Understanding sperm development and fertilization: Scientifically protecting men's health
Sperm Development and Maturation
As the saying goes, "You reap what you sow," and male sperm is an integral part of the seed of life. Where are sperm produced, and how do they mature?
The testes contain many seminiferous tubules, each containing numerous reproductive cells called spermatogonia. These are the "embryos" that produce mature sperm. During childhood, these cells are in a dormant state. Only after sexual maturity do they "awaken," undergoing multiple divisions to develop into primary spermatocytes, then secondary spermatocytes, and finally, after a second division, spermatids, which continue to develop into sperm. Therefore, the testes are the "factory" for sperm production. The sperm production process takes approximately 74 days and is regulated and controlled by a series of hormones secreted by the hypothalamus and pituitary gland.
Adult testes have a tremendous spermatogenic capacity, producing an average of 10 million sperm per gram of tissue per day. This capacity gradually declines after age 40, although there are reports of men as old as 90 still possessing spermatogenic ability.
After sperm formation, sperm are collected in the rete testis via the seminiferous tubules, then pass through 8-12 efferent ductules to the epididymal duct. The epididymal duct is winding and coiled, reaching a total length of 5-6 meters, before migrating into the vas deferens. Sperm typically remain in the epididymis for 19-25 days. During their journey to and residence in the epididymis, sperm gradually acquire motility and fertilization capacity. This is the process of sperm maturation.
Sperm possess an inherent maturation capacity, meaning they do not necessarily mature in the epididymis. Sperm can also mature under in vitro culture conditions. However, if sperm that do not mature in the epididymis are used for artificial insemination, the embryo mortality and malformation rates are very high. Therefore, the epididymis is not merely a passageway; its normal function provides the necessary and suitable conditions for sperm maturation.
After sperm maturation, during intercourse and ejaculation, sperm travel through the 35-45 cm long vas deferens to the enlarged ampulla at its end, and finally through the ejaculatory duct into the posterior urethra, expelling semen from the body. Under normal circumstances, an adult male ejaculates 2-6 ml of semen each time, containing 100-200 million sperm per milliliter. When the sperm count is less than 20 million per milliliter, the chance of fertilization is significantly reduced; when it is less than 4 million, fertilization is difficult.
Because sperm are delicate and easily damaged, care must be taken in daily life.
Alcohol and tobacco are major enemies of sperm. Some men's bodies are quite sensitive to the toxins in cigarettes, especially the reproductive cells in the testes, which are more easily harmed. Nicotine in cigarettes can reduce the secretion of sex hormones and kill sperm. Alcohol abuse not only leads to decreased reproductive gland function and inhibits sperm formation, but also causes about 70% of sperm to be underdeveloped and lose their vitality, resulting in chromosomal abnormalities in sperm and thus fetal malformations. Sperm growth requires a supply of nutrients. Some men have monotonous diets, are picky eaters, or dislike animal products, leading to a decline in zinc levels over time. 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. Sperm development requires low temperatures; therefore, the temperature inside the scrotum must be at least 1–1.5°C lower than body temperature, and the temperature inside the testicles must be 0.5–1°C lower than the scrotum temperature, otherwise, sperm will die. If a man has a habit of taking hot baths, it can raise the temperature of the scrotum, thereby reducing sperm count and leading to infertility. Wearing tight pants like jeans can constrict the scrotum, making it difficult for heat to dissipate, and the local temperature of the testicles will also rise, which is detrimental to sperm growth. Furthermore, frequent sexual intercourse can lead to impotence and reduce the amount of sperm in each ejaculation. Frequent use of sedatives and hormones can also cause sperm growth disorders. High doses of radiation exposure can cause sperm chromosome abnormalities. Therefore, men of reproductive age should avoid prolonged and excessive exposure to harmful substances and should not use medications indiscriminately.
The Secret of Sperm-Egg Union
The union of sperm and egg is not an easy process; it requires passing through at least four hurdles: through the vagina, through the cervix, through the uterine cavity, and finally into the fallopian tube to meet the egg.
Within two hours in the vagina, 90% of sperm die, and most survive for no more than eight hours. 1%–5% of sperm reach the uterine cavity. Most sperm that enter the uterus can survive for up to six hours, while those in the fallopian tubes can survive for 1–3 days.
After intercourse, the amount of semen ejaculated by the male into the vagina ranges from 2–3 ml to 5–6 ml, with each milliliter containing anywhere from 60 million to 100 million–200 million sperm. This massive "army" marches deep into the female reproductive tract.
Whether they can enter the cervix is a challenge, related to the composition of cervical mucus. During ovulation, cervical mucus is abundant and thin, facilitating sperm passage. Simultaneously, the cervical mucus contains fewer white blood cells, reducing the chance of white blood cells engulfing sperm. In addition, at this time, the cervical mucus is more alkaline, and sperm happen to prefer alkaline environments and dislike acidic ones, thus protecting the sperm. At other times, the cervical mucus is less abundant and viscous, containing fewer nutrients, making it difficult for sperm to pass through. It also contains a large number of white blood cells that engulf the sperm. The cervical mucus also filters sperm because it contains countless protein fibers that form a series of "barriers." Only strong and vigorous sperm can overcome these obstacles, while abnormal or weak sperm cannot. Therefore, the sperm that pass through the cervical mucus and enter the uterine cavity and fallopian tubes are the best of the best.
After entering the uterine cavity through the cervix, the sperm continue their journey with the help of the fluid within the uterine cavity, reaching the fallopian tubes. Although the distance is only a few centimeters, for tiny sperm, the journey is not only long but also fraught with obstacles. While passing through the narrow fallopian tube, some sperm get stuck. Only 15-20 sperm ultimately reach the ampulla of the fallopian tube. At this point, the egg, which has been waiting there, uses its estrogen to adhere to and attract the sperm. The sperm rush in together, their heads secreting hyaluronidase to dissolve the outer corona radiata membrane of the egg, but only the strongest sperm survive. Once the sperm enters the egg, the egg immediately closes its doors, refusing any "third party" intrusion. Thus, the union of sperm and egg is finally complete.