Sperm that comes out of the testicles can move, but once it enters the head of the epididymis, it loses its ability to move. During the process of sperm running within the epididymis, they gradually acquire motility. First, there is a swing in place, followed by a circular movement, and finally, there is the unique swing forward movement of mature sperm. Therefore, observing the movement of sperm is also an indicator of whether sperm is mature.
In addition, during the process of sperm maturation, the permeability of the sperm membrane changes, such as increased permeability to potassium ions, leading to the function of sodium excretion, which has an important impact on enzyme activity and metabolism. During the movement of the epididymis, the surface negative charge of sperm increases, which can prevent them from aggregating into clusters due to the repulsive effect of charges when stored in the epididymis.
It should also be pointed out that some secretions from the epididymis cover the surface of sperm, including a glycoprotein containing sialic acid. Various signs indicate that sialic acid transferred to the surface of sperm has important physiological functions:
(1) Glycoprotein containing sialic acid can stabilize the cell membrane of the acrosome precursor region, thereby inhibiting the acrosome reaction of sperm. In this sense, this glycoprotein should also belong to the de energizing factor.
(2) From an immunological perspective, sialic acid has the function of masking sperm specific antigens, which prevents sperm from being recognized by immune active cells and causing autoimmune reactions during the long process of maturation and operation. Congenital deficiency of sialyltransferase or the action of bacterial sialyltransferase can lead to immune infertility.
(3) Sialic acid is a sugar containing carboxyl groups and carries a negative charge. Therefore, the main reason for the increase in surface load during sperm maturation is the increase in sialic acid.
The surface of ejaculated sperm is attached to a type of disenergizing factor secreted by the epididymis and seminal vesicles. The so-called acquisition process refers to the process of removing factors on the surface of sperm and obtaining fertilization ability, which will cause drastic changes in the structure and function of the sperm membrane. The clearance of de energizing factors mainly relies on the presence of β Amylase, trypsin β Glucosidase and sialidase, the latter also have high levels in follicular fluid.
Therefore, sperm directly taken from the tail of the epididymis do not have fertilization ability, and these sperm are placed in the female reproductive tract for a certain period of time before being taken out for in vitro fertilization test to have fertilization ability. After Capacitation, the instability of sperm membrane increases and the factors inhibiting acrosome reaction are removed, so acrosome reaction may occur in sperm.
The apical system is a special structure located at the head end of the sperm nucleus, which surrounds two-thirds of the head end of the nucleus. The acrosome is a specific lysosome, enveloped in a single layer of membrane and presented as a flat sac. The part near the cell membrane is called the outer membrane of the acrosome, and the part near the nuclear membrane is called the inner membrane of the acrosome. Homogeneous acrosome contents exist in the narrow cavity between the inner and outer membranes, containing glycoproteins and a variety of hydrolases, such as hyaluronidase, sialidase, acid phosphatase β- N-acetylglucosaminidase, aryl sulfatase, and trypsin like enzymes.
When sperm begin to undergo acrosome reaction, the outer membrane of the sperm acrosome merges with the cell membrane adjacent to it at multiple points and ruptures, leading to the formation of small pores. Acrosin is released through these small pores until the inner membrane of the acrosome is completely exposed.
The completion of the acrosome reaction is marked by the complete fusion of the acrosome inner membrane and the sperm cell membrane. For mammalian sperm, calcium ions are a necessary condition for triggering acrosome reactions. During the acrosome reaction, a large amount of calcium ions can be seen flowing in. Under physiological conditions, the concentration of calcium ions in epididymal fluid is low, while the concentration of calcium ions in fallopian tube fluid is high, indicating that the latter is beneficial for acrosome reactions. Many sperm in the fallopian tube undergo an acrosome reaction and release a large amount of hydrolases, dispersing the cumulus cells around the egg and removing the corona radiata. This allows some sperm that have not yet undergone an acrosome reaction to approach the zona pellucida and recognize each other. It needs to be added here that in the process of Capacitation, the removal of sialic acid at the end of the sugar chain can significantly change the membrane structure of sperm, which is manifested in the phenomenon of protein rearrangement in the membrane clustering, which may be closely related to sperm and egg recognition. After the sperm binds to the zona pellucida of the egg, an acrosome reaction occurs, releasing acrosin, breaking down the zona pellucida, and gradually penetrating.