Defective Oocytes as a Cause of Subfertility in Old Mares1

Abstract

Oocytes were collected by transvaginal, ultrasonically guided follicular aspirations from young (6–10 yr) and old (20–26 yr) mares at 21–26 h after the administration of hCG. Oocytes were cultured for 16-20 h (tissue culture medium + 10% fetal calf serum, 39°C, 5% CO2 and air) and transferred into the oviducts of young recipients (1–4 oocytes/recipient), which were inseminated with semen from one stallion. More (p < 0.001) oocytes from young (11 of 12, 92%) than from old donors (8 of 26, 31%) developed into embryonic vesicles as imaged by ultrasound on Day 12. To obtain additional information, the mares were artificially inseminated during a subsequent ovulatory cycle. The resulting pregnancy rate for young mares (15 of 18, 83%) was higher (p < 0.005) than for old mares (3 of 16, 19%) on Day 12. The percentage of transferred oocytes that developed into embryos was not different from the natural pregnancy rate on Day 12 within an age group. The results of this study suggest that oocytes from old mares are defective and that transfer of oocytes from old mares into young recipients does not eliminate age-associated subfertility and that the GIFT procedure as modified was effective.