New insights into oogenesis, oocyte vitrification and rescue in-vitro maturation were among the highlights of this comprehensive ESHRE workshop in Ghent organised by SIG Embryology.
Professor Helen Picton, from the University of Leeds, opened the campus with a presentation on oocyte biology, detailing areas of consensus and where more research is required to address unanswered questions. Other highlights on day one included a presentation from Professor Karl Swann, from Cardiff University, about mitochondria, their role in embryo arrest, and the interaction with adenosine triphosphate (ATP) and calcium oscillations.
The first day ended with a talk by Professor Roger Sturmey, from Hull York Medical School, who discussed how the environment that the oocyte experiences in vivo is complex, dynamic and only beginning to be understood. In an age of micro-plastics, the presentation outlined current evidence demonstrating the presence of environmental chemicals in the ovary and their negative impact on oogenesis.
Day two began with Johan Smitz, from Vrije Universiteit Brussel, providing insights into CAPA-IVM. This methodology is potentially revolutionary for those undergoing fertility treatment because it removes the need for ovarian stimulation. Instead, immature oocytes are collected from follicles of 2 to 8mm and cultured in the laboratory to maturity. This process has been used in around 3000 cycles and resulted in around 1000 live births. However, some aspects of CAPA-IVM need to be refined such as the needle used for collection, the culture environment in the laboratory for achieving maturity, and how these oocytes should be cryopreserved.
Clinical embryologist Dr Sofia Makieva, from the University Hospital of Zurich, then gave an overview of the literature on rescue-IVM. Her message was that there is a lack of standardisation with practice in this area and that the protocol needs to be determined for rescue-IVM in order to maximise results. What is clear based on current evidence is that oocytes should be vitrified at the metaphase II stage as these have superior survival and ongoing competence compared to GV or metaphase I oocytes.
The importance of calcium oscillations and of the sperm-specific protein PLC-zeta in achieving fertilisation was emphasised in a second presentation by Professor Swann. He described experiments aimed at inducing calcium oscillations using PLC-zeta. However, PLC-zeta is very unstable as a molecule so is unsuitable as a treatment in these cases. The compounds that are currently available only produce one sustained peak of calcium in oocytes, even following multiple exposures, and there needs to be 10-20 calcium oscillations to achieve fertilisation. Professor Swann reminded the attendees that culture media contains calcium chloride. As such, when an oocyte is injected, a calcium spike is created by injecting calcium chloride. Therefore, embryologists must not inject too much culture media when releasing the sperm because an excess of calcium can cause embryo arrest. There are research groups actively trying to find a compound that will mimic the activity of PLC-zeta in the hope to get the 25 to 30% of oocytes that don’t fertilise, to fertilise.
Enrica Bianchi, then delivered a presentation on the fusion of the a sperm with an oocyte during fertilisation. Dr Bianchi from the University of Rome Tor Vergata described how many new proteins have been identified in the last few years but, to date, there has only been one binding pair identified, Izumo on the sperm and Juno on the oocyte. She also described how AI is now helping to model how these proteins interact. The discovery of new proteins in this critical process could lead to more treatment options for patients.
Next, Giovanni Coticchio explored the topic of fertilisation assessment, both present and future. Dr Cottichio, scientific director of 9.Baby, presented compelling data on the ploidy status of oocytes that are considered to have no pronuclei (0PN), those with one pronucleus (1PN), and those with two pronuclei and a micro-pronucleus (2.1PN). The evidence demonstrates that the percentage of these that were diploid was 95%, 70% and 25%, respectively (1). Dr Coticchio also described how AI is now being used to detect cytoplasmic movements in oocytes exposed to sperm that do not show conventional signs of fertilisation in order to predict their capacity to produce a blastocyst.
The final day opened with a session on fertilisation with topics on unification errors and pronuclei transfer. In his talk on parental genome unification errors, Professor Tommasso Cavazza from the University of Zurich described how the first cleavage plane is a very good indicator for the chance of a live birth while described how spindle transfer can be used successfully to treat those with mitochondrial diseases. However, due to lack of publications, there’s only been one baby born in the world following mitochondrial disease therapy using spindle transfer. Professor Bjorn Heindryckx, from the University of Ghent, also described an interesting but unfortunate case where there was reversion from an embryo that had <1% mitochondrial mutation load but the resulting child had 40 to 60% mutation load.
The final session of the course included an overview of key performance indicators (KPIs) associated with oocyte vitrification as well as a presentation on oocyte epigenetics by Julie Barberet who focused on the impact of vitrification on the epigenome. Reassuringly, there seems to be no effect of length of time in storage on gene expression but it’s less clear if oocyte vitrification affects the epigenome. Dr Barberet, from the University of Dijon, posited that if there is an effect it’s unclear if this is related to proteomic expression and concluded that there is more work to do.
References 1 Capalbo A, Cimadomo D, Coticchio G, Ottolini CS. An expert opinion on rescuing atypically pronucleated human zygotes by molecular genetic fertilization checks in IVF. Hum Reprod. 2024 Sep 1;39(9):1869-1878. doi: 10.1093/humrep/deae157. PMID: 39043217.
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