Serum and follicular fluid steroid levels as related to follicular development and granulosa cell apoptosis during the estrous cycle of goats

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The relationship between the concentrations of progesterone (P 4), estradiol (E 2), granulosa cell apoptosis in follicles of different sizes and between serum and follicular fluid levels of steroid hormones was investigated in does at various stages of the estrous cycle. While the number of large (≥5 mm) follicles on the ovary did not change and that of medium-sized (3–5 mm) follicles changed moderately during the estrous cycle, the number of small (≤3 mm) follicles varied markedly and dominated the total number of follicles. The number of small follicles was significantly ( P < 0.05) higher on days 0 (30.3 ± 0.4 per ovary), 5 (19.5 ± 2.6 per ovary), 12 (28.8 ± 2.6 per ovary) and 15 (24.3 ± 6.0 per ovary) compared to day 9 (2.6 ± 2.9) and 18 (2.2 ± 2.7 per ovary) of the estrous cycle. Although granulosa cell apoptosis occurred in follicles of all sizes, the ratio of apoptotic cells varied with the stage of the cycle and size of the follicles. Thus, while the ratio of apoptotic granulosa cells in large follicles was much higher on day 9 (0.44 ± 0.05) and 18 (0.45 ± 0.08) of the cycle than at other stages, that in the medium follicles was lower on day 9 (0.23 ± 0.05) and 18 (0.95 ± 0.01) than at other stages. However, the ratio of apoptotic cells in small follicles did not change significantly during the estrous cycle. Serum P 4 level changed in relation to the formation and regression of corpus luteum, having a minimum level on day 0 (0.2 ± 0.2 ng/ml) and maximum level on day 9 (5.6 ± 1.3 ng/ml) post estrus. While P 4 levels in large follicular fluid were significantly lower on days 0 (0.5 ± 0.5 ng/ml), 5 (0.23 ± 0.13 ng/ml) and 15 (0.14 ± 0.02 ng/ml) than on days 9 (3.5 ± 1.5 ng/ml) and 18 (2.5 ± 1.7 ng/ml), that of the medium follicles were significantly higher on days 0 (1.9 ± 1.3 ng/ml), 5 (1.6 ± 0.9 ng/ml) and 15 (2.1 ± 1.6 ng/ml) compared to days 9 (0.2 ± 0.08 ng/ml), 12 (0.12 ± 0.08) and 18 (0.3 ± 0.1 ng/ml) of the estrous cycle. Serum E 2 level was consistent with estrous behavior, with a maximum level on day 18 (3.5 ± 0.8 pg/ml) of the cycle significantly higher than the other stages of the cycle. E 2 concentration in large follicles tended to decrease on days 9 (1.2 ± 0.4 ng/ml) and 18 (1.7 ± 0.2 ng/ml) after estrus, whereas that in the medium follicles tended to increase at the same time (0.4 ± 0.1 ng/ml and 1.3 ± 0.5 ng/ml, respectively). Furthermore, E 2 concentration in the large follicles increased on day 15 (2.4 ± 0.2 ng/ml), but that in the medium follicles decreased at this time (0.08 ± 0.03 ng/ml). The ratio of E 2/P 4 in large follicles was higher on days 0 (90.8 ± 51.5), 5 (36.8 ± 16.6) and 15 (33.7 ± 5.4) than on days 9 (0.4 ± 0.1), 12 (0.7 ± 0.1) and 18 (1.5 ± 0.6) of the cycle, however this ratio in the medium follicles was lower on days 5 (0.4 ± 0.2) and 15 (0.2 ± 0.2) than on days 9 (9.2 ± 6.7) and 18 (7.0 ± 4.2). Overall, these results suggested that in the three categories of follicles studied, only the number of small follicles changed significantly and dominate the total number of follicles during the estrous cycle. P 4 and E 2 levels in the follicular fluid changed independently of that in the serum during the estrous cycle, but changes in concentration were consistent with the degree of atresia and development of follicles. The levels of E 2 and P 4 and the ratio of apoptotic granulosa cells in the large follicles tended to change in opposition to those in the medium follicles, indicating that dominant follicles inhibited the growth of subordinate ones. These findings will contribute to the understanding of the mechanisms of follicular development and atresia, and to the improvement of superovulation in goats.