TIMING OF MEIOTIC PROGRESSION IN BOVINE OOCYTES AND ITS EFFECT ON EARLY EMBRYO DEVELOPMENT

This study was designed to investigate the effect of the kinetics of n uclear maturation in bovine oocytes on early embryo development and to examine whether the time of insemination of mature oocytes affects th e oocytes' ability to support events of early embryo development, The time required for completion of nuclear maturation was influenced by g onadotropins used to supplement the maturation medium. Luteinizing hor mone (LH) enhanced the speed of nuclear maturation when compared to fo llicle-stimulating hormone (FSH). Oocytes completing their nuclear mat uration early (by 16 hours after the initiation of culture) were more likely to complete the first embryonic cell cycle (78% in LH vs. 43% i n FSH) and develop to the blastocyst stage (47% in LH vs. 34% in FSH). As the age of the oocytes at the time of MII arrest increased (extrus ion of the polar body by 20 or 24 hours), a decrease in their ability to cleave and develop to the blastocyst stage was observed. Difference s in the oocyte's ability to decondense chromatin and form pronuclei w ere also observed. Early maturing oocytes started forming pronuclei ea rlier than their later maturing counterparts. The time of insemination of mature oocytes played an equally important role. Generally, when i nsemination of mature oocytes was delayed for 8 hours, higher proporti ons of fertilized oocytes developed to advanced preimplantation stages than did the oocytes inseminated immediately after metaphase II arres t. (C) 1997 Wiley-Liss, Inc.