Cytologic correlates of chemical carcinogens in respiratory tracts of dogs.

During canine respiratory carcinogenesis studies with benzo[a]pyrene (BP) and N-methyl-N-nitrosourea (NMU), a stereotypic pattern of cytomorphology characteristic for each carcinogen was observed. In the early stage, BP induced changes primarily in the cytoplasm. These changes consisted of incresaed size, basophilia, and vacuolization. In contrast, NMU induced primarily nuclear enlargement and enhanced the prominence of the nucleolus. Subsequently, cells exposed to BP demonstrated pleomorphism of the cytoplasm and nucleus, whereas cells exposed to NMU formed bizarre spindle-shaped cells. Cells were generally more scattered from one another following BP treatment than after NMU treatment. When the mucosa was exposed to NMU after the previous use of BP, the cytologic findings promptly became those characteristic of NMU.