Dysfunction of embryo transport causes ectopic pregnancy which affects approximately 2% of conceptions in the US and Europe, and is the most common cause of pregnancy-related death in the first trimester. Embryo transit involves a valve-like tubal-locking phenomenon that temporarily arrests oocytes at the ampullary-isthmic junction (AIJ) where fertilisation occurs, but the mechanisms involved are unknown. Here we show that female mice lacking the orphan adhesion G-protein coupled receptor Adgrd1 are sterile because they do not relieve the AIJ restraining mechanism, inappropriately retaining embryos within the oviduct. Adgrd1 is expressed on the oviductal epithelium and the post-ovulatory attenuation of tubal fluid flow is dysregulated in Adgrd1-deficient mice. Using a large-scale extracellular protein interaction screen, we identified Plxdc2 as an activating ligand for Adgrd1 displayed on cumulus cells. Our findings demonstrate that regulating oviductal fluid flow by Adgrd1 controls embryo transit and we present a model where embryo arrest at the AIJ is due to the balance of abovarial ciliary action and the force of adovarial tubal fluid flow, and in wild-type oviducts, fluid flow is gradually attenuated through Adgrd1 activation to enable embryo release. Our findings provide important insights into the molecular mechanisms involved in embryo transport in mice.

Mitochondria are fundamental organelles in eukaryotic cells that provide ATP through oxidative phosphorylation. During this process, reactive oxygen species (ROS) are produced, and an imbalance in their concentrations can induce oxidative stress (OS), causing cellular damage. However, mitochondria and ROS play also an important role in cellular homeostasis through a variety of other signaling pathways not related to metabolic rates, highlighting the physiological relevance of mitochondria–ROS interactions. In reproduction, mitochondria follow a peculiar pattern of activation, especially in gametes, where they are relatively inactive during the initial phases of development, and become more active towards the final maturation stages. The reasons for the lower metabolic rates are attributed to the evolutionary advantage of keeping ROS levels low, thus avoiding cellular damage and apoptosis. In this review, we provide an overview on the interplay between mitochondrial metabolism and ROS during gametogenesis and embryogenesis, and how OS can influence these physiological processes. We also present the possible effects of assisted reproduction procedures on the levels of OS, and the latest techniques developed to select gametes and embryos based on their redox state. Finally, we evaluate the treatments developed to manage OS in assisted reproduction to improve the chances of pregnancy.

We observed that in vitro transcriptomic response of bovine oviduct epithelial cells (BOEC) to the early embryo could be the result of a contact-dependent signalling effect or interactions with embryo secretions. In order to determine this, BOEC were co-cultured directly with embryos or indirectly with embryo-conditioned media (CM); BOEC from the isthmus of oviducts at early luteal phase were cultured with TCM-199+10% fetal calf serum (FCS) in 4-well plates in 5% CO2 in air at 38.5°C for 6 days until confluence. In vitro 2- and 8-cell embryos as well as their CM were produced in parallel. A day before co-culture, BOEC medium was replaced with SOF+10% FCS. Groups for 2- and 8-cell embryos were established: BOEC in direct contact with embryos; BOEC in the same well as embryos but not in indirect contact; BOEC with embryo CM; and BOEC without embryos, as a control. Polyester mesh was used to maintain embryos position on top of the cells. After 48 h of co-culture, BOEC were recovered for gene expression analysis (4 replicates). The relative abundance of candidate genes previously shown to be affected by the presence of embryo in vivo (Maillo et al. 2015 Biol Reprod. 92, 144) [SMAD6 (BMP signalling pathway); ROCK1, ROCK2 (cytokinesis); SOCS3 (inflammatory response); PRELP (extracellular matrix)] or in vitro (Schmaltz-Panneau et al. 2014 Anim. Reprod. Sci. 149, 103-106) [GPX4, NFE2L2 (oxidative stress); SCN9A (sodium ion binding); EPSTI1 (tissue remodelling); IGFBP3 (insulin-like growth factor binding); TDGF1 (BMP signalling pathway); AGR3 (regulation of ciliary beating)] was assessed by RT-qPCR. H2A.Z and ACTG1 were used as housekeeping genes. Statistical analysis was assessed by ANOVA. The BOEC responded to the presence of 2-cell embryos only when in direct contact by significantly decreasing abundance of NFE2L2. Both direct and indirect embryo contact or culture with CM significantly decreased GPX4, ROCK2, and SCN9A transcripts compared with control. The presence of 2-cell embryos irrespective of being in direct or indirect contact reduced the expression of SMAD6 compared with the control and CM groups. In the case of CM, expression of IGFBP3 was enhanced compared with the control but was similar to the presence of the 2-cell embryos. In the presence of 8-cell embryos, direct contact with BOEC significantly down-regulated the expression for GPX4 and SOCS3, whereas expression of SCN9A was up-regulated. The opposite was observed when compared with control. The presence of 8-cell embryos down-regulated the expression of SMAD6 and ROCK2 compared with the CM group, whereas direct or indirect contact with BOEC or culture with CM down-regulated the expression of PRELP compared to control. In conclusion, these results provide evidence for a differential affect on the transcriptome of BOEC in vitro depending on embryo stage. These changes may be related either with direct embryo contact or embryo secretions released into the media. Research supported by Spanish MINECO-AGL2015-70140-R; AGL2015-66145-R; OECD-Co-operative Programme TAD/CRP JA00092482.

Dysfunction of oviductal embryo transport can lead to ectopic pregnancy which affects 1 to 2% of all conceptions in the United States and Europe, and is the most common cause of pregnancy-related death in the first trimester 1, 2 . Ectopic pregnancies almost always occur in the Fallopian tube, emphasizing the critical role of oviductal transport in human reproduction 3 .Oviductal transit is regulated and involves a valve-like "tubal-locking" phenomenon that temporarily arrests oocytes at the ampullary-isthmic junction (AIJ) where fertilization occurs 4 .Here, we show that female mice lacking the orphan adhesion G-protein coupled receptor Adgrd1 are sterile because they are unable to unlock the restraining mechanism at the AIJ, inappropriately retaining embryos within the oviduct. Adgrd1 is expressed on the oviductal epithelium and the post-ovulatory attenuation of tubal fluid production is dysregulated in Adgrd1-deficient mice. We identified Plxdc2 as an activating ligand for Adgrd1 displayed on the surface of cumulus cells. Our findings suggest that regulating oviductal luminal fluid production by Adgrd1 controls embryo transit, and provides important insights into the genetic regulation and molecular mechanisms involved embryo tubal transport.

Study question Does oocyte donation improve reproductive outcomes in recurrent pregnancy loss (RPL) patients? Summary answer Oocyte donation increases live birth and reduces miscarriage rates in RPL women older than 35, but it does not improve reproductive outcomes in younger patients. What is known already Recurrent pregnancy loss (RPL) is defined as 2 or more pregnancy losses prior to 20-24 weeks of gestation and has an incidence of approximately 1 to 2% in couples trying to conceive. Known risk factors for RPL are embryo aneuploidy, advanced maternal age (AMA), previous miscarriages, uterine abnormalities, parental chromosomal abnormalities, antiphospholipid syndrome, endocrine factors, such as thyroid function or obesity, and thrombophilia. However, 50-70% of patients suffering from RPL do not present any known risk factor, making the management of idiopathic RPL patients particularly challenging for ART clinicians. Study design, size, duration This is a cohort retrospective study involving 2 centers. Charts from 18,273 women undergoing IVF treatment between 2011 and 2020 were reviewed. A total of 912 patients (5%) met the definition of RPL and were classified into study groups based on their age and oocyte origin: ≤35 years old receiving oocyte donation (n = 39) or using their own oocytes (n = 35), and AMA women using donor eggs (n = 716) or their own (n = 122). Participants/materials, setting, methods Demographic variables analysed included known risk factors, number of transferred embryos, day of embryo transfer (3 vs 5) and sperm origin (partner/donor). Differences in biochemical, clinical, ongoing pregnancy, miscarriage and live birth rates were assessed by Pearson’s Chi-squared or Fisher’s exact test. P-values <0.05 were considered significant. Main results and the role of chance RPL patients had 2.77±1.27 pregnancy losses overall (2.76±1.37 in women ≤35 and 2.77±1.26 in patients >35). Most RPL patients (91.9%) were of AMA. Other RPL-associated risk factors, including uterine malformations, previous pregnancy losses, chromosomal abnormalities and thrombophilia were identified in 207/912 patients (22.7%). Interestingly, these were more frequent in patients <35 (37.8% vs 21%, p = 0.001). Young RPL women presented higher rates of karyotype abnormalities than older patients (17.6% vs 3.8%, p < 1x10-04), while showing a similar incidence of uterine abnormalities (14.9% vs 12.9%, p > 0.63) and thrombophilia (4.1% vs 4.7%, p > 0.81). RPL patients >35 preferentially underwent oocyte donation (85.4% vs 52.7%, p < 1x10-04), which led to significantly higher biochemical (52.5% vs 17.1%, p < 1x10-04), clinical (42.4% vs. 8.5% p < 1x10-04), ongoing pregnancy (37.9% vs 5.1%, p < 1x10-04), live birth (32% vs 4%, p < 1x10-04) and lower miscarriage rates (10.5% vs 40%, p = 0.0186) than in autologous cycles. In contrast, RPL patients <35 had similar reproductive outcomes: biochemical (41% vs. 52.9), clinical (28.2% vs. 37.1%), ongoing pregnancy (25.6% vs 29.4%), live birth (25.6% vs 26.4%) and miscarriage rates (9% vs 23%), regardless of oocyte origin (donated vs own, p > 0.05 for all cases). Importantly, this was also true for a small subgroup of idiopathic young RPL patients (n = 13). Limitations, reasons for caution The main limitation of this study is its retrospective nature, which does not allow for full elucidation of all potential confounders. The number of young RPL patients undergoing oocyte donation may be too low to draw significant conclusions. Wider implications of the findings RPL can be resolved in AMA patients by using donor oocytes, which points to key roles of oocyte quality and aneuploidy underlying RPL aetiology. However, RPL is not ameliorated by oocyte donation in young women, suggesting an endometrial/systemic origin and highlighting the need to further study mechanisms driving this disorder. Trial registration number Not applicable