There are two reasons why you should know just where you are on your biological clock. The first is that you can plan your life better if you know when you need to worry. The second is that if you find you do not have much time left, you can actually have your eggs (or even a portion of one of your ovaries) retrieved and frozen for later use. Thus, despite the otherwise inexorable ticking of your biological clock (the major cause of infertility in the modern era), you can now preserve your fertility for when you are truly ready to have a baby.

Although embryo freezing has been around for decades, up until recently egg freezing has been very problematic.

This is briefly how it works: 70 percent of the human body is water. The only reason we cannot freeze living organisms (like us) and hold them in suspended animation indefinitely is that the water in our cells would become ice, which crystallizes and expands (like milk in a freezer) and would destroy the integrity of every cell. That is how freezing to death occurs. But if we can avoid that formation of ice crystals inside the cells, then lowering the temperature to – 196 degrees Celsius just suspends all metabolic processes, and can do so indefinitely without harm.

Individual cells (like white blood cells and embryos) can be frozen quite successfully on a routine basis by a process called “slow freeze,” in which the water content of the cell is removed and antifreeze solution (like in your car’s radiator) replaces it.

There are three components of this slow-freeze process. A cryoprotectant antifreeze solution contains sucrose, a sugar that osmotically pulls water out of the cell. At the same time the antifreeze solution itself (either dimethylsulfoxide [DMS0] or propylene glycol) penetrates into the cell just like in your car’s radiator. Finally, this antifreeze solution, which contains the embryos, is placed in a computer-controlled freezing machine that lowers the temperature slowly (–0.3 degrees Celsius per minute). That way, as ice crystals form preferentially outside of the cell, the concentration of solutes increases on the outside, gradually drawing more and more water out of the cell by increasing osmotic pressure. Thus, by using these three separate processes, the embryo can be more or less freeze-dried, usually without harm.

However, eggs have rarely survived this slow-freeze technique that worked so well for embryos in IVF patients and for stem cells in cancer patients. The reason is that mature eggs have their chromosomes lined up very precisely on a delicate platelike structure called the spindle, and are therefore much more sensitive to temperature drops. A cell that has completed cell division (with a discernible nucleus) has its DNA in a much more stable arrangement than either a mature egg or any cell in the process of dividing. Therefore, the slightest residual intracellular ice formation can kill the mature egg, even though it does not kill an embryo.

A new technique called vitrification avoids this damage not by trying to pull every molecule of water out of the cell, but by using such a super-high concentration of antifreeze that the water inside the cell never becomes ice no matter how low the temperature. This is a rapid technique that completely avoids the use of a slow-freezing machine. Most important, because the temperature drops so quickly and no intracellular ice at all is allowed to form, this technique finally makes freezing eggs a viable way to preserve fertility. The specific procedure was developed in Japan, and thus far is yielding remarkably good results. However, another solution (even better for some women) is now feasible. An entire ovary can also be preserved, an approach that may even surpass egg freezing in efficacy. The reason for the poor success of egg freezing in the past is that the eggs retrieved through normal IVF-type processes are undergoing chromosomal division. The chromosomes of retrieved eggs are highly organized on a complex spindle, which is very susceptible to minor crystal damage from freezing. However, “resting” eggs in primordial follicles within an unstimulated ovary are undergoing minimal cellular activity and have no such complex spindle formation. Therefore, these immature, resting eggs are not easily damaged during an appropriately administered freezing procedure. Furthermore, all these resting eggs, called “primordial follicles,” are located in the thin outer one-millimeter crust of the ovary. Thus, we can remove an entire ovary, perform bench microsurgery to remove the inside (to allow successful diffusion of the cryoprotectant into the outer crust), and then successfully freeze this ovarian tissue in the same way we have been freezing embryos for decades (see figure below). The frozen ovary can be stored safely for many years, and then can be transplanted back and function normally.

In 2004, in St. Louis, we performed the first whole-ovary transplant using this technique, and a young woman who was prematurely menopausal began to have normal periods. She conceived several months later, and is now able to have her own genetic offspring. This landmark case proved that an ovary can be transplanted back and forth through a minimally invasive outpatient procedure, completely restoring normal fertility, as well as hormone production, to an otherwise sterile, menopausal woman. This means that women can now put off childbearing until later years, keeping their ovary young until they decide later to have it transplanted back. Thus, menopause can be delayed and reproductive life span lengthened.

The actual procedure to remove the ovary, as well as the procedure for transplanting it back, has been refined to a one-day outpatient approach using a very minimally invasive incision. The reason this is possible is the same reason it is possible to freeze an entire ovary. All of the eggs are located in the one-millimeter outer shell (called the cortex) of the ovary. The entire inside of the ovary contains only blood supply, with no real structure, and is completely dispensable. Therefore, the thin outer shell of the ovary can be placed on any raw surface, just like a routine skin graft, and survive and function normally. This simplicity allows both the ovary removal and the ovary transplant to be performed through a tiny incision on an outpatient basis.

This technology can be used not only to preserve a woman’s declining fertility until she is finally ready to have a baby, but also to prevent young cancer patients from becoming sterile as a result of chemotherapy and radiation. We can actually take out a cancer patient’s entire ovary and freeze it for grafting back later, or we can retrieve and freeze her individual eggs before she receives an otherwise sterilizing treatment.

Until recently it was feared that breast cancer, the most common malignancy in women, would not be amenable to IVF and freezing of eggs or embryos, because of the increased estrogen level resulting from ovarian stimulation. The fear was that the breast cancer would be accelerated by that brief elevation in estrogen. However, by taking tamoxifen during the ovarian stimulation and NF cycle, the breast can be protected from this brief estrogen surge. Then the eggs (or embryos derived from fertilizing those eggs with the partner’s sperm) can be frozen. Immediately thereafter, the woman can undergo her chemotherapy and radiation, which has at least a 50 percent chance of rendering her sterile. But she will now have frozen eggs or embryos that can be returned to her once she is cured so that she can still have her own genetic child. Alternatively, the embryos could be transferred to a surrogate who could carry her baby for her.

If there is not sufficient time (four to six weeks) available to stimulate the ovary and retrieve eggs before going on chemotherapy or having radiation, the woman can have her ovary removed and frozen safely without delaying her treatment. In fact, since 1996 we have been freezing and saving ovaries for young women who have undergone potentially sterilizing chemotherapy or radiation for a variety of different cancers. If careful examination of sample tissue from the ovary reveals no cancer cells, then at some time in the future the ovary can be transplanted back to restore their fertility.

Killer cells