Blastocyst freezing.

Blastocyst are preimplantation embryos that have successfully passed the critical step of genomic activation and have so a high developmental potential, thus allowing to reduce the number of embryos transferred. Blastocyst culture and freezing have really started in IVF programs in the early nineties, when it was possible to obtain good blastocysts (essentially using coculture) on a large scale. Blastocysts freezing has been performed since the beginning using glycerol as cryoprotectant in slow protocols. The transition from coculture to sequential media has been a little bit delicate with a drop in the results. Few modifications including a change in the freezing curves and thawing at 37 degrees C have allowed to reach the same results with the two culture techniques. Vitrification with ethylene glycol (EG) has been proposed recently. However, the toxicity of the metabolites of EG for rat embryos has to be considered before moving to this technology on a large scale.