Reproduction Abstracts

Introduction: Reproductive ageing is associated with a reduced ovarian reserve and suppression of oocyte developmental competence (quality) as evidenced by increased meiotic segregation errors, epigenetic alterations and reduced fertilisation rates. Metabolic indicators of oocyte quality such as oocyte amino acid profile (AAP) and mitochondria DNA (mtDNA) copy number may provide insights into the mechanism of declining oocyte quality with age. This study investigated the effect of adult age on oocyte AAP and mtDNA copy number in sheep.

Methods: Ovaries were collected from young (9–12 months, n=6), and old (6–8 years, n=6) Greyface ewes. Cumulus–oocyte complexes from 2 to 10 mm diameter follicles underwent individual maturation for 18 h in serum-free IVM medium before denudation and culture for 6 h in AAP assay medium. Oocyte maturity was assessed before and after AAP incubation. Amino acid turnover was measured in spent AAP assay medium by HPLC; mtDNA copy number was quantified in individual oocytes by real-time PCR.

Results: A total of 57 follicles (9.5±0.6/animal) and 64 follicles (10.7±1.4/animal) were collected from young and old ewes, respectively. Age had no significant effect on the capacity of oocytes to mature to MII in vitro (young: 91.7% MII, n=48 vs old: 94.2% MII, n=52; P>0.05). AAP revealed significantly (P<0.05) lower depletion of glutamine and leucine, as well as lower appearance of tyrosine, higher appearance of glutamic acid and lower overall amino acid depletion from/to the media of young MII oocytes (n=42) compared to old counterparts (n=48). Oocyte mtDNA copy number was significantly higher (P<0.01) in young (2.4±0.4×10⁶, n=24) vs old oocytes (1.6±0.4×10⁶, n=24).

These results suggest that oocyte nuclear maturation capacity in vitro is not affected by adult age. In contrast, the observed increase in amino acid depletion and decline in mtDNA copy number with age may be related to oocyte quality.