Whether you are a frog or a human, reproduction is an occasion for cellular celebration. New research reveals that in species separated by millions of years of evolution, fertilized egg gives off metal fireworks atoms.
This microscopic shower of sparks was discovered in human eggs in 2014, leading researchers to wonder what the sudden release of metals was for. Now, the same researchers who discovered the original fertilization fireworks have found that the same thing happens in fertilization in frogs – organisms that are separated from humans by 300 million years of evolution. Both manganese and the zinc atoms explode from the membranes of the eggs just after the sperm make contact.
“It is to underline that zinc fluctuations are an old and preserved part of the regulatory mechanisms controlling development, ”said study leader Thomas O’Halloran, a chemist at Northwestern University in Illinois. He and his colleagues are still working on the details, but they believe that manganese and zinc can both prepare the egg for its future development and protect it from fertilization by more than one sperm.
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O’Halloran and his team learned in 2014 that when a human egg enters the final stages of development for fertilization, it begins to “swell” with zinc, attracting 50% more metal from its surroundings than it does. early in its development. . They have since learned that if an egg fails to incorporate all of that zinc, it cannot be fertilized. It is not clear what exactly zinc does at this pre-fertilization stage, but researchers have some preliminary data to suggest that zinc prime other proteins in the egg prepares the egg to become pluripotent, which means that the egg is able to divide. in every cell type in the body of a new organism.
At this point, O’Halloran told Live Science, zinc appears to play a communicative role, allowing parts of the cell to signal to each other. But at the time of fertilization, the zinc seems to do something else: first, it collects in tiny compartments called vesicles in the cell membrane, and then it releases in what researchers call a “zinc spark”, with billions of metal atoms exploding outwards. at once.
Since finding this zinc spark in human eggs, researchers have found it in monkeys, cows and mice. Turning to lineages older than mammals, they have now found a similar spark in the African clawed frog (Xenopus laevis). They reported their findings on June 21 in the newspaper Chemistry of Nature.
Frog eggs are fun to study, O’Halloran said, because they are much larger than mammalian eggs. This means that they are made up of more atoms, which makes it easier to use microscopy to see metal atoms.
“The frog really gave us special information that we couldn’t get from other systems,” O’Halloran said.
Nutrients for Fertility
A surprise was that when a frog’s egg is fertilized, it doesn’t just release zinc; it also releases manganese. O’Halloran and his team are unsure whether this release of manganese is specific to frogs or if it could also occur in mammals. Their metal detection threshold in mammalian egg studies was not high enough to say for sure, given that mammalian eggs are so much smaller than amphibian eggs. It could be that manganese is a peculiar quirk of amphibian eggs, which are fertilized and develop outside the mother’s body. Or it may be that a manganese spark is also occurring in humans and more detailed research is needed to observe it.
Either way, the rash of manganese and zinc from the egg during fertilization could be a way to prevent what O’Halloran has called the “polyspermia catastrophe.” Polyspermia refers to the fertilization of an egg by several spermatozoa. This leads to the problem of too many genomes and kills the developing zygote.
In the new study, researchers have shown that zinc and manganese levels prevent sperm from fertilizing an egg, leading them to suspect that fireworks are actually a protective mechanism that may prevent further sperm from entering the egg.
Zinc and manganese are interesting nutrients, O’Halloran said, because the body cannot synthesize these metals or break them down. He needs to get them from the diet, store just the right amount, and excrete any excess that he is not using.
“These are more complex nutrients than most vitamins, proteins or fats,” he said.
Zinc is found throughout the body and was once thought to simply help enzymes in cells complete chemical reactions. More recently, said O’Halloran, it has become clear that zinc is sometimes used as a cellular messenger. Manganese, on the other hand, has yet to be shown to play a large messenger role, but research is limited, O’Halloran said.
The main goal of research is simply to unravel the complex ways in which eggs work. After all, said O’Halloran, these cells begin to grow when the female is still a fetus and must remain in a state of stasis for years before they suddenly grow very quickly in preparation for fertilization. Ultimately, the role of zinc and other metals in this process could raise questions for fertility researchers, such as whether a lack of zinc plays a role in infertility.
“I think this indicates that you need to have a very complex array of elements available to cells during reproduction,” O’Halloran said.
Originally posted on Live Science.