There is no evidence, nor need, for a fast block to polyspermy in animal oocytes. The idea that oocytes have evolved a mechanism to allow the entry of one spermatozoon and repel all others has, however, gained consensus over the last century. The main culprit is the sea urchin, which has been used for over a century in in vitro studies of the fertilization process. Images of sea urchin oocytes with thousands of sperm attached to the surface are commonplace in textbooks and appeal to the nature of the reader implying an intriguing surface mechanism of sperm selection despite these oocytes being fixed for photography (Figure 1). The abundance of gametes in this marine invertebrate and the ease of experimentation have given us the possibility to elucidate many aspects of the mechanism of fertilization, but has also led to ongoing controversies in reproductive biology, one being polyspermy prevention. Kinetic experiments by Rothschild and colleagues in the 1950s led to the hypothesis of a fast partial block to polyspermy in sea urchin oocytes that reduced the probability of a second spermatozoon from entering the oocyte by 1/20th. In the 1970s, Jaffe and colleagues suggested, with circumstantial evidence, that this partial block was due to the sperm-induced depolarization of the oocyte plasma membrane. However, the fate of supernumerary spermatozoa is determined well before the plasma membrane of the oocyte depolarizes. Transmembrane voltage does not serve to regulate sperm entry. Scholastic texts have inadvertently promulgated this concept across the animal kingdom with no logical correlation or experimentation and, as of today, a molecular mechanism to regulate sperm entry in oocytes has not been identified.

Semen sexing is among one of the most remarkable inventions of the past few decades in the field of reproductive biotechnology. The urge to produce offspring of a desired sex has remained since traditional times. Researchers have tried many methods for accurate semen sexing, but only the flow cytometry method has proved to be effective for commercial utilization. However, there were always concerns about the effects of sexed semen, especially on fertility and the rate of genetic gain. Some concerns were genuine because of factors such as low semen dosage in sexed semen straws and damage to sperm during the sorting process. Various researchers have conducted numerous studies to find out the effect of sexed semen on fertility and, in this article, we reflect on their findings. Initially, there were comparatively much lower conception rates (∼70% of conventional semen) but, with refinement in technology, this gap is bridging and the use of sexed semen will increase over time. Concerning genetic gain with use of sexed semen, a positive effect on rate of genetic progress with the use of sexed semen has been observed based on various simulation studies, although there has been a mild increase in inbreeding.

Zygotic genome activation (ZGA) is a critical event in early embryonic development, and thousands of genes are involved in this delicate and sophisticated biological process. To date, however, only a handful of these genes have revealed their core functions in this special process, and therefore the roles of other genes still remain unclear. In the present study, we used previously published transcriptome profiling to identify potential key genes (candidate genes) in minor ZGA and major ZGA in both human and mouse specimens, and further identified the conserved genes across species. Our results showed that 887 and 760 genes, respectively, were thought to be specific to human and mouse in major ZGA, and the other 135 genes were considered to be orthologous genes. Moreover, the conserved genes were most enriched in rRNA processing in the nucleus and cytosol, ribonucleoprotein complex biogenesis, ribonucleoprotein complex assembly and ribosome large subunit biogenesis. The findings of this first comprehensive identification and characterization of candidate genes in minor and major ZGA provide relevant insights for future studies on ZGA.

Spermatogenesis is a developmental process driven by interactions between germ cells and Sertoli cells. This process depends on appropriate gene expression, which might be regulated by transcription factors. This study focused on Rreb1, a zinc finger transcription factor, and explored its function and molecular mechanisms in spermatogenesis in a mouse model. Our results showed that RREB1 was predominantly expressed in the Sertoli cells of the testis. The decreased expression of RREB1 following injection of siRNA caused impaired Sertoli cell development, which was characterized using a defective blood–testis barrier structure and decreased expression of Sertoli cell functional maturity markers; its essential trigger might be SMAD3 destabilization. The decreased expression of RREB1 in mature Sertoli cells influenced the cell structure and function, which resulted in abnormal spermatogenesis, manifested as oligoasthenoteratozoospermia, and we believe RREB1 plays this role by regulating the transcription of Fshr and Wt1. RREB1 has been reported to activate Fshr transcription, and we demonstrated that the knockdown of Rreb1 caused a reduction in follicle-stimulating hormone receptor (FSHR) in the testis, which could be the cause of the increased sperm malformation. Furthermore, we confirmed that RREB1 directly activates Wt1 promoter activity, and RREB1 downregulation induced the decreased expression of Wt1 and its downstream polarity-associated genes Par6b and E-cadherin, which caused increased germ-cell death and reduced sperm number and motility. In conclusion, RREB1 is a key transcription factor essential for Sertoli cell development and function and is required for normal spermatogenesis.

Men with diabetes frequently experience spermatogenic dysfunction, which is the most significant sign that diabetes has harmed their ability to reproduce. The effect of various doses of the hydro-alcoholic extract of Nerium oleander leaves on the pituitary–gonadal axis, sperm motility and number, antioxidant system, changes in testicular tissue structure, and spermatogenesis in healthy and diabetic rats has been examined in the current study. Eighty male rats that had been streptozotocin-induced diabetic and healthy were divided into eight groups: (1) control, (2) Nerium (50 mg/kg), (3) Nerium (100 mg/kg), (4) Nerium (200 mg/kg), (5) DM (6) DM+Nerium (50 mg/kg), (7) DM+Nerium (100 mg/kg) and (8) DM+Nerium (200 mg/kg) and were administered orally for 48 days consecutive. Following the studies, analysis of the testicular tissues’ antioxidant capacity as well as sperm parameters, Johnsen’s scoring and morphometric evaluation, histology, biochemical and stereology studies were performed.

The outcomes showed that Nerium 50 and 100 mg/kg considerably enhanced the testicular morphology, sperm parameters, and reproductive organs to varying degrees in diabetic rats. After Nerium 50 mg/kg administration, glutathione peroxidase (GPX) and catalase (CAT) levels in the testicular tissue were increased whereas malondialdehyde (MDA) levels were markedly decreased. Nerium may help protect against diabetic-induced spermatogenic dysfunction in male rats by enhancing the activities of antioxidant enzymes in lower dosages.

Electromagnetic radiation (EMR) has deleterious effects on sperm motility and viability, as well as oocyte membrane and organelle structure. The aim was to assess the effects of cell phone radiation on preimplantation embryo morphokinetics and blastocyst viability in mice. For superovulation, 20 female mice were treated with intraperitoneal (IP) injections of 10 IU pregnant mare’s serum gonadotropin (Folligon® PMSG), followed by 10 IU of human chorionic gonadotropin (hCG) after 48 h. The zygotes (n = 150) from the control group were incubated for 4 days. The experimental zygotes (n = 150) were exposed to a cell phone emitting EMR with a frequency range 900–1800 MHz for 30 min on day 1. Then, all embryos were cultured in the time-lapse system and annotated based on time points from the 2-cell stage (t2) to hatched blastocyst (tHDyz), as well as abnormal cleavage patterns. Blastocyst viability was assessed using Hoechst and propidium iodide staining. Significant increases (P < 0.05) were observed in the cleavage division time points of t2, t8, t10, and t12 of the experimental group compared with the controls. In terms of blastocyst formation parameters, a delay in embryo development was observed in the experimental group compared with the controls. Data analysis of the time intervals between the two groups showed a significant difference in the s3 time interval (P < 0.05). Also, the rates of fragmentation, reverse cleavage, vacuole formation, and embryo arrest were significantly higher in the experimental group (P < 0.05). Furthermore, the cell survival rate in the experimental group was lower than the control group (P < 0.05). Exposure to EMR has detrimental consequences for preimplantation embryo development in mice. These effects can manifest as defects in the cleavage stage and impaired blastocyst formation, leading to lower cell viability.

Infertility affects 15% of all couples worldwide and 50% of cases of infertility are solely due to male factors. A decrease in motility in the semen is considered one of the main factors that is directly related to infertility. The use of supplementation to improve the overall sperm quality has become increasingly popular worldwide. The purpose of this study was to evaluate whether sperm motility was affected by the combination of serotonin (5-HT), selenium (Se), zinc (Zn), and vitamins D, and E supplementation. Semen samples were incubated for 75 min at 37°C in medium containing varying concentrations of 5-HT, Se, Zn, vitamin D, and E. 5-HT (200 μM), Se (2 μg/ml), Zn (10 μg/ml), vitamin D (100 nM), and vitamin E (2 mmol) have also been shown to increase progressive sperm motility. Three different mixtures of supplements were also tested for their combined effects on sperm motility and reactive oxygen species (ROS) production. While the total motility in the control group was 71.96%, this was found to increase to 82.85% in the first mixture. In contrast the average ROS level was 8.97% in the control group and decreased to 4.23% in the first mixture. Inclusion of a supplement cocktail (5-HT, Se, Zn, vitamins D and E) in sperm processing and culture medium could create an overall improvement in sperm motility while decreasing ROS levels during the incubation period. These molecules may enhance the success of assisted reproduction techniques when present in sperm preparation medium.

Environmental hypoxia adversely affects reproductive health in humans and animals at high altitudes. Therefore, how to alleviate the follicle development disorder caused by hypoxia exposure and to improve the competence of fertility in plateau non-habituated female animals are important problems to be solved urgently. In this study, a hypobaric hypoxic chamber was used for 4 weeks to simulate hypoxic conditions in female mice, and the effects of hypoxia on follicle development, proliferation and apoptosis of granulosa cells, reactive oxygen species (ROS) levels in MII oocyte and 2-cell rate were evaluated. At the same time, the alleviating effect of melatonin on hypoxic exposure-induced oogenesis damage was evaluated by feeding appropriate amounts of melatonin daily under hypoxia for 4 weeks. The results showed that hypoxia exposure significantly increased the proportion of antral follicles in the ovary, the number of proliferation and apoptosis granulosa cells in the follicle, and the level of ROS in MII oocytes, eventually led to the decline of oocyte quality. However, these defects were alleviated when melatonin was fed under hypoxia conditions. Together, these findings suggest that hypoxia exposure impaired follicular development and reduced oocyte quality, and that melatonin supplementation alleviated the fertility reduction induced by hypoxia exposure.

Oocytes with excessively large first polar bodies (PB1) often occur in assisted reproductive procedures. Many times these oocytes are discarded without insemination and, as a result, the application of this portion of oocytes has scarcely been reported to date. Few studies have examined large PB1 oocytes in infertile women and have virtually entirely studied genetic variations for large PB1 oocyte abnormalities. Here, we describe an unusual case of a live birth from a remarkably large PB1 oocyte in a frozen embryo transfer (FET) cycle. This is the first instance of a successful live birth resulting from a PB1 oocyte with an extremely large polar body measuring 80 μM × 40 μM in size. The large PB1 oocyte was performed by an early rescue intracytoplasmic sperm injection (r-ICSI) and was formed into a blastocyst on day 5. Following FET, a healthy boy baby weighing 3100 g was finally delivered by caesarean section at 37 weeks and 5 days after conception. Additionally, there were no complications throughout the antenatal period or the perinatal phase of this following full-term delivery. In this study, it is revealed for the first time that a huge PB1 oocyte can be fertilized, resulting in the growth of a blastocyst, a subsequent pregnancy, and a live birth. This new information prompts us to reconsider the use of large PB1 oocytes. More insightful talks should be given attention to prevent the waste of embryos because not all oocytes with aberrant morphology are unavailable.

Intracytoplasmic sperm injection (ICSI) is a technique that directly injects a single sperm into the cytoplasm of mature oocytes. Here, we explored the safety of single-sperm cryopreservation applied in ICSI. This retrospective study enrolled 186 couples undergoing ICSI-assisted pregnancy. Subjects were allocated to the fresh sperm (group A)/single-sperm cryopreservation (group B) groups based on sperm type, with their clinical baseline/pathological data documented. We used ICSI-compliant sperm for subsequent in vitro fertilization and followed up on all subjects. The recovery rate/cryosurvival rate/sperm motility of both groups, the pregnancy/outcome of women receiving embryo transfer, and the delivery mode/neonatal-related information of women with successful deliveries were recorded. The clinical pregnancy rate, cumulative clinical pregnancy rate, abortion rate, ectopic pregnancy rate, premature delivery rate, live birth delivery rate, neonatal birth defect rate, and average birth weight were analyzed. The two groups showed no significant differences in age, body mass index, ovulation induction regimen, sex hormone [anti-Müllerian hormone (AMH)/follicle-stimulating hormone (FSH)/luteinizing hormone (LH)] levels, or oocyte retrieval cycles. The sperm recovery rate (51.72%-100.00%) and resuscitation rate (62.09% ± 16.67%) in group B were higher; the sperm motility in the two groups demonstrated no significant difference and met the ICSI requirements. Group B exhibited an increased fertilization rate, decreased abortion rate, and increased safety versus group A. Compared with fresh sperm, the application of single-sperm cryopreservation in ICSI sensibly improved the fertilization rate and reduced the abortion rate, showing higher safety.