![]() Additionally, SEMG peptides are also involved in other biological functions such as increasing sperm hyaluronidase activity, antibacterial activity, hyperpolarization, and permeability of sperm plasma membrane. ![]() also indicated that patients with higher number of SEMG-unbound spermatozoa can achieve successful pregnancy, making total SEMG-unbound sperm count a relevant parameter for in vivo fertilization. ![]() In this context, a study by Yamakasi et al. reported that recombinant human SEMG inhibits sperm progressive motility. In addition, the C-terminal fragment of SEMG1 containing Cys 239 (164–283 amino acids) was found to have significant inhibitory effects on both motility and progressive motility of intact live human spermatozoa. The N-terminal fragment of SEMG1 was originally identified as the region of seminal plasma motility inhibitor (SPMI). As a result, sperm are entrapped within the seminal coagulum. ![]() Upon ejaculation, semen immediately turns into a gelatinous meshwork of crosslinking SEMGs. SEMG1, a predominant 52 kDa protein, contains a single cysteine residue at position 239 (Cys 239) and forms intermolecular disulfide bridges with the less abundant SEMG2 (exist as non-glycosylated 71 kDa and glycosylated 76 kDa) at Cys 159 and Cys 360 residues, resulting in high molecular weight complex SEMGs (reviewed in ). SEMG1 and SEMG2 are the two major proteins of the seminal coagulum and represent 20–40% of the seminal plasma proteins. Semenogelin proteins (encoded by SEMG1 and SEMG2 genes) are secreted from seminal vesicles. Here, we will discuss the function of key factors present in the seminal plasma that are involved in the semen liquefaction process. The seminal plasma proteins play an important role in semen coagulation, sperm motility, capacitation, acrosome reaction, and immune activity suppression in the female reproductive tract (reviewed in ). Secretions from bulbourethral glands (contain mucins, galactose, sialic acid) contribute to ~1% of semen volume and act as lubricants enabling efficient sperm transfer (reviewed in ). The semen has an alkaline pH (7.2–8.0) from seminal vesicles and prostate secretions containing basic polyamines such as spermine, spermidine, and putrescine, which counteract the vaginal acidity and are important for sperm survival. Seminal vesicles contribute to ~65% of the semen volume and are rich in semenogelins (SEMGs), fibronectin, prostaglandins, cytokines, and fructose, while the prostatic secretions are rich in proteolytic enzymes, citrate, and lipids and contribute to ~25% of the total volume of seminal fluid ( Figure 1). The seminal plasma is rich in sugars, glycans, lipids, inorganic ions, metabolites, cell-free DNA, microRNAs, peptides, and proteins, which are secreted from seminal vesicles, prostate, epididymis, and bulbourethral glands (reviewed in ). Therefore, before describing the process, we will discuss necessary protein components present in the seminal plasma that are involved in the liquefaction process. The liquefaction process requires proteins present in the acellular fraction (seminal plasma) of the semen. Human semen consists of approximately 2–5% spermatozoa and 98–95% seminal plasma, have a minimum volume of 2 mL, a pH of 7.2–8.0 and contain 200–500 million spermatozoa. In humans, the semen is a fluid conglomerate consisting of two major components: the cellular fraction (consisting of spermatozoa, migrating leucocytes, immature germ cells, and epithelial cells) and acellular fraction consisting of seminal plasma and extracellular vesicles (epididymosomes and prostasomes) ( Figure 1) (reviewed in ). In humans, however, the ejaculate is deposited in the anterior wall of the vagina, which later liquefies, and the sperm gain their motility to transport to the upper female reproductive tract for fertilization (reviewed in ). Male mice and rats ejaculate sperm and accessory gland secretions (e.g., seminal vesicle, prostate) directly into the uterus and produce a copulatory plug, which is not liquefied in vivo. The fate of ejaculated spermatozoa in humans is very different from that in rodents.
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