In vitro maturation of equine oocytes: Effect of follicular size, cyclic stage and season

Four trials for in vitro maturation (IVM) of equine oocytes were performed (May, June, August and September, respectively). Oocytes were obtained from the slaughterhouse ovaries. The cyclic stage of each mare was determined from plasma progesterone concentrations, degree of the dilation of the cervix and the presence of large follicles and/or corpora lutea (CL) on the ovaries. Follicular contents were collected by aspiration using a suction pump and follicle sizes were recorded. A total of 587 oocytes was aspirated from 38 mares resulting in an overall recovery rate of 50.0%, yielding 7.7 oocytes per ovary. The oocyte recovery rate was significantly (P<0.0001) higher for follicles measuring 11 to 20 mm in diameter (61.3%) than for follicles less than or equal to 10 mm (43.7%). The distribution of nuclear maturation stages in uncultured oocytes (n=48) did not differ in relation to season, follicle size and reproductive status of the mare. During 4 maturation trials, 207 oocytes were individually cultured for 36 h, and 39.7% (27/68), 38.9% (14/36), 34.0% (16/47) and 37.5% (21/56), respectively, reached the metaphase II stage. The maturation rate of oocytes from follicles 11 to 20 mm in diameter (45.7%) was significantly higher (P=0.01) than that of oocytes from follicles less than or equal to 10 mm in diameter (26.7%). There was no difference in the maturation rate to metaphase II after 36 h of culture in relation to the stage of cycle of the mare or season.