Changes in histone methylation during human oocyte maturation and IVF- or ICSI-derived embryo development

Objective: To characterize the histone methylation pattern during human oocyte maturation and embryo development after conventional IVF and ICSI.

Design: Experimental study.

Setting: Reproductive center of hospital.

Patient(s): Women underwent IVF or ICSI.

Intervention(s): Immature and mature oocytes were collected from patients undergoing ICSI. Tripronuclear and normally fertilized embryos were obtained from patients undergoing IVF or ICSI.

Main outcome measure(s): The distribution patterns of dimethylated histone H3 lysine 9 (H3K9) and histone H4 arginine 3 (H4R3) in oocytes and embryos were observed by indirect immunofluorescent staining and scanning confocal microscopy.

Result(s): H3K9 and H4R3 were dimethylated throughout the meiotic maturation of human oocytes and the tripronuclear embryo development from two-cell to blastocyst stage. However, at the pronuclear stage, approximately half of the tripronuclear IVF zygotes displayed strong staining of MeH3K9 in one pronucleus, whereas MeH4R3 staining was always uniform in all three pronuclei. In the other half of the tripronuclear zygotes, all three pronuclei were strongly stained with MeH3K9 in some cases, and the remaining zygotes were completely unstained. Moreover, with progressively increasing fragmentation of blastomeres in ICSI-derived 2PN embryos with low morphological grade, H3K9 dimethylation decreased when compared with that of IVF embryos.

Conclusion(s): Asymmetric distribution of the dimethylation form of H3K9 exists in human zygote pronuclei. The ICSI-derived embryos with low morphological grade are more likely to display H3K9 demethylation than their IVF counterparts.