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In the bitch, oocytes are ovulated at the germinal vesicle (GV) stage and mature in the isthmus of the oviduct around 3 days after ovulation, it is not known what elements trigger the release of this meiotic arrest. Canine IVM has shown limited success with maturation rates, usually around 20% (MII) (Farstad W, 2000 Anim. Reprod. Sci. 60–61, 375–387). Estrogen and progesterone are suggested to play a significant role in causing oocyte resumption of meiosis and progression to MII stage. The purpose of this study was to investigate the role of estradiol-17β (E2) and progesterone (P4) during in vitro maturation of canine oocytes in serum-free tissue culture medium (TCM)-199. Canine oocytes collected from bitches were categorized into three groups based on estrous stages, follicular, luteal, or anestrus, at routine ovariohystrectomy. Oocytes were cultured in vitro in TCM-199 supplemented with E2, P4 or E2+P4 according to experimental design at 39°C in 5% CO2 and O2. After 72h of maturation culture, oocytes were denuded, fixed in a 3.7% paraformaldehyde solution for 10min, stained with Hoechst 33342 in glycerol, and observed under the UV light. Three groups of oocytes were cultured in TCM-199 supplemented with different concentrations (0, 0.1, 1.0 or 2.0μgmL−1) of E2 (Experiment 1, n=898, replications: 5) or P4 (0, 0.5, 1.0 or 2.0μgmL−1, Experiment 2, n=734, replications: 5). Multiple comparisons were implemented using Generalized Linear Models in the SAS 8.12 program. The rates of oocyte maturation to MII stage were higher (P&amp;lt;0.05) in follicular stage oocytes cultured with 2μgmL−1 E2 (17.9%) compared to other supplement groups (0 to 7.6%). No differences (P&amp;lt;0.05) in rate of MII stage oocytes among P4 supplement groups were observed. In Experiment 3, to investigate the combined effects of E2 and P4 on in vitro maturation, three groups of oocytes were cultured in TCM-199 supplemented with 2μgmL−1 E2 and various concentration of P4 (0, 0.5, 1.0 or 2.0μgmL−1, Experiment 3, n=1613, replications: 5). The rate of oocyte maturation to MII stage (11.5%) was higher (P&amp;lt;0.05) in follicular stage oocytes cultured with 2μgmL−1 E2+2.0μgmL−1 P4 supplement compared to other supplement groups (0 to 6.4%). In conclusion, the present study demonstrated that E2 supplement in the culture medium increased maturation of canine oocyte to MII stage and that supplement of P4 alone did not promote oocyte maturation. However, P4 supplemented with E2 further promoted oocyte maturation in the follicular stage compared to E2 supplement alone, indicating that P4 acts synergistically with E2 on canine oocyte maturation in the presence of E2. From our results, we conclude that canine oocytes are exposed to high levels of P4 during maturation due to the preovulatory luteinization of canine follicles which gives rise to high intrafollicular as well as intratubal P4 concentrations-this is very different from the situation in oocytes from other domestic animal species. This study was supported by Biogreen 21-10005200301.

316EFFECTS OF ESTRADIOL-17² AND PROGESTERONE SUPPLEMENT ON THE RESUMPTION OF MEIOSIS OF CANINE OOCYTES MATURED IN VITRO