Freezing human eggs has proved one of the most difficult techniques to master in reproductive medicine and only about thirty babies have been born worldwide since the first attempts in 1986. But research published today*(Tuesday 27 February) in Europe's leading fertility journal, Human Reproduction, could lead to a significant improvement in success rates.
Italian fertility experts lead by Dr Rafaella Fabbri from the IVF Centre, Human Reproductive Medicine Unit at the University of Bologna, have identified crucial factors in the freeze-thaw process which contribute to the difference between success and failure. Putting their findings into practice in a laboratory study, they achieved a much higher than normal rate of egg survival and fertilised 57% by using ICSI (the injection of a single sperm into the egg). This is a figure comparable to fertilisation of fresh oocytes.
The key to their success was to increase the time the oocytes were exposed to the chemical protectants, propanediol plus sucrose, from around 10 minutes to 15 minutes and to double or even triple the normal amount of sugar (sucrose) in the freezing solution.
The main reason that human eggs disintegrate in the freezing process is inadequate dehydration. Longer exposure to the protectant and more help from the extra sucrose meant that the eggs were dehydrated gradually and more completely. This gentler freezing process minimised the formation of ice crystals, which can pierce the egg membrane and kill the cell.
The researchers further established that one factor thought to be important in successful cryopreservation - the presence of the protective nourishing cells surrounding the oocyte (cumulus oophorus) - did not actually have a bearing on success. There was no real difference in survival between eggs frozen with their cumulus oophorus partially removed or totally removed.
But, increasing the exposure time to the protectant and doubling the sucrose solution in the freezin
Contact: Margaret Willson
European Society for Human Reproduction and Embryology