Selection, dominance and atresia of follicles during the oestrous cycle of heifers

Abstract

This study examined the correlation between measurement of follicle growth by ultrasound, and measurement of intrafollicular ratios of oestradiol and progesterone concentrations and the serum concentrations of FSH during selection, dominance and atresia or ovulation of dominant follicles in heifers. Heifers were ovariectomized on days 0 (before LH surge), 1 (after LH surge, preovulation), 1 (postovulation), 3, 6 and 12 of the oestrous cycle. Blood samples were collected at 4-6 h intervals. After ovariectomy all follicles ≥ 5 mm were measured and follicular fluid was aspirated. Follicles were classified by size according to ultrasound (F1, largest; F2, second largest; F3, all remaining follicles ≥ 5 mm) and by the ratio of oestradiol:progesterone concentrations. During the follicular phase, a single dominant oestrogen-active follicle increased in diameter while serum concentrations of LH increased and FSH decreased (P < 0.05). On day 1 (after LH surge, preovulation), serum LH and FSH decreased to pre-surge concentrations (P < 0.0001), while follicle size and intrafollicular progesterone concentration increased and oestradiol concentration decreased (P < 0.05). A dominant nonovulatory follicle, classified as oestrogen-active on days 1, 3 and 6 and oestrogen-inactive on day 12, increased in size from day 1 to day 7 and lost dominance during days 10-12, coincident with the growth of multiple oestrogen-active follicles. The serum FSH concentration increased transiently (P < 0.05) before each new wave of dominant follicular growth. The overall correlation of ultrasound measurements of follicle diameter with measures of follicle size after ovariectomy was high. The ratio of oestradiol:progesterone concentrations, but not of size, reliably distinguished potential dominant from atretic follicles. The size of the follicle and the oestradiol concentration were not determinants of subsequent dominance during a selection phase. We conclude that: (1) ovarian follicles go through selection, dominance and atresia phases coincident with transient increases and decreases in FSH; and (2) ultrasound is an accurate measure of follicle growth, but that size alone is not a sufficient measure to ascribe dominance and both ultrasound and the intrafollicular ratio of oestradiol progesterone concentrations are needed to monitor selection, dominance and atresia of follicles accurately.