Sparks really do fly when the magic happens. At the exact moment a sperm penetrates an egg, the latter releases billions of zinc atoms from its surface which spark off. Using fluorescent microscopy, these sparks emit a light, which can then be filmed. The discovery that human eggs cells undergo this process could lead to new ways for doctors to identify viable eggs for in vitro fertilization (IVF).
“It was remarkable,” explained Teresa Woodruff, one of the two senior authors of the paper published in Scientific Reports. “We discovered the zinc spark just five years ago in the mouse, and to see the zinc radiate out in a burst from each human egg was breathtaking. All of biology starts at the time of fertilization, yet we know next to nothing about the events that occur in the human.”
The scientists were able to image the zinc sparking off the human eggs by injecting them with an enzyme released by the sperm when they hit the egg in normal circumstances. They weren’t allowed to use actual sperm due to restrictions on human research under federal law, but previous studies using mouse eggs and sperm have shown the same thing happen. The enzyme from the sperm triggers the calcium in the egg to increase and the subsequent release of zinc.
This zinc then attaches to small molecules that in turn emit light in fluorescence microscopy experiments. This means that when the sperm hits the eggs and it releases billions of zinc atoms, there can even be a tiny flash of light. This is the first time that researchers have established that this process not only happens in other mammals, but also occurs during human fertilization.
In the development of an embryo, the level of zinc contained within an egg can have important implications in how the newly fertilized embryo grows. With the discovery that the sparks produced by the eggs are a direct marker of how much zinc they contain, it could give fertility doctors a new method in which to choose the best eggs with the highest chance of survival to be used for IVF.
“There are no tools currently available that tell us if it’s a good quality egg,” says Dr. Eve Feinberg, one of the paper's co-authors. “Often we don’t know whether the egg or embryo is truly viable until we see if a pregnancy ensues. That’s the reason this is so transformative. If we have the ability up front to see what is a good egg and what’s not, it will help us know which embryo to transfer, avoid a lot of heartache and achieve pregnancy much more quickly.”