The production of sperm is a complex and wonderful physiological process, and to understand this process, we need to first understand the anatomy and organization of the male reproductive system. Generally speaking, the male reproductive system is divided into internal and external genitalia. The internal genitalia include the gonads (testes), the reproductive ducts (epididymis, vas deferens, ejaculatory ducts and male urethra) and the accessory glands (seminal vesicles, prostate and urethral bulb glands). The external genitalia include the scrotum and the penis.
The testes are located in the scrotum, one on the left and one on the right, and are slightly flattened ovoid organs with smooth surfaces, divided into upper and lower ends, inner and outer sides, and anterior and posterior edges. The upper end of the testis is covered by the head of the epididymis, and the lower end is free; the inner side is flatter and attached to the scrotal septum, and the outer side is more convex and attached to the scrotal wall; the anterior edge is free, and the posterior edge has the blood vessels, nerves and lymphatic vessels of the testis in and out, and is in contact with the epididymis and the testicular part of the vas deferens. The adult testis is about 4 cm × 2.5 cm × 3 cm in size and weighs about 12 g.
The surface of the testis is surrounded by a layer of membranous tissue, which is called the testicular peritoneum, and it consists of three layers from outside to inside: the dirty layer of the sphincter, the white membrane and the vascular membrane. The testicular sheath is the plasma membrane, which covers the surface of the testis. The deeper side of the plasma membrane is the leukoplast, which is thick and tough and consists of dense connective tissue rich in elastic fibers. The white membrane thickens at the posterior border of the testis to form the longitudinal testicular septum, from which many radial testicular septa emanate into the testicular parenchyma and connect with the white membrane, dividing the testicular parenchyma into many cone-shaped testicular lobules, each containing 1-4 highly coiled seminiferous tubules. The seminiferous tubules converge near the longitudinal septum of the testis to form the seminiferous tubules, which enter the longitudinal septum of the testis and anastomose with each other to form the testicular network. The testicular reticulum assembles into 10-15 testicular output tubules, which exit the upper part of the posterior border of the testis into the head of the epididymis. The vascular membrane is located on the inner surface of the white membrane, thin and loose, and consists of branches of testicular arteries and their accompanying veins, which are closely connected with the testicular parenchyma and penetrate deeply into the seminiferous tubules, and the vessels within the vascular membrane are the main source of blood supply to the testicular parenchyma. The connective tissue between the seminiferous tubules is the testicular interstitium.
In adults, each seminiferous tubule is 30-70 cm long and 150-250 µm in diameter, and the wall of the tubule is composed of a compound spermatogenic epithelium. The spermatogenic epithelium consists of two types of cells with different morphological structures and functions, one is the spermatogenic cells and the other is the supporting cells. There is a basement membrane below the epithelium, and the outer side of the basement membrane has collagen fibers and some spindle-shaped myxoid cells, the contraction of which facilitates the discharge of spermatozoa.
Spermatogenic cells include spermatogonia, primary spermatocytes, secondary spermatocytes, spermatocytes and spermatozoa. The process of developing spermatogenic cells into spermatozoa through a series of successive proliferation and differentiation is called spermatogenesis, which includes three stages: proliferation and differentiation of spermatogonia, meiosis of spermatocytes and spermatozoa formation, which we will not go into details here. In humans, it takes about 64 ± 4.5 days to develop from spermatogonia to spermatozoa.
Modern scientific studies have proven that environmental estrogens and other hormones can interfere with spermatogenesis in the seminiferous tubules of the testis in adult men, leading to a decrease in sperm count. Environmental estrogens are chemicals that can interact with estrogen receptors to produce estrogenic effects after entering the body, including organochlorine pesticides, certain synthetic detergents, disinfectants and food additives. These substances can accumulate in the organism after entering the body, interfere with the spermatogenesis process in adult males, and affect the development of the male fetal reproductive system or even cause malformations. In addition, the spermatogenic function of the testes gradually decreases with age, mainly because the seminiferous tubules tend to atrophy, but a small amount of sperm is still produced.
The epididymis is a pair of elongated, flattened organs that are located immediately behind the testes. The upper end is enlarged as the head of the epididymis, the middle is the body of the epididymis, and the lower end becomes slender as the tail of the epididymis. The tail of the epididymis turns sharply and moves inward and upward into the vas deferens. The head of the epididymis is formed by the coiled output ducts of the testes, and the end of the output ducts converge into an epididymal duct, which meanders and coils to form the body and tail of the epididymis.
The functions of epididymis are mainly as follows.
1.Promote sperm maturation
The spermatozoa formed by the seminiferous tubules of the testes have no motility of their own and are transported to the epididymis by the contraction of muscle-like cells outside the seminiferous tubules and the flow of testicular fluid.
2.Transportation of sperm
The spermatozoa move toward the vas deferens under the action of various factors such as the cilia of the epithelium of the output tubules in the head of the epididymis, the rhythmic contraction of the smooth muscle of the epididymis and the flow of the epididymal fluid.
3.Storage of sperm
A small amount of sperm can be temporarily stored in the tail of the epididymis.
4.Disposal of excess sperm
Aged or dead sperm may be engulfed by the epithelium of the epididymis with the macrophages in the lumen of the duct.
Let’s get to know the sperm itself as well as the seminal plasma.
Spermatozoa are shaped like tadpoles, about 60 microns long and divided into two parts: the head and the tail. The head is embedded in the cytoplasm at the top of the supporting cells, and the tail is free in the seminiferous tubules. After the spermatozoa are formed, they detach from the wall of the duct and enter the lumen. The sperm head is flat and pear-shaped and consists of a highly concentrated nucleus and an acrosome covering the first 2/3 of the head. The acrosome is a special lysosome, containing a variety of hydrolytic enzymes such as acrosomalin and hyaluronidase. The sperm tail, also known as the flagellum, is the motility organ of the sperm, and can be divided into four parts: the neck segment, the middle segment, the main segment and the end segment.
Semen is milky white, usually about 3-6 ml per ejaculation, and consists of two parts: spermatozoa and seminal plasma.
Semen is a weakly alkaline fluid with a pH value of 7.2-7.8. The pH of semen is an important factor affecting sperm survival, metabolism and viability.
After fresh semen is discharged from the body and comes into contact with air, it will soon condense into jelly, but will generally liquefy after 5-30 minutes. In the solidified semen, it is difficult for sperm to move, but it can prevent loss in the vagina, and sperm can move freely again after liquefaction.
The coagulation of sperm is related to the coagulation factor secreted by the seminal vesicle gland.
(1) The normal sperm count is 20-400×106/ml. The number of sperm per ml of semen <20 million does not easily fertilize the egg.
(2) Seminal plasma is mainly composed of the secretion of the seminal vesicle gland and the prostate gland, both of which account for about 90% of seminal plasma, and the rest is the secretion of the epididymis as well as the urethral bulb gland and urethral gland.
The physiological functions of seminal plasma are mainly.
(1) to serve as a carrier of sperm from the male reproductive tract to the female reproductive tract, and has the function of transporting sperm.
(2) to neutralize the acidity of the vagina and facilitate the activity of spermatozoa
(3) Providing energy substances.
(4) Participating in the coagulation and liquefaction of semen.
In conclusion, mature males are stimulated and maintained by androgens to generate and develop into sperm cells in the testes, which then undergo a complex evolutionary process to eventually develop into mature spermatozoa. A complete spermatogenic cycle as described above takes about 74 days. Therefore, the treatment of infertility often requires a spermatogenic cycle as a single course of treatment, and the treatment of infertility is relatively long.