ULTRASONOGRAPHIC STUDY OF OVARIAN FOLLICULAR DYNAMICS IN EWES DURING THE ESTROUS-CYCLE

Transrectal ovarian ultrasonography was performed daily in eight ewes during one interovulatory interval, using a 7.5 MHz, rigid, human pros tate transducer, and a realtime B-mode scanner to record the numbers, diameters and position of all follicles greater than or equal to 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, progeste rone 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 t he numbers of follicles emerging was seen on days 2 and 11. The ovulat ory follicle (6.9 +/- 0.1 mm diameter) was retrospectively traced to e mergence on day 11.1 +/- 0.3 and grew over a period of 4.1 +/- 0.1 day s at a growth rate of 1.2 +/- 0.04 mm day(-1). The largest nonovulator y follicles of the same period grew at the same rate as ovulatory foll icles 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 a gain from day 11 to the day before ovulation (6.9 a 0.1 mm; P < 0.05). The mean number of antral follicles greater than or equal to 2 mm in diameter increased over the oestrous cycle from 4.5 +/- 0.4 on day 3 t o 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). T he 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 inc reased to 13.3 +/- 0.6 mm on day 5, and declined from day 11 to a diam eter of 7.5 +/- 0.3 mm on the day of ovulation. Apart andom increases before ovulation, there were no clear associations between serum conce ntrations of FSH or oestradiol and the pattern of follicular growth an d regression. We concluded that follicle emergence appeared on many da ys of the oestrous cycle of ewes, with two phases of increased emergen ce. There was no discernible connection between follicle emergence and FSH secretion; the overall pattern of growth and regression of follic les was not as distinctly wave-like as in cattle. Follicular dominance was noted only just before ovulation, again, in contrast to cattle.