Pathology of the oropharyngeal cavity in six strains of rats: predisposition of Fischer 344 rats for inflammatory and degenerative changes.

The sagittolongitudinal histopathological evaluation of the oropharyngeal cavity in 6 strains of rats revealed a clear predisposition of the Fischer 344 strain for degenerative and inflammatory lesions in this region. A degeneration and calcification predominantly of the laryngeal cartilage is evident in 72%, granulomas of the seromucinous glands in 38%, and an inflammation with bacterial colonies or plant material in the basal part of the laryngotracheal junction in 26% of the Fischer 344 rats. The incidences of these types of lesions were negligible in the other rat strains evaluated. The comparison of control Fischer 344 rats from 2 carcinogenicity studies (untreated group; gavaged group with water, pH 10; gavaged group with propylene glycol/methocel) showed an influence of the gavage technique with the solvent on the survival rate in the female Fischer 344 rats. The lower survival rate was accompanied by the occurrence of food impaction in the oropharyngeal cavity. This increased mortality in female Fischer 344 rats was also associated with a degeneration of the laryngeal cartilages, whereas the formation of granulomas and inflammation seemed to have no direct relationship to the mortality. The gavage technique with the applied solvent was an enhancing factor in the female Fischer 344 rats on mortality, on cartilage degeneration, but not on inflammatory changes. In contrast to the females, male Fischer 344 rats had a higher incidence of granulomas and inflammation, which, however, did not influence the survival rate. In conclusion, Fischer 344 rats, especially the female sex, exposed to chronic daily irritation by gavage, are predisposed to have a high mortality rate in chronic toxicologic tests. An explanation lies in the disposition of the Fischer 344 rat to a high incidence of cartilage degeneration, which presumably leads to a dysfunction of the larynx.