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 micron 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.

Total follicular populations and peripheral plasma concentrations of LH, FSH, prolactin, oestradiol-17 beta and progesterone during the preceding cycle were studied in two breeds of sheep (Romanov and Ile-de-France) which differed widely in their ovulation rates (3.2 and 1.5 respectively). No LH parameters could be correlated with the follicular details measured. The second peak of FSH occurring 20-30 h after the preovulatory surge of LH was significantly larger in the Romanov ewes and the area under this peak was correlated (P less than 0.01) with the number of antral follicles present in the ovary 17 days later. This suggests that formation of the antrum during the follicular growth phase is under the control of FSH. The discharge of prolactin preceding the LH peak, although not significantly different between breeds, was correlated with several of the follicular classes measured, including the number of preantral follicles. The peak value of oestradiol-17 beta measured before the LH peak was significantly higher (P less than 0.05) in the Romanov ewes and was correlated with the number of the largest follicles present. There was no significant difference between breeds in the concentration of oestradiol at the onset of oestrus. The progesterone concentration during the luteal phase was highly correlated with the number of preovulatory follicles.

Summary. Five pony and 5 saddle-type mares were used to compare total ovarian follicular populations between breeds. The animals were hemi-ovariectomized at the preovulatory stage during the breeding season and the ovary bearing the large preovulatory follicle was studied using histological techniques.Pony and saddle-type mares did not differ as to mean number of primordial follicles, mean number of growing follicles, the variability of these numbers and follicular distribution into various size classes.No difference was detected either in the initiation processes of follicular growth, oocyte growth, granulosa cell multiplication and follicular atresia. However, significant differences were found in the pattern of antral formation (P < 0.01). ).The present data suggest that pony mares are a valuable tool for the study of follicular populations in equines.The main features of the equine ovary are the small number of follicles (35 950 primordial and 100 growing follicles) and the high individual variability of this number (5 600 to 75 000 primordial and 20 to 300 growing follicles).Introduction.

were collected by flushing the oviducts 28 h after the LH surge, and were fertilized and cultured in vitro for 7 days. Ovulation and cleavage rates were not significantly different among the three groups but a higher rate of blastocysts (P < 0.01) was obtained after Antarelix treatment when LH pulsatility was re\ x=req-\ established (group B). Oestradiol concentration was strongly depressed (P < 0.0003) after Antarelix treatment in group A, but was maintained after injection of LH pulses in group B, although at a lower value than before the preovulatory surge in the control group. In conclusion, inhibition of endogenous LH pulses 1 day before the preovulatory surge was not essential for ovulation and in vitro fertilization but was associated with a decrease in plasma oestradiol concentrations and inferior embryo development both in vivo and in vitro. When LH pulsatility was re-established, oestradiol concentrations increased and embryo development was restored.

The short-term action of PMSG on the population of growing follicles in cattle was studied using histological methods. On Day 7 of a synchronized oestrous cycle 10 Friesian heifers were unilaterally ovariectomized. The remaining ovary was immediately stimulated by an injection of PMSG (2000 i.u.) and was removed 48 h after the preovulatory discharge of LH. Control animals did not receive any injection of PMSG. In all ovaries, follicles greater than 70 micron diameter were counted, measured and checked for atresia. The mitotic index in granulosa cells of follicles of different sizes was estimated in both ovaries of all the PMSG-injected animals. Unilateral ovariectomy alone had no significant effect on follicular populations. In the interval between PMSG injection and removal of the second ovary (148 +/- 22.7 h), PMSG significantly increased the number of normal preantral follicles but did not change the number of normal antral follicles. The mitotic index doubled in preantral and early antral follicles but remained unchanged in large antral follicles. PMSG stimulated slightly the growth of the antrum in large antral follicles but did not stimulate its formation in preantral follicles. The incidence of atresia among antral follicles, particularly the largest ones (diam. greater than 1.7 mm), was significantly reduced after PMSG, suggesting some 'rescue' of follicles from atresia.