Relationship between oocyte apoptosis and ovarian tumours induced by high and low LET radiations in mice

PURPOSE: To investigate the biological effectiveness of neutrons at the energy below 1 MeV on apoptosis and carcinogenesis in the mouse ovary. MATERIALS AND METHODS: Female mice were exposed to 1.0 Gy monoenergetic neutrons (0.317, 0.525 and 1.026 MeV), (252)Cf fission neutron (2.13 MeV) or (137)Cs gamma-rays at 7 days of age. Apoptosis of the oocyte and pregranulosa cells, and ovarian carcinogenesis were compared between the radiations. The efficiency of gamma-rays for granulosa cell tumorigenesis was tested by transplantation of the irradiated ovaries into non-irradiated mice. RESULTS: The cumulative apoptotic index of oocytes was 77.9%, 65.6% and 41.6% for the 0.525 MeV neutron, 2.13 MeV neutron and gamma-rays, respectively. Follicles with apoptotic pregranulosa cells were 53.0%, 18.3% and 22.8% of cumulative index for the three groups. Tubular adenomas developed in the groups of monoenergetic neutrons (26.1%) and gamma-ray (35.5%), whereas granulosa cell tumours developed only in the gamma-ray groups (3.2% for 1.0 Gy and 15.6% for 3.0 Gy). Partial-body irradiation with 3 Gy gamma-rays to the ovaries induced granulosa cell tumours with an incidence of 27.3%. CONCLUSION: Effectiveness of neutrons to cause apoptosis was higher for 0.525 MeV than for 2.13 MeV. The pregranulosa cell apoptosis occurred in an oocyte-prone manner. The higher effectiveness of neutrons than gamma-rays to induce oocyte and pregranulosa cell apoptosis correlates with the inhibition of granulosa cell tumour development.