Oxygen consumption and ROS production are increased at the time of fertilization and cell cleavage in bovine zygotes

Abstract

BACKGROUND Oxygen consumption is a key indicator of metabolic activity within embryos. Increased oxidative activity and REDOX changes at the time of fertilization have been suggested to signal Ca²⁺ oscillations after sperm penetration. The objective of the present study was to determine the oxygen consumption and the REDOX status of zygotes and early embryos at the time of sperm penetration and cell cleavage and to investigate how metabolism relates to key temporal events and developmental competence. METHODS Individual oxygen-consumption rates of bovine in vitro matured oocytes and presumptive zygotes (n = 101) were measured using the Nanorespirometer at 0, 7, 12, 17 and 24 h after IVF. Using the Embryoscope, oxygen-consumption profiles of individual oocytes and embryos (n = 75) were recorded repeatedly from 6 h until 30 h after IVF and time-lapse images were acquired, at intervals of ∼36 min. Oocytes and embryos were stained with Hoechst 33342 and visualization of nuclear stage was performed by fluorescence microscopy. To determine the REDOX status, cohorts of oocytes and zygotes (n = 55) were individually stained with REDOX-Sensor Red CC-1 and Hoechst 33342 at 0, 7, 12, 17 and 24 h after IVF and subsequently imaged by confocal microscopy. RESULTS A peak of oxygen consumption was observed at the time of fertilization and a smaller rise and fall in oxygen consumption could be detected prior to the first cell cleavage. Increased reactive oxygen species production was also observed at 7 h and then at 24 h after IVF, just preceding the first embryonic cleavage. CONCLUSIONS There are specific events during embryo development that appear to be associated with a change in oxygen consumption and REDOX state, indicating that both have a role in sperm-mediated oocyte activation and cell cleavage in bovine embryos.