What Exactly Happens When Sperm Meets Egg?

When sperm fertilizes an egg, sparks literally ignite. How can these flashy signals be useful in non-traditional forms of fertilization?

When sperm meets egg for the first time, it's an exciting moment for everyone concerned. You might even say that sparks fly. Actually, according to a recent breakthrough study at Northwestern University, sparks really do fly. Crystal Dilworth has the details in today's DNews report.

The Northwestern research shows that human oocytes -- or egg cells -- give off a a shower of zinc sparks when activated by a sperm enzyme. In a kind of biological fireworks show, the eggs launch a barrage of zinc ions that signal it's time to start the nine-month assembly process of making a baby.

Scientists have known about zinc sparks for a while -- the phenomenon has been observed in other mammals. But the Northwestern study marks the first time that zinc sparks have been documented in a human egg. The study has genuine and immediate practical value, in that it can help doctors choose the best eggs to transfer during in vitro fertilization.

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Some background on the zinc thing: When mammalian eggs are released by the ovaries, they start to absorb billions of zinc atoms. The zinc acts as a signal to halt the cell division process known as meiosis and wait around to be fertilized.

When a sperm does enter the picture, and fertilization is achieved, the egg cell needs to eject all those zinc ions and start dividing again. Those ejections are figuratively called sparks, but the Northwestern team took an admirably literal approach to the term.

They ran an experiment in which human oocytes were put into a petri dish of fluorescent dye. The dye, however, only emitted light when zinc was present. So long as the zinc in the oocytes was contained within the cell wall -- no light. But when the oocytes were fertilized, the dye lit up, revealing that the zinc was indeed being released. For the first time, scientists could actually see zinc sparks. You can see it for yourself here.

(By the way, the process only emulated the process of fertilization. No actual baby-making took place in the lab. At least not in the petri dishes -- we can't know what the grad students get up to after hours.)

Anyway, the particularly cool aspect of the experiment is that only some of the oocytes triggered the "spark of life" indicating embryonic progression. As such, doctors will be able to select for these and increase the odds of success for women undergoing IVF treatment. Since IVF success rates are only around 40 percent under the best of circumstances, it's a Pretty Big Deal.

-- Glenn McDonald

Learn More:

NCBI: Zinc Sparks Are Triggered By Fertilization And Facilitate Cell Cycle Resumption In Mammalian Eggs

CDC: Assisted Reproductive Technology: Fertility Clinic Success Rates Report