Association of equine oocyte and cleavage stage embryo morphology with maternal age and pregnancy after intracytoplasmic sperm injection

Abstract

In this retrospective study the morphological characteristics of oocytes and cleavage stage embryos were associated with pregnancy results from clinical intracytoplasmic sperm injection (ICSI) in mares. Oocytes were collected from preovulatory follicles, and images (×200; n=401) were captured for measurements of ooplasm, the perivitelline space and zona pellucida. After ICSI and before transfer into recipients’ oviducts, cleavage stage embryos were imaged (n=178). Oocyte donor ages (3–13, 14–19, 20–23, 24–27 years) were compared, as were mares aged 3–13 years without versus with recent histories of performance or injury stress. Cleavage rates did not differ with age. However, pregnancy rates declined and pregnancy loss rates (11–50 days gestation) increased with mare age. Young mares with performance or injury stress had significantly lower pregnancy rates than young mares under management typical for broodmares. No morphological oocyte characteristic was consistently associated with age or pregnancy outcome. Cleavage stage embryo morphology was not associated with pregnancy outcome; however, the rate of embryo development before oviductal embryo transfer was faster (P<0.05) for embryos that resulted in an early pregnancy (≤17 days) and tended (P ≤ 0.1) to be higher for embryos that produced a 50-day pregnancy. Embryonic vesicles that had a more rapid increase in diameter were more often (P<0.05) maintained until 50 days gestation.