The guinea-pig differs from other rodents in that the ovary of the post-natal animal contains two distinct populations of oocyte at diplotene. A `large' oocyte, comparable to that of other mammals, is found in some primordial follicles; in others a second, `contracted', type occurs. As the animal ages the contracted cell predominates. The radiosensitivity of these two cell-types has been investigated.
Ninety-eight animals received unilateral ovarian X-irradiation (300 to 15,000 r). The numbers of germ cells in the irradiated ovary were counted from 3 to 32 days later, and compared with those found in the untreated, contralateral ovary of each animal.
Irradiation killed large oocytes, and induced contracted cells either to degenerate or undergo transformation to the large nuclear type. Contracted oocytes were consistently and significantly more sensitive to irradiation, even over a short (3 days) post-irradiation interval. Over a longer interval (16 or 32 days) a dose-dependent response was revealed: the higher the dose, the smaller the number of surviving oocytes. Although the LD50/32 for oocytes in primordial follicles was as low as 500 r, 15,000 r was required to eliminate all the oocytes.