Bovine oocyte diameter in relation to developmental competence

This study was conducted to determine the diameter of bovine oocytes that were able to attain their full developmental competence to blastocysts. Oocytes were recovered by aspiration of surface-visible follicles (1 to 7 mm in diameter) from slaughterhouse ovaries. Only healthy-looking cumulus-oocyte complexes were used for in vitro maturation, and they were divided into six groups based on diameter: <110 mu m, 110 to <115 mu m, 115 to <120 mu m, 120 to <125 mu m, 125 to <130 mu m and greater than or equal to 130 mu m. Oocytes were processed through standard procedures for in vitro maturation, fertilization and culture. Following in vitro maturation or fertilization, some oocytes were stained to assess nuclear maturation and penetration rates. The numbers of embryos that cleaved at 42 h post insemination and developed to blastocysts and hatched blastocysts after 8 days of culture were recorded. The mean oocyte diameters were 114.0 +/- 4.8 mu m. The oocytes displayed size-related ability to undergo meiotic maturation. The rates of nuclear maturation of oocytes in the greater than 115-mu m size range were significantly higher than those of oocytes with diameters <115 mu m. In the <120 mu m diameter groups, the polyspermic fertilization rates of oocytes <115 mu m were significantly higher than those of oocytes 115 to <120 mu m in diameter. The rates of cleavage and development to blastocysts and hatched blastocysts rose as oocyte diameter increased. Among oocytes with a diameter >110 mu m, oocytes <120 mu m were found to have significantly lower developmental competence than oocytes 120 to <130 mu m in diameter. These results suggest that bovine oocytes have acquired full meiotic competence ata diameter of 115 mu m but not yet attained full developmental competence to blastocysts, and that oocytes have acquired full developmental competence at a diameter of 120 mu m. (C) 1997 by Elsevier Science Inc.