How is sperm produced? Why is the testicle so small that it can produce hundreds of millions of sperm every day? To answer this question, we must start with the anatomical morphology and histological structure of testicles. If you have a chance to look at sperm under the microscope, you will definitely sigh. The dense sperm looks like a tadpole, with a small head (6 microns long) and a long tail (60 microns). It is this long tail that enables sperm to swim forward vigorously to find eggs and combine with them. However, sperm activity is very fast. If you have the chance to see the sperm penetrate into the egg, hundreds of sperm attach their heads to the egg (150 microns) which is much larger than it, and the tail is outward, swinging desperately, and struggling to drill in, you will be even more surprised! Due to the movement of sperm, the fertilized egg and other sperm around it quickly turn, and they seem to be dancing in the dance of life.
After watching these wonderful scenes, you must ask, how is sperm produced? Why is the testicle so small that it can produce hundreds of millions of sperm every day? To answer this question, we must start with the anatomical morphology and histological structure of testicles.
Testis is the male gonad, which is the place for producing sperm and secreting sex hormones. From the appearance, the testicles are one on the left and one on the right, slightly flat oval, and located in the scrotum. In the early stage of the fetus, they fall into the scrotum in the abdominal cavity before birth. The adult testicle is about 4 cm long, 3 cm wide, and weighs about 20 g. In clinic, testicular volume is often used to express its size (15-20 ml). The testicles of newborn babies are relatively large, and shrink about 1-2 months after birth. Before sexual maturity, the testicles are basically in a static state and develop rapidly after puberty. By the age of 50, it gradually shrank and became smaller.
The testicles look like the color of fresh shrimp. They are white and red. There are also a few blood vessels on them. The outer surface of the testis is covered with a slightly thick and strong white film, which plays an important role in protecting the testicular essence. If the white membrane is cut, the testicular tissue will collapse into a pile and cannot maintain its original shape. The tunica albuginosa is the tunica vaginalis of the testis, which is the peritoneal component brought down when the testis descends in the embryonic period. There is a small amount of liquid between the tunica albuginea and the tunica vaginalis, which keeps lubrication and makes the testicles have certain activity. The white membrane extends inward to form a radial mediastinum, which divides each testicle into about 200~300 intervals from the inside. We call them "testicular lobules", and each lobule is surrounded by curved microtubules, called "seminiferous tubules".
In adults, its pipe diameter is 150~250 microns, and its length varies from 30~70 cm to 150 cm at most. There are about 300 to 1000 such small tubes in each testicle. If they are connected, they will be about 200 to 300 meters long. All seminiferous tubules anastomose and converge at the terminal to form a testicular network. One end of the net is divided into a dozen or so output tubules, which pass through the white membrane outside the testis and finally converge into the epididymis. Sperm is constantly produced in the seminiferous tubules. The structure of the small testicles is so fine and complex that the testicles play a large role in spermatogenesis, which is why the testicles can produce so many sperm every day.
There are two types of cells in the epithelium of seminiferous tubules: one is spermatogenic cells at different stages of development, and the other is sertoli cells. The spermatogenic cells are embedded in the irregular pits on the surface of the supporting cells. The closest to the basement membrane is the most immature spermatogenic cells, while the closest to the central lumen is the mature spermatogenic cells. With the continuous development of spermatogenic cells, they move from the base to the lumen. A spermatogonia can form 256 sperm cells with only half of the chromosomes after multiple mitosis and two mature divisions (meiosis), 50% of which contain X chromosome and the other 50% contain Y chromosome. The shape of spermatogonia and spermatocyte is round, without tail. The tail gradually appears in sperm cells.
Sperm cells undergo a series of changes and finally mature to form tadpole-like sperm, which is discharged into the seminiferous tubules. Sertoli cells have many functions and can secrete a variety of active substances, which play a role in regulating and helping the differentiation and maturation of spermatogenic cells, sperm discharge, and phagocytosis of degenerated spermatogenic cells. In adult testicles, sperm is produced every moment. It has been estimated that every gram of adult testicular tissue can produce 300 to 600 sperm per second, and hundreds of millions of sperm can be produced in bilateral testicles every day. This is one of the reasons why the male contraceptive method of controlling sperm production is much more difficult than the female ovulation.