Cryopreservation of egg cells

 

Detailed information and a list of references can be found in Chapter 3.2 of the FertiPROTEKT book, "Indications and fertility preservation methods for oncological and non-oncological disorders", which can be downloaded free of charge.

 

Background

Before the start of chemotherapy and / or radiotherapy, egg ells (oocytes) can be frozen (cryopreserved) in a fertilized or unfertilized state. Cryopreservation of unfertilized oocytes or embryos after in vitro fertilization (IVF) and / or intracytoplasmic sperm injection (ICSI) are established reproductive medicine techniques that can be used for fertility preservation in patients before chemotherapy.

Two different methods are used: the slow freezing method and vitrification, which is ultra-fast freezing. 

 

Egg cell collection

So that enough egg cells (oocytes) can be cryopreserved (unfertilized and / or fertilized), ovarian stimulation is necessary, which can start at any point during the cycle and requires a time frame of about two weeks. On average, about 13 oocytes can be collected before chemotherapy. However, the number of collected oocytes decreases significantly above 35 years of age. A detailed presentation of stimulation techniques, their effectiveness, risks, and costs can be found here.

 

Cryopreservation of egg cells

+ Pronuclear stage (fertilized egg cells)

In contrast to the cryopreservation of unfertilized oocytes, cryopreservation of fertilized oocytes with 2 pronuclei (pronuclear stage), i.e. one day after egg collection, is a long-established successful method and is an essential part of assisted reproductive medicine techniques. The highly reliable thawing rate of 73-95% and a pregnancy rate of 20 to 32% after cryopreservation and thawing are probably due to the absence of the spindle apparatus after the completion of meiosis. Cryopreservation can either be performed as a slow freezing process, which is characterized by simple and well reproducible handling using an automatic freezing programme, or as part of vitrification.

+ Cryopreservation of unfertilized egg cells

After the establishment of freezing unfertilized eggs as a clinical procedure had failed for over 2 decades, vitrification brought the desired breakthrough. Neither the American Society for Reproductive Medicine (ASRM) nor the European society (ESHRE) guidelines describe any reservations about this method, since neither an increased rate of chromosomal disorders (disorders of the genetic material) nor congenital abnormalities have been identified up to now.

However, it should be noted that in the age range of 25 to 42 years, the fertilization rate decreases continuously and cryopreservation of unfertilized oocytes should be performed in patients under 35 years of age since the number of oocytes decreases due to the age-related reduction in the ovarian reserve.

If unfertilized oocytes are later thawed as part of a fertility treatment, conventional in vitro fertilization (IVF) is no longer possible because the cumulus cells (surrounding cells) are removed before cryopreservation to assess the maturity of the oocyte. Intracytoplasmic sperm injection (ICSI) is therefore used for fertilization, even if is the semen analysis is normal.

Practice summary

The introduction of new stimulation protocols and the establishment of vitrification allow the collection of egg cells within about 2 weeks, which can then be cryopreserved in an unfertilized and fertilized state. Because it is recommended that at least some of the egg cells should be frozen in an unfertilized state, even for couples in a stable relationship, vitrification is an indispensable prerequisite in the implementation fertility preservation techniques.