We tend to picture fertilization as a simple, cinematic moment: one sperm cell reaches the egg, slips inside, and the genetic material from the father meets the genetic material from the mother already present in the egg. That description is accurate, but incomplete, because the egg is not a passive participant. It activates a sophisticated, highly effective defense system that blocks additional sperm from entering – something that could disrupt embryonic development.

Most human cells carry 46 chromosomes – 23 pairs – DNA molecules that carry most of our genetic information. But sperm and egg cells carry only half that number: 23 chromosomes each – only one set, not 23 pairs. So when sperm and egg fuse, they form an embryo with the full complement of 46 chromosomes: 23 from the mother and 23 from the father.

If a second sperm cell were to enter after the first, it would add an extra set of 23 chromosomes to the fertilized egg. That kind of chromosomal imbalance would make the earliest rounds of cell division go badly wrong and prevent normal embryonic development. Across animals, eggs have evolved multiple safeguard mechanisms that make it much less likely that more than one sperm will succeed.

One of these mechanisms responds to the entry of a single sperm by rapidly hardening the protein coat surrounding the egg, so the remaining sperm cells encounter what is effectively a sealed barrier. We still do not fully understand how this hardening process works. In a 2024 study published in Cell, researchers from Sweden have determined the three-dimensional structure of the protein ZP2 – an important component of the egg’s protective coat in mammals.

 
The entry of an additional sperm cell after the first has already entered adds a third set of 23 chromosomes to the egg, greatly complicating cell division and preventing the embryo from developing. Eggs fertilized by two sperm cells; the three nuclei containing the genetic material can be seen in yellow | Professor P.M. Motta et al. / Science Photo Library

Shape Matters

Proteins are small, complex molecular machines responsible for carrying out most biological processes in the body. For a protein to function properly, it must be assembled correctly – which makes its three-dimensional structure crucial. If a protein folds or is built incorrectly, its function can change, potentially contributing to disease or disrupting vital processes. Determining a protein’s 3D structure is no simple task: with today’s faster tools, some structures can be solved relatively quickly, but challenging proteins can still take months—or even years—to pin down.

The egg’s outer coat is a tangle of proteins: a dense, branching network that protects the egg from unwanted intruders. To determine the structure of these proteins, the researchers aimed X-rays at them and analyzed the resulting diffraction patterns. They found that when a sperm cell enters the egg, the protein ZP2 is cleaved. This cleavage alters ZP2’s structure and gives it a new ability: to bind to another cleaved ZP2 molecule. As many such units bind together, they form an increasingly complex, intertwined shape, drawing closer and closing in a manner reminiscent of a zipper. In this way, the network around the egg becomes denser and stiffer, preventing additional sperm from entering.

“It was known that ZP2 is cleaved after the first sperm has entered the egg, and we explain how this event makes the egg coat harder and impermeable to other sperm,” explained molecular biologist Luca Jovine, of the Karolinska Institutet, who led the study. “This prevents polyspermy – the fusion of multiple sperm with a single egg – which is a fatal condition for the embryo.”

When many units of the protein bind together, they form a dense, rigid network around the egg that blocks additional sperm. Illustration of the network in its open (right) and closed state | Joana C. Carvalho

Preventing Pregnancy Without Hormones?

Hormonal contraceptives are widely used, and oral birth control pills remain one of the most common reversible options. The pill is highly effective when used correctly and is entirely under a woman’s control, but it also has drawbacks. Because it relies on added hormones, it can affect mood and other aspects of well-being, and it isn’t the safest choice for everyone—for example, some women with elevated risk factors for hormone-sensitive cancers may be advised to avoid certain formulations.

A better and more detailed understanding of how the egg’s outer coat changes in response to fertilization could have implications for developing non-hormonal contraceptives that mimic the egg’s own natural “block.” In principle, a drug might trigger parts of this protective switch in advance, for example by using a molecule that cleaves the ZP2 protein and makes the coat resistant to sperm entry even before fertilization occurs. Developing such a contraceptive would, of course, require extensive additional studies in animals and humans to test the safety and effectiveness of this approach.

The growing understanding of how the egg’s protective coat works also sheds light on some forms of female infertility, which may be linked to faults in the complex protein network that surrounds and protects the egg.  “Mutations in the genes encoding egg coat proteins can cause female infertility, and more and more such mutations are being discovered,” Jovine added. “We hope that our study will contribute to the diagnosis of female infertility and, possibly, the prevention of unwanted pregnancies.”

Even so, key questions remain. Researchers are still piecing together the sequence of molecular events, and the players involved, in the rapid remodeling (“hardening”) of the egg’s protective coat after fertilization.