Stopping your Biological Clock: Options for Elective Egg Freezing, Oocyte Cryopreservation
Oocyte Cryopreservation, Egg Freezing Options
Although we have been able to
effectively and successfully freeze sperm and
embryos for many decades, successful freezing of
oocytes has been hampered by many challenges. The
first pregnancy obtained from frozen mature eggs was
reported in 1986. Since that time, various
protocols, equipment, and methodologies have been
suggested to improve the ultimate outcome.
Unfortunately, despite a significant amount of
research in this area, pregnancy rates that would
push egg freezing into the mainstream have remained
elusive.
There are many reasons why
patients are interested in freezing oocytes. One
major reason is to preserve future fertility for
women who have been diagnosed with life-threatening
illnesses such as cancer or other types of chronic
disease requiring intensive chemotherapy, radiation,
or surgical therapy. The concern is that treatment
of the underlying disease would damage or destroy
the patient's remaining eggs. Therefore, these
patients frequently desire to have their eggs
removed and stored in the hope of being able to use
them after successfully completing treatment for
their disease. A second population of patients is
represented by women who are not yet ready to
conceive. They desire to freeze eggs at a younger
age, in the hope of being able to use them years
later while still maintaining a higher chance for
pregnancy. This is theoretically possible, as we
believe that the integrity of the egg and the
likelihood of it generating a successful pregnancy
are related to the age of the woman at the time that
the egg was obtained – rather than the woman’s age
at the time that the pregnancy actually delivers. A
third group, representing a small subset of
prospective egg freezing patients, has religious or
legal concerns centering around embryo
cryopreservation. They therefore prefer to freeze
eggs instead.
In general, eggs are much more
easily damaged during the freezing and thawing
process than are embryos. Due to a variety of
biochemical reactions, including hardening of the
membrane around the egg and damage to the
chromosomes inside that can be caused by freezing,
the egg is much more fragile. For these reasons,
the percent of mature eggs that survive freezing and
thawing is substantially lower than that of
embryos.
A variety of different
laboratory protocols have been described in the hope
of improving the survival and subsequent successful
use of frozen/thawed oocytes. The most commonly
employed protocol at this time is “slow freezing”.
This process involves gradually dehydrating the
oocyte and replacing the water that is normally
found inside the egg with a type of “antifreeze”.
This antifreeze is designed to protect the egg as it
is slowly frozen. The eggs are placed in a solution
in a programmable freezer while the temperature
around them is gradually lowered. Eventually, the
temperature is lowered all the way down to
approximately 350 degrees F below zero, where they
can be subsequently stored in liquid nitrogen. A
newer methodology, called vitrification, is an
ultra-rapid process during which liquids turn into a
glass-like solid state, avoiding crystal formation
(which could damage the egg’s chromosomes). Recent
data suggests that this technique may be just as
effective, if not more so, than the slow freezing
methodology.
Although the amount of data
concerning the effect of egg freezing on subsequent
offspring is limited, it does appear that
cryopreserved oocytes do not have an increased risk
of chromosomal abnormalities or other types of birth
defects. As of the end of 2009, less than 2000
babies have been born worldwide using frozen
oocytes, so you can see why it may still be too
early to answer this question just yet.
Although oocytes
cryopreservation offers a substantial amount of
promise, there is still a lot of work that must be
done in order to bring this technology into the
mainstream treatment of infertility.
