Serial transplants of 7,12-dimethylbenz[a]anthracene-induced mammary tumors in Fischer rats as model system for human breast cancer. 3. Quantitative ultrastructural studies of the pinealocytes and plasma melatonin concentrations in rats bearing an advanced passage of the tumor.

The aim of the present study was to investigate the ultrastructure of pinealocytes and plasma melatonin concentrations in Fischer rats bearing an advanced (14th) passage 7,12-dimethylbenz[a]anthracene-induced mammary tumor. Quantitative ultrastructural analysis of the cell size and relative volumes of various cell organelles as well as the number of dense-core vesicles was performed in pinealocytes of the animals killed either during the daytime (15.00 h) or at night (3.00 h) 1 month after tumor transplantation. No significant differences between control and tumor-bearing animals were observed either during the daytime or at night. However, plasma melatonin concentrations in tumor-bearing rats killed at 3.00 h were suppressed by 35% (p < 0.025) when compared to the control animals killed at the same time.