Ovarian antral follicular dynamics in sheep revisited: Comparison among estrous cycles with three or four follicular waves

In this study, the characteristics of ovarian follicular waves and patterns of serum concentrations of follicle-stimulating hormone (FSH), estradiol, and progesterone were compared between cycles with three (n = 9) or four (n = 10) follicular waves in Western White Face (WWF) ewes (Ovis aries). Transrectal ultrasonography and blood sampling were performed daily during one cycle. Estrous cycles were 17.11 ± 0.3 and 17.20 ± 0.2 d long in cycles with three and four waves, respectively (P > 0.05). The first interwave interval and the interval from the emergence of the final wave to the day of ovulation were longer in cycles with three waves compared with those in cycles with four waves (P < 0.05). The growth phase (5.1 ± 0.5 vs. 3.1 ± 0.4 d) and life span (5.67 ± 0.3 vs. 4.3 ± 0.3 d) of the largest follicle growing in the last or ovulatory wave was longer in cycles with three waves compared with that in cycles with four waves (P < 0.05). The maximum diameter of the largest follicle was greater in the first wave and the ovulatory wave compared with that in other waves of the cycle (P < 0.05). The regression phase of the largest follicle growing in the first wave was longer in cycles with three waves compared with that in cycles with four waves (4.44 ± 0.4 vs. 3.4 ± 0.4 d; P < 0.05). The length of the life span, regression phase, and, although not significant in every case, FSH peak concentration and amplitude decreased across the cycle (P < 0.05). We concluded that estrous cycles with three or four follicular waves were confined within the same length of cycle in WWF ewes. In this study, there were no apparent endocrine or follicular characteristics that could explain the regulation of the different number of follicular waves (three vs. four) during cycles of similar length.