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Long considered one of the most difficult techniques to master in reproductive medicine, attempts to use frozen eggs have resulted in only about 30 births since scientists first attempted the process in 1986.
But by increasing the amount of time the eggs are bathed in chemicals and by doubling or tripling the amount of sugar in the freezing solution, the number of eggs that survive after thawing can be tripled, the new research claims.
The secret is finding the right chemical solution to dehydrate human eggs, says Raffaella Fabbri, a biologist and embryologist in the department of obstetrics and gynaecology at the University of Bologna in Italy.
"[Eggs] have a lot more water inside the cytoplasm, [the part of the cell outside the nucleus,] and so they need to dehydrate more and more during freezing," Fabbri explains. "Without dehydration, a lot of ice crystals can be formed in cytoplasm, and during the thawing process, the ice can burst the oocytes [the developing egg cells]. That is the reason we looked at the concentration in the protective solution we use to freeze the eggs."
Dr Kevin Winslow, director of the Florida Institute of Reproductive Medicine in Jacksonville, says freezing eggs has "tremendous potential." The Florida institute has had more pregnancies - 19 - using frozen eggs than any other facility.
"It has enormous potential for women faced with chemotherapy, radiation or for women losing their ovaries," Winslow says. "Women who need to postpone having their children could also benefit from the technique."
But more importantly, he says, frozen eggs do not have the same ethical problems associated with frozen embryos. A fertilised egg is considered an embryo until the beginning of the third month of pregnancy, when it's termed a foetus.
"The big problem when you go through an in vitro process is the fertilisation of extra embryos [because] a lot of people have problems with abandoning or thawing frozen embryos," Winslow says. "This technology alleviates that problem."
Frozen embryos are used in about 14 percent of artificial reproduction techniques, according to the Centres for Disease Control and Prevention. In 1998, doctors used 11 228 frozen embryos in attempts to create a pregnancy, but only 19.3 percent resulted in the birth of a baby, the CDC says.
In the latest research, Fabbri and her colleagues used human eggs culled from 96 patients undergoing in vitro fertilisation. They prepared solutions with increasing amounts of sucrose and propanediol, a chemical that allows sucrose to penetrate the egg cell membrane, allowing water to be "pulled out of the cytoplasm," as Fabbri explains it. The researchers also increased the time eggs were bathed in the chemical solution to about 15 minutes.
"The results were better survival rates of the oocytes after thawing," Fabbri says. "Before, when we used less concentrations of sucrose, we had a 34 to 60 percent survival rate of the eggs. Now we have an 80 to 83 percent survival rate."
Injecting a single sperm into the egg fertilized 57 percent of the eggs, a figure comparable to natural fertilisation, she adds. This process is called intracytoplasmic sperm injection, or ICSI.
"Our next step will be to improve, even further, the oocyte survival rate after thawing," Fabbri says. "But our study has established that it is possible to cryopreserve human oocytes and that ICSI could be an efficient method of achieving satisfactory fertilization." Findings appear in the current issue of the journal Human Reproduction.
Fabbri hopes many reproductive centres will adopt the technique, making it routine.
"It is very important that we improve the possibilities for women to become pregnant after having their eggs frozen," she says. "It will offer hope to women who lose their fertility through medical treatments or who may be at risk of premature menopause."
