293 TWO-STEP MATURATION OF BOVINE OOCYTES WITHOUT CDK INHIBITORS: AN ALTERNATIVE TO AFFECT THEIR SUBSEQUENT DEVELOPMENTAL COMPETENCE

Abstract

The acquisition of meiotic and developmental competence seems to correlate not only with the size of follicles and oocytes but also with the morphology and transcriptional activity of the oocyte nuclei and nucleoli. To secure or increase the fertilization and the developmental competence of bovine oocytes, we have developed a two-step culture system using the specific cyclin dependent kinase inhibitors (Butyrolactone I, Bohemine). However, these drugs have several side effects during the prolonged time of culture. To avoid this disadvantage, we have used in the present experiments modified culture conditions simulating the intrafollicular block of meiosis. In the first step of culture, bovine oocytes isolated from small, medium, and large follicles (2–3, 3–4, and 4–6 mm in diameter, respectively) were kept under conditions that secured for at least 48 h the intact germinal vesicle stage (GV) in more than 90% of oocytes. The second step represented the subsequent 20–22 h in conditions stimulating resumption of meiosis. The effectiveness of this model depended mainly on medium composition: reduced NaHCO₃, substitution of serum with serum albumin, addition of antioxidants (curcumin), increased viscosity of a medium by agar (0.3%), and reduction of oxygen concentration (within 6–9%). The reduction of the proportion between the number of cumulus-oocyte complexes (COC) and the amount of medium (within 6–7 mL per COC) should amplify the GVBD-inhibiting effect of oocyte-surrounding granulosa cells. The COC were situated in clots of 6–7 COC per clot. The effectiveness and reversibility of GVBD inhibition depends also on the duration of COC isolation. The full reversibility of GVBD inhibition was controlled morphologically and also by measuring histone H1 and MAP kinase activities. The two-step versus one-step (24 h) maturation technique was evaluated by the percentage of total and hatched Day 9 blastocysts. When compared with one-step maturation, the two-step culture showed a slightly increased proportion of total and hatched blastocysts developed from the smallest follicular category (13.9 vs. 7.1% and 9.2 vs. 3.3% for total and hatched blastocysts, respectively). No significant difference was noticed between between one- and two-step culture when oocytes from large healthy follicles were used. However, the two-step maturation of oocytes from regressing follicles substantially reduced the blastocyst yield (9.7 vs. 39.1% and 4.9 vs. 26.7% for total and hatched blastocysts, respectively).

This study was supported by grant of GA CR No. 524/02/0674.