How Do You Like Your Eggs in the Morning? Frozen and Dipped in Liquid Nitrogen

B0002100 Human egg with coronal cells - goldIf you’ve been watching the news or reading the paper recently, you’ll probably have heard about Apple and Facebook’s announcement that they will fund egg freezing procedures for their female employees; what you may not know is how this relatively unheard-of technique is carried out, how effective it is, or the reasons women opt to undergo such an expensive and novel procedure in order to delay motherhood.

Egg freezing has been used since the 1980s to help women have children after becoming infertile due to chemotherapy, but only within the last five years or so has it been used for non-medical purposes, to allow women to delay motherhood for personal reasons. ‘Social’ egg freezing has recently been declared no longer an experimental procedure in the USA, with the UK likely to follow, due to the development of better freezing and storing techniques and higher conception and birth rates from frozen eggs. To date, over 1500 children have been born as a result of egg freezing.

There are various non-medical reasons that women choose to have their eggs frozen to delay motherhood: not having found the right partner, focussing on their career, and financial instability for example. Fertility starts to decline sharply after the age of 35, so egg freezing allows women to spend their twenties and early thirties focussing on work and dating without worrying about the decline in the quality of their eggs making it difficult or impossible to have a baby. Most women who have their eggs frozen are aged 30-35, single, well-educated professionals.

In the early days of egg freezing, the slow cooling method was used to freeze the extracted eggs. This method was not ideal however, as it allowed ice crystals to form in the egg, which resulted in cell death. Cryoprotective additives (CPA) were added to the freezing solution and dehydrated the cell to reduce ice crystal formation. Two types of (CPA) were used: intracellular and extracellular CPA. Intracellular CPA permeated the cell membrane and replaced the water in the cell so it couldn’t freeze, extracellular CPA caused water to leave the cell by increasing the osmolarity outside the cell. Dehydration carried on until the cells reached -30°C. After dehydration, eggs were cooled to -30 – 40°C at a rate of around 0.3°C/min, then to -80 – 150°C at a rate of 50°C/min. The eggs were then plunged into liquid nitrogen. When the eggs were needed, they were thawed in warm water and rehydrated ready for use.

In the past 10 years or so, a new technique call vitrification has been used instead. Vitrification (a process that solidifies a solution by increasing the viscosity) stops ice crystals from forming by completely dehydrating the cells with CPA prior to rapid cooling. During vitrification, the cells are cooled at a rate of 10°C/min or higher – much faster than the old method. The thawing process is similar to the one used in slow cooling. The drawback of vitrification is that it is much more difficult to perform than slow cooling. The eggs can only be handled for around a minute at most, and the solutions used in the process are very viscous and therefore difficult to handle. Slow cooling on the other hand is a longer process in which the eggs can be handled for longer.

While still a relatively new technique, frozen eggs have been shown to result in good rates of fertilisation and birth. Studies have reported frozen eggs to have a fertilisation rate of 76%, compared to 83% using fresh eggs. Implantation rates are also similar: 7% for frozen eggs and 11% for fresh eggs. Embryos that form from frozen eggs seem to develop more slowly. On Day 2, 64% of embryos formed from frozen eggs were still at the 2-cell stage, compared to only 25% of the fresh egg embryos. The frozen egg embryos develop normally however. There is no difference in instances of chromosomal abnormalities and congenital defects, organ formation, birth weight and intellectual development between babies born from frozen or fresh embryos.

Egg freezing is a promising method of allowing women who want children to have more flexibility over when they become mothers. It allows women to establish themselves in their careers, achieve financial stability, and find an appropriate partner before starting a family. Unsurprisingly, it is an expensive procedure – it can cost up to £20,000 and several hundred pounds per year to store the eggs, and the NHS doesn’t fund it. There is also no guarantee that the eggs harvested will be of good enough quality for a successful pregnancy. However, as women who have chosen to freeze their eggs say, it can relieve the pressure of a ticking biological clock and provide a back-up plan if they find themselves unable to conceive naturally.

Waldby C (2014). ‘Banking time’: egg freezing and the negotiation of future fertility. Culture, health & sexuality, 1-13 PMID: 25247927
Konc J, Kanyó K, Kriston R, Somoskői B, & Cseh S (2014). Cryopreservation of embryos and oocytes in human assisted reproduction. BioMed research international, 2014 PMID: 24779007

wellcome images via photopin cc

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