Among human cells, germ cells can be said to be full of the most mysterious, for example, scientists have found that eggs will choose their favorite sperm, and sperm in order to obtain the favor of eggs, they need to pass layers of tests.
For this reason, human sperm can use the tail to gain strength as they swim towards the egg. Researchers from the UK found that the tail of healthy sperm is covered with a reinforcing layer, which allows them to make a powerful and rhythmic swing of the tail, thus breaking through the cervical mucus barrier.
According to statistics, of the 55 million sperm released by men during conception, only about 15 sperm are able to pass through the reproductive tract. Cervical mucus is 100 times more viscous than water, making the union of eggs and sperm one of nature's toughest choice challenges.
The caudal part of the sperm is made up of about 1,000 components, including a structure called tubulin, which is the main component that makes up the microtubules. Attached to these microtubules are a class of molecules that can move, called motorprotein. The movement of these molecules contributes to the stretching and bending of the tail of the sperm, causing the sperm to swim. The energy in the caudal part of the sperm comes from the mitochondria. It is well known that mitochondria are power stations that produce energy in cells.
However, since there is no central nervous system to decide how and when to exercise, the factors that control sperm motility remain a scientific puzzle. "We know that, like our arms and legs, sperm also have tiny muscles that allow them to bend the tail, but no one knows how all this in the tail is carefully organized at the nanoscale."
And when the challenged sperm comes to the egg, in fact, the victory or defeat has long been revealed, the egg will pick its favorite sperm, and then immediately release the follicle fluid (the follicle fluid is a nutrient-rich liquid that surrounds the follicle during the development and release of the egg, and the sperm must swim through the follicle fluid to reach the unfertilized egg), it induces the sperm with strong genetic compatibility to approach it and end the mate selection process, while other sperm will be hindered by the progress of the follicle fluid and fail miserably.
By taking follicle fluid and sperm samples from 16 couples undergoing fertility treatment and then putting them together in different combinations, the scientists found that although humans themselves chose a mate, it did not mean that the sperm of the spouse was also able to get the egg, and the egg did not always coincide with the woman's choice of partner. When comparing the sperm of two men, the eggs attract 18 to 40 percent more sperm from the man they want.
Because only one sperm can bind to the egg, this greatly improves the chance of the egg being conceived by its favorite sperm, which can be said to be a grand "backdoor". And what exactly is the standard by which an egg likes a man? Scientists are still working on it.
This also leads to some people who have normal fertility but are unable to conceive.
After selecting the preferred mate, the egg needs to identify the sperm to determine whether it is from humans, the sperm is the same, the sperm membrane surface and the egg zona pellucida surface have a special mutual recognition device, sperm and egg meet, recognize each other and fuse into a new generation of life, is one of the most important events that will occur in the human body's 60 trillion cells. Scientists have long been looking for important factors in sperm-ovival fusion events. In 2005, scientists discovered a new protein lzumo1, which is to make sperm and egg fuse when fertilized, and this protein also determines the reproductive ability of males, and British researchers found the receptor folr4 that binds egg cells to the protein lzumo1 on the surface of sperm cells, and it is the binding of lzumo1 and folr4 that completes the fertilization process of eggs.
After successful binding, they release "zinc fireworks" to celebrate, and in 2011, researcher teresa Woodruff and colleagues at Northwestern University, through chemical and physical probes, first discovered that eggs in mice absorb nearly 2 billion zinc ions as they mature, and store them in vesicles (also known as cortical particles) in the egg's zona pellucida. When a fertilized egg forms, these zinc ions will spray outward, triggering a stunning "zinc firework." Using a new fluorescent sensor that tracks zinc movement within living cells, they get a glimpse into the ability of eggs to store zinc — about 8,000 zinc storage zones within eggs, each containing about 1 million zinc atoms. After fertilization, these little "fireworks" lasted about two hours.
After 10 years of research, in 2021, a paper published in the top international scientific journal Nature Chemistry revealed this phenomenon in detail.
The researchers found that from the point where sperm entered the egg, the calcium ions inside the fertilized egg were repeatedly raised, a process also known as "calcium oscillation" that activates the egg cells. At the same time, zinc ions begin to be released from cortical particles. The billions of zinc atoms in the vesicle will be ignited like fireworks in an instant, emitting a terrifying flash of light.
Why is zinc so special? The researchers found that egg cells control the development of healthy embryos by allocating zinc, and high concentrations of zinc ions activate the cytostatic factor (csf) emi2 to maintain the activity of mature, unfertilized egg cells. Only when zinc ions are excreted and the concentration is reduced, emi2 is inactivated, and this process activates the fertilized egg, causing it to detach from the state after meiosis and enter mitosis, continuing the subsequent growth and classification process. This condition is common in mammals.
That is, zinc plays an important role in controlling the "decision" of egg cells to develop into embryos. Therefore, the more zinc is released, the brighter the light, and the stronger the developmental ability of egg cells. First, the egg cell must store zinc; then, it must release part of the zinc in order to successfully guide maturation with fertilization and begin to form an embryo.
The study also invented that amphibians have the same "spark splashing" process when fertilizing, just like humans, mice and other animals. This also means that this "fireworks" show has lasted for 300 million years.
The scientists' study could be used to predict the quality and development of embryos. To this day, germ cells are still mysterious for humans, waiting for scientists to reveal the secret.