The intricacy with which an early embryo divides, compacts, hatches out of the zona pellucida, ingeniously secretes molecules that penetrate the cells lining the uterine wall in order to implant in the womb, and then recruits blood vessels to nourish the placenta and the developing fetus marks one of the most awe-inspiring metamorphoses in all of nature.
But here’s the rub: It’s horribly inefficient in humans.
Much more often than not, the process fails. Although the statistics on the failure rate of human fertilization are not entirely robust, given the biological and ethical delicacy of conducting research in this area, the numbers consistently suggest that, at minimum, two-thirds of all human eggs fertilized during normal conception either fail to implant at the end of the first week or later spontaneously abort. Some experts suggest that the numbers are even more dramatic. John Opitz, a professor of pediatrics, human genetics, and obstetrics and gynecology at the University of Utah, told the President’s Council on Bioethics last September that preimplantation embryo loss is “enormous. Estimates range all the way from 60 percent to 80 percent of the very earliest stages, cleavage stages, for example, that are lost.” Moreover, an estimated 31 percent of implanted embryos later miscarry, according to a 1988 New England Journal of Medicine study headed by Allen Wilcox of the National Institute of Environmental Health Sciences.
In some respects, less scientifically sophisticated cultures may have come to terms with this conundrum in the way they grappled with the knotty question of when life begins. The medieval etymology of the word conception, said Harvard biologist John Biggers, traces it to the Latin root capio, which means to grasp, take hold, or receive into the body. In 1615 an obscure writer named Cooke noted, “Conception is nothing els but the wombs receiuing and imbracing of the seede,” suggesting that centuries-old notions of conception referred, perhaps wisely, to when an embryo survived its perilous first week and was “imbraced” by the womb.
Nonetheless, the high failure rate begs challenging ethical questions. If life begins at conception, as many believe, why are so many lives immediately taken? If, as some ethicists argue, nascent life must be protected, how do we assess the degree of moral entitlement due a nascent entity that fails to pass nature’s own muster perhaps 80 percent of the time? And if the fate of an organism is indeed inscribed in the earliest biological inklings of an egg, does life begin with the gametes?
From a purely scientific, not to mention pragmatic, point of view, the main question is more straightforward: Why do so many embryos fail to grasp the womb? That question has bedeviled developmental biologists for decades, and more recently, it has vexed clinicians who practice assisted reproductive medicine. Studying early human development in the academic setting is extremely difficult, in part because of political constraints on embryo research in the United States, so a certain amount of our knowledge is limited to inferences from animal studies.
Nonetheless, it has become increasingly clear that the fate of an embryo may be cast in the ovarian follicles, where egg cells are built. “Much of the developmental biology and ability of the human embryo is determined even before it’s fertilized,” Van Blerkom said. “This all happens by the one-cell stage, which is when the fate of the embryo is determined.”
Such thinking upends long-held assumptions in the world of biology. Mammalian development was once thought to be essentially different from embryological development in fruit flies, frogs, worms, and other laboratory organisms, where well-defined polarities in the egg—higher concentrations of a protein in one part of the egg than in another, for example—ordained such fundamental aspects of body plan as head and tail, or back and belly. Mammals seemed exempt from these rules for building a body. In the mouse, it had been shown in the 1970s and 1980s that if you split an embryo at the two-cell stage, each resulting cell had the ability to develop into a full organism. If the egg were indelibly etched with asymmetric information that unequivocably determines development, the argument went, how could two embryonic cells be separated and still produce whole, intact, normal individuals? “Animal experiments led to the conclusion that mammalian eggs do not have polarity, but I think that’s a huge fallacy,” said David Albertini, a developmental biologist at Tufts University in Boston. One possible answer, he added, is that mammalian embryos are similarly shaped by polarity but retain a certain developmental flexibility as well.
These days, as biologists like Van Blerkom, Albertini, and a superb school of British embryologists based in Oxford and Cambridge have started to look at the early embryo, they have begun to catalog a number of very early polarities that affect both the competence of the egg and the form of later embryonic development. The implications of polarity reverberate far beyond the confines of academia. For example, Van Blerkom and Albertini have a gentlemanly disagreement about recent research that may spill out into the public discourse soon because it raises the possibility that some popular IVF techniques might have subtle but long-term health implications for children conceived in a dish. Indeed, on the night that Van Blerkom inspected the fertilized eggs at the Denver clinic, he made this disagreement clear at one point by holding up a sharp micropipette for my benefit. He remarked over his shoulder, “This is what I use to take off the cells that David Albertini says I shouldn’t take off.”
And with that, he began prying away the granulosa cells clinging to the eggs, in order to get a better microscopic view of the nascent embryos to see if they were developing properly. Within three days or so, those denuded embryos would be implanted in a woman’s womb.
The Sperm Cell
Polarity begins in the sex cells. The female egg cell is a huge biochemical universe unto itself, with a complex and sophisticated cytoplasm. The sperm cell, by contrast, is little more than DNA strapped to an outboard motor. Nonetheless, of the 15 percent of couples experiencing infertility problems, about half the trouble can be traced to the male, mostly in the genetic qualities of the sperm.
Immature sperm cells form during the fourth week of embryological development but remain unfinished until puberty. At that point, the male begins to churn out haploid sperm cells—that is, sex cells with half the normal complement of 46 chromosomes. Thus, when a sperm cell delivers its genetic cargo at fertilization, the one-celled egg again possesses the full 46 chromosomes. Sperm dysfunction can arise from the way these cells are built. The sperm has an acrosome (the head and sheath), a nucleus, and a tail. Sometimes a club-shaped profile on the head disturbs the proper construction of the tail. These tail abnormalities can include looping, folding, and fusion, all of which can result in reduced motility (ability to swim).
While assisted reproductive techniques such as intracytoplasmic sperm injection (ICSI)—which involves the direct injection of sperm into the egg cell—can overcome head or tail abnormalities in sperm, recent animal research suggests that fertility doctors must use these techniques with care. Abraham Kierszenbaum of the City University of New York Medical School has conducted experiments in mice showing that even normal-looking sperm from a mutant mouse “is likely to create infertile offspring.” Hence, selection of donor sperm, he said, cannot be based on appearance alone.
Biologist Jonathan Van Blerkom of the University of Colorado published a paper in 1996 suggesting that some cases of male infertility derive from defects in a tiny structure in the sperm cell called the centrosome. When a sperm penetrates the egg, it unwraps the centrosome, an organelle that acts like a construction foreman overseeing the creation of microtubules in the cell. Sperm DNA uses these microscopic highways to find the female DNA and merge into a zygote. If a sperm has centrosome defects, Van Blerkom speculates, it can get inside the egg but then is destined to wander in the desert of the egg’s cytoplasm, unable to find its way to the female’s DNA.
—S. S. H.