Effect of follicular diameter, time of first cleavage and H3K4 methylation on embryo production rates of Bos indicus cattle

Abstract

This study aimed investigate the relationship between epigenetics, follicular diameter and cleavage speed, by evaluating the developmental potential and occurence of H3K4 monomethylation of early-, intermediate- and late-cleaving Bos indicus embryos from in vitro fertilized oocytes originating from follicles up to 2 mm in diameter or between 4 and 8 mm in diameter. Oocytes (n = 699) from small follicles (? 2 mm) and 639 oocytes from large follicles (4-8 mm) were punched from 1,982 Bos indicus’ slaughterhouse ovaries. After maturation and in vitro fertilization (IVF), the cultured embryos were separated into early (? 28 h post-IVF), intermediate (> 28 h and ? 34 h post-IVF) and late (> 34 h and ? 54 h post-IVF) cleavage groups. Blastocysts were subjected to an immunofluorescence assessment for H3K4me investigation. The blastocyst rate for large follicles (36.3%) was higher than that for small follicles (22.9%, P < 0.05). In addition, blastocyst rates for early and intermediate cleavage groups (45.3% and 33.8%, respectively) were higher than that for late cleavage group (13.5%, P < 0.05). The blastocysts from all groups displayed H3K4me staining by immunofluorescence, particularly intense in what seemed to be trophectoderm cells and weak or absent in cells seemingly from the inner cell mass. For the first time for indicus embryos, data from this study demonstrate that higher blastocyst embryo rates are obtained from embryos that cleave within 34 h after fertilization and from those produced from follicles of 4-8 mm in diameter, indicating a greater ability of these embryos to develop to the stage of embryonic preimplantation. This is the first article demonstrating the occurrence of H3K4me in cattle embryos; its presence in all the evaluated blastocysts suggests that this histone modification plays a key role in maintaining embryo viability at preimplantation stage.