Preovulatory Maturation of Ovarian Follicles and Oocytes in Unstimulated and Superovulated Mares

Oocytes and follicular fluid (FF) were collected from preovulatory follicles in nonstimulated and superovulated pony mares in order to describe the nuclear and cytoplasmic (cortical granules [CG]) maturation of the oocyte and to compare it with follicular maturity as measured by the steroid composition of the fluid (progesterone [P4], estradiol [E2], testosterone [T], and androstenedione [A]). In superovulated mares, 2.39 ± 0.29 follicles reached the preovulatory stage vs. 1.07 ± 0.05 in nonstimulated (control) mares. The rate of oocytes reaching metaphase II (MII) was not different between control and treated mares (14 of 16 vs. 21 of 26) nor was the rate of oocytes with complete and partial CG migration (13 of 14 vs. 20 of 25). In all follicles punctured, the concentration of steroids in FF was related to 1) recovery of an oocyte in the follicle (the proportion of follicles with low E2 and A concentrations was increased in follicles in which no oocyte was collected compared to follicles producing an oocyte [p < 0.01 and p < 0.05]), and 2) the superovulation treatments irrespective of whether or not an oocyte was collected (a significantly higher frequency of low A concentration was found in FF of all treated mares vs. controls [p < 0.01]). In follicles that yielded an oocyte irrespective of treatment, hormone concentrations were not related to the maturation of the oocyte: the frequency of low concentration of the four steroids was not significantly different in follicles producing immature nuclear oocytes (metaphase I [MI], germinal vesicle stage [GV], and degenerative stage [D]) vs. mature (MII) oocytes or in follicles producing immature cytoplasmic oocytes (internal CG localization) vs. mature (complete and partial migration of CG) (p > 0.05). In follicles that yielded an MII oocyte, the steroid concentrations were related to treatment; there was a significantly higher frequency of low T and A concentrations in the FF of treated vs. control mares (p < 0.02 and p < 0.01). It was concluded that oocyte maturation is reached in unstimulated as well as in superovulated mares but that the follicular environment appears to be different in stimulated compared to unstimulated mares.