Summary. The C3H/Tw mice were those showing regular 5-day cycles. A population of small follicles (100–149 μm in diameter) started growing at oestrus, but the majority underwent atresia during metoestrus, dioestrus and pro-oestrus. The remaining healthy follicles continued to grow during the next cycle, although several large ones eventually became atretic and failed to ovulate. Finally about 10 follicles survived and attained the preovulatory stage, measuring more than 400 μm in diameter by the time of the ovulatory surge of gonadotrophins. Two oestrous cycles, i.e. about 10 days, are therefore required for small follicles to complete their growth until ovulation.
At oestrus, atretic follicles bearing oocytes in the resting phase were rarely encountered, showing that few follicles underwent atresia at this stage. Smaller atretic follicles, 100–199 μm in diameter, were numerous at metoestrus, while larger ones, 250–399 μm, increased in number during dioestrus 1 and dioestrus 2. Corpora lutea took 4 successive oestrous cycles from the time of formation to complete disintegration.
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