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The present study was performed to assess the survival of immature denuded or cumulus-covered porcine oocytes (COCs). Immature porcine oocytes were collected from 2–6 mm follicles of slaughterhouse ovaries and subjected to open pulled straw (OPS) vitrification, according to the method of Vajta et al. (1998 Mol Reprod. Dev. 51, 53–58). After vitrification, oocytes were matured in vitro for 48 h at 39°C, 5% CO2 in air. The maturation medium was TCM199 supplemented with 10% pig follicular fluid, 1.25 mM L-glutamine, 0.9 mM Na pyruvate, 150 μM cysteamine, 0.1 mg/mL streptomycin sulfate, 100 IU/mL PG penicillin g potassium, 10 IU/mL PMSG, and 25 IU/mL hCG. After IVM, to assess nuclear stage, all oocytes were fixed with acetic acid–alcohol (1:3) for at least three days and then stained with 0.1% orcein and examined under a phase-contrast microscope at 100× magnification. All data were analyzed by χ2 test (P &lt; 0.05). Immediately after collection, all oocytes were at the germinal vesicle (GV) stage with an intact GV membrane. After vitrification, significantly fewer oocytes had normal morphology (intact plasma membrane) in the denuded and COC groups (4.7% and 8.5%, respectively) than did the denuded and COC control groups (95% and 92%, respectively). By the end of IVM, significantly fewer oocytes were surrounded by expanded cumulus after vitrification of COCs than were the COC controls (28.1% and 63.5%, respectively). After IVM, more of the COC control oocytes underwent germinal vesicle breakdown than did the denuded controls (95% and 78.2%, respectively); the rate of MII oocytes was higher for the COC controls than for the denuded controls (80% and 54.5%, respectively). After vitrification, the number of oocytes that underwent GVBD was significantly less for both the denuded and the COC groups (2.0% and 7.0%, respectively); the percentage of oocytes that reached MII was also lower (0.64% and 2.78%, respectively). Most of the vitrified oocytes had a damaged GV with disrupted membrane and cluster-like or scattered chromatin in both the denuded and the COC groups (96.4% and 90.7%, respectively). These data suggest that vitrification of cumulus-enclosed immature porcine oocytes is preferable compared to vitrification of denuded ones. Loss of cumulus cells compromises competence of oocytes to resume meiosis, which might result in a lower maturation rate after IVM.This research was supported by the grants of the Hungarian Scientific Research Fund (T 031758), the Hungarian National Committee of the Technical Development at the Ministry of Education (00796/2003), and the Ministry of Education (OM-KMUFA; BIO-00086/2002).

78 SURVIVAL OF PORCINE OOCYTES AT GERMINAL VESICLE STAGE AFTER VITRIFICATION WITH OPEN PULLED STRAW METHOD