were assessed. Immunization led to elevated antibody levels to B. macacae while ligation increased plaque, gingival inflammation, and bone loss. Following the 6 month ligation period, B. macacae comprised 1.7% of the cultivable flora in the immunized, non-ligated monkeys, 2.1% in the immunized, ligated monkeys, and 5.6% in the sham-immunized, ligated monkeys. Similar differences between the immunized, ligated and the sham-immunized, ligated groups were not seen for B. intermedius. nor B. melaninogenicus. These results suggest a heightened humoral response to B. gingivalis reduces subgingival re-colonization by this organism and modulates the course of ligature-induced periodontitis.

Subcritical crack propagation and time dependent deformation in the transducer ceramic, PZT, have been studied using the double torsion and stress relaxation techniques. Slow crack growth in PZT is sensitive to both chemical and electrical testing environments. Moisture and high ac or dc fields applied perpendicular to the crack plane enhance crack propagation in unpoled PZT. The stress relaxation data can be treated in terms of an analysis of thermally activated deformation. A dislocation model of the domain processes responsible for deformation is consistent with the experimental data.

The thrombospondins are a family of extracellular calcium binding proteins that are involved in cell proliferation, adhesion, and migration. We have sequenced full-length human thrombospondin-4 and characterized the recombinant protein. In contrast to Xenopus laevis thrombospondin-4, the human protein contains an RGD cell binding sequence in the third type 3 repeat. Transfection of mouse NIH3T3 fibroblasts or C2C12 myoblasts with a full-length human thrombospondin-4 cDNA results in the expression of a polypeptide with a reduced molecular weight of 140,000. In the absence of reducing agent, the expressed protein has an apparent molecular weight of 550,000. Recombinant thrombospondin-4 has been purified from the culture supernatant by heparin-Sepharose and anti-thrombospondin-4 antibody-Affi-gel affinity chromatography. Electron microscopy indicates that thrombospondin-4 is composed of five subunits with globular domains at each end. The observation of a calcium-dependent change in the electron microscopic appearance of thrombospondin-4 is consistent with limited tryptic digestion data that indicate that thrombospondin-4 is resistant to digestion in the presence of calcium. These data indicate that thrombospondin-4 is a pentameric protein that binds to heparin and calcium.

To assess the ability of food frequency methods to measure current dietary calcium intake in elderly women, the authors administered two types of food frequency instruments to 37 randomly selected women who attended two senior citizens centers in San Francisco, and they compared those responses to seven-day food records. A 34-item food frequency instrument (with portion sizes rated as small, medium, or large) correlated well (r = 0.76) with the estimated calcium intake from seven-day records. Limiting the instrument to the top 15, 10, or five foods that contribute to dietary calcium intake had little effect on this correlation (r = 076, 0.75, and 0.67, respectively). Rating all portions as "medium" reduced these correlations somewhat. An 18-item instrument, which asks respondents to rate portion sizes in ounces or cups, did not correlate as well with the seven-day records (r = 0.49). The authors conclude that brief food frequency instruments which rate portion sizes on a simple qualitative scale may be suitable for many clinical uses and adequate for some types of epidemiologic studies of calcium intake in elderly women.