SDF was produced in sperm cells during spermatogenesis and maturation, which reflects the damage of chromosome and impaired DNA integrity [11–12]. The SDF index (DFI) is used to assess the degree of sperm DNA destruction. Damage to sperm chromatin can directly affect the sperm’s normal functions [13], also leads to the incorrect transmission of genetic information to the offspring. Currently, three factors are considered to be the main cause of sperm damage, including abnormal sperm chromatin assembly, aberrant apoptosis of sperm cells, and excessive oxidative stress [14]. During sperm maturation, histones are gradually replaced by the arginine-rich, cysteine-rich and smaller protamine (HP), this process reduces the self-repair ability of sperm DNA in response to changes of internal and external environment. Furthermore, under the action of torsion-tension generated by the double-stranded DNA helix, the misfolding of DNA supercoiling structures in the chromosome can also lead to aberrant DNA repair, which result in abnormalities of the chromatin structures and increased SDF [15]. Otherwise, Inflammation in the external genital tract and varicocele can also increase the production of SDF by inducing free radical-mediated oxidative stress in the sperm [16].
Many studies have proposed hypotheses on the mechanism of sperm DNA damage. The main possible reasons are as follows: 1) Antioxidants in semen can protect the oxidative damage of sperm DNA, and a small amount of appropriate ROS can help sperm capacitation and acrosomal reaction. Excessive ROS removal ability and defensive ability, more than itself lead to sperm DNA double chain rupture, produce single, its principle is ROS may through direct oxidation of sperm DNA bases, may through covalent binding of lipid peroxidation products and DNA, causing the sperm DNA chain rupture, leading to a biological structure of sperm, sperm DNA damage 2)Abnormal sperm chromatin assembly can lead to sperm DNA double strand break. The main link leading to sperm nuclear DNA damage is the abnormal appearance of protosomal proteins when they replace histones [17–20]. During the sperm formation process, nucleocapsid proteins of chromatin undergo the transformation from histones to transition proteins to protosomal proteins to concentrate chromatin and maintain the normal morphology and DNA integrity of sperm. In damaged or immature sperm, a large amount of sulfhydryl (SH) of protamine cannot be oxidized into disulfide bonds, so it cannot bind closely with DNA. As a result, loose chromatin is formed and its structure is unstable. Its DNA is denatured into single strand under the action of acid, which eventually leads to sperm DNA damage. Therefore, sperm DFI can be detected to reflect the defect degree of genetic material in sperm nucleus and sperm DNA maturity status.
In this study, The low DFI group had an obviously lower seminal MDA level and an obviously higher seminal TAC level than high DFI groups, indicating that too much MDA was produced during seminal lipid peroxidation and that the drop of TAC level triggered oxidative stress reaction and destroyed the spermatic membranes [21]. According to Ni et al.[22] and Fu et al.[23], sperm DNA damage could be caused by ROS in patients with varicocele. Shang et al.[24] and Greco et al.[25] have reported that antioxidant can decrease the rate of DNA fragmentation, suggesting that the seminal ROS participates in the process of sperm DNA damage.
The results of this study showed that sperm DFI level was significantly negatively correlated with sperm survival rate and PR%, which was consistent with relevant research reports [26–28], and there were statistical differences among groups I, II and III. The survival rate and PR% of sperm decreased significantly with the increase of DFI level, but this does not mean that the lower percentage of the former means that sperm DNA is damaged. However, damaged sperm DNA can affect sperm fertilization ability and even inhibit embryo development. This study showed that there was a statistically significant difference in Fru content between group II and III and group I, while there was no statistically significant difference between group II and group III. However, most PR% of these two groups was lower than the lower limit of WHO reference value[29], which was classified as asthenospermia. However, spermatozoa Fru is closely related to sperm motility. Fru provides energy source for sperm motility, and the reduced content of Fru can reduce sperm motility [30]. The seminal plasma Fru originates from the seminal vesicle and mainly reflects its secretion function. In this study, inflammation and infection have been excluded, which may be related to the endocrine function of the seminal vesicle and other factors. In summary, sperm DFI was low, sperm Fru was high, and sperm PR% was high. The results also showed that the percentage of normal sperm in group III was statistically different from that in group II, which was consistent with some reports [31], and the former was significantly lower than the latter. Although there was no statistical difference between group I and the other two groups, the percentage of normal sperm was higher than that of group III and lower than that of group II .In this study, no correlation was found between patient age, abstinence days, semen volume, sperm concentration, spermatozoal G-glu and spermatozoal Zn and sperm DFI. However, it can be seen from Table 2 that DFI increases with age, and it has been reported that age is positively correlated with DFI(With the aging of male organs such as testis, prostate and epididymis, it leads to the increase of reactive oxygen species (ROS) and the decline of antioxidant capacity. Too much ROS produces a large amount of lipid peroxides, which attack the sperm cell membrane, causing the sperm DNA strand to break and destroy it integrity)[32], which may be due to insufficient sample size in this study. Semen volume and sperm concentration were mainly related to the secretion volume of accessory gonads and total sperm count, but no correlation was found between them and DFI. About 60% of the seminal plasma is from the seminal vesicle, 30% from the prostate, and the rest from epididymis, paraurethral gland, paraurethral gland, testis, etc. The spermatic a-Glu and spermatic Zn are mainly from the secretion of epididymis and prostate. It has been reported that too high free Zn has toxic effects on sperm, and its oxide inhibits sperm fermentation and oxidation process [33–35]. It has also been suggested that Zn can delay cell membrane lipid oxygenation. In this study, the level of spermatic Zn in each group was basically within the normal reference range, and it was not clear whether it affected sperm quality [36].