Thousands of frozen human embryos, floating in culture fluid in hundreds of thin plastic vials, are stored in insulated tanks at Boston IVF, one of the nation's largest fertility clinics. Each embryo consists of only eight cells and is considerably smaller than a period on this page.

HHMI investigator Douglas Melton refers to the eight-cell clumps as "pre-embryos." At the time they are frozen, three days after in vitro fertilization, they are still several steps away from becoming the 100-cell blastocysts that, under the right circumstances, are implanted in a woman's body about 10 days later. Yet these microscopic balls of cells are an essential resource for Melton and his colleagues at Harvard, who plan to produce new colonies of human embryonic stem (ES) cells for research. Melton hopes to turn such ES cells into pancreatic beta cells, which secrete insulin, to replace those that are missing in people with juvenile diabetes.

The embryos come from the many eager couples, about 20 a day, who stream into Boston IVF's clinic in Waltham, Massachusetts, for help in having a baby—specifically, by the in vitro fertilization process from which the clinic takes its name. Despite repeated attempts, all these couples have failed to achieve a pregnancy "the old-fashioned way," as R. Douglas Powers, Boston IVF's scientific and laboratory director, puts it. Now they rely on the clinic to unite their sperm and eggs in a dish.

The couples are greeted by a team of blue-smocked young technicians and embryologists who pad about in blue paper shoe covers and caps as they move in and out of labs and operating rooms. One of the most important procedures the lab carries out is semen analysis. If the semen proves suitable, the husband's sperm may be processed.

First, however, the woman's eggs must be removed from her ovaries, which requires a minor operation. Only one egg normally matures in the ovaries every month, but a single egg is extremely unlikely to result in a successful pregnancy. Thus, to avoid repeat operations, the woman receives hormones that stimulate her ovaries to produce many more eggs. The physicians can then collect 10 to 20 eggs at one time.

"Assume we have obtained 15 eggs," says Powers. "We put the eggs in a culture medium in a dish, and then we drop the sperm on top of the eggs. The sperm swim around; about 80 percent of the time they actually attach to the eggs and penetrate them. So maybe 13 eggs will get fertilized in vitro. Any eggs that were not fertilized are discarded right away.

"Probably 10 of the 13 eggs will go on to develop into fairly good looking embryos," says Powers, explaining that he chose the term "good looking" deliberately. "We have no test that can tell us reliably which embryos are the best," he says. "So we just look at them with a microscope and try to judge them on their appearance."

Several of these 10 embryos will then be put into the woman's uterus. "With the current state of infertility treatments," Powers explains, "we can't guarantee that if you put only one embryo back she'll have a high probability of getting pregnant. So usually, depending on her age, somewhere between two and four embryos are placed back—fewer embryos for younger women and more for older women.

"Assuming you put 3 of the 10 embryos back, you've still got 7 left over," he says. "That's where the freezing comes in. If the woman doesn't get pregnant the first time, she can come back for another attempt. At that point, we thaw out some more of her embryos and try again."

If the couple is lucky, the woman will get pregnant after one or two attempts of in vitro fertilization and have a healthy baby. She might even have a second child later. But eventually the couple may not want any more children, even though some of their frozen embryos remain stored in liquid nitrogen. Or, if the woman does not get pregnant, the couple may sooner or later cease trying. "That's why the number of embryos we have in storage slowly grows and grows," Powers explains. "We freeze about 8 embryos a day, and we probably thaw out 3 embryos every day. Since 1989, we have built up a large bank of embryos." In fact, he says, the clinic now has about 3,000 frozen embryos under lock and key.

Powers teaches embryology at Boston College and was one of the founders of Boston IVF. "We've been responsible for the birth of 9,000 babies," he notes proudly. In the past, when a couple decided they had finished their fertility treatments, they were given the choice of leaving the remaining embryos in storage or discarding them. Now, Powers says with satisfaction, "we can offer them an additional choice: donating their embryos for research."
—MP

Photo: Kathleen Dooher

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Reprinted from the HHMI Bulletin,
March 2002, pages 10-17.
©2002 Howard Hughes Medical Institute