Ultra-Rapid Freezing and Rapid Freezing Methods in Clinical ICSI Program: Effects on Sperm Biological Characteristics, DNA Methylation Stability, DNA Methyltransferase Activity, and Embryo Morphokinetics

Abstract

The purpose of this study was to evaluate the differences in sperm function and embryo morphokinetics following sperm cryopreservation by ultra-rapid freezing or rapid freezing methods compared to fresh spermatozoa. Thirty samples of normozoospermia were divided equally into fresh, ultra-rapid freezing, and rapid freezing groups. In the rapid freezing, sperm suspension was placed horizontally on nitrogen vapor. In the ultra-rapid freezing, sperm suspension with a straw-in-straw system was directly immersed in liquid nitrogen. Sperm function was assessed in terms of motility, morphology, viability, mitochondrial membrane potential, sperm DNA fragmentation, and acrosome reaction status. Also, the effects of two cryopreservation methods were assessed on global DNA methylation and DNA methyltransferase activity. Moreover, 730 embryos in three groups were cultured using time-lapse imaging until day 6 for embryo morphokinetics. Progressive motility (38.80 ± 4.21 vs. 34.86 ± 4.19; $p<0.001$) and viability (64.30 ± 6.24 vs. 58.10 ± 8.69; $p<0.01$) in ultra-rapid freezing were significantly higher than rapid group. DNA fragmentation and acrosome reaction were significantly increased in both cryopreserved groups ($p<0.001$). However, DNA fragmentation (16.30 ± 1.14 vs. 14.33 ± 2.94; $p<0.01$) was significantly higher in the rapid than the ultra-rapid freezing group. No significant differences were noted in global DNA methylation ($p>0.05$) and DNA methyltransferases activity ($p>0.05$) in fresh compared to cryopreservation groups. The kinetic times, including tPB2, tPNa, tPNf, t2, t3, t4, t5, t6, t7, t8, and tM, showed a significant delay in cell divisions in both cryopreservation groups. Furthermore, tPNa, tPNf, and t8 occurred with a significantly higher delay in embryos fertilized by sperm from the rapid freezing compared to the ultra-rapid freezing group. In addition, blastocysts formation was similar in both cryopreservation groups. Ultra-rapid freezing preserved the sperm biological integrity and lead to better embryo morphokinetics compared to the rapid freezing method. However, both methods of sperm cryopreservation were epigenetically safe.