Ovarian and endocrine determinants of superovulatory responses in anestrous ewes

Ovarian follicular status and secretory function at the outset of the superovulatory treatment in cyclic ewes are closely related to the resulting ovulation rate and embryo yields. The objective of this study was to examine if the size and developmental pattern of the two largest follicles, antral follicle numbers (follicles ≥2 mm in diameter) and circulating estradiol-17β (E2-17β) concentrations at the onset of gonadotropin stimulation are related to superovulatory responses in anestrous ewes. Twenty-four anestrous (May–June) Rideau Arcott ewes were treated with medroxyprogesterone acetate (MAP) intravaginal sponges (60 mg) for 14 days. The superovulatory regimen consisted of six i.m. injections of pFSH (Folltropin®-V) administered twice daily (1× 2.5 ml and 5× 1.25 ml), from 48 h before to 12 h after sponge removal, followed by a bolus injection of GnRH (50 μg i.m.). A single i.m. dose of 500 IU eCG was given concurrently with the 1st pFSH injection. Jugular blood samples were collected and the number of ovarian antral follicles ≥2 mm in diameter recorded, using transrectal ovarian ultrasonography, at the time of the 1st and the 2nd pFSH injection. The size and developmental status (i.e., growing, static or regressing) of the two largest follicles had no influence on ovarian responses and embryo yields. The numbers of medium-sized follicles (4 mm in diameter) at the time of the 2nd pFSH injection were positively correlated to the numbers of luteal structures and the numbers of viable embryos (both r = 0.44, P < 0.05). Serum E2-17β concentrations at the time of the 1st and the 2nd pFSH dose were positively correlated to the numbers of non-viable embryos (r = 0.41, P < 0.05 and r = 0.60, P < 0.01, respectively). It can be concluded that the number of gonadotropin-responsive small antral follicles and follicular estrogenicity appear to be major determinants of the ovarian response and embryo quality in superovulated anestrous ewes.