New techniques for embryo freezing

May 2008


Vitrification – the rapid freezing of embryos
in liquid nitrogen – could supersede
current cryopreservation methods, an expert
has advised.
“I think that vitrification of embryos will
one day become routine practice instead of
conventional slow freezing,” said Professor
Shee-Uan Chen of the National Taiwan University
Hospital in Taipei.
The majority of in-vitro fertilization (IVF)
centers in South East Asia currently preserve
embryos and blastocysts using slow
freezing, which takes around 3 hours. Vitrification,
whereby embryos are directly solidified
into a glass-like state, is more costeffective
and can take as little as 6 minutes,
but has prompted safety concerns regarding
the high concentrations of cryoprotectants
involved.
Chen says, however, that sufficient data
is now accumulating to support the widespread
use of vitrification. The technique is
already being adopted at his center, where
10 babies have been born from vitrified embryos,
to date.
A study published last year demonstrated
an 89 percent survival rate among vitrified
zygotes and a subsequent pregnancy
rate of 37 percent – three times higher than
that achieved using slow frozen cells. [Reprod
Biomed Online 2007 14(3):288-93]
“Both slow freezing and vitrification are
highly effective for pronuclear, early-stage
and blastocyst embryos, but vitrification
is time-saving and also appears to have a
higher survival rate,” said Chen. He added
that toxicity to the embryo can be reduced
by using aseptic liquid nitrogen and lowering
the concentrations of cryoprotectants.
The apparent superiority of vitrification
was also demonstrated by a recent metaanalysis
which found that post-thawing
survival rates were significantly higher
for vitrified blastocysts and cleavage stage
embryos than for those preserved by slow
freezing [Fertil Steril 2007, online 1 November].
Only four studies were eligible for the
analysis, however, and further trials are
needed to provide an assessment of pregnancy
rates.
Advances in cryopreservation have
made storing surplus embryos an increasingly
common practice. Successfully finetuning
these techniques can help to reduce
embryo wastage, and thereby maximize the
benefits of an IVF cycle.
“The trend is moving towards vitrification,”
added Dr. Kwang-Ryul Cha of the
Cha Research Institute in Seoul, where some
50 babies have been successfully born from
vitrified embryos. Cha is also confident that
this technique will one day replace conventional
embryo freezing techniques. “We need
more time to accumulate the data but I think
it’s not a problem,” he concluded. - DB