Effect of Follicle Size on In Vitro Maturation of Pre‐Pubertal Porcine Cumulus Oocyte Complexes

Contents Very small follicles (<3.0 mm diameter) are over‐represented on the surface of ovaries of non‐cycling pigs, and the oocytes collected from these follicles generally have reduced developmental competence in vitro . This study examined the effect of follicle size on the nuclear maturation (n = 608), the potential of parthenogenetic activation (n = 243) and the cyclic AMP ( cAMP ) content of pre‐pubertal porcine oocytes (n = 480). In addition, the influence of follicle size on steroid hormone synthesis was analysed. Cumulus oocyte complexes ( COC s) flushed from small (2.5–4.0 mm) or large (4.5–6.0 mm) ovarian follicles were cultured for 0, 28 and 46 h. After 46 h of IVM , a greater proportion of oocytes from 4.5‐ to 6.0‐mm follicles reach metaphase II ( MII ) compared with those from follicles with 2.5–4.0 mm of diameter (96.1 vs 77.0%, respectively; p < 0.001). Parthenogenetic activation of oocytes from large follicles produced higher developmental rates than oocytes from large follicles (p < 0.05). At 28 h, the IVM medium with oocytes from large follicles contained significantly more 17ß‐oestradiol (E 2 ) than the medium with oocytes from small follicles (5.55 vs 3.45 ng/ml, respectively; p < 0.05) and at 46 h, the medium with oocytes from small follicles contained significantly more progesterone (P 4 ) than the medium with oocytes from large follicles (276.7 vs 108.2 ng/ml, respectively, p < 0.05). Porcine oocytes from large follicles have higher nuclear and cytoplasmic maturation capacities, but the differences did not appear to be cAMP ‐mediated. Our findings also suggest that COC s from small follicles undergo more intensive luteinization than COC s from large follicles. The results show that oocytes from follicles with a diameter greater than 4.0 mm are more suitable for in vitro studies.