Effects of gonadotrophin deprivation on follicular growth in gilts.

The endocrine and ovarian effects of hypophysectomy (n = 5) and gonadotrophin-releasing hormone (GnRH) antagonist administration (Antarelix) (n = 5) were studied in gilts by comparison with control animals (n = 6). All gilts were given Regumate (20 mg/d for 18 d). The last day of Regumate was day 0. Hypophysectomy and initiation of Antarelix administration (0.6 mg iv twice daily for 7 d) were performed on day 5. All ovaries were obtained at slaughter on day 12. Blood samples were obtained daily from all Antarelix-treated and control sows to measure luteinizing hormone (LH) and follicle stimulating hormone (FSH) concentrations. Frequent sampling was performed on day 10 on control and Antarelix-treated gilts to assess pulsatile LH secretion. Oestrus or LH surge was initiated before the beginning of treatment in 2 hypophysectomized, 4 Antarelix-treated, and 4 control females. Gonadotropins were undetectable in the blood of hypophysectomized sows on day 6. On day 10, pulsatile LH release was blocked in Antarelix-treated gilts. At no time were FSH concentrations significantly affected. Histological observation of the ovaries demonstrated that: (i) similar populations of healthy or total (healthy + atretic) follicles < 1 mm were found in the 3 groups of females; (ii) healthy follicles 1 to 2 mm in diameter were present in Antarelix-treated but not in hypophysectomized gilts; and (iii) healthy follicles > 2 mm were absent in Antarelix-treated and hypophysectomized gilts. The present study suggests the existence of 3 subgroups amongst antral follicles (gonadotropin independent: 0.19 to 1.1 mm; FSH dependent: 1.1 to 2 mm; LH pulses dependent: > 2 mm in diameter).