We’ve all felt that metaphorical spark when we connect with someone, but when a sperm connects with an egg, it seems that sparks quite literally do fly. Using a combination of sophisticated imaging and mapping techniques, a team of researchers in the US has, for the first time, captured and measured the molecular firework display that occurs in mammals shortly after fertilization, offering new insight into these events.
As described in Nature Chemistry, the twinkling show comes from billions of zinc atoms that are released in a wave from the egg’s surface. Since these events, or “zinc sparks,” are required for successful egg-to-embryo transition, the team believes that their work could help improve in vitro fertilization (IVF) treatments.
“The amount of zinc released by an egg could be a great marker for identifying a high-quality fertilized egg, something we can’t do now,” study author Teresa Woodruff said in a news release. “If we can identify the best eggs, fewer embryos would need to be transferred during fertility treatments.”
For the study, researchers from Northwestern University developed a new fluorescent sensor that enabled them to track the movement of zinc atoms in live cells. Using sensitive imaging techniques, the scientists revealed the presence of some 8,000 packages, or vesicles, in the egg—each one jam packed with around a million zinc atoms.
They showed that as the egg matures, these tiny parcels undergo dynamic movement before simultaneously erupting their zinc cargo from the cell surface at the time of fertilization. This process of material release into the surrounding environment is called exocytosis and is the same way that brain cells release signaling molecules, or neurotransmitters, as a means of cellular communication.
The researchers were able to gather fine details of the event using high-energy X-ray imaging, which allowed them to map the bursts of zinc atoms in both 2D and 3D. This revealed that, rather than all of the pouches releasing their contents at the same time, the sparks came in intermittent waves, with around two thousand vesicles dumping their load simultaneously before a short stretch of silence.
“Each egg has four or five of these periodic sparks,” study author Thomas O’Halloran said in a news release. “It is beautiful to see, orchestrated much like a symphony. We knew zinc was released by the egg in huge amounts, but we had no idea how the egg did this.”
This work builds on previous studies in the same lab which found that eggs require a huge amount of zinc in order to reach maturity. That’s because zinc is a crucial part of a master switch that controls the decision to kickstart embryogenesis, or the formation and development of an embryo. After hoarding zinc atoms, they found that around a sixth of the 60 billion zinc atoms in the cell are released during the two hour period after the sperm and egg connect. This ensures successful navigation of the fertilization process and initiates embryo formation.
The researchers are now taking this work forward by investigating whether these zinc release events are associated with the quality of the egg, which could assist IVF treatments.
Credit: Northwestern University. Top left: zinc labelled in a live egg cell, middle: electron microscope image of zinc, left: X-ray image. Bottom images show release of pockets.