Thymomas from 277 Fischer 344/N (F344/N), 10 Sprague Dawley (HSD:Sprague Dawley SD) (SD), 129 Wistar Han [Crl:WI(Han)] (WH), and 4 Wistar Outbred (WO) rats were reviewed from long-term studies in the National Toxicology Program (NTP) database. The incidence of thymomas in F344/N rats was slightly higher in males than in females, while the incidences in SD and WH rats were higher in females than in males. Only male WO rats were used in NTP studies. Of the 277 thymomas in F344/N rats, 235 (84.8%) were benign and 42 (15.2%) malignant, 14 of which exhibited metastasis. Of the 10 thymomas in SD rats, 5 (50%) were benign and 5 (50%) were malignant, one of which exhibited metastasis. Of the 129 thymomas in WH rats, 126 (98%) were benign and 3 (2%) were malignant, 1 with metastasis. Of the 4 thymomas in WO rats, 3 (75%) were benign and 1 (25%) was malignant, with no metastases. Malignant thymomas in F344/N and WH rats showed a propensity to be the cause of death and to result in early mortality, whereas the benign thymomas were associated less often with decreased survival. No occurrences of this neoplasm were reported to be related to exposure to any test articles.

Cystic degeneration of the adrenal cortex is a common age-related finding in the Sprague-Dawley (SD) rat strain occurring more frequently in females. Compression of the adjacent cortex, a common hallmark of benign adrenal cortical tumors, often accompanies foci of cystic degeneration, creating a diagnostic challenge. Accurately differentiating these relatively common degenerative changes from proliferative lesions is critical in safety assessment studies. Cystic degeneration typically arises in the zona fasciculata of the adrenal cortex and often causes compression along the margin of the lesion. The degenerating cells are large, with abundant eosinophilic cytoplasm, or contain clear cytoplasmic vacuoles. Mitotic figures are generally uncommon. In many cases, cystic degeneration appears to arise in areas of hypertrophy in the zona fasciculata. In contrast, adrenal cortical hyperplasia and adrenal cortical adenoma are frequently comprised of smaller cells that cause compression of adjacent cortex, and in some cases mitotic figures are observed. Cytological detail and growth patterns should be considered more useful criteria than compression alone for separating degenerative cystic lesions from proliferative lesions in the adrenal cortex of SD rats.

Rare tumors were observed in chronic studies in F-344 rats that were purely or largely free in the mediastinal cavity, yet had the histological architecture of alveolar bronchiolar tumors. They had originally been diagnosed as either pulmonary alveolar bronchiolar tumors, mediastinal mesotheliomas, or thymomas. The authors described these tumors, estimated the fraction of thoracic tumors that they represented, and carried out a preliminary immunohistochemical investigation of whether they were of pulmonary or mesothelial origin. Sections of 715 thoracic tumors originally diagnosed as alveolar bronchiolar tumors, mesotheliomas, or thymomas from control or treated F-344 rats in NTP two-year studies were reviewed. Thirty (4%) were found to be purely or largely mediastinal, yet to have an alveolar bronchiolar histological pattern. A subset of these tumors and some typical intrapulmonary alveolar bronchiolar carcinomas and pleural mesotheliomas were immunostained for Clara cell secretory protein (CCSP), beta-tubulin IV, and Wilm's tumor 1 susceptibility gene products (WT1). The tumors with the histological architecture of alveolar bronchiolar tumors immunostained positive for CCSP and negative for WT1, implying they might have been of alveolar bronchiolar origin, despite their predominantly mediastinal location, although more certain identification would require the use of a larger panel of antibodies.

Benzene is an important industrial chemical. At certain levels, benzene has been found to produce aplastic anemia, pancytopenia, myeloblastic anemia and genotoxic effects in humans. Metabolism by cytochrome P450 monooxygenases and myeloperoxidase to hydroquinone, phenol, and other metabolites contributes to benzene toxicity. Other xenobiotic substrates for cytochrome P450 can alter benzene metabolism. At high concentrations, toluene has been shown to inhibit benzene metabolism and benzene-induced toxicities. The present study investigated the genotoxicity of exposure to benzene and toluene at lower and intermittent co-exposures. Mice were exposed via whole-body inhalation for 6 h/day for 8 days (over a 15-day time period) to air, 50 ppm benzene, 100 ppm toluene, 50 ppm benzene and 50 ppm toluene, or 50 ppm benzene and 100 ppm toluene. Mice exposed to 50 ppm benzene exhibited an increased frequency (2.4-fold) of micronucleated polychromatic erythrocytes (PCE) and increased levels of urinary metabolites (t,t-muconic acid, hydroquinone, and s-phenylmercapturic acid) vs. air-exposed controls. Benzene co-exposure with 100 ppm toluene resulted in similar urinary metabolite levels but a 3.7-fold increase in frequency of micronucleated PCE. Benzene co-exposure with 50 ppm toluene resulted in a similar elevation of micronuclei frequency as with 100 ppm toluene which did not differ significantly from 50 ppm benzene exposure alone. Both co-exposures – 50 ppm benzene with 50 or 100 ppm toluene – resulted in significantly elevated CYP2E1 activities that did not occur following benzene or toluene exposure alone. Whole blood glutathione (GSH) levels were similarly decreased following exposure to 50 ppm benzene and/or 100 ppm toluene, while co-exposure to 50 ppm benzene and 100 ppm toluene significantly decreased GSSG levels and increased the GSH/GSSG ratio. The higher frequency of micronucleated PCE following benzene and toluene co-exposure when compared with mice exposed to benzene or toluene alone suggests that, at the doses used in this study, toluene can enhance benzene-induced clastogenic or aneugenic bone marrow injury. These findings exemplify the importance of studying the effects of binary chemical interactions in animals exposed to lower exposure concentrations of benzene and toluene on benzene metabolism and clastogenicity. The relevance of these data on interactions for humans exposed at low benzene concentrations can be best assessed only when the mechanism of interaction is understood at a quantitative level and incorporated within a biologically based modeling framework.

AbstractVakuum‐Kurzzeitpyrolyse von (Ia) führt zu dem Isomeren (IIa), das durch "Deck‐Verschiebung" aus der inneren in die endständige Position in ihrer Umlagerung entsteht, bei der gleichzeitig mit dem Bruch einer C‐C‐Dreifachbindung eine andere gebildet wird.

In Puerto Rico, as in the United States, a very large number of students drop out of school. For the states, recent studies suggest that the dropout problem is much greater than usually acknowledged, with perhaps as many as one-third of all students failing to complete high school in the normal four years. The data for Puerto Rico tell a similar story, but in Puerto Rico the problem appears greater because the data indicate that a large number of students leave the public schools system before even entering high school. This report examines the school dropout issue in Puerto Rico, provides new estimates of dropout rates, and discusses the economic context and the implications of those estimates. The examination of the Puerto Rican school data and the estimates of dropout rates lead to the following observations: • Puerto Rico has a serious public school dropout problem. Even though officially reported public school dropout rates are extremely low, enrollment figures indicate that actual dropout rates are very high. • Although dropout rates are high in Puerto Rico, they appear to have fallen significantly during the 1990s and into the new century. Moreover, given that Puerto Rico's level of income is substantially below that of the states, we might have expected much higher dropout rates. • The Puerto Rican dropout problem appears to be especially severe for young students, those at the 7th and 8th grade levels. Whereas high school dropout rates appear to be no higher for Puerto Rico than for the average of the 50 states (and thus significantly better than for many individual states), the dropout rate in Puerto Rico for pre-high school students appears to be higher than in almost all states. Overall, then, from the 6 th grade to high school graduation, Puerto Rico is below all or virtually all of the states in its ability to engage and retain students. • Extremely high dropout rates before the mid-1990s have left a legacy of adults without high school degrees. Forty percent of adults over 24 lack a high school degree, significantly higher than in any state.