Bradford
Kolb, M.D., F.A.C.O.G., Board
Certified, Reproductive Endocrinology and Infertility
and Christy Jones, CEO, Extend Fertility
For
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.
TechniquesGiven
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. The cryoprotectant acts as"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 cryoprotectants 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 non-permeating 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 ultra
rapid 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.
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. |
