Effects of Ewe Age on Embryo Viability and Morphokinetics: A Potential Ovine Model of Human Reproductive Ageing

The main objective of this study was to assess the effects of donor age on the development of in vitro-derived sheep embryos. Ovaries were obtained after slaughter from cycling Polish Longwool ewes aged 1.5-3 years (Group Y-'young'; n = 14) or 8-9 years (Group O-'old'; n = 16). Cumulus-oocyte complexes were collected and subjected to in vitro maturation (IVM), followed by in vitro fertilisation (IVF) with fresh capacitated ram semen. The resultant embryos were then cultured and monitored with a time-lapse (TL) imaging system for up to 8 days (Group Y, n = 64 and Group O, n = 48). The timing of key developmental stages relative to the moment when the oocytes and sperm were combined and including cleavage divisions as well as morula and blastocyst formation was recorded. Both the cleavage (68.75%) and blastocyst formation (26.6%) rates were significantly higher in younger ewes compared with their older counterparts (50.0% and 10.4%, respectively), with Group Y zygotes showing fewer (p < 0.05) incidences of abnormal cleavage and morphology (fragmentation, direct cleavage or asymmetrical cleavage) compared with Group O (10.9% vs. 33.3%, respectively). The first cleavage division occurred earlier (25:42 ± 3:43 vs. 29:20 ± 6:59 [hours: minutes post-insemination]; mean ± SD; p < 0.05) and the duration of the second cell cycle (time between the first and second mitotic division) was greater for Group Y compared with Group O (11:20 ± 9:51 vs. 4:14 ± 6:40; p < 0.05). No significant differences were observed between the times of the following mitotic divisions or formation of morulae and blastocysts. This study documents the specific differences in embryo morphokinetics between donor ewes varying in age and highlights the usefulness of TL imaging for assessing the influence of maternal ageing on embryogenesis in sheep as a model for different mammalian species including humans.