223 RELATIONSHIP BETWEEN THE LENGTH OF CELL CYCLES, CLEAVAGE PATTERN, AND DEVELOPMENTAL COMPETENCE DURING IN VITRO CULTURE OF IN VITRO-MATURED/IN VITRO-FERTILIZED BOVINE OOCYTES

Abstract

In in vitro embryo production systems, there is a need to select embryos with good developmental competence at the early stages. This study was conducted to determine whether there was any relationship between the duration of the first 3 cell cycles, the cleavage pattern of the first cell division, and the developmental competence of embryos during in vitro culture. A total of 320 in vitro-matured and in vitro-fertilized bovine oocytes were cultured in microdrops of CR1aa medium supplemented with 5% calf serum covered by mineral oil in 5% CO₂ and 20% O₂ at 38.5°C. The kinetics of embryo development were measured by time-lapse cinematography. Embryos were classified according to their cleavage pattern at the first cell division. Of 285 cleaved embryos, 119 had 2 blastomeres of the same size (normal cleavage: NC), 49 had 2 blastomeres with multiple small fragments (multiple fragments: MF), 34 had 2 blastomeres and a protrusion (protrusion: PT), 45 showed direct cleavage from 1 cell to 3 or 4 blastomeres (3–4BL), and 60 oocytes cleaved to 2 blastomeres of different sizes (unequal blastomeres: UB). (Twenty-two embryos belonged to 2 classes.) After 175 h of culture, blastocysts were either subjected to differential inner cell mass/trophectoderm (ICM/TE) staining or karyotyped. The first and second cell cycles (mean ± SEM) of viable embryos (that could develop to the blastocyst stage) were significantly shorter than those of nonviable embryos (24.9 ± 0.3 h and 8.7 ± 0.1 h v. 26.6 ± 0.7 h and 10.0 ± 0.1 h, respectively); however, the length of the third cell cycle did not differ (P < 0.05, paired t-test). The duration of 1 cell stage in the NC group was significantly shorter than that of MF, PT, 3–4BL, and UB groups (24.7 ± 0.4 h, 26.6 ± 0.5 h, 26.3 ± 0.6 h, 26.0 ± 0.2 h, and 27.7 ± 0.9 h, respectively). The length of the second and third cell cycles did not differ among the groups. The percentage of NC embryos that developed to the blastocyst stage was similar to that of the 3–4BL group (66.9 and 56.7%, respectively) but was significantly higher than those of the MF, PT, and UB groups (40.5, 26.5, and 35.6%, respectively; P < 0.05, ANOVA). The mean cell numbers of NC blastocysts did not differ from those of the MF, 3–4BL, and UB groups but were higher than those of PT embryos (147.1, 155.6, 121.6, 146.4, and 115.1, respectively). There was no difference in ICM/TE rates between the groups. Unlike NC, MF, PT, and UB embryos, most (6 of 8 karyotyped) 3–4BL blastocysts had abnormal ploidy, such as haploid, triploid, mixoploid, or chaotic chromosome numbers, in blastomeres. Our results revealed that not only the length of the first cell cycles, but also the cleavage pattern during first cell division can be a marker of developmental competence and should be considered for the selection of good-quality embryos for embryo transfer.

This work was supported by the Research and Development Program for New Bio-industry Initiatives.