Growth hormone exerts no effect on the timing of the first zygotic cleavage in cattle

Most of the protocols used for oocyte and embryo quality assessments are invasive and thus reduce embryo viability. Special interest has recently been placed on a search for non-invasive markers related to embryo quality. The characteristics of a non-invasive marker are met by the timing of the first zygotic cleavage (FZC). Therefore, the aim of the present study was to investigate whether growth hormone added to IVM medium influences the timing of the FZC in cattle and also the quality of resulting blastocysts. The novelty of this manuscript concerns two findings: 1) no effect of GH supplementation to IVM medium on the timing of the first zygotic cleavage in cattle, and 2) differences in the relative transcript abundance in bovine day 7.5 blastocysts derived from early and late cleaved zygotes. Cumulus-oocyte complexes aspirated from slaughterhouse ovaries were matured in TCM199 medium supplemented with growth hormone (GH+ 100 ng/ml, GH- control), inseminated, and cultured in sequential media for 7.5 d. Embryo selection was done at 30 hpi (early cleavers EC) and 48 hpi (non-early cleavers NEC). The blastocyst quality was assessed by the total and ICM:TE cell counts, dead cell index, and relative transcript abundance (RA) of five genes affecting embryo development (p66(shc), bax, bcl-2, survivin, Hsp 70.1). The results of this study showed that although GH added to the IVM medium significantly improved the quality of blastocysts on day 7.5 pi, it had no effect on the timing of the first zygotic cleavage expressed by the rate of EC zygotes. The quality of the four categories of blastocysts investigated in this study can be ranked as follows: GH+ EC, followed by GH+ NEC, and further by the GH- EC and GH- NEC embryos. It has to be mentioned, however that the quality of blastocysts derived from the NEC zygotes was significantly improved by the GH supplementation. This is particularly relevant, since those blastocysts were very few and usually characterized by an impaired morphology (e.g., presence of fragmented blastomeres, smaller size).