Research progress on the effect of oocyte smooth endoplasmic reticulum clusters on early embryo development and pregnancy outcome

HUANG Han, YI Hong-yan, MA Yan-lin✉

1.Hainan Provincial Key Laboratory for Human Reproductive Medicine and Genetic Research, Department of Reproductive Medicine, Hainan Provincial Clinical Research Center for Thalassemia, the First Affiliated Hospital of Hainan Medical University, Haikou 570102, China

2.Key Laboratory of Tropical Translational Medicine of Ministry of Education, Hainan Medical University, Haikou 571199, China

3.Haikou Key Laboratory for Preservation of Human Genetic Resource, the First Affiliated Hospital of Hainan Medical University, Haikou 570102,China

Keywords:

ABSTRACT With the clinical development and application of intracytoplasmic sperm injection (ICSI)technology in human assisted reproduction, the influence of oocyte quality on embryo development has been paid more and more attention.So far, there have been many reports on oocyte morphology affecting embryo development.It has been found in some works that the appearance of smooth endoplasmic reticulum clusters (SERC) in oocytes may affect the fertilization and embryo development of oocytes.However, with the increasing reports of SERC-containing oocytes obtained by in vitro fertilization and healthy offspring in recent years, there is still some controversy on whether to continue to use SERC-containing oocytes for the following assisted reproductive therapy in clinical practice.Based on this, this review aims to review the research progress of SERC in oocytes in recent years.

With the increasing incidence of infertility, in vitro fertilization -embryo transfer technology as an important measure for infertility treatment, has been rapidly developed in recent years.ICSI is an improvement in conventional in vitro fertilization (IVF), has been widely used in clinical practice.Specifically, individual sperm are injected into the cytoplasm of oocytes, which gives men with low sperm count and quality, including men with azoospermia,the opportunity to have children of their own.In order to improve the success rate of injection when assisted reproduction personnel perform ICSI, and then improve the fertilization rate and pregnancy rate, high-quality oocytes should be selected as the injection object under equipment such as an inverted microscope before sperm injection, and sperm injection into the area where oocytes appear morphological abnormalities should be avoided during the injection process[1].Therefore, the influence of oocyte quality on embryonic development has received more and more attention, and the morphological evaluation of oocytes has become an important factor in predicting the successful implantation of embryos, and reaching an international consensus on oocyte morphological evaluation has become one of the important goals in reproductive medicine.

1.The effect of SERC on oocytes

In clinical practice, Oocyte evaluation is usually based on various intraplasmic and cytoplasmic morphological features, and oocyte morphological malformations have a large impact on pregnancy outcomes.Seven cytoplasmic deformation phenotypes have been identified in human oocytes including: black/granular, clustered organelles, SERCs, vesicles, necrotic regions, organelle polarized cortical depletion, vacuoles[2].Among them, in 1997, Serhal et al.first discovered SERCs in oocytes.Subsequent studies have shown that intra-oocyte SERC is a more severe oocyte malformation associated with lower fertilization rates, cleft rates, implantation rates, and clinical pregnancy[3-4].Given the high risk of fetal malformations from SERC-positive (SERC+) oocyte sources[5] in 2011, a panel of embryology experts from the European Society of Human Reproduction and Embryology (ESHRE) reached a consensus at a symposium on embryo evaluation in Istanbul that the use of SERC+ oocytes for assisted reproductive therapy was not recommended[6].However, at present, the molecular mechanism of SERC in oocytes and its influence on embryo implantation rate, pregnancy rate, abortion rate, teratology rate and clinical pregnancy outcome has not been clarified, and if SERC+ oocytes are completely abandoned in clinical work, it will not be conducive to auxiliary pregnancy for patients with insufficient ovarian function reserve.As a result, most reproductive centers still selectively use SERC+ oocytes for assisted reproductive therapy[7].In 2017,the panel revised the Istanbul Consensus and recommended that clinicians should consider whether to use SERC+ oocytes for assisted reproductive therapy on a case-by-case basis, and recommended rigorous follow-up after in vitro fertilization embryos derived from SERC+ oocytes[8].

The endoplasmic reticulum (ER) is a tubular cytoplasmic structure consisting of pairs of membranes attached to the nuclear membrane,almost all cells contain endoplasmic reticulum structures, and new research evidence suggests that the endoplasmic reticulum plays a vital role in the stress response of oocytes or embryos and the regulation of meiosis spindles[9].Highly developed in cells with protein secretory functions, the endoplasmic reticulum (RER) with a large number of ribosomes attached to them is called the rough endoplasmic reticulum (RER).The endosomal reticulum (SER)to which no ribosomal attaches is called the smooth endoplasmic reticulum (SER).As one of the most common organelles in the oocyte plasm, SER is usually a small membrane tube and membrane sac-like structure, and its most important physiological function in oocytes is the storage and redistribution of calcium.During fertilization, sperm entry into oocytes triggers the release of a series of free intracellular calcium, which is the result of a series of calcium releases and reuptakes in the SER.After the sperm enters the oocyte,it triggers fluctuations in free calcium within the cell through a series of calcium release and reuptake processes, where calcium release is mainly mediated by the oocyte’s phosphate inositol pathway,which may be induced by the interaction of the oocyte receptor Juno with the izumo 1 sperm protein, and the tyrosine kinase-activated tyrosine kinase activated by phospholipase C.Phospholipase C results in the hydrolysis of phosphatidylositol 4,5-bisphosphate(Phos-Phatidylinositol 4,5-Bisphosphate, PIP2) to inositol 1,4,5-triphosphate (inositol triphosphate, IP3) and diacylglycerol.Another hypothesis is that after sperm-egg fusion, the soluble phospholipase C subtype zeta (PLC) spreads from sperm to the omegame and begins to lyse PIP2.In both hypotheses, the signaling cascade ends with the binding of IP3 to its receptors on the surface of the SER and subsequent induction of calcium release[10].SERC is an intracytoplasmic dimorphism that generally occurs in meiosis II oocytes.Current research suggests that it can interfere with calcium storage and oscillation during fertilization, the emergence of SERC may be related to the clinical outcome of abnormal embryonic development, the emergence of SERC in oocytes will affect the storage and release of intracellular calcium ions, and the abnormal distribution pattern formed by SERC may be involved in the abnormal regulation of intracellular Ca2+signaling, but its mechanism and principle have not been elucidated[11].Based on statistical analysis by Shaw-Jackson et al., based on existing research results, the formation of SERCs is not directly related to the age of patients, the average number of oocytes obtained, the presence of ovarian endometriosis, or the thickness of the endometrium[12].

2.The molecular mechanism of SERC in oocytes

Although the causes and mechanisms of SERC in oocytes have not been elucidated, the current study found that the serum estradiol level on the day of hCG administration in the SERC+ oocyte cycle was higher than that of oocytes that did not have SERC, so most scholars believe that the appearance of SERC is related to the increase in serum estradiol concentration.SERC production may also be a marker of delayed cytoplasmic maturation, so elevated estradiol levels in oocytes may be an indicator of SERC susceptibility[13-15].The longer ovulation stimulation of Oocytes with SERC, the greater initial and total amount of follicle-stimulating hormone (FSH), and the early peak of luteinizing hormone (LH) will lead to cytoplasmic hypermaturation of oocytes, aggregation of organelles, and excessive growth of oocytes and accelerated proliferation of granulocytes[16-17].The duration of ovarian stimulation and the dose of the drug may be risk factors for the development of SERC during the cycle[18-,19].In addition, because Ca2+released by SER plays a key role in oocyte maturation, fertilization, and early embryonic development, the study of Ca2+signaling in SERC+ oocytes may be useful in understanding the causes of SERC formation and its effects on oocyte maturation and pregnancy[20].

3.Effect of Oocyte SERC on embryonic quality

In studies of the effects of SERC on embryonic development,different research teams often come to different conclusions.Previous studies have shown that there is no significant difference in the fertilization rate between the SERC+ cycle and the SERCcycle fertilization rate in the ICSI cycle, hiromitsu Hattori et al.found that in the SERC positive cycle and the SERC negative cycle after analyzing 2,158 patients who performed ICSI for 3578 cycles,the oocyte fertilization rate (71.2% vs 70.3%, P=0.47), blastocyst formation rate (43.0 % vs 42.6 % P=0.84) was found, No significant differences were observed in implant rates (16.6 percent vs 16.6 percent, P=1.00), clinical pregnancy rates for embryo transfer (20.0 percent vs.17.8 percent, P=0.75 percent), and miscarriage rates(30.3 percent vs 30.2 percent, P=0.72).In the same year, Shaw-Jackson C et al.analyzed and compared the data of patients receiving ICSI at the IVC Affiliated Center of a private university in Brazil from July 2011 to June 2014 and found that there was no statistical difference in fertilization rate and second and third day eubembrate rate between the SERC positive cycle and the SERC negative cycle,and the blastocyst formation rate, the fifth eubembrate rate and the number of transferable embryos were also similar, but SERC+ in the SERC+ cycle Implantation rates in oocyte-derived embryos were lower than in SERC-oocyte-derived embryos (14.8% vs 25.6%,P＜0.001)[12].However, in the study of Xue Wang et al., the presence of SERC in oocytes was found to significantly affect the rate of blastocyst development (73.3 vs.55.9%, P＜0.05).It is also believed that the presence of SERCs in oocytes may have a negative impact on blastocyst quality and blastocyst development rate[22].

In 2017, Itoi F et al.had similar findings, with statistical results showing no significant differences between SERC+ oocytes and SERC-oocytes in fertilization rates (64.2%～81.8%) and D3 embryonic rates (37.6%～46.5%)[23].

In another study, fertilization rates in both SERC-negative and SERC-positive cycles in traditional IVF were compared, and both suggested a significant reduction in fertilization rates during SERCpositive cycles in both IVF modalities[18].It can be seen that the effect of the SERC positive cycle on embryonic development cannot be conclusively concluded.

A 2014 Hatteri et al.study found that the fertilization rate of SERC+ oocytes in the ICSI cycle was lower than that of SERCoocytes, and the difference was statistically significant [20].Otsuki and colleagues used a jet lag imaging system to find that SERC+ oocytes are more susceptible to rupture failure, and the difference between the two groups is also statistically significant[15].In 2016, Itoi et al.used a jet lag imaging system to observe that the D5 preferential embryo rate of SERC+ oocyte-derived blastocysts was significantly reduced compared with SERC-oocyte-derived blastocysts[13].Several other studies have reported adverse effects of SERC on embryo implantation, with SERC+ oocyte-derived embryos having lower implantation rates[21-23].Clinical selection of high-quality embryos is more conducive to subsequent embryo implantation and clinical pregnancy, and the above findings suggest that SERa+ oocytes have an adverse effect on embryonic development.

4.The effect of the emergence of oocyte SERCs on fetal outcomes

Most studies have shown no significant differences in clinical pregnancy rates between SERC+ and SERC-cycles, as well as SERC+ and SERC-oocyte-derived embryos[24-25].Shaw-Jackson C et al.found that SERC+ oocyte-derived embryos in the SERC+ cycle not only showed no significant differences in clinical pregnancy rates, but also similar miscarriage rates between the two groups compared to SERC-oocyte-derived embryos[12].However, the risk of SERC teratogenicity reported in previous studies has been the focus of attention in the field of assisted reproduction at home and abroad[26-27].

Akarsu et al.reported a rare case in which SERCs were found in all oocytes recruited during three consecutive ICSI cycles, and three D3 grade I embryos were transferred in the first cycle without clinical pregnancy.Transfer of 4 D3 first-degree embryos in the second cycle to obtain clinical pregnancy, but ultrasound screening at 18 weeks of gestation suggests fetal panencephalopathy.Induction of labor is followed by induction of labor to terminate the pregnancy, and postoperative fetal autopsy reveals that the fetus is brainless and has a single umbilical artery.Clinical pregnancy was also obtained in the third cycle, and ultrasound screening at 18 weeks’ gestation again revealed abnormalities, manifested by enlarged fetal intracranial ventricles, fist-clenching hands, and high intestinal echoes.Caesarean section was performed at 36 weeks, a live baby boy was delivered, and after birth, he observed patent ductus arteriosus,sulcus hypoplasia, right external ear canal closure, polycystic kidney,etc.Subsequently, newborns die 14 d after birth due to respiratory depression[28].These studies report that the presence of intraoocyte SERCs may be associated with abnormal fetal outcomes.It is also based on these findings that the Alpha/ESHRE Consensus,developed in 2011, explicitly states that in vitro insemination of SERC+ oocytes is not recommended, but it is unclear whether SERC+ oocytes can eventually give birth to healthy newborns[6].

Shortly after the consensus was published, in 2013, Matizel et al.reported the first delivery of healthy infants from SERC+ oocyte sources[29].In 2017, the expert group revised the 2011 consensus to recommend that SER+ oocytes be used for the next step of IVF on a case-by-case basis during clinical assisted reproduction, and that strict follow-up records of the pregnancy process and the offspring given to childbirth should provide more convincing evidence for subsequent clinical decisions on whether to abandon SER+ oocytes[8].

But then in 2018, researchers reported another case of teratogenicity.It found that reports of complex chromosomal rearrangement of SERC+ oocyte embryos involved 3 fracture sites on two chromosomes, which eventually led to mutual translocation of chromosomes and implied loss of the 2q31 band, and the pregnant woman ruptured the amniotic cavity one week after amniocentesis and gave birth to a stillborn fetus.Although the cause of stillbirth may be complications of amniocentesis, the chromosomal rearrangement of the fetus does not exclude the possibility of birth defects even if the fetus is born smoothly in the end[30].

5.Conclusion

At present, SERC as a kind of reported oocyte phenotype mutation is causing widespread concern in the field of research, combined with the current domestic and foreign research results, it is recommended that clinical work can be SERC + oocytes for in vitro fertilization, but should still try to avoid the use of SERC + oocyte source embryos for transplantation, the use of SERC + oocytes for assisted reproductive therapy should be the corresponding risks and uncertainties should be fully informed of the patient and their families, and obtain their informed consent before in vitro transplantation.Given the current lack of reports on SERC+ oocytederived embryo transfer outcomes, larger sample sizes and longer follow-up of SERC+ oocyte-derived embryos and offspring in future research efforts are needed to better understand the reasons for SERC+’s presence and its impact on pregnancy outcomes.With the development of clinical experimental techniques and the advancement of research methods, it is expected to further elucidate the mechanism of SERC production in oocytes and its impact on fertilization and pregnancy at the molecular mechanism level in the future.

Authors’ Contribution

HUANG Han: Proposed research directions, topic selection,collection of relevant literature information, and thesis writing; MA Yan-lin: thesis review; YI Hong-yan: thesis revision.

Author conflict of interest

There is no conflict of interest for all authors of this article.