Increased apoptosis in late-developing in vitro fertilized bovine blastocysts decreases successful pregnancy

Abstract

Objective: Pregnancy in cattle after embryo transfer (ET) is influenced by several factors, including embryo quality. Therefore, preparing high-quality embryos with the greatest developmental potential is essential for achieving a successful pregnancy after ET. Meanwhile, blastocysts produced by <i>in vitro</i> fertilization (IVF) procedure have different developmental speed during <i>in vitro</i> culture (IVC) and they exhibited different competence in the establishment of pregnancy.Methods: This study aimed to identify the comparative features of early-, mid-, and late-developing bovine IVF blastocysts, when they first appeared at Day 7, 8, and 9 during IVC, respectively. In addition, the correlations between their molecular features and pregnancy ability were analyzed.Results: The results showed no difference in the morphological characteristics, including total cell count and diameter, between the Day 7, 8, and 9 blastocysts. However, the pregnancy rate post-ET was significantly different between the groups at 51.7%, 36.7%, and 17.8% for Day 7, 8, and 9 blastocysts, respectively. During early embryo development, late-developing blastocysts demonstrated a reduced cell count in the inner cell mass and decreased expression of the early embryo developmental genes (<i>Oct4</i> and <i>Sox2</i>) compared with the early- and mid-developing blastocysts. In addition, the number of apoptotic cells and apoptosis-related gene expression (increased <i>Bax</i> and decreased <i>Bcl2</i>) gradually elevated from the Day 7 to Day 9 blastocysts. However, there was no difference in mitochondrial activity and mitochondria-relevant gene expression (<i>Tfam</i> and <i>Cox1</i>) between the groups. Correlation analysis identified a significantly negative correlation between the pregnancy rate and the blastocysts’ degree of apoptosis, indicating that the low pregnancy ability of late-developing blastocysts was mainly caused by increased apoptosis.Conclusion: This study’s results may contribute to the field of animal biotechnology by assisting in establishing an improved strategy for bovine ET with IVF embryos.