What is the most idoneous developmental stage for embryo-freezing?

Mammalian preimplantation embryos can be cryopreserved by a variety of methods involving different cryoprotectants. In the present study, we ultrarapidly froze 299 excess embryos at various early cleavage stages (zygotes, 2-cell embryos, 4-cell embryos and > 5-cell ones) after a brief exposure to high concentrations of dimethyl sulfoxide (DMSO; 3.5 M) and 0.25M sucrose. One-hundred seventeen of them were thawed in a warm water bath. Thirtyone embryos were completely destroyed after thawing but only 6% of them were originally at the pronuclear stage. Regarding the morphological aspects, 94% of pronucleate embryos were intact whereas just 12% of > 5-cell embryos showed more than 50% intact blastomeres after thawing. We froze also 60 embryos at the blastocyst stage by a slow freezing procedure. Eighteen blastocysts were thawed and 12 of them fully re-expanded were transferred in four patients.