Like Albertini, Van Blerkom sensed that the most important information in the embryo was not confined to the nucleus but embedded in the cytoplasm. “If I’ve done anything in this field,” he said, “it’s to deemphasize the embryo and emphasize the egg cell. Our work has shown that it all begins with the oocyte, which can have subtle cytoplasmic defects that are actually very profound. But,” he added hastily, “you have to be careful. It’s like looking at canals on Mars. Unless you can show a consistent pattern [of polarity] and then an effect that is different as cells divide, it doesn’t have meaning.”
Van Blerkom had been seeing hints of polarity since the 1970s, but one of the major turning points occurred in 1996 when, by accident, his lab discovered that cells surrounding the developing egg—the same granulosa cells that had piqued Albertini’s interest—possessed a receptor very similar to the leptin receptor. Leptin made front-page news when it was discovered in 1994 because the molecule appeared to regulate fat metabolism and obesity. What was it doing in egg cells?
The Colorado lab discovered that granulosa cells—the cells that surround maturing eggs in the ovarian follicles—were pumping out leptin and shipping it into the egg. What’s more, the researchers showed that leptin is polarized in the egg in such a way that, after fertilization, the protein is allocated primarily to the cells that become the placenta, while it is virtually undetectable in the cells destined to become the fetus.
At first, many embryologists resisted the notion that leptin was segregated in certain parts of the egg and that this asymmetry had any significance for the fate of the embryo. “For a long time, no one believed it,” Van Blerkom said. But mice in which the leptin gene has been erased are incapable of producing embryos—the fertilized eggs die almost immediately. And various experiments tracking leptin inside the mammalian egg clearly showed a more prominent distribution in one hemisphere than in the other. It is now believed that this protein acts as a delayed silencer; it hangs around in the egg and keeps certain genes from turning on in certain parts of the embryo until days after fertilization. Again, the appearance of a protein in a certain part of the egg cell may affect embryonic development or the formation of organs days and weeks later.
Lately Van Blerkom has been intrigued by another form of polarity: the way mitochondria, the cell’s little power plants, migrate in the maturing egg cell. “It’s kind of like a lava lamp,” he says, “with these blobs of cytoplasmic elements moving up and down in the cell.” Typically, mitochondria arrange themselves along the outer edge of the egg cell. But at certain points in the reproductive cycle, they migrate en masse toward the nucleus. Wherever they gather, mitochondria change the local chemical microenvironment: They cause a lower pH, and that small change, Van Blerkom believes, can affect the local activity of certain enzymes. “It’s not a bag of cytoplasm,” he said. “It’s highly structured, and that structure is changing.”
Finally, Van Blerkom has conducted extensive work on the internal structural organization of the human oocyte. First the oocyte constructs the scaffolding of connections known as microtubules, which allow molecules to move around inside the cell. Then, toward the end of fertilization, the egg provides a kind of highway that allows the sperm to make its final approach to the female pronucleus. “There’s something in that cytoplasm that allows the sperm to know where it’s going,” he said. One of the compelling messages—and central paradoxes—to emerge from these studies of polarity is that even bad eggs can be fertilized to create an embryo, but only good eggs seem to create a successful pregnancy. The politics of embryo research, however, is one reason we don’t know more about what distinguishes good eggs from bad. Federally funded research on human embryos, although sanctioned by a congressionally mandated national bioethics commission in 1975, has faced unrelenting opposition from right-to-life groups. In 1996 Congress banned NIH funding outright for any research in which an embryo is destroyed. Van Blerkom calls the issue of when life begins the “third rail” of developmental biology. “You can find whatever you want in the embryo to support any position you have on when life begins,” he said. “A lot of people believe that life begins at conception. But life also ends at conception or shortly thereafter—hours after, a day after, four or five days after. We don’t know why that happens, and what’s gone wrong. We’d like to know the answers to those questions,” Van Blerkom said, “but we can’t do those experiments.”
If polarity and the forces that shape it play a determining role in the fate of a human egg, it’s not difficult to see the implications for making babies, whether through assisted reproductive technologies or the old-fashioned way. It becomes a particularly nettlesome question because basic research of the sort done by Van Blerkom and Albertini has historically been adapted—snatched, really—for use in IVF clinics, often before all the biological ramifications are clear.
Indeed, this is where the polite disagreement between Albertini and Van Blerkom becomes a matter of intense public and medical interest. If you believe, for example, that granulosa cells and other very early features of ovarian ecology set up the polarities that ultimately determine the quality of a human egg, as Albertini does, then certain techniques widely used in IVF may be subtly perturbing the very mechanisms that eggs use to establish a plan to build an embryo and maximize the chances that it will develop properly. “We recognized in the 1980s that many culture techniques used by assisted reproduction were reducing the quality of those eggs,” Albertini said. “My own skepticism has been growing that we therefore may be damaging things with what we’re doing to these eggs prior to embryogenesis.” Other researchers—notably Alan Handyside in England—have begun to express similar concerns.
Albertini cites a popular IVF technique known as intracytoplasmic sperm injection, or ICSI, in which sperm is injected by needle right into the middle of an egg cell. If his polarity research in mice is true for humans, with its suggestion that sperm are biased toward entering the egg at the opposite pole from the cell’s nucleus for important reasons, then ICSI injections might subtly disrupt patterns of polarity in the egg. Moreover, ICSI requires the removal of the cells surrounding the egg; Albertini thinks that might deprive the egg and early embryo of important signals or alter the time course of fertilization. Several rare, so-called imprinting disorders, including Beckwith-Wiedemann syndrome, a form of gigantism, have been found in children produced by ICSI, although the extent and significance of these links is unclear. “Ten years ago, we wouldn’t have thought about the polarity thing,” said Albertini. “It wasn’t even on the radar. But now we’re looking at how we’re making these babies.” Albertini hastened to add, “I’m certainly a proponent of human-assisted reproductive medicine, but I’m concerned that we’re rushing technologies before we’re certain they’re safe and effective.”
Van Blerkom respects Albertini’s research but expresses reservations about his clinical ruminations. “If there were really problems with manipulating eggs, you’d see it, and in fact you’d have seen it 10 or 15 years ago,” said Van Blerkom. “In the literature, there are only 26 reported cases of imprinting-associated disorders with IVF, and that is out of 1.2 million IVF births.” In some hands, he added, ICSI is now achieving fertilization rates of between 60 percent and 70 percent, even though the technique requires the removal of surrounding cells. “If these cells were so important,” he said, “you shouldn’t get such high pregnancy rates.”
Albertini replied that there might be subtle health effects, such as early onset of adult diseases like diabetes and cancer, that won’t appear until 15 or 20 years after IVF, and he pointed out that there is very little follow-up data on the health of children created through assisted reproductive medicine. Even Van Blerkom conceded that point. “There’s no systematic, organized mechanism for follow-up,” he said. “And the reason for that is that people don’t want it.”
It may seem like an arcane debate, but it has life-and-death ramifications every day, when IVF practitioners peer at egg cells through microscopes and try to predict the fate of the embryos they might become. IVF remains, at best, a hopeful art driven by the best of intentions and less than complete knowledge. About two weeks after he sorted through those eight human eggs late one moonlit night, Van Blerkom called to report, happily, that his initial hunch had been wrong.
“I’ve got good news,” he announced. “She’s pregnant.” It was a particularly felicitous way of acknowledging that, until biology provides a better crystal ball, pregnancy remains the best—and perhaps only—way to find out if an egg is good.
