4. Sperm production, characteristics, and the process of fertilization.

The Source of Life-About Sperm

Sperm production

Sperm, the male reproductive "messenger," is produced in the testicles. But how did humans discover sperm? And how is sperm produced in the testicles? Let's uncover the secrets.

Since ancient times, humankind has relentlessly explored the mysteries of reproduction. More than two thousand years ago, the famous ancient Greek physician Hippocrates proposed a view on human propagation, believing that the semen ejaculated by men during intercourse contained something that could enable women to conceive-sperm. Aristotle, another ancient Greek scholar, went even further, stating, "In procreation, the embryo forms within the uterus from the coagulation of menstrual blood, and the man's semen has the ability to give life to the embryo." In later years, the medical community developed even more ingenious hypotheses about sperm, proposing the so-called preformation theory. This theory suggests that the reason a man's sperm can enable a woman to bear children is because the sperm has a specific, tiny human form. Within it lies an even smaller human form. And within this smaller form are even smaller forms, and so on, endlessly. In the late 17th century, Hamm, an assistant to the Dutch scholar Levenhoek, discovered tiny bodies in semen under a microscope. People were astonished to realize that this was the sperm that medical scientists had been pondering for centuries. This discovery shocked the scientific community at the time.

So, what do sperm look like under a microscope? A mature human sperm is about 60 micrometers long. Despite its small size, a sperm can be divided into four parts: head, neck, body, and tail. The head is relatively large, about 4.6 micrometers long, 2.6 micrometers wide, and 1.5 micrometers thick. The head and body together are roughly the same length as the head. The tail is about 10 times the length of the head, reaching about 40 micrometers. Its overall shape is very similar to a tadpole.

So, if we compare the testicles to a "factory" that produces sperm, then the seminiferous tubules inside the testicles are the "workshop" for producing sperm. In other words, this is where sperm are born, but the process of sperm production is quite complex.

First, sperm production requires a "driving force," namely the regulation of the endocrine system. Specifically, certain sex hormones directly command and manipulate sperm development and growth. It's worth noting that the hypothalamus in the human brain secretes two types of gonadotropins: one is called follicle-stimulating hormone (FSH), which acts on the pituitary gland, causing it to secrete more FSH. This FSH then acts on the seminiferous tubules of the testes, promoting sperm production. The other is called luteinizing hormone-releasing hormone (LH), which acts on the pituitary gland, causing it to secrete interstitial cell-stimulating hormone (ILS-C), which in turn acts on the interstitial cells in the testes, causing them to secrete testosterone. Testosterone, in turn, promotes sperm production. Therefore, sperm production relies on the action of a series of sex hormones.

Next, let's look at how sperm are produced in the seminiferous tubules. The seminiferous tubules in the testes are composed of countless germ cells and supporting cells. Germ cells include spermatogonia, spermatocytes, and spermatids. Spermatogonia are attached to the basement membrane of the seminiferous tubules, constantly dividing and differentiating, and gradually moving from the base into the lumen. Therefore, spermatogonia are the "ancestors" of sperm. Each spermatogonia can eventually develop into four sperm cells, and the seminiferous tubules in the testes contain hundreds of millions of spermatogonia. At a ratio of 1 to 4, the sperm production is enormous. Data shows that the total weight of a man's two testes is approximately 20-40 grams, and each gram of testicular tissue produces 10 million sperm cells per day, totaling approximately 200-400 million sperm cells daily-an astonishing number.

However, sperm production requires two stringent conditions: one is nutrition. The division of spermatogonia into sperm, especially the process of one cell dividing into four, requires a large amount of nutrients, particularly proteins, which are known as the "building materials" of the human body. The other is low temperature. Sperm growth requires a low temperature; the temperature inside the scrotum must be at least 1 to 1.5°C lower than body temperature, and the temperature inside the testes must be 0.5 to 1°C lower. Otherwise, sperm growth will fail halfway.

Finally, it's worth mentioning that the sperm produced from the seminiferous tubules are still quite delicate and immature. Therefore, they need to stay in the epididymis for a while, allowing the epididymis to help the sperm undergo further processing and gradually mature.

In summary, the process of sperm production in the seminiferous tubules takes about 74 days, and then about 16 days for them to continue maturing in the epididymis, so the whole process takes about 90 days.

Six things sperm fear

Millions of sperm can be considered a vast army. People might think they must be incredibly powerful, but that's not the case. Despite their enormous number, sperm are actually quite fragile and vulnerable to various things. In summary, they have "six fears".

1. Afraid of "smoking and alcohol"

Smoking and excessive alcohol consumption are major enemies of sperm. Sperm are highly sensitive to the toxins in cigarettes. Nicotine in cigarettes can reduce the secretion of sex hormones and kill sperm. If an adult male smokes 30 cigarettes a day, the sperm survival rate is only 40%, and the rate of sperm malformation increases. Long-term heavy drinking can cause chronic or acute alcohol poisoning, which can cause 70% of sperm to be underdeveloped or lose their vitality.

2. Fear of being hungry

Some men have a monotonous diet, are picky eaters, and do not like to eat animal products (such as meat, eggs, poultry, fish, and dairy products). Over time, this can lead to a decrease in the zinc content in their bodies. Zinc, a trace element, is known as the "harmony element for couples." Zinc deficiency in men can reduce libido and sexual function, decrease sperm count by 30-40%, and even cause infertility.

3. Afraid of heat

The scrotum acts as a "temperature regulator" for the testicles, maintaining a temperature 1-2 degrees Celsius lower than body temperature for optimal sperm production. If a man has a habit of taking hot baths, the scrotum temperature can rise, potentially reducing sperm production and leading to infertility. Wearing tight-fitting jeans or similar garments that constrict the scrotum hinders heat dissipation, causing the local temperature of the testicles to rise. Furthermore, tight pants can impede venous blood flow in the scrotum, which is also detrimental to sperm production.

4. Fear of "frequent"

Frequent sexual activity can lead to impotence and reduce the number of sperm in each ejaculation. If there are fewer than 20 million sperm per milliliter of semen, the chances of getting pregnant will be very low, and if there are fewer than 4 million, it will be almost impossible for the wife to conceive.

5. Fear of "worry"

If family matters cause discord between husband and wife, leading to mutual blame and both parties being constantly depressed and troubled, these negative mental states will directly affect the function of the nervous and endocrine systems, causing testicular spermatogenesis disorders and resulting in infertility after marriage.

6. Fear of "medicine" and "threads"

Frequent use of sedatives, antitumor drugs, and chemical drugs such as malaria, furosemide, and hormones can cause sperm growth disorders, sperm chromosome damage and breakage. Excessive radiation exposure can also cause sperm chromosome aberrations.

Given that sperm are delicate and easily damaged, people should not take them lightly in their daily lives.

The union of sperm and egg

After intercourse, a man ejaculates 2-5 ml of semen containing 120-300 million sperm. This massive "army" races upstream within the female reproductive tract. Sperm propel themselves forward rapidly by the movement of their tails, moving at a rate of 2-3 millimeters per minute. To fertilize an egg, they swim upstream against the current. The female reproductive tract is only 16-20 centimeters long, but for sperm, it is a very arduous journey, fraught with dangers that can affect their progress and lifespan. For sperm to reach the egg, they must overcome at least four hurdles: passing through the vagina; traversing the cervix; traveling within the uterine cavity; and finally entering the fallopian tube to meet the egg.

During male ejaculation, most semen is ejaculated into the upper part of the vagina and the cervix, entering the cervix within minutes. Of this large number of sperm, only 1-5% reach the uterine cavity, and only a few thousand reach the fallopian tubes. Poor-quality sperm lose their vitality because they cannot reach the uterine cavity quickly. A large number of sperm become exhausted and die. The number of sperm visible at the fundus of the uterus, where the uterine cavity connects to the fallopian tubes, is even lower. This phenomenon demonstrates that the cervix plays a crucial role in preventing a large number of sperm from entering the uterine cavity.

Sperm can only survive for 1-3 days in the female reproductive tract, and no more than 8 hours in the vagina. For sperm to meet the egg, they must overcome a significant hurdle: whether they can penetrate the cervix. This hurdle is related to the composition of cervical mucus. During ovulation, a woman's estrogen levels rise, causing cervical mucus to become thinner, clearer, and more abundant, resembling egg white. This increased volume contains nutrients such as sugars, vitamins, and salts, providing the energy needed by sperm, maintaining their motility, and facilitating their ascent. Therefore, sperm can most easily pass through the cervix during ovulation. At other times, however, cervical mucus is thinner and thicker, containing fewer nutrients and a large number of white blood cells, making it much more difficult for sperm to pass through.

Sperm enter the uterine cavity through the cervix and continue their journey with the help of uterine fluid. After passing through the uterus, the sperm reach the fallopian tubes and continue forward. The epithelial cells of the fallopian tubes contain cilia. The cilia bend from the fimbriae towards the uterine cavity, which would normally hinder the sperm's forward movement. However, sperm possess the remarkable property of "retrograde movement," swimming upstream and generally reaching the ampulla of the fallopian tube where they meet the egg, finally achieving their "reunion."

How does fertilization occur? Inside the fallopian tube, when a sperm and an egg approach, many sperm surround the egg. The egg cell is surrounded by a zona pellucida composed of protein, and further surrounded by radiating corona cells. The sperm acrosome releases two enzymes called neuraminidase and hyaluronidase. These enzymes digest the zona pellucida surrounding the egg, break through the cells surrounding the egg, and dissolve the acidic environment between these cells, creating a pathway for the sperm. The sperm then enters the fallopian tube and combines with the egg cell; this process is called fertilization, and the fertilized egg is called a zygote.

The fertilized egg rapidly divides and multiplies while moving from the fallopian tube into the uterine cavity, reaching it approximately 4-5 days after fertilization. Once in the uterine cavity, the fertilized egg secretes an enzyme that breaks down proteins, eroding the uterine lining. At this time, the uterine lining undergoes secretory changes under the influence of ovarian hormones, becoming like well-cultivated and fertilized soil-rich and soft. The fertilized egg, like a seed planted in this prepared soil, undergoes implantation, a process generally occurring 7-8 days after fertilization. Embedded in the uterine lining, the fertilized egg receives nourishment from the glands of the uterine lining, continuously growing and developing into a fetus.