EGG FREEZZING – STOPPING THE BIOLOGICAL CLOCK
Tuesday, March 28th, 2006For decades, sperm and embryos (fertilized eggs) have been
successfully frozen for the purposes of fertility preservation
and donation for men and couples. Although both sperm and embryo
cryopreservation have become commonplace, the freezing of
unfertilized oocytes (or eggs) for similar applications in women
has not historically delivered the acceptable success rates
necessary to drive adoption across the board.
Unlike sperm and embryos, oocytes did not survive the
freeze/thaw process well, primarily because the egg is the
largest cell in the human body and comprised mostly of water.
The water inside the cell forms ice crystals that destroy the
egg during the traditional freezing process. Prior to 2002, the
success rate of live births from frozen eggs was 1-3% globally,
with few babies born from frozen eggs over decades of attempts.
Fortunately, a number of advances in our knowledge of oocyte
physiology and laboratory techniques are rapidly changing this
dream into a reality.
The ability to preserve unfertilized oocytes is profound if one
looks at the potential benefits and some of the controversies
surrounding reproductive medicine. Egg-freezing should appeal to
a broad range of women. Ultimately, the common factors that link
all of these women are the strong desire to have a family and
the willingness to take proactive steps to give themselves the
best odds possible.
1. While some couples are comfortable with the concept of embryo
freezing, many have moral and ethical dilemmas regarding this
issue. For those that believe that life is created at the moment
of conception, each frozen embryo represents a life and, if
unused, a life unfulfilled. Oocyte cryopreservation, like sperm
cryopreservation, presents us with the possible opportunity to
preserve one’s fertility while avoiding these ethical dilemmas.
2. Oocyte cryopreservation provides young women facing
chemotherapy or irradiation for treatment of life-threatening
disease, such as cancer, the opportunity to preserve their
fertility. The importance of this cannot be understated.
Treatment regimens for many of the malignancies faced by
adolescents and young adults result in the destruction of their
gametes (sperm or eggs). The advancements in cancer treatments
are also ensuring that many of these cancer survivors are living
long, productive lives. Thus, the opportunity to preserve their
ability to have children later in life is critical.
3. The professional and personal opportunities for women have
exploded over the past 30 years, encouraging many women to
postpone motherhood. Unfortunately, the biology of female eggs
hasn’t kept pace and women often face challenges starting their
families later in life. Women, who are born with a limited
supply of eggs, start to experience diminished fertility rates
in the late twenties and this rapidly accelerates as they reach
their mid-thirties. Childlessness is one of the biggest concerns
for professional women. Studies show that the majority of the
33% of high-achieving women that are childless at ages 41-55 did
not choose to be childless. In fact, more than a quarter of
high-achieving women in the 41-55 year old age group said they
would still like to have children. For women in this category,
the ability to preserve their eggs (and thus their future
fertility) gives them more flexibility as to when they can start
their families.
4. Oocyte cryopreservation will allow those who need to resort
to egg donation (the use of someone else’s eggs due to the
diminished fertility potential of their own eggs) more
affordable treatment options. Today, when a couple chooses to
use donor eggs, they must bear the cost of the entire donor’s
IVF cycle alone. The ability to use only the limited number of
eggs necessary while freezing the unused eggs will allow couples
using donor’s eggs to cut their expenses dramatically. This will
allow individuals who were unable to afford such services the
opportunity to pursue having children.
Techniques Given the magnitude of the need, clinicians around
the world have raced to develop a technique for successful
egg-freezing, and beginning in 2002, promising results ranging
from 20-40% successful pregnancy rate (on par with a woman’s
natural peak fertility rate) were published. The key difference
over previous techniques was the change in cryoprotectants used
to protect the egg during the freezing process. Cryoprotectant
acts as an “antifreeze” to protect the delicate egg as the
temperature drops.
Before attempting to understand how our ability to cryopreserve
unfertilized eggs has been achieved, it is important to
understand how sperm and embryo (fertilized oocytes)
cryopreservation has long been commonplace. The major problem
faced in freezing a cell is to minimize damage to the membranes
induced by ice crystal formation. Intracellular ice formation is
dangerous because it may rupture the cell membranes causing
cellular destruction. The smaller the cell, the less likely ice
crystal formation will occur.
Sperm cells are about 180th the size of a mature egg and thus
can be easily preserved. Embryos, which are eggs that have been
fertilized, are approximately the same size as mature eggs, but
are much more likely to survive the freezing/thawing cycles.
This is due to the fact that the eggs’ membranes undergo
dramatic changes during fertilization, making them more likely
to tolerate the stresses associated with freezing. The use of
cyroprotectants and highly controlled freezing/thawing rates
have dramatically improved the survival rates of frozen sperm
and frozen embryos. In many ways, the lessons learned from
freezing sperm and embryos are being applied to freezing
oocytes. However, the unique nature of the female egg has
required additional study and technological developments.
A number of approaches have been taken in order to maximize the
survival rates of frozen oocytes. The greatest success has been
achieved with protocols that use slow freezing/rapid thaw
protocols. Critical to any freezing protocol is the use of
cryoprotectants.
Cryoprotectants act by a variety of means to reduce the amount
of water that crystallizes within the cell and protects the cell
during the freezing process. Common cryoprotectants include an
alcohol (1,2-propranediol), a carbohydrate (sucrose) and a
solvent (DMSO). The concentration and the duration of exposure
to most cryoprotectants (alcohols and solvents) are critical, as
exposure to high concentrations or exposure for prolonged
periods of time can result in damage to the cell. We have found
that increasing the concentration of sucrose (a relatively safe
cryoprotectant that works by pulling water out of the cell)
results in significantly improved survival rates, fertilization
rates and pregnancy rates for frozen oocytes.
We also have found that the removal of the cryoprotectant with
progressive dilution is a critical step in the thawing process.
If oocytes are placed directly in a medium without
cryoprotectant after thawing, they can swell and burst. The use
of nonpermeating molecules (molecules that do not enter in the
thawing cell) such as sucrose, act to oppose the inflow of water
into the cell and thus prevent the membrane from bursting.
Others have recently undertaken investigations using a process
called vitrification; a process that utilizes ultrarapid
freezing techniques. While some pregnancies have been achieved
utilizing this technique, it has not been shown to be more
efficacious than slow freeze/rapid thaw protocols and is more
susceptible to human error.
This process exposes the egg to potentially damaging levels of
cryoprotectant and direct exposure to liquid nitrogen. Exposure
to liquid nitrogen is a critical factor in this age of concern
over infectious agents. The infectious agents, while rare, can
result in life-threatening illnesses. This is a critical concern
as the cryopreserved cells are stored in common tanks and a
single tank may contain thousands of cells.
Regardless of the freezing technique, the oocyte goes through a
number of changes that make it less likely to fertilize using
standard co-incubation techniques (the mixing of eggs and sperm
together).
The understanding that the zona pellucida (an exoskeleton that
covers the outside of the egg) undergoes changes due to the
premature release of the cortical granules (these are normally
released at the time of fertilization and prevent multiple sperm
from fertilizing the egg) is an important factor that has lead
to improved success with frozen eggs.
This has led to changes in how frozen oocytes are fertilized.
With the introduction of intracytoplasmic sperm injection
(ICSI), the results for fertilization, embryo development, and
for implantation rates (attachment of the embryo to the uterus)
are approaching those obtained with fresh embryos.
Conclusions
We are just reaching a time where it is becoming feasible to
preserve unfertilized, mature oocytes. These are harvested after
taking fertility medications to induce the maturation of a
number of oocytes.
Women of this generation want more options and power when it
comes making life decisions and plans. This is an exciting next
step in the long line of developments in the field of women’s
reproductive health – on par with the introduction of the birth
control pill.
Dr. Bradford Kolb is a reproductive endocrinologist and OB/GYN
at Huntington Reproductive Center in Pasadena, CA Phone: (626)
440-9161 or Toll Free (866) HRC-4IVF Website:
http://www.havingbabies.com Christy Jones is the CEO and founder
of Extend Fertility in Boston, MA Phone: (800) 841-7197 Email:
info@extendfertility.com Website: http://www.extendfertility.com
REFERENCES: U.S. Census Bureau. “Distribution of Women by
Average Number of Children Ever Born, by Race, Age, and Marital
Status.” Fertility of American Women Current Population Survey
(June 2000). National Parenting Association. “Groundbreaking
Study Exposes A Crisis Among Successful Women: The Survey Behind
Sylvia Ann Hewlett’s ‘Creating a Life.’” National Parenting
Association Web site.
About the author:
Bradford Kolb, MD, F.A.C.O.G.
Undergraduate -University of California, Irvine Medical School
University of California, Irvine
Residency Obstetrics & Gynecology-Northwestern University
Fellowship Reproductive Endocrinology & Infertility-University
of Southern California
Board Certifications-Reproductive Endocrinology and Infertility
Obstetrics and Gynecology
http://www.havingbabies.com/staff_kolb.html
