Collection and in vitro maturation of equime oocytes from estrus, diestrusand pregnant mares

Two experiments were conducted to evaluate oocyte collection rates and in v itro nuclear maturation rates of equine oocytes obtained during diestrus an d pregnancy, and to compare these rates with maturation rates in oocytes de rived from preovulatory follicles. In Experiment I, transvaginal ultrasound -guided aspiration of follicle was performed during estrus and diestrus in 14 mares over four consecutive cycles. Follicular aspirations during estrus were performed 24 to 27 hours after injection of 2500 IU of hCG given when the largest follicle reached 35 mm in diameter. Oocyte recovery rate from preovulatory follicles was 51% (33/65) in 49 aspiration sessions. Cumulus-o ocyte complexes from preovulatory follicles were cultured for 12-15 hours i n TCM199 + 10% (NCS) at 38.5 degreesC in 5% CO2 in air, and 22/33 (67%) wer e in metaphase II. During diestrus, mares were treated (Group I) or not tre ated (Group II) daily with equine pituitary extract (EPE) during alternate cycles from days 1 to 14 after the preovulatory aspiration. Diestrous folli cles were aspirated when four or more follicles greater than 12 mm in diame ter were present. EPE had no effect on the number of follicles that develop ed during estrus or diestrus (p>0.05). Oocytes were recovered from 119 of 3 83 diestrous follicles (31%)in 75 sessions. There was no difference in reco very rates between Groups I and II (p>0.05). Maturation rates for oocytes c ollected during diestrus, after 42 hours of culture in TCM 199 + 1 mug/ml o f Estradiol and 1nl/ml of EPE, were not significantly different (p>0.05) be tween Groups I and II (49% vs. 53%). In Experiment II, mares between 50 and 85 days of pregnancy were used as oocyte donors. The oocyte recovery rate was 53% (66/125). After in vitro maturation for 40 hours (compact COG) or 1 5 hours (expanded COC), 22% (7/32) and 22% (7/32), respectively, of the ooc ytes were in metaphase II. It was concluded that: 1) Preovulatory follicles yield a higher percentage of oocytes with a higher rate of maturation to m etaphase II than follicles of diestrus and pregnant mares. 2) Diestrous fol licles yielded fewer oocytes than follicles of pregnant mares but with a hi gher percentage of oocyte maturation. Further studies are necessary to dete rmine if oocytes recovered from diestrous follicles and matured in vitro ca n be fertilized successfully.