Age, FSH dose and follicular aspiration frequency affect oocyte yield from juvenile donor lambs
Experiments were conducted to determine the effects of lamb age, frequency of follicular aspirations, and hormone stimulation by fixed or variable FSH dose, on the number of collected oocytes and their maturational competence. In trial 1, the characteristics of follicular population (number and diameter of follicles) were studied in 40 lambs which were slaughtered at the age of 30 days (S1), 42 days (S2), 60 days (S3) and 5-6 months (S4), each n = 10. In trial 2, 27 lambs were divided into four groups. group MF lambs (n = 6) had follicular aspiration (OPU) in four monthly intervals commencing from the age of 8-9 weeks (sessions MF1, MF2, MF3 and MF4). In groups SF2, SF3 and SF4 (each n = 6), OPU was conducted once during the 12-13, 16-17 and 20-21 week of age, respectively. Ovarian stimulation was conducted with fixed FSH dose (3.52 mg/animal). In trial 3, 10 lambs (group MV) were treated as those of group MF apart from the FSH dose, which was administered according to the body weight in a dose of 0.27 mg/kg. The number and the size of follicles, the number and the quality of collected oocytes and the maturational competence of the oocytes were compared between and within groups. In trial 1, the total number and the number of small follicles were greater in groups S1 and S2 compared with those of S3 and S4 (p < 0.01). Similarly, the follicular population was greater in group MF1 than in group SF3 (p < 0.01). In sessions MF2, MF3, MV2, MV3 and MV4, more oocytes were collected in comparison with those from the respective once-aspirated age mates (groups SF2, SF3 and SF4). In total, more (p = 0.02) oocytes per donor were collected from group MV (15.2 +/- 5.5) than from group MF (9.0 +/- 3.2). An absolute maturational failure was observed in oocytes collected from groups SF2 and SF3. Maturational competence varied between 16.7% and 58.3% (p = 0.017) among sessions of group MF, but it was more uniform among sessions of group MV (range 12.5-42.9%, p > 0.05). Our results indicate that firstly, the number and the quality of harvested oocytes from juvenile lambs can be much improved if follicular stimulation regime is adjusted to the body weight. Secondly, in terms of follicular population and oocyte quality, 3 and 4-month-old lambs are naturally bad oocyte donors, but this characteristic can be reversed by a previous follicular ablation.