Effect of fibrous glass on rat pleural mesothelium. Histopathologic observations

Spontaneous Primary Pleural Mesothelioma in Fischer 344 (F344) and Other Rat Strains: A Retrospective Review

Spontaneous primary pleural mesotheliomas in Fischer 344 (F344) or other rat strains have rarely been reported. The objectives of this retrospective study were to develop historical incidence data and better characterize the light-microscopic morphology of these naturally occurring neoplasms in a large cohort of rats of several strains. A retrospective review was performed of National Toxicology Program (NTP) studies in rats conducted between 1980 and 2019 and comprising a total of 104,029 rats (51,326 males, 52,703 females), predominantly (90%) of the F344 strain. Of the 94,062 F344 rats surveyed, there were 30 cases of primary pleural mesotheliomas (22 males, 8 females). Of the 2998 Wistar Han rats surveyed, primary pleural mesotheliomas were present in 2 male rats. No primary pleural mesotheliomas were noted in male and female rats of other strains (6669 Sprague Dawley; 300 Osborne-Mendel). All primary pleural mesotheliomas in control and treated F344 and Wistar Han rats were considered spontaneous and unrelated to treatment. Based on light-microscopic evaluation of paraffin-embedded hematoxylin and eosin stained sections, only epithelioid and biphasic histologic subtypes were observed: 18 and 12 in F344 rats, respectively, and one each in Wistar Han rats. No sarcomatoid subtype cases were noted in any strain of rat.

The So-called Short-Fiber Controversy: Literature Review and Critical Analysis

Context

Numerous articles in the scientific literature indicate that pathogenic fibers with respect to asbestos-related diseases are those that exceed 5 μm in length. Nonetheless, some authors have expressed concerns regarding pathogenicity of shorter fibers.

Objective

To review the scientific evidence regarding pathogenicity (or lack thereof) of fibers less than or equal to 5 μm in length, with particular attention to publications indicating that such fibers might be hazardous.

Data Sources

The scientific literature was reviewed for experimental animal studies and human studies that address the role of fiber size in causation of disease. Sources included original studies, as well as review articles related to the topic.

Conclusions

Experimental animal studies involving inhalation of fibers have demonstrated that fibers greater than 5 μm in length are associated with both pulmonary fibrosis (ie, asbestosis) and malignancies (carcinoma of the lung and mesothelioma). There is no convincing evidence for a pathogenic effect for fibers that are 5 μm or less in length. Fiber analyses of human lung tissue samples provide further support for pathogenicity of long fibers, particularly the more biopersistent amphibole fibers. Similar observations have been reported for nonasbestos mineral fibers. Concerns expressed by some authors (eg, the greater abundance of short fibers) do not alter these conclusions. Similarly, in vitro studies demonstrating biological activity of short fibers do not override inhalational studies of whole animals or the epidemiological findings in humans.

Asbestos fiber length as related to potential pathogenicity: a critical review

BACKGROUND

Asbestos inhalation is recognized as an exposure that increases the risk for the development of lung disease. It is unique among dusts in that it is both a carcinogen and capable of inducing extrapulmonary responses such as pleural thickening and fibrosis as well as malignancy. One feature of asbestos suggested as crucial in its pathological activity is its fibrous morphology. Long fibers that have been inhaled are cleared less readily and are thus more persistent in the body. Furthermore certain experimental models link fiber length to levels of risks for development of certain diseases. The present review will survey the data on this subject.

METHODS

The review considers experimental models that have been used to assess the response to various lengths of fibers in animal models in addition to data obtained from studies of human materials. The review also emphasizes the importance in defining the method by which a sample is categorized.

RESULTS

Data are offered which support the potential for longer fibers as well as shorter fibers to contribute to pathological responses.

CONCLUSIONS

The data presented argue that asbestos fibers of all lengths induce pathological responses and that caution should be exerted when attempting to exclude any population of inhaled fibers, based on their length, from being contributors to the potential for development of asbestos-related diseases.

Effect of erionite on the pleural mesothelium of the Fischer 344 rat

OBJECTIVE

This study sought to assess the fibrogenic and carcinogenic potential of erionite (a fibrous zeolite) on the pleural mesothelium of the Fischer 344 rat (n = 24).

DESIGN

The study was designed to examine rat pleural mesothelial changes by three independent observers at timed intervals, ranging from 1 to 480 days postinoculation using erionite from the Pine Valley, Nevada (USA) area. The mean length and width of the erionite fibers were 2.29 and 0.48 microns, respectively. Only microscopic observations made by majority (2/3) or unanimity (3/3) were accepted for final diagnosis.

RESULTS

Pleural and lung tissue were available for examination in 21 of the 24 rats. Fibrosis, chronic inflammation, and foreign body reaction occurred in 6 of 21 rats. Mesothelial hyperplasia and dysplasia occurred in 9 and 3 of the 21 rats, respectively. A single mesothelioma was identified at 434 days in a rat that had gross nodular pleural lesions.

CONCLUSIONS

The findings reported herein confirm the strong fibrogenic potential of erionite but are at variance with previous studies reporting much higher yields of mesothelioma. The reasons for the low yield of mesothelioma in this study are not known, but may be related to the study design, the strict criteria used for histopathologic diagnosis, and/or possible differences in erionite physicochemical properties, associated with its geographic distribution, most previous animal studies having used erionite from the Rome, Oregon (USA) area.

Pulmonary response of mice to fiberglass: cytokinetic and biochemical studies

It has been suggested that glass fibers in the respirable size range may pose a health hazard similar to asbestos because of the similarities in physical characteristics. To compare the pulmonary cell response with that described earlier with crocidolite asbestos, we administered a milled fiberglass sample to mice by intratracheal instillation. Little effect was seen at a dose of 0.1 mg, but at 1 mg there was epithelial injury and an inflammatory cell response concentrated at bronchiolar-alveolar duct regions. Cellular incorporation of tritiated thymidine showed that repair of both bronchiolar and alveolar epithelium occurred rapidly. This was followed by an extended increase in cell labeling, particularly in peribronchiolar fibroblasts, from 2 to 8 wk after fiber instillation. Granulomas formed at this site and later there was morphologic evidence of fibrosis, which was confirmed biochemically by a significant increase in lung collagen at 4-16 wk. Although 10 times higher dose is required, the results show that the lung response to fiberglass in this experimental system is similar to that described previously for crocidolite asbestos; the sites of cell injury and repair are the same, and the subsequent fibrotic response produces small airway disease.