BOVINE OOCYTE DIAMETER IN RELATION TO DEVELOPMENTAL COMPETENCE

This study was conducted to determine the diameter of bovine oocytes t hat were able to attain their full developmental competence to blastoc ysts. Oocytes were recovered by aspiration of surface-visible follicle s (1 to 7 mm in diameter) from slaughterhouse ovaries. Only healthy-lo oking 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 g reater than or equal to 130 mu m. Oocytes were processed through stand ard procedures for in vitro maturation, fertilization and culture. Fol lowing in vitro maturation or fertilization, some oocytes were stained to assess nuclear maturation and penetration rates. The numbers of em bryos that cleaved at 42 h post insemination and developed to blastocy sts 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 nucle ar maturation of oocytes in the greater than 115-mu m size range were significantly higher than those of oocytes with diameters <115 mu m. I n the <120 mu m diameter groups, the polyspermic fertilization rates o f oocytes <115 mu m were significantly higher than those of oocytes 11 5 to <120 mu m in diameter. The rates of cleavage and development to b lastocysts 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 ha ve acquired full meiotic competence ata diameter of 115 mu m but not y et attained full developmental competence to blastocysts, and that ooc ytes have acquired full developmental competence at a diameter of 120 mu m. (C) 1997 by Elsevier Science Inc.