Table 2.

Impact of oocyte vitrification on DNA methylation and histone modifications in animals

References	Animal model	Materials	Number of oocytes	Number of embryos	Technology of assessment	Studied sequences	Conclusions
Chen et al. [112]	Bovine	Fresh MII, vitrified MII or embryos (D2-D8) from fresh or vitrified MII after IVM	10–15 oocytes or embryos per group		Immunofluorescence	5mC, H3K9ac, H3K9me3	Decrease in the global DNA methylation and H3K9me3 levels and increase in H3K9ac for vitrified MII oocytes. No difference observed specifically in the ICM. Decrease in the level of DNA methylation and H3K9ac in trophectoderm after oocyte vitrification.
Cheng et al. [113]	Murine	Blastocysts (D4) from fresh or vitrified MII		30–45 blastocysts per condition	Bisulfite treatment + sequencing	H19, Peg3, Snrpn	No significant differences in oocytes. Decrease in DNA methylation levels for H19, Peg3, and Snrpn in blastocysts after oocyte vitrification.
Zhao et al. [114]	Murine	Fresh MII, vitrified MII	100 oocytes per group		Bisulfite treatment + sequencing	Dnmt1o, Hat1, promoteur de Hdac1	No significant differences.
Hu et al. [115]	Bovine	Fresh MII after IVM, vitrified MII after IVM	150 oocytes per group		Immunofluorescence	Global analysis	Decrease in methylation levels after oocytes slow freezing or after use of DMSO. Increase in methylation levels after using PROH.
Spinaci et al. [116]	Porcine	Fresh MII after IVM, vitrified MII after IVM	H4K5ac 282 fresh oocytes, 192 vitrified oocytes; H3K9me 98 fresh oocytes, 121 vitrified oocytes		Immunofluorescence	H3K9 methylation and H4K5 acetylation	Increase in H4ac level and significant modifications of H3K9me2 levels (decrease or increase) after oocyte vitrification.
Milroy et al. [117]	Murine	Fresh MII, fresh MII after IVM, MII after IVM from vitrified GV	200 oocytes per group		Bisulfite treatment + sequencing	Pluripotency promotors Oct4, Nanog, Foxd3, Sox2	Increase in the methylation levels of Oct4 (25%) and Sox2 (4.5%) promoters after vitrification of in vitro matured oocytes compared to in vivo matured and fresh MII.
Yan et al. [118]	Murine	Fresh MII, vitrified MII	66 fresh, 70 vitrified		Immunofluorescence	H3K9me, H4K5ac	Increase in the H3K9me and H4K5ac levels after oocyte vitrification.
Suo et al. [119]	Murine	Fresh MII, vitrified MII	At least 78 oocytes per group		Immunofluorescence	H4K12ac	Increase in the H4K12ac levels after oocyte vitrification. Zygotes from vitrified MII have disturbed levels before and after appearance of pronuclei.

D day of embryo culture, GV oocyte at germinal vesicle stage, ICM inner cell mass, IVM in vitro maturation, 5mC 5-methylCytosine, MII oocyte at metaphase II stage