<P> The egg retrieval process for oocyte cryopreservation is the same as that for in vitro fertilization . This includes one to several weeks of hormone injections that stimulate ovaries to ripen multiple eggs . When the eggs are mature, final maturation induction is performed, preferably by using a GnRH agonist rather than human chorionic gonadotrophin (hCG), since it decreases the risk of ovarian hyperstimulation syndrome with no evidence of a difference in live birth rate (in contrast to fresh cycles where usage of GnRH agonist has a lower live birth rate). The eggs are subsequently removed from the body by transvaginal oocyte retrieval . The procedure is usually conducted under sedation . The eggs are immediately frozen . </P> <P> The egg is the largest cell in the human body and contains a high amount of water . When the egg is frozen, the ice crystals that form can destroy the integrity of the cell . To prevent this, the egg must be dehydrated prior to freezing . This is done using cryoprotectants which replace most of the water within the cell and inhibit the formation of ice crystals . </P> <P> Eggs (oocytes) are frozen using either a controlled - rate, slow - cooling method or a newer flash - freezing process known as vitrification . Vitrification is much faster but requires higher concentrations of cryoprotectants to be added . The result of vitrification is a solid glass - like cell, free of ice crystals . Indeed, freezing is a phase transition . Vitrification, as opposed to freezing, is a physical transition . Realizing this fundamental difference, vitrification concept has been developed and successfully applied in IVF treatment with the first life birth following vitrification of oocytes achieved in 1999 . Vitrification eliminates ice formation inside and outside of oocytes on cooling, during cryostorage and on warming . Vitrification is associated with higher survival rates and better development compared to slow - cooling when applied to oocytes in metaphase II (MII). Vitrification has also become the method of choice for pronuclear oocytes, although prospective randomized controlled trials are still lacking . </P> <P> During the freezing process, the zona pellucida, or shell of the egg can be modified preventing fertilization . Thus, currently, when eggs are thawed, a special fertilization procedure is performed by an embryologist whereby sperm is injected directly into the egg with a needle rather than allowing sperm to penetrate naturally by placing it around the egg in a dish . This injection technique is called ICSI (Intracytoplasmic Sperm Injection) and is also used in IVF . </P>

Where are the eggs stored in a female