Ultrastructure of nitrosoheptamethyleneimine-induced lung tumours in Fischer rats.

Male F344 rats were given N-nitrosoheptamethyleneimine (NHMI) twice a week by gavage for 20 weeks. They developed peripheral squamous cell carcinomas of the lungs. Electron microscopy revealed the presence of mature keratin in the tumor centers and the formation of keratohyalin granules in the more peripheral tumor cells. Basal cells were identified as the actively growing component of the neoplasms. In contrast to NHMI-induced lung carcinomas in European hamsters, neither Clara, nor APUD-type, cells were found in the rat tumors. These data indicate that the mode of development of NHMI-induced lung tumors is different in these two species.