Persisting long-term effects of a single carcinogenic dose of methylnitrosourea on epidermal growth in mice

The cell proliferation in hairless mouse epidermis was studiedbefore tumor development following a single application of acarcinogenic dose (2 mg) of N-methyl-N-nitrosourea (MNU). Thenumber of basal and suprabasal cells, the mitotic index (MI)and the mitotic rate (MR) were scored in histological sections.The [³H labeling index (LI) and the mean grain count (MGC) werescored in histological sections or in nears of basal cells.Flow cytometric two-parameter analyses of cellular DNA and proteincontent were performed on isolated epidermal basal cells. Anincreased MR and a slight but consistent epidermal hyperplasiawere found. A 24-h study performed 25 weeks after MNU applicationshowed that the MR and MI were altered in a circadian stage-dependentmanner with considerably increased values around noon when thecircadian rhythms had their peaks, followed by normal valuesaround midnight. The LI was generally increased, but showeda normal circadian rhythm with high values at night and lowvalues during day. The MGC was reduced at night and in the morningwhen the LI values were high. The results show that a singlecarcinogenic dose of MNU caused alterations in the epidermalgrowth kinetics that persisted until tumor development. Thealtered growth parameters, however, had circadian rhythms thatwere in phase with control rhythms. Assuming a constant sizeof the proliferative compartment, the increased mitotic activityindicated a considerable shortening of the mean cell cycle time.