Oxygen consumption of single bovine embryos probed by scanning electrochemical microscopy

Oxygen consumption of individual bovine embryos was noninvasively quantified by scanning electrochemical microscopy (SECM). A probe microelectrode was used to scan near a single embryo surface in a culture medium to monitor the oxygen reduction current at 37 °C, under a water-saturated atmosphere of 5% CO₂ and 95% air. The oxygen concentration profiles near the embryos were in good agreement with the theoretical spherical diffusion. When an embryo reached the stage of a morula with a 74-μm radius on day 6 after in vitro fertilization, the oxygen concentration difference (ΔC) between the bulk solution and the morula surface was 6.90 ± 1.35 μM. The oxygen consumption rate (F) of the single morula was estimated to be (1.40 ± 0.27) × 10^-14 mol s^-1. After the SECM measurement, the embryo was continuously cultured for another 2 days and grew to the stage of a blastocyst with a 100-μm radius. For the blastocyst, the ΔC values for the inner cell mass side and the trophoblast side were 16.40 ± 1.83 and 9.14 ± 1.68 μM, respectively. The oxygen consumption rate of the blastocyst was found to be in the range of (2.50 ± 0.46) × 10^-14 mol s^-1 > F > (4.49 ± 0.50) × 10^-14 mol s^-1. We have carried out SECM measurements for 19 embryos, and the results were compared in detail with these from an Optical microscopic observation. The ΔC values for the morulae on day 6 after in vitro fertilization were strongly related to the morphological embryo quality. The morulae showing a larger ΔC value developed into blastocysts of a larger size, and the ΔC value after the subsequent 2 days of cultivation was found to be increased.