Canine oocyte diameter in relation to meiotic competence and sperm penetration

This study was conducted to determine the diameter of canine oocytes that a re able to attain full meiotic competence and sperm penetration. Oocytes we re collected from ovaries of bitches at various stages of the estrous cycle . Only healthy-looking cumulus-oocyte complexes were used for in vitro matu ration, and were divided into four groups based on diameter: <100, 100 to < 110, 110 to <120 and <greater than or equal to>120 mum. Following in vitro maturation or fertilization, oocytes were stained to assess nuclear maturat ion and penetration rates. The mean oocyte diameter was 108.5 +/- 0.4 mum. The oocytes displayed size-related ability to undergo meiotic maturation. A fter culture for 72 h, the rates of oocytes that remained at the germinal v esicle stage in the <110 <mu>m groups were significantly higher (P<0.01) th an in the <greater than or equal to>110 mum groups. None of the oocytes <11 0 <mu>m reached metaphase II (MII), but 4.9 and 21.5% of the oocytes that w ere greater than 110 and 120 mum, respectively, progressed to MII. After in vitro fertilization for 20 h, 10 to 25% of oocytes were penetrated by sper matozoa, but there were no clear relationships between oocyte diameter and penetration rates of the oocyte by sperm. In the <120 <mu>m groups, sperm p enetration was mostly found in oocytes arrested at the germinal vesicle sta ge. However, a total of eight oocytes greater than or equal to 120 mum in d iameter were penetrated by spermatozoa, of which five oocytes reached MII. These results suggest that there is a clear relationship between oocyte dia meter and meiotic competence, but no relationship between oocyte diameter a nd sperm penetration. Canine oocytes may have acquired meiotic competence o nce they reach at a diameter of 120 mum, but the oocytes may allow the entr y of spermatozoa into the ooplasm irrespective of oocyte diameter. (C) 2000 by Elsevier Science Inc.