Reproductive competency and mitochondrial variation in aged Syrian hamster oocytes

The hamster is a useful model of human reproductive biology because its oocytes are similar to those in humans in terms of size and structural stability. In the present study we evaluated fecundity rate, ovarian follicular numbers, ova production, mitochondrial number, structure and function, and cytoplasmic lamellae (CL) in young (2-4 months) and old (12-18 months) Syrian hamsters (Mesocricetus auratus). Young hamsters had higher fertilisation rates and larger litters than old hamsters (100 vs 50% and 9.3 +/- 0.6 vs 5.5 +/- 0.6, respectively). Ovarian tissue from superovulated animals showed a 46% decrease in preantral follicles in old versus young hamsters. There was a 39% reduction in MII oocyte number in old versus young hamsters. Young ova had no collapsed CL, whereas old ova were replete with areas of collapsed, non-luminal CL. Eighty-nine per cent of young ova were expanded against the zona pellucida with a clear indentation at the polar body, compared with 58.64% for old ova; the remaining old ova had increased perivitelline space with no polar body indentation. Higher reactive oxygen species levels and lower mitochondrial membrane potentials were seen in ova from old versus young hamsters. A significant decrease in mitochondrial number (36%) and lower frequency of clear mitochondria (31%) were observed in MII oocytes from old versus young hamster. In conclusion, the results of the present study support the theory of oocyte depletion during mammalian aging, and suggest that morphological changes of mitochondria and CL in oocytes may be contributing factors in the age-related decline in fertility rates.