As it turns out, this little fireworks display is what causes the egg (now fused with the sperm) to start dividing again and develop into a baby, reigniting a process of cellular division that has been on hold most of your life.1 This exhibition is visualized in the laboratory via x-ray fluorescence microscopy.
Zinc is a transition metal that is a crucial part of most of the cells in your body, helping to catalyze many of the enzymatic reactions that occur on a day-to-day basis. Another crucial role it plays in mammals is halting eggs at this critical time point in their development.
Dr. Teresa K. Woodruff of the Department of Obstetrics and Gynecology at the Feinberg School of Medicine at Northwestern University in Chicago has been studying this zinc spark for nearly a decade. She and her lab discovered this zinc spark back in 2010 in an NIH-funded study.2 Up until April 2016, the published works regarding this zinc spark had been on non-human eggs, due to the ethical and legal limitations.
A long time ago when you were still inside your mother’s womb, your own eggs were developing in your tiny little ovaries. However, those 1-2 million eggs that you are born with were frozen mid-process (at the Prophase I stage for those who remember their high school biology) and would not resume their meiotic maturation until shortly before ovulation. This freezing occurred because the egg (technically called an “oocyte” at this stage) takes up over 20 billion zinc atoms.2
Shortly before ovulation, a massive surge of hormone (LH) signals for the egg to momentarily “unfreeze.” During the 12-14 hours between this surge and ovulation, the egg will go through the next stage of meiotic maturation, called metaphase II, where it halts once again until fertilization occurs.
Because zinc is responsible for holding the egg at these arrest points, the easiest way for it to resume its maturation is for the egg to release all of its extra zinc. So, if and when the egg is fertilized, within minutes it purges itself of all this now unnecessary zinc and starts developing into a baby. 3
But how does the fertilized egg ignite this zinc spark? Calcium.
When the sperm finally breaks through the outer layers and fuses with the egg, there is a release of calcium inside the egg that is triggered. The role of this calcium flood is multifaceted: it triggers the slow block to polyspermy, which prevents multiple sperm from fertilizing the same egg, and is involved in the fertilized egg resuming development.
They found that each zinc spark was preceded by a fluctuation in the amount of calcium present in the fertilized egg. Therefore this increase in the amount of calcium in the egg triggers the release of the zinc that is keeping the egg on hold, allowing the cell cycle to resume and develop into the little baby you meet 9 months later.4
In a really incredible video you can actually see the spark the moment the egg is fertilized.
What they have now come to understand after studying human eggs (studied via parthenogenesis – or the asexual development of an egg when there is not a sperm to fertilize it) is the intensity of this microscopic light show may distinguish good, healthy, viable eggs from eggs that are less likely to survive.
This could be an incredibly useful tool to those undergoing in vitro fertilization. The number of heartbreaks due to the cost and shattered hopes of failed IVF treatments may be minimized if they can visually see the best candidates and increase the likelihood of a successful pregnancy.
Read the full article.
1Kong, B. Y., Bernhardt, M. L., Kim, A. M., Ohalloran, T. V., & Woodruff, T. K. (2012). Zinc Maintains Prophase I Arrest in Mouse Oocytes Through Regulation of the MOS-MAPK Pathway1. Biology of Reproduction,87(1). doi:10.1095/biolreprod.112.099390
2Kim, A. M., Vogt, S., Ohalloran, T. V., & Woodruff, T. K. (2010). Zinc availability regulates exit from meiosis in maturing mammalian oocytes. Nature Chemical Biology,6(9), 674-681. doi:10.1038/nchembio.419
3Duncan, F. E., Que, E. L., Zhang, N., Feinberg, E. C., O’Halloran, T. V., & Woodruff, T. K. (2016). The zinc spark is an inorganic signature of human egg activation. Scientific Reports,6(1). doi:10.1038/srep24737
4Kim, A. M., Bernhardt, M. L., Kong, B. Y., Ahn, R. W., Vogt, S., Woodruff, T. K., & O’Halloran, T. V. (2011). Zinc Sparks Are Triggered by Fertilization and Facilitate Cell Cycle Resumption in Mammalian Eggs. ACS Chemical Biology,6(7), 716-723. doi:10.1021/cb200084y