120 Effects of antral follicle count in ovaries on follicular development and endocrine dynamics of follicle-stimulating hormone and steroid hormones in cattle

Abstract

The antral follicle count (AFC) in mammalian ovaries positively correlates with female fertility. We have reported previously that cumulus-oocyte complexes derived from high-AFC ovaries produce more oestradiol-17β (E₂) and have higher developmental competence than those from low-AFC ovaries in in vitro growth (Sakaguchi et al. WCRB2017, Okinawa, Japan; 1-15). We also indicated that follicular growth was different between high- and low-AFC cows, and that follicular deviation occurred earlier in high-AFC cows (Nagai et al. 2015 Anim. Reprod. Sci. 163, 172-178). To clarify the causes of the higher E₂ production and earlier follicular deviation in high-AFC cows, we investigated circulating steroid hormones and FSH during the oestrous cycle, as well as steroidogenesis in follicles at different phases of the oestrous cycle. Four high-AFC cows (>40 follicles) and 3 low-AFC cows (<20 follicles) based on the peak AFC were used. We ablated their follicles 8 days after oestrus and administered prostaglandin F_2α on Day 12. After 40 h of prostaglandin F_2α injection, follicular fluid of a dominant follicle (preovulatory phase) was collected and subordinate follicles were ablated. Gonadotropin-releasing hormone was then injected to induce LH surge. Seven days after gonadotropin-releasing hormone, follicular fluids were collected from dominant follicles (luteal phase) and all follicles were ablated. Four days later, follicular fluids were again collected from the largest follicles (selection phase). Steroid hormones in follicular fluids were measured by enzyme immunoassay. Throughout the study, we daily examined ovaries by ultrasonography and collected blood for hormone measurements. We analysed data using two-way ANOVA followed by Tukey-Kramer or Student’s t test. The number of small follicles (<4 mm) was greater in high-AFC than low-AFC groups (low v. high = 7.6 v. 46.9; P < 0.05), whereas that of large follicles (>8 mm) was similar between groups (0.8 v. 0.8) at any time. Although the number of medium-sized follicles (4-8 mm) was not affected in the low-AFC group (4.2-6.7) after follicle ablation, the number increased from 1 to 4 days (10.5 to 17.8) then decreased 6 days after follicle ablation (11.5) in the high-AFC group, which means that degradation of follicles occurs at selection phase in high-AFC cows. Peripheral FSH concentration was higher (low v. high: 4.7 v. 2.1 ng/mL), but E₂ concentration was lower in the low-AFC (2.4 pg/mL) than high-AFC group (3.3 pg/mL) during the selection phase (P < 0.05). Although follicular diameter was similar in both groups at any phase, E₂ (low v. high = 333.8 v. 1127.4 ng/mL; P < 0.05) and testosterone (low v. high = 16.7 v. 35.0 ng/mL; P = 0.07), but not progesterone, concentrations were higher in the high-AFC than low-AFC group in the preovulatory phase. These findings suggest that a lower response to FSH causes low E₂ production in the low-AFC group, resulting in high FSH concentration and a consistent development of medium-sized follicles. Conversely, higher E₂ concentration suppresses FSH secretion, resulting in obvious degradation of follicles in the high-AFC group at the selection phase.