Morphological changes of the nucleolar organizer regions induced by 7,12‐ dimethylbenz[a]anthracene in the hamster cheek pouch

The staining of nucleolar organizer regions (NORs), which equate to rDNA transcription, was applied to chemically induced‐lesions of the hamster cheek pouch. The check pouches of 16 male golden Syrian hamsters were treated three times a week with 0.5% 7,12‐dimethylbenz[a]anthracene (DMBA) in mineral oil for 16 weeks. The percentage of gathered‐type NORs with high activity nucleoli increased in the pouch epithelium during DMBA treatment and reached the highest values in malignant tumors. The percentage of disperscd‐type NORs also increased in the malignant lesions. However, the absolute number of NORs was not affected by DMBA treatment. These results suggest that DMBA induces modification of NOR activity at the early stages of carcinogenesis and shows the potential of this model for studying NOR alterations in neoplasia.