Female Swiss mice (Cr:NIH(S)) developed bronchiolar cell
hyperplasia, dysplasia,
metaplasia, and various morphologic types of bronchiolar cell
tumors after topical (skin) application of N-nitroso-methyl-bis-chloroethylurea (NMBCU) or
N-nitroso-tris-chloroethylurea (NTCU). These compounds are the first found to induce systemically bronchiolar cell
tumors in mice in high incidence. Twice a week, with a 3-day interval, a 25-microliter drop of 0.04 mol/l (molar) NMBCU or NTCU in
acetone was applied to the shaved interscapular integument for a maximum of 35 to 40 weeks. The earliest
lung neoplasms were seen in mice that died after 23 weeks of treatment and affected 11 of 19 with NMBCU and 14 of 19 with NTCU treatment.
Tumor growth pattern was nodular or the neoplastic tissue was frequently disseminated throughout the parenchyma, starting from multicentric peribronchiolar foci. The most common
tumor types were
squamous cell carcinomas and
adenosquamous carcinomas, followed by
adenocarcinomas with or without secretory cells, and a single ciliated-cell
tumor. Histochemical and immunohistochemical studies were carried out on
paraffin-embedded lungs using the
avidin-
biotin immunoperoxidase complex procedure and
antisera against
keratin,
Clara cell antigen,
surfactant apoprotein,
neuron-specific enolase,
bombesin, and
chromogranin A. In several mice from both groups,
hyperplasias and
tumors were composed of cells expressing
Clara cell antigen. No
tumor cells were found expressing alveolar type II or neuroendocrine cell markers. It appeared that bronchiolar cells, in particular Clara cells, had migrated from terminal bronchioles or invaded bronchiolar walls to extend into the alveolar parenchyma. Squamous cell
metaplasia with
keratin expression was seen within airways or associated with glandular
tumors, especially at the periphery. A unique cell type, with large eosinophilic globules and associated eosinophilic crystals, was seen lining airways or forming hyperplastic and neoplastic lesions. N-nitroso-methyl-bis-chloroethylurea- and NTCU-induced mouse bronchiolar cell alterations could be an interesting new model to study mechanisms of bronchiolar cell differentiation and
tumor formation.