Oestrogen retards the development of spontaneous thymomas in BUF/Mna rats

BUF/Mna rats develop spontaneous thymomas with nearly 100% incidence in both sexes. While the thymomas in males develop from around 9 months of age, those in females start from 13–15 months of age. To clarify the mechanism of the delay of thymomagenesis in females, the effect of sex hormones on the development of thymomas was examined after either gonadectomy or oestrogen treatment. Prepubertal ovariectomy accelerated the thymoma development in females, whereas orchiectomy did not affect it. An intraperitoneal injection of oestriol (20 mg) into males at 2 months of age remarkably diminished the thymic weight to about one-tenth of age-matched controls at 16 months of age. These results suggest that oestrogen can actually retard the onset of thymoma in spite of genetic control of its incidence. However, oestrogen did not cause thymic involution when it was injected into rats over 9 months of age. Immunohistochemically, there seemed to be no distinct difference in distribution of oestrogen-receptor-bearing epithelial cells between thymomas and 2- to 3- month-old thymuses. The oestrogen sensitivity of the thymus might be destined to be lost, as the thymic epithelial cells start neoplastic changes with the impairment of oestrogen-receptor function.