Ultrasonographic study of ovarian follicular dynamics in ewes during the oestrous cycle

in Reproduction
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Transrectal ovarian ultrasonography was performed daily in eight ewes during one interovulatory interval, using a 7.5 MHz, rigid, human prostate transducer, and a realtime B-mode scanner to record the numbers, diameters and position of all follicles ≥ 2 mm in diameter and the corpora lutea in both ovaries. Blood samples were taken once a day and were analysed for concentrations of FSH, progesterone and oestradiol. During the interovulatory interval of 17.2 ± 0.4 days, antral follicles (follicles > 2 mm in diameter) emerged on all days except for days 1, 5, 15, 16 and 17. A significant increase in the numbers of follicles emerging was seen on days 2 and 11. The ovulatory follicle (6.9 ± 0.1 mm diameter) was retrospectively traced to emergence on day 11.1 ± 0.3 and grew over a period of 4.1 ± 0.1 days at a growth rate of 1.2 ± 0.04 mm day−1. The largest nonovulatory follicles of the same period grew at the same rate as ovulatory follicles and regressed over a period of 2.6 ± 0.2 days at a rate of 1.2 ± 0.07 mm day−1. The mean diameter of the largest follicles seen on each day of the oestrous cycle was lowest on the day of ovulation (2.9 ± 0.2 mm), increased from day 3 to day 5 (4.1 ± 0.4 mm) and again from day 11 to the day before ovulation (6.9 ± 0.1 mm; P < 0.05). The mean number of antral follicles ≥2 mm in diameter increased over the oestrous cycle from 4.5 ± 0.4 on day 3 to 7.2 ± 0.7 on day 11 and showed a sharp decline starting on day 15, to a low of 3.5 ± 0.3 on the day of ovulation (day 17.2 ± 0.4). The corpus luteum could be identified by day 3 of the oestrous cycle in all the ewes, at a mean diameter of 11.5 ± 0.3 mm. The diameter increased to 13.3 ± 0.6 mm on day 5, and declined from day 11 to a diameter of 7.5 ± 0.3 mm on the day of ovulation. Apart from increases before ovulation, there were no clear associations between serum concentrations of FSH or oestradiol and the pattern of follicular growth and regression. We concluded that follicle emergence appeared on many days of the oestrous cycle of ewes, with two phases of increased emergence. There was no discernible connection between follicle emergence and FSH secretion; the overall pattern of growth and regression of follicles was not as distinctly wave-like as in cattle. Follicular dominance was noted only just before ovulation, again, in contrast to cattle.

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