138 THE RELATIONSHIP BETWEEN THE NORMALITY OF FIRST CLEAVAGE, THE GENE EXPRESSION IN BLASTOMERES, AND THE ABILITY TO DEVELOP TO THE BLASTOCYST STAGE IN IVF-DERIVED BOVINE 2-CELL STAGE EMBRYOS

Abstract

Early first and second cleaved embryos after IVF associated with even blastomeres without fragments or protrusions were found to be a potent criterion for the selection of embryos with high developmental competence (Sugimura et al. 2012 PLOS ONE 7, e36627). The aim of this study was to examine the relationship between an early normal first cleavage pattern, the transcript abundance, and their development to the blastocyst stage in each blastomere in 2-cell stage bovine embryos. The IVF-derived bovine embryos were cultured individually in well-of-the-well culture dishes in CR1aa medium supplemented with 5% calf serum and 0.25 mg mL⁻¹ linoleic acid albumin at 38.5°C in 5% CO₂, 5% O₂, and 90% N₂. The first embryonic cleavage was categorized as being either normal (occurring within 28 h after IVF with 2 even blastomeres without fragment or protrusion) or abnormal (2 uneven blastomeres, with/without fragment/protrusion and/or later than 28 h after IVF). Then, cleaved embryos were placed in 0.5% actinase-E in Ca- and Mg-free PBS and blastomeres were separated by pipetting (n = 85; 4 replicates). In each embryo, one blastomere was subjected to quantitative RT-PCR to analyse the expression of developmentally important genes. The remaining blastomere was subsequently cultured in an individually identifiable manner to verify their ability to develop to the blastocyst stage. Primers were designed for 12 target genes related to pluripotency, cell cycle, metabolism, pregnancy reorganization, placentation, and fetal growth (OCT4, ATP1A1, CCNB1, CDH1, COX1, CTNNB1, GLUT8, MNSOD-3, SOX2, DYNLL1, IGFBP3, and PMSB1) and a reference gene (PPIA). Transcript abundance of target genes in individual blastomeres was compared between embryos showing normal and abnormal cleavage. Values were normalized to the average values of the reference genes and all the means were compared by the Student t-test. Blastomeres resulted from normal cleavage developed to the blastocyst stage on Day 7 to 8 (Day 0 = IVF) at significantly higher rates than those resulted from abnormal cleavage (65.7% v. 37.5%, respectively, P < 0.05). Transcript abundance of OCT4 was significantly higher in blastomeres associated with all abnormal cleavage than in those associated with normal cleavage (P < 0.05). The expression of CCNB1, COX1, ATP1A1, GLUT8, and PMSB1 in blastomeres associated with normal cleavage and blastocyst development was higher than that in those of abnormal cleavage (P < 0.05). However, the level of OCT4, CCNB1, COX1, ATP1A1, and PMSB1 was lower in blastomeres associated with normal cleavage but failure of blastocyst development than those in blastomeres showing abnormal cleavage (P < 0.05). Our results reveal that significantly higher expression of CCNB1, COX1, ATP1A1, and PMSB1 in blastomeres at the 2-cell stage in bovine embryos with superior developmental competence compared with those showing abnormal cleavage and low competence.

Research was supported by JSPS KAKENHI (26450388).