Egg Freeze (human oocyte cryopreservation)
Oocyte cryopreservation, also known as egg freezing, is now a reality in the specialty of reproductive endocrinology and infertility. It has opened many new and exciting avenues of treatment options for assisting the infertile population but also has new applications for women wishing to preserve their fertility for medical indications or personal reasons.
The Egg Freezing Process
Progress in egg freezing remained stagnant because early trials of egg freezing used suboptimal freeze/thaw protocols, which did not address the structural differences between oocytes and embryos. The intent of most freezing protocols is to achieve simultaneous exchange of intracellular water with protective solutions in a limited amount of time, under special lab conditions, without causing injury to the target tissue. Since the human egg is the largest cell in the body and is composed mostly of water, the formation of ice during freezing can disrupt the egg’s internal microscopic contents and its outer membrane. Therefore cryoprotectants, which act as “antifreeze solution" are used to dehydrate the water and replace it with the protective solution to reduce internal ice formation.
It is a known fact that osmosis causes an exchange of the egg’s inner cell water with cryoprotectant fluids by diffusion across the egg membrane. To facilitate the transference of the fluids, a delicate balance of the solution’s concentration, the time exposure to the solutions and the temperature of the culture system are needed to achieve optimal levels of water replacement. However, the dilemma is that these cryoprotectants tend to be cytotoxic and as such they may damage the egg if exposure is too long at “high” temperatures (4 to 37 degrees Celcius). So the science of egg freezing (cryobiology) is all about compromise: how to remove water as quickly as possible, without damaging the egg.