The human fertilization process is usually described as a massive swimming marathon, and only the fastest and strongest sperm can win the reward of combining with the egg cells. However, the real process of human fertilization is very different from what people imagine. In this article, the famous biological anthropologist Robert Martin tells us a real but unfamiliar fertilization process.
Before science can explain the reproduction of human beings, most people think that new life is spontaneously produced by non-living matter. In the mid-17th century, natural philosophers were barely able to see female eggs with the naked eye. They also created a theory called pre-formation that all life is produced when God created the world, and new life exists in the female egg, like the Russian doll.
Some may think that with the advancement of science, the emergence of clear biological lenses will overturn this theory. But on the contrary, when the microscope finally allowed the researchers to see the sperm and the egg cells, the theory of the first theory became a new, more patriarchal political thought: philosophers and some students who studied reproduction believed that the egg was only A passive, waiting for viable sperm to start a developing container. And what about sperm? Every sperm has a tiny, already shaped person on the head. In 1695, the Dutch mathematician and physicist Nicolas Hartsoeker first painted the image of the gnome in the sperm when he observed human sperm under the microscope. Hatsock admitted that he did not see the gnome at the time, but he believed that it was there.
Nicholas Hatsock believes that there are small people living in the sperm.
A clearer microscope eventually overturned this theory. But to some extent, there have not been many changes. The residue of the first generation persisted, and the egg was regarded as a passive participant in fertilization, waiting for the active sperm to come to life after hardship. Whether in the mass media or scientific media, human fertilization is often described as a massive swimming marathon, and only the fastest and strongest sperm can win the reward of combining egg cells. However, the real process of human fertilization is very different from what people imagine.
Why are there so many sperm?
High-precision microscopy shows that in humans, the average half teaspoon of ejaculation contains about 250 million sperm. But it also leaves a key question: “Why are there so many sperm?” In fact, studies have shown that once the number of sperm in a semen shot by a male is less than 100 million, the probability of a woman becoming pregnant is reduced.
A common explanation for this is sperm competition. Just like buying a lottery ticket, the more lottery tickets you buy, the higher the chance of winning. By the same token, natural selection also increases the number of sperm, like an arms race for the reward of birth.
Cases of sperm competition do exist in the animal world. Our close relatives, chimpanzees, often engage in chaotic mating, and the same female may mate with several males in turn. This mammal has the ability to produce large amounts of sperm quickly. A large amount of semen condenses in the vagina, temporarily blocking the entry of semen from competitors.
For humans, despite all sorts of sensational claims, there is no convincing evidence that human males are physiologically adapted to sperm competition. Among the social units of many primate primates, there is only one male that can breed, and the testicles are often smaller due to lack of direct competition. Humans are extremely similar to these primates. Human testes are only the size of a walnut, one-third of chimpanzees, and the proportion of abnormal sperm in human semen is much higher than that of chimpanzees. Due to the lack of direct sperm competition, humans seem to have relaxed control of semen quality.
Compared with chimpanzees, humans lack direct sperm competition.
For species that do not directly experience sperm competition, the only possible explanation is that high sperm count is associated with genetic diversity. In several rarely cited papers published more than 40 years ago, Jack Cohen, a biologist at the University of Birmingham in the United Kingdom, pointed out that the number of sperm is related to the number of copies of the chromosomes in its production. During meiosis, pairs of chromosomes exchange large amounts of genetic material by cross-interchange. Cohen found that the more crossovers between different species, the more sperm there are.
One billionth of good luck
Other findings are also different from the public opinion. For example, most mammalian sperm cannot actually swim through the female reproductive tract, and a large part needs to be transported through the movement of the uterus and fallopian tubes. Surprisingly, on average, small mammals have longer spermatozoa than larger mammals, so for small mammals, it may be feasible for sperm to swim through the shorter reproductive tract. But for the smaller blue whale sperm, it is obviously impossible to swim in the reproductive tract of the female blue whale 100 times that of the mouse without help. Convincing evidence also shows that human sperm are passively transported a considerable distance in the way through the uterus and into the fallopian tube. This is not the same as the Olympic runners described earlier!
In fact, only a few hundred of the 250 million sperm that a human ejaculate shoots can reach the fertile tube for fertilization. Rather than a swimming competition, the road to fertilization is more like a challenging military obstacle course. The number of sperm decreases as they move within the female reproductive tract. All abnormally shaped sperm cannot complete this journey, and only random intact sperm can reach the egg and become survivors.
The process of sperm swimming to the egg cell requires a lot of obstacles.
Many sperm can’t even reach the cervix. Sperm cannot survive in the acidic environment of the vagina for a long time. When passing through the cervix, many sperm that successfully leave the vagina will be trapped in the mucus of the cervix, and all the deformed sperm cannot pass through here. In addition, thousands of sperm will enter the tubule structure called the crypt and store it for a few days. Only a small number of sperm can pass directly through the uterine cavity, and the number of sperm will be further reduced during the process of entering the fallopian tube. Once in the fallopian tube, the sperm is temporarily bound to the inner surface of the fallopian tube, and only part of it will be released, close to the egg.
The idea of comparing sperm with successful egg cells to the Olympic champion masks the fact that a ejaculation can contain too much sperm. If too much sperm surrounds the egg, the risk of multiple sperm being inseminated at the same time (multiple spermatozoa) increases, which can lead to catastrophic consequences. More spermatozoa occur occasionally, especially when the number of sperm in men is very high. Most of the results are that two sperms enter the same egg, which causes the embryonic cells to contain 69 chromosomes instead of the usual 46 chromosomes, which usually leads to fatal consequences of miscarriage. In order to avoid multiple spermatozoa, the female reproductive tract gradually evolves a series of barriers that strictly limit the number of sperm around the egg cells.
The possibility of multiple spermatozoa provides new clues to the evolution of sperm count. Discussions about sperm competition usually focus only on maximizing the number of sperm. But as is common in biology, some kind of trade-off exists. If there is direct competition in the male, natural selection leads to an increase in the number of sperm, but females also enhance certain mechanisms to limit the number of sperm around the egg. For example, the special case of chimpanzees in primates, their mating is more open, sperm competition is more intense, so the number of sperm produced by males is also more, the corresponding females evolved longer fallopian tubes to limit the number of sperm close to the eggs. This also shows that females do not be as passive in their reproductive function as they usually think.
There is also a deep-rooted view that only the best sperm can win. The various conjectures that have arisen suggest that some sort of choice has occurred, but it is hard to imagine how these choices occur. The DNA located in the sperm head is highly condensed and almost crystalline. How can females detect the nature of these DNAs from the outside? For example, mouse experiments have shown that individuals do not choose whether to have a Y chromosome or an X chromosome in sperm. Human fertilization seems to be more like a draw from 250 million lottery tickets. For healthy sperm, the essence of successful fertilization is just the luck of surpassing other sperm.
People have always believed that after ejaculation, sperm will swim wildly to the egg, and that human sperm can only survive for two days in the female reproductive tract. However, since the mid-1970s, there has been increasing evidence that human sperm can survive at least five days in perfect condition. Nowadays, it is widely accepted that sperm can survive for 10 days or more.
Sir Robert Edward won the 2010 Nobel Prize for research in in vitro fertilization. In his more than 1,000-page textbook, Human Women (1980), cervical crypts are mentioned in one sentence. After this, many other authors also briefly mentioned the sperm stored in the cervix. Sperm may survive for 10 days or longer, which completely impacts the idea of the so-called “natural” contraceptive method.
Another dangerous misconception is that older men still maintain full fertility. This is in stark contrast to the sudden disappearance of fertility in menopausal women. There is a lot of evidence that the quantity and quality of male sperm decreases with age. In addition, recent studies have reported that the rate of sperm mutation is four times that of egg cells, so the semen of older men actually contains high risk factors.
Many articles have written that in industrial society, women’s first birth age is getting bigger and bigger, which brings a series of fertility problems. However, the increasingly severe reproductive problems of older men, especially the rapid accumulation of sperm mutations, have largely not been taken seriously.
Today, the story of the “little man” in Hart Sok’s sperm seems to have been hidden in the fog of time, just to be mentioned as a wrong and interesting illustration of the early exploration of human germ cells. But its influence and the resulting male prejudice have also remained in the cultural stereotypes of reproductive problems, continuing to exist in a more subtle form.
About the Author:
Robert Martin, biological anthropologist, honorary curator of the Field Museum of Natural History, and an adjunct professor at the University of Chicago, the University of Illinois at Urbana-Champaign, and Northwestern University.