Post Date December, 03 2023
Sperm is the fundamental source of human reproduction. The process of sperm formation begins with the seminiferous epithelium located in the inner convoluted tubules of the testis, and undergoes a series of complex changes before it can develop and mature.
The development and maturation of a sperm undergoes a complex process that takes approximately 3 months. The main process is carried out in the seminiferous tubules of the testis, which can be roughly divided into three stages:
1. The proliferative and mitotic stage of spermatogonia: The most primitive stage of sperm is called spermatogonia, which is the stem cell that produces sperm and is located in the seminiferous epithelium of the seminiferous tubules. Initially, spermatogonia proliferate in the form of mitosis, with one dividing into two and two becoming four. After six divisions, one spermatogonial cell proliferates to 64, which is called primary spermatocyte.
After spermatogenesis in the seminiferous tubules of the testis, morphologically, tadpole shaped spermatozoa have been formed, but experiments have shown that spermatozoa in the testis do not have the ability to fertilize. Testicular sperm also undergoes sperm maturation in the epididymis and capacitation in the female reproductive tract.
2. The meiosis of spermatocytes into sperm cells: Primary spermatocytes continue to divide. However, this time it is meiosis, which means that one primary spermatocyte divides into two secondary spermatocytes, but it is different from the proliferative division of spermatogonia because the chromosomes in the nucleus do not replicate. Therefore, each secondary spermatocyte: The cell carries only half the original number of chromosomes, that is, 23 chromosomes, including one sex chromosome.
The cell volume is also smaller than that of primary spermatocytes. Following this process, the secondary spermatocyte undergoes another maturation division, becoming two spermatocytes. As a result, one primary spermatocyte divides into four spermatocytes, each of which carries a haploid number of chromosomes. At this point, one spermatogonial cell becomes 256 sperm cells.
3. Sperm formation stage: During the above cell division, sperm cells have gradually moved closer to the lumen of the seminiferous tubules. At this time, sperm cells continue to develop, but they no longer undergo division, but undergo complex morphological changes and become sperm with heads and tails, which enter the lumen. At this point, the development of sperm in the testicle is complete, lasting approximately 64 days. Sertoli cells located in the epithelium of the seminiferous tubules play an important role in supporting, protecting, and nourishing sperm formation.
Sertoli cells also secrete a globulin that specifically binds to androgens, resulting in a significantly higher concentration of androgens in the seminiferous tubules than in the blood. Only in this suitable microenvironment can spermatogenic cells differentiate into sperm. Sperm then enters the epididymis along the seminiferous tubules and remains at the head of the epididymis for about 2-3 weeks before developing into mature sperm that ultimately has the ability to move and fertilize. Therefore, it takes about 90 days for a spermatogonial cell to develop into mature sperm.
4. Sperm maturation: Changes in the permeability of the sperm membrane, such as increased permeability to potassium ions, and the emergence of a function of sodium excretion, have an important impact on enzyme activity and metabolism. During the movement of the epididymis, the negative surface charge of sperm increases, which can prevent sperm from aggregating into clusters due to the repulsive effect of charges when stored in the epididymis.
During the maturation process, the volume of sperm becomes slightly smaller and its water content decreases. The most obvious change is that the cytoplasmic droplets in the body of immature sperm move backward and eventually detach. In the semen of some infertile patients, a large number of immature sperm without shedding cytoplasmic droplets can be seen.
Sperm from the testicle can move, but when it enters the head of the epididymis, it loses its ability to move. During the process of sperm movement within the epididymis, they gradually acquire the ability to move. First, there is a wobble in place, then there is a circular motion, and finally there is the unique wobble forward motion of mature sperm. Therefore, observing the movement of sperm is also an indicator of whether sperm is mature.
(Intern Editor: Liu Jinhao)
