Amelioration of mitochondrial dysfunction and apoptosis of two-cell mouse embryos after freezing and thawing by the high frequency liquid nitrogen infusion.

Liquid nitrogen (LN2) infusions are currently used in a slow controlled-rate freezing during cryopreservation. The effects of two different LN2 infusion frequencies (conventional, slow 50 infusions/min and high 120 infusions/min) were studied with frozen-thawed two-cell mouse embryos and their subsequent development to blastocysts. The embryos that were subjected to the high frequency LN2 infusion (HFLI) showed a significantly higher survival rate over the low frequency LN2 infusion (LFLI) (50.7 vs. 34.6%, P < 0.05). The blastocyst formation was also higher in HFLI (76.7%) than LFLI (44.0%, P < 0.05) with respective to the number of cells in a blastocyst of 71.6 8.0 (n = 20) and 62.5 +/- 4.7 (n = 20) (P < 0.05). The relative amount of H2O2 in an embryo that was assessed by a fluorescence intensity of 2',7'-dichlorofluorecein (DCF) showed a difference between the procedures with 16.6 +/- 1.6 (n = 21) and 23.4 +/- 1.8 (n = 24) for HFLI and LFLI, respectively (P < 0.05). Mitochondrial staining by Rhodamine 123 showed that the number and distribution of viable mitochondria were similar in both procedures, but fewer mitochondria were observed with a marked aggregation in the arrested embryos, indicating a mitochondrial disintegration. The mitochondrial membrane potential was visualized by a membrane potential-sensitive fluorescent probe, 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolcarbocyanine iodide (JC-1). There was a decrease in the number of mitochondria that had a high membrane potential, and they showed a peripheral redistribution along the cell membrane in LFLI. A fluorescent staining of the actin filaments revealed a discontinuity that was noticeably at the peripheral "actin band" in LFLI. The DNA fragmentation was assessed by the dUTP nick end-labeling (TUNEL). The results showed a higher DNA fragmentation of blastocyst nuclei in LFLI compared to HFLI (65.6 vs. 36.0%, P < 0.05). Based on these observations, it was concluded that HFLI was better than LFLI in the case of freezing the mouse 2-cell embryos for preserving cytoskeletons and mitochondrial integrities. This could subsequently lead to a higher survival and developmental rate of the cryopreserved mouse embryos.

Characteristics of the cell membrane fluidity, actin fibers, and mitochondrial dysfunctions of frozen-thawed two-cell mouse embryos.

Physical and chemical alterations caused by the freezing and thawing and their effects on survivals/developments in vitro were investigated. Of a total of 452 two-cell mouse embryos, the overall survival rate of the frozen-thawed embryos was 76.1% (344/452). The blastocyst formation of the frozen-thawed embryos was 32.6% (44/136) compared to 74.5% (117/157) in the fresh embryos (P<0.05). The total number of cells in a blastocyst also decreased from 96.0 +/- 19.0 (n=26) in the fresh embryos to 42.0 +/- 11 .34 (n=30) in the frozen-thawed embryos (P<0.05). Fluorescence recovery after photobleaching (FRAP) measurement revealed about 5-fold decrease in the cell membrane fluidity with a characteristic time constant (tau) of 1.46 +/- 0.13 sec (n=5) in the frozen-thawed embryos as opposed to 0.28 +/- 0.04 sec (n=5) in the fresh embryos (P<0.05). The relative amount of H(2)O(2) in an embryo as quantified by the fluorescence intensity of 2',7'-dichlorofluorescein (DCF) showed 62.8 +/- 23.5 (n=24) and 34.2 +/- 14.5 (n=20) in the frozen-thawed embryos and in the fresh embryos, respectively (P<0.05). The distribution of actin filaments in the frozen-thawed embryos revealed an uneven distribution, particularly discontinuities at the "actin band," which contrasted to an even distribution shown in the fresh embryos. Mitochondrial staining by Rhodamine 123 showed that there was no significant difference between the two treatments in the number and in the distribution of viable mitochondria, but a marked aggregation was seen in the arrested embryos. No Annexin V binding was detected in two-cell or four-cell embryos while the binding was positive in the arrested embryos. The mitochondrial membrane potential measured by a membrane potential-sensitive fluorescent probe 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazol- carbocyanine iodide (JC-1) revealed a marked depolarization in the frozen-thawed embryos. Finally, terminal deoxynucleotidyl transferase (TdT)-mediated dUTP-digoxigenin nick end-labeling (TUNEL) was employed to quantify the DNA fragmentation. In 75.0% cells of blastocysts (n=24) in the frozen-thawed embryos, the DNA fragmentation was detected as opposed to 37.0% in the fresh embryos (n=20) (P<0.05). Taken together, it is proposed that during the cryopreservation, two-cell mouse embryos are subjected to physical and chemical alterations, including destruction of the cell membrane integrity, redistribution of actin fibers, mitochondrial depolarizations, and increased reactive oxygen species (ROS) productions, which then may trigger the apoptotic cascade leading to a decrease in the survival rate and in the developmental rate of the embryos.