The difference between freezing eggs vs. freezing embryos is that, after ovarian hyperstimulation and eggs retrieval, the eggs are fertilized using IVF before they’re frozen, and develop, over a period of several days, into embryos, which are then flash frozen. The possibility of freezing embryos has been proposed for nearly 40 years. Embryos stored in liquid nitrogen (-196 ° C) have been shown to have evolutionary potential.
The embryo, at this stage, consists of more than 90% water. It is therefore necessary to avoid the formation of ice crystals which by volume increase tear the cell membranes during freezing. So water is eliminated by replacing it with a cryoprotectant, a real "antifreeze" cell. The embryo is immersed in successive baths containing the cryoprotectant and sucrose at different concentrations. Sucrose makes the medium hypertonic with respect to the embryo, resulting in the release of water and the return of the cryoprotectant. The embryo is placed in a plastic straw, part of the straw contains all the indications necessary for a good identification.
Here, the principle of supernumerary embryos also comes into consideration. A supernumerary embryo is an embryo conceived during in vitro fertilization in the context of medically assisted procreation (MAP). These embryos can be frozen at the parents' written request, in order to be implanted later in the womb of the mother. If the number of embryos obtained is greater than the number of embryos transferred, supernumerary embryos may only be frozen provided they are of very good quality. The freezing is carried out, at the 4-cell stage, in liquid nitrogen at -196 ° C.
The existence of supernumerary embryos is explained by the fact that the fertilization technique has a random nature: therefore, several fertilization trials are carried out simultaneously. Among successful fertilizations, two to three embryos are usually chosen to grow and give a child. Other embryos can be kept for parents to have a new child later. It is when they give up the project that the remaining embryos become "supernumerary embryos".
