Vitrification and in-straw warming do not affect pregnancy rates of biopsied bovine embryos

In the cattle industry, in vivo or in vitro embryo production combined with genotyping and cryopreservation technologies allows the selection and conservation of embryos carrying genes for desirable traits. This study aimed to assess the efficiency of a vitrification method suitable for in-straw warming of biopsied in vivo derived (IVD) bovine embryos. Three experiments were carried out using two methodologies: the Cryotop®, the gold standard vitrification and 3-step warming methodology, or the VitTrans, a vitrification/in-straw 1-step warming method that enables direct embryo transfer to the uterus. In experiment 1, intact and biopsied in vitro produced (IVP) day 7 expanded blastocysts were vitrified using the Cryotop® and warmed in 1- or 3-steps. No differences in survival rates were recorded at 24 h after warming for intact or biopsied IVP blastocysts irrespective of the warming procedure. In experiment 2, the effect of the time from trophectoderm (TE) biopsy to vitrification/in-straw warming on post-warming survival rate was assessed. No significant differences in survival were observed when blastocysts were vitrified/in-straw warmed immediately after biopsy or after 3 h in culture when compared to intact blastocysts. In experiment 3, IVD embryos were vitrified 3 h after biopsy using the Cryotop® or the VitTrans method and pregnancy rates were assessed at day 60 after transfer. Fresh, biopsied embryos served as control. Similar pregnancy rates were observed when IVD biopsied embryos were transferred fresh or vitrified/warmed by the Cryotop® or VitTrans method. No significant effect of the embryo quality or developmental stage was detected on the percentage of pregnant recipients when IVD biopsied embryos were transferred fresh or after vitrification. While fresh female IVD embryos produced significantly higher pregnancy rates than male embryos, there were no differences in pregnancy rates when male or female vitrified/warmed embryos were transferred. About 81% from the biopsies analyzed successfully determined the embryo sex, confirming that DNA was there, and it was efficiently amplified. To conclude, our findings indicate that both vitrification methodologies produced similar post-warming outcomes for both intact and biopsied IVP embryos. Besides, vitrification/in-straw warming of biopsied IVD bovine embryos did not affect the viability to originate pregnancy, being a useful option for their direct transfer in field conditions.

Embryo biopsy and development: the known and the unknown

Preimplantation genetic diagnosis (PGD) has been introduced in clinical practice as a tool for selecting ‘healthy’ embryos before their transfer in utero. PGD protocols include biopsy of cleaving embryos (blastomere biopsy (BB)) or blastocysts (trophectoderm biopsy (TB)), followed by genetic analysis to select ‘healthy’ embryos for transfer in utero. Currently, TB is replacing the use of BB in the clinical practice. However, based on the European Society of Human Reproduction and Embryology Preimplantation Genetic Diagnosis Consortium reports, BB has been used in >87% of PGD cycles for more than 10 years. An exhaustive evaluation of embryo biopsy (both BB and TB) risks and safety is still missing. The few epidemiological studies available are quite controversial and/or are limited to normalcy at birth or early childhood. On the other hand, studies on animals have shown that BB can be a risk factor for impaired development, during both pre- and postnatal life, while little is known on TB. Thus, there is an urgent need of focused researches on BB, as it has contributed to give birth to children for more than 10 years, and on TB, as its application is significantly growing in clinical practice. In this context, the aim of this review is to provide a complete overview of the current knowledge on the short-, medium- and long-term effects of embryo biopsy in the mouse model.