Egg Freezing Process
Both embryos and eggs can be frozen for future use. An embryo consists of many cells and is formed after the egg is fertilized. Embryos have the genetic makeup of both parents whereas eggs only have the mothers DNA.
The major challenge in freezing embryos and eggs is that water must be removed from the cell before freezing as water crystal formation can destroy the cytoskeletal structure. The cell membrane of the egg (zona pellucida) is more delicate than the embryo making it more susceptible to damage.
Embryos are frozen by sequentially immersing them in special solutions to “withdraw” water from the cells while lowering the temperature. Eggs are frozen using the “vitrification” technique where the cell is stabilized in various “cell protectants” and instantly frozen in liquid nitrogen vapor. This rapid freezing avoids formation of ice crystals that would destroy the egg.
Another challenge in freezing eggs was that thawed eggs often did not fertilize due to hardening of the cell membrane. This issue was solved in the mid-nineties with the advent of intracytoplasmic sperm injection (ICSI) .This is a procedure where a single sperm is injected through the membrane directly into the egg.
The ovaries are stimulated with FSH to cause the development of many eggs. Once mature, the eggs are retrieved using transvaginal guided ultrasound and are frozen.
Thawing and Using Eggs
When a woman decides to use her eggs in an IVF cycle, they are thawed, fertilized using ICSI, and the embryos are incubated. She receives medications to cause the development of the uterine lining (synchronization) to support an embryo. After the embryos mature, they are transferred to the uterus.