Blastocysts are the top embryos, what have successfully passed the critical step of genomic activation and have a high developmental potential. Their advantage is containing numerous small cells; thus, the loss of some cells during freezing and thawing is probably less harmful for future development of the embryo. Furthermore, during extended cultivation, embryos with worse viability are arrested in development and will not be cryopreserved.
Blastocyst presents special challenge to cryopreservation. Excessive water in the blastocoel may lead to ice formation and subsequent damaging of cellular structures. To minimize this risk, removal of some of the blastocoel fluid has been attempted. Removal of blastocoel fluids can be done by perforating the blastocoel and letting the fluid flow passively out. The process called assisted shrinkage can be performed in a variety of ways, including microneedle puncture, repeated micropipetting of the blastocoel or laser-pulse opening of zona pellucida
Laser blastocoel puncture (assisted shrinkage): human expanded blastocyst before puncture; (a) laser pulse open zona pellucida (red circle) and make a small defect in the trophectoderm (b), which resulted to blastocyst shrinking (c). Scale bars represent 30 μm.