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L. P. Cahill and P. Mauléon

Summary. The population of primordial and small follicles in adult ewes of high (3·1) and low (1·6) ovulation rate was studied by histological methods. The small follicle population which has a skewed frequency distribution in relation to either follicle or oocyte size can be divided according to morphological features into dormant, transitory and growth phases.

It is suggested that initiation of follicle growth involves firstly the passage of follicles from the dormant to the transitory category and (if it occurs) is independent of gonadotrophins; and secondly, the passage from the transitory category to the growth phase. The second step is dependent on gonadotrophins and the rate of such passage is higher in ewes with a high ovulation rate.

The overall mean follicle size of small follicles is larger in ewes with a low ovulation rate, suggesting that either they have structurally different small follicles or their reserve of small follicles is less depleted than in ewes with a high ovulation rate.

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L. P. Cahill and P. Mauléon

Summary. Follicular growth was studied in 16 ewes of different breeds (Romanov, mean ovulation rate 3·0, and Ile-de-France, mean ovulation rate 1·6), stage of cycle (Day 0 or 7) and season (December and June).

The follicular growth rates, determined by measuring the mitotic index before and 2 h after colchicine treatment, varied greatly between animals studied and did not vary significantly between breeds, time of cycle or season. From 3 layers of granulosa cells until antrum formation the mitotic index increased slowly but then the follicles grew rapidly reaching maximum growth rate at a follicular diameter of 0·85 mm; thereafter the mitotic index decreased almost to zero in preovulatory follicles. The mean time for a follicle to pass from 3 layers of granulosa cells (200 cells) to preovulatory size (3 × 106 cells) was estimated to be about 6 months.

The total number of normal follicles with >3 layers of granulosa cells in Ile-de-France ewes was similar in the anoestrous (3 ovaries studied) and breeding (3 ovaries) seasons, but there were more antral follicles in the latter.

Highly significant correlations existed in each ewe between the number of follicles and the mean mitotic index per class, suggesting the existence of an intra-ovarian mechanism regulating folliculogenesis.

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J. Dufour, L. P. Cahill and P. Mauléon

Summary. The effects of hypophysectomy and unilateral ovariectomy on the total number of follicles with >3 layers of granulosa cells were determined at 4 and 70 days following treatment.

The population of preantral follicles (<0·23 mm diam.) was found to be under the control of gonadotrophins but such control was only evident on a long-term basis. At 70 days after unilateral ovariectomy there was a large increase in the number of preantral follicles but at 70 days after hypophysectomy there was a large decrease.

The population of antral follicles (>0·23 mm diam.) was under the immediate control of gonadotrophins. By 4 days after hypophysectomy all large antral follicles had become atretic and the number of antral follicles was further decreased at 70 days after treatment. At 70 days after unilateral ovariectomy there was an increase in the number of antral follicles.

The follicular growth rates at 70 days following treatment were decreased in hypophysectomized ewes but increased in ewes after unilateral ovariectomy.

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R. A. S. Lawson and L. P. Cahill

Summary. When 25 mg progesterone/day were injected into ewes on Days 0–3 of the oestrous cycle, (i) the subsequent cycle was shortened by 4 days and (ii) on Day 6 such ewes provided an acceptable uterine environment for the survival of 10-day-old embryos. We suggest that exposure of the non-pregnant uterus to approximately 8 days of normal luteal concentrations of progesterone may be necessary to initiate luteolysis.

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M. A. Driancourt and L. P. Cahill

Summary. To investigate the number, size and fate of the follicles > 2 mm in diameter involved in the differentiation of the preovulatory follicle in-vivo monitoring of growth and regression of individual follicles was performed at different times after a PG-induced follicular phase.

In Exp. 1, the 3 largest follicles of each ovary were labelled with ink in the peripheral stroma at 0,4,8,12,24 or 48 h after a PG injection. At a second laparotomy, it was assessed which follicles had ovulated. In Exp. 2, a similar procedure was applied in 12 ewes which underwent repeated laparotomies at 6, 30 and 54 h after PG and after ovulation.

At all times, there was a huge variability in the size at the time of ink labelling of the follicles that finally ovulated ('ovulatory' follicles). A single measurement of follicle size is therefore a poor prediction of the ovulatory follicles. However, a significant reduction in the proportion of non-ovulatory follicles within the size range of ovulatory follicles occurred at 12–24 h after PG, together with a change in follicle hierarchy at 8-24 h after PG.

The results of repeated laparotomies showed that only 60% of follicles > 2 mm at 6 h after PG had grown by 30 h. Selection occurred between 30 and 54 h after PG as indicated by a significant drop in the number of growing follicles to a level at 54 h that equalled the ovulation rate. The growth rate of the ovulatory follicles was 1·4 and 0·4 mm per day between 6 and 30 h and 30 and 54 h after PG respectively. The shrinkage rate of non-ovulatory follicles was 1·6 mm/day.

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L. P. Cahill, J. C. Mariana and P. Mauléon

Summary. The total ovarian follicular populations were studied in two breeds of ewes which differed greatly in their ovulation rates. Thus 8 Romanov (mean ovulation rate 3·1) and 12 Ile-de-France ewes (mean ovulation rate 1·4) were ovariectomized at oestrus during the breeding season. Each right ovary and 3 left ovaries were sectioned at 7 μm and examined microscopically. The number of small follicles, i.e. with 2 or less layers of granulosa cells, was estimated by a tested sampling procedure whilst all larger follicles were measured and arranged into classes. There were half as many small follicles but 1·5–2 times more large follicles in the ovaries of the Romanov ewes compared to those of the Ile-de-France ewes. The number of atretic follicles was approximately the same in both breeds and does not explain the difference observed in ovulation rate.

It is concluded that the higher ovulation rate in the Romanov ewe is due to the greater number of large follicles available to be stimulated for ovulation.

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R. A. S. Lawson, R. A. Parr and L. P. Cahill

Summary. The fate of embryos transferred asynchronously in the ewe was investigated when the functional life of the corpus luteum was prolonged by both hemi-hysterectomy and by the presence of a second synchronously transferred embryo. The development of asynchronously transferred embryos was assessed at progressively later stages after transfer. Prolongation of luteal function did not enable asynchronously transferred embryos to persist. Embryos from Day 4 donors were found to be retarded in their rate of development when placed in 'younger' Day 1 or 2 uteri and appeared unable to develop beyond the early blastocyst stage. Conversely, embryos from Day 4 donors placed in 'older' Day 6 or 7 uteri showed accelerated growth and development which was maintained until the uterus reached Day 12. Thereafter further growth of the asynchronously transferred embryos was retarded, although synchronously transferred embryos then entered the phase of rapid blastodermic vesicle elongation. Asynchronously transferred embryos disappeared from the uterus when the ewe entered pro-oestrus.

The experiments demonstrate the existence of an active relationship between the embryo and the maternal environment during mid-cycle and an apparent lack of association between embryo size, growth rate and physiological maturation.

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M. A. Driancourt, L. P. Cahill and B. M. Bindon

Summary. To investigate the factors contributing to the different ovulation rates observed in two strains of sheep (Booroola 5·2, Merino 1·2), in-vivo monitoring of follicular kinetics followed by histological examination of both ovaries was performed during the late luteal and follicular phases. Ewes of both strains were either ovariectomized at Day 13, or had the 3 largest follicles of each ovary ink-labelled at Day 13 and were ovariectomized at Day 15, or had the 3 largest follicles of each ovary ink-labelled at Days 13 and 15 and were ovariectomized 16 h after the beginning of oestrus (N = 6 per time per strain). In another experiment, the age effects on the follicular populations of these two strains were also studied.

There were 2–4 times more primordial follicles and 1·–2 times more preantral follicles in the ovaries of Booroola than in control Merino ewes, although the number of antral follicles was the same. The percentage of normal follicles in this population was higher in Merino than Booroola ovaries. In Booroola ewes, there was no correlation between the number of antral follicles per ovary and the ovulation rate at the previous cycle (r = 0·22). This suggests that follicle numbers do not play a key role in the high ovulation rate of the Booroola strain. The number of follicles initiating growth from the primordial pool, the number of growing follicles disappearing at the preantral stage, the pattern of antrum development, granulosa cell multiplication and appearance of atresia differed between strains.

The reasons for the high ovulation rate of the Booroola strain became clear when preovulatory enlargement was followed by ink labelling. An extended period of time during which recruitment of ovulatory follicles takes place, together with a low incidence of selection and the ability of the follicles to wait for ovulation are the features involved in this high ovulation rate.

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R. C. Fry, I. J. Clarke and L. P. Cahill

Summary. Ewes were unilaterally ovariectomized and/or hypophysectomized and treated with PMSG and hCG. For a given gonadotrophin treatment the ovulation rate per ewe was maintained, i.e. the ovulation rate of the remaining ovary was significantly increased (P < 0·05), after the removal of one ovary in hypophysectomized and in pituitary-intact ewes. It is concluded that compensation of ovulation rate in the remaining ovary after unilateral ovariectomy in the sheep may be independent of feedback from the ovary and the release of gonadotrophins from the pituitary gland.

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L. P. Cahill, M. A. Driancourt, W. A. Chamley and J. K. Findlay

Summary. Manipulation of circulating concentrations of hormones and ovarian follicle status was carried out on Day 11–12 of the oestrous cycle in sheep. All follicles visible on the ovary were ablated by cautery and ewes were treated with oestradiol or ovine follicular fluid (oFF) to suppress FSH or with PMSG to increase circulating gonadotrophic activity. One group underwent unilateral ovariectomy which greatly increased endogenous FSH and was the only treatment which significantly affected LH pulse frequency. The size distribution of antral follicles, the extent of atresia and the mitotic index of granulosa cells of follicles on Day 15 showed that (a) treatment with oFF inhibited the growth of follicles beyond 2 mm diameter by suppressing the mitotic index of the granulosa cells and (b) the concentration of FSH in peripheral plasma was related to the ability of small antral follicles to grow during the late luteal—early follicular phase of the cycle.

Subsequently, it was demonstrated that oFF inhibits, in a dose-dependent manner, folliculogenesis sustained by PMSG in ewes on Days 12–15. Inhibition of folliculogenesis was represented by a decrease in those follicles > 4 mm, an increase in the relative proportion of follicles < 2 mm, and minimal change in the average number of follicles visible on the ovarian surface, and a decrease in the mitotic index of granulosa cells of follicles < 2 mm. There was no change in the extent of atresia. This supports the concept of a mitotic inhibitor present in charcoal-treated oFF, which can act at the ovarian level to prevent follicles passing beyond 2 mm diameter, even in the presence of excess exogenous gonadotrophin. Such a factor could be involved in selection of the follicle(s) destined to ovulate.