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Summary. Ovarian follicles ( ≥ 100 × 10⁵ μm³ or a mean diameter of ≥ 275 μm) in adult rats were classified as non-atretic and atretic during the oestrous cycle and recorded in 5 volume classes. The atretic follicles were also categorized in several stages according to the progress of atresia. The degeneration of the entire granulosa wall until the induced changes in the oocyte took at least 24 h. Another 24 h elapsed before the oocyte became denuded. Therefore the % of atretic follicles, i.e. follicles in all stages of atresia, could not be used as indicator for the rate of atresia. The atretic portion in the follicle population ≥ 100 × 10⁵ μm³ increased from early dioestrus 1 to early dioestrus 3, reached a plateau during dioestrus 3 and pro-oestrus, and declined at late oestrus to the level of early dioestrus 1. The sudden decrease in number of atretic follicles after late pro-oestrus was caused by the discard of many atretic follicles in the advanced stages due to various deformities as revealed by histological observation. By using the % of atretic follicles in the earliest stage as indicator of atretic rate, two waves of atresia were found affecting the population of antral follicles during their growth, the first at dioestrus 1 amounting to 15–20% and then at dioestrus 3, affecting 35% of the population. The present study also shows the extension of atresia in the various volume classes of follicles during the oestrous cycle. A pool of ∼ 7 follicles in the smallest volume class was maintained after ovulation, grew further in the next cycle with a new cohort of 20 follicles, and seemed to provide the required number of follicles destined to ovulate. This suggests that the follicles that ovulate were already present at an antral stage in the preceding cycle and needed two cycles for their growth to ovulation.

Summary. Morphometric analysis of the follicle population ≥ 100 × 10⁵ μm³ or a mean diameter of ≥ 275 μm and assessment of the rate of atresia in ovaries of pregnant and pseudopregnant rats revealed no evidence for the presence of rhythmic follicular maturation during the prolonged dioestrous period. During the first 4–5 days of the dioestrous period, follicles developed to preovulatory size (volume class 5, i.e. ≥ 1000 × 10⁵ μm³ = diam. ≥ 576 μm) reaching the normal number of ovulating follicles in cyclic animals in pregnant rats, but only half that number in pseudopregnant rats. These follicles collapsed on the 5th to 8th days of the dioestrous period and full numbers of preovulatory follicles were not found thereafter until the end of pregnancy and pseudopregnancy. Follicles of smaller sizes (classes 1–4: 100–999 × 10⁵μm³), however, were present throughout the prolonged dioestrous period. The rate of atresia in the follicle population had increased by the 2nd day and remained from then on at 26·5 ± 4·5% in the pregnant and 34·3 ± 1·9% in the pseudopregnant rats. Atretic follicles in the advanced stages of atresia, mostly derived from follicles of classes 1–3, persisted and accumulated at the end of the dioestrous period.

The continuous presence of follicles and the constant rate of atresia during the dioestrous period indicate continuous follicular replacements and refute the idea of follicular quiescence during pregnancy and pseudopregnancy. Copulation and electrical stimulation of the cervix seemed to reduce the formation of the new crop of follicles the next morning and the pool of small antral follicles normally maintained after oestrus in cyclic animals. Nevertheless, the smaller crop and pool of follicles seemed able to provide a sufficient number of preovulatory follicles at the end of pregnancy and a sufficient number of ovulations at the end of pseudopregnancy.

Department of Anatomy, Medical Faculty, Erasmus University, Rotterdam, The Netherlands

Indomethacin has been shown to block ovulation in rats (Armstrong & Grinwich, 1972; Tsafriri, Lindner, Zor & Lamprecht, 1972a; Orczyk & Behrman, 1972) and other mammals (O'Grady, Caldwell, Auletta & Speroff, 1972; Grinwich, Kennedy & Armstrong, 1972; Saksena, Lau & Shaikh, 1974). The release of LH is not affected in indomethacin-treated rats (Tsafriri et al., 1972b), and it has been suggested that indomethacin exerts its ovulation-inhibiting action at the ovary (Grinwich et al., 1972; Tsafriri et al., 1972a; Saksena et al., 1974). It is not clear, however, how indomethacin affects the mechanism of ovulation, and in the present study the ovaries of rats treated with indomethacin at pro-oestrus were examined histologically.

Adult female rats of the Wistar substrain R'Amsterdam and weighing 180-200 g were used after two consecutive 5-day cycles. Indomethacin (Indocid capsules: Merck, Sharp & Dohme) was suspended in

Summary. The ovaries of pro-oestrous rats (pubertal and adult) and hamsters were explanted in toto at various times after the preovulatory gonadotrophin surge to obtain endogenously stimulated ovaries. Subnormal numbers of ovulations from pubertal and adult rat ovaries were found when incubation was started during the period from 17:00 to 24:00 h on the day of pro-oestrus. Normal numbers of ovulations only occurred from rat ovaries in vitro when incubation was started at 01:00 h, i.e. when ovulation had already begun in vivo. In contrast, full ovulation from hamster ovaries was observed in vitro after incubation at 22:00 h and later, although the ovulatory process had not started in vivo. This difference in the ability of rat and hamster follicles to ovulate in vitro could be due to a different role of the accumulation of tissue fluid for the mechanism of ovulation.

Antral follicles were counted in ovaries from young adult Wistar rats, collected on the 5 days of the ovarian cycle. Follicles were classified as healthy, early atretic or late atretic and divided into five volume classes. From these data, a model was developed in which the inflow of healthy follicles into the various size classes was quantified. This model describes the follicle dynamics during a normal 5-day cycle. It was concluded that the stage of early atresia takes between 20 and 24 h. The inflow of follicles into the antral stage (volume ≥ 100 * 10⁵ μm³) was continuous but not constant. The highest inflow was found during pro-oestrus and oestrus, at about the time of the first and second FSH surge. The total inflow during each cycle was about 120 follicles of which only 10% ovulated. These ovulating follicles were recruited during the previous pro-oestrus and oestrus. Follicle selection took place in volume classes 1 and 2 (volume 100–350 * 10⁵ μm³) during oestrus and dioestrus 1. At dioestrus 2, the follicles that will ovulate have been selected and can be recognized on the basis of their bigger size.