All people dream from time to time about the possibility of immortality. Science-fiction novelists frequently toy with the idea of human beings being placed in a deep freeze just prior to the moment of death, to be revived perhaps two hundred years later, at which time science may have better treatments for illnesses and a way of prolonging life indefinitely. Life is, in a sense, a series of chemical events proceeding irreversibly toward death, and these chemical events cannot take place at –196 degrees Celsius. Thus, if an organism could be “safely” placed in a deep freeze, it could be preserved until a future century, and revived with subsequent warming.

Of course, freezing large animals would kill them immediately because of damage created by crystallization of water within their cells during the freezing process. However, it has been known since 1776 that human sperm are remarkably resistant to the damaging effects of freezing (because there is so little water in them). In that year, an Italian scientist exposed spermatozoa to freezing temperatures and noted that, after warming, some of them regained their motility. It was speculated then that frozen semen might be used not only in breeding the finest farm animals but also for saving the sperm of a man going off to war so that his wife might have a child from him even though he had already died on the battlefield. Although these crude, early studies established that sperm could survive freezing and thawing, the sperm were so terribly damaged that there was no possibility of practical application.

But in 1949, British scientists discovered, completely by accident, that if a relatively common chemical, glycerol, is added to the semen before it is frozen, the majority of the sperm survive freezing and thawing uneventfully. The researchers who made this discovery were so surprised to find live, healthy sperm in large concentrations after thawing that they had to go back to their laboratory shelf to find out which of the chemicals accidentally added to their sperm suspension was the one that protected the sperm against freezing. They finally discovered that it was glycerol. It took very little time after their remarkable discovery for frozen-sperm banks to find acceptance in the field of cattle breeding, and today, the vast majority of calves born in the world are the result of artificial insemination from frozen bull semen. In 1953, four years later, it was demonstrated that frozen and thawed human sperm could result in pregnancy and the delivery of normal babies. The first human sperm bank was established the next year.

With the advent of the ICSI technique, no matter how poorly sperm might thaw out after the freeze, they can be used quite successfully unless there is absolutely zero motility whatsoever, which is quite unusual. Therefore, although sperm of the majority of men do not freeze well enough for a subsequent pregnancy to occur with simple insemination, the sperm of almost all men will freeze adequately for subsequent pregnancy with ICSI.

Sperm freeze better than most other cells because there is so little cellular water content. The sperm head is basically an extremely compact, dense arrangement of DNA with much less water content than any other cell. Therefore, there is very little intracellular ice-crystal formation to damage it. Nonetheless, even sperm require some sort of cryoprotectant, in this case glycerol, whose function is to pull water out of the cell and to get inside it to act as a sort of antifreeze, preventing ice formation from any water still remaining inside. Subsequently, there was further improvement in cryoprotection by adding test yolk buffer to the glycerol and diluting the sperm 50 percent instead of 10 percent in the cryoprotectant.

There is no increased risk of birth abnormalities from frozen sperm. Whatever harm may come to sperm from freezing, either in the sperm’s structure or ability to fertilize, there does not appear to be any increased risk of defective children. Extensive experience using both cattle and humans has now documented that frozen sperm from sperm banks is safe. Literally hundreds of thousands of normal pregnancies and births in humans using donor sperm have been reported in the scientific literature.

Perhaps the most exciting benefit for sperm-banking in the modern era is that because of the development of the ICSI technique we can freeze the sperm of anyone who is about to undergo a vasectomy, chemotherapy or radiation for cancer, or a sperm aspiration procedure, and who doesn’t want to have multiple surgeries for future attempts at pregnancy. For all these conditions, we can freeze the man’s sperm now, no matter how poor its quality, and offer a high chance of pregnancy once this sperm is thawed, if it used in conjunction with ICSI. Freezing of sperm has been attempted in the past with men who were about to undergo cancer chemotherapy, but it wasn’t used very frequently because the chance for pregnancy from any one insemination would be very low. To have any kind of assurance, the patient might have to collect twenty or thirty samples of sperm to freeze prior to undergoing cancer treatment, and this would be too long a delay for his cancer treatment. Now, with ICSI, it is simply a matter of having one single sperm sample frozen and saved, and then the man can have whatever treatment he needs that may subsequently make him sterile, without the fear that he will not be able to have his own genetic child someday in the future.

Why is it so hard