Overcoming poor in vitro nuclear maturation and developmental competence of domestic cat oocytes during the non-breeding season

The domestic cat experiences circannual variations in ovarian activity and intrafollicular oocyte quality. One result is poor nuclear and cytoplasmic maturation during in vitro maturation (IVM) conducted during the annual non-breeding season (July through November). In an attempt to overcome this seasonal phenomenon immature oocytes were collected from July through November and cultured in a conventional IVM medium (IVM1) or in IVM1 supplemented with different FSH concentrations and antioxidant (ascorbic acid or cysteine). Nuclear status of oocytes was assessed after IVM or IVF. Embryo stage and blastocyst quality were evaluated after 7 days of in vitro culture. Although the addition of antioxidant alone had no effect, the presence of 10 mug FSH ml(-1) improved nuclear maturation (75.4 +/- 4.1% versus 48.7 +/- 8.8% in IVM1; P<0.05) and fertilization success (47.9 +/- 3.2% versus 35.0 +/- 5.1% in IVM1; P<0.05). Furthermore, developmental competence of fertilized oocytes was enhanced (P < 0.05) only in the presence of ascorbic acid (30.6 +/- 6.7%) or cysteine (33.6 +/- 5.1%) compared with IVM1 (8.1 +/- 8.8%). Consequently, blastocyst yield (17% of total oocytes cultured) was highest when oocytes were matured in medium containing higher FSH concentration and antioxidants. The results of this study demonstrate that meiotic and developmental competences are inherent to the immature cat oocyte collected during the non-breeding season. However, appropriate mechanisms (perhaps seasonal variation in FSH receptors or lack of antioxidant capacity of the cumulus-oocyte complex) are inadequate during this period of gonadal quiescence. Regardless, this compromised oocyte function during the non-breeding season can be overridden by altering in vitro culture conditions to include supplemental FSH and antioxidant.