Published 22 July 2019
Fertilisation was the subject of an invited session eloquently bringing to life its very beginnings. Two outstanding speakers reviewed events documented in research and clinical laboratories covering the fusion of gametes and the 24 hours thereafter.
Dr Enrica Bianchi, a researcher from the Wellcome Sanger Institute in Cambridge, presented her group’s breakthrough discovery of a fundamental mechanism required for the sperm-egg recognition. Hitherto, it was unknown whether IZUMO1, a protein located on the sperm head with a role in gamete fusion, was indeed interacting with a receptor on the egg membrane (the oolemma). It was Bianchi’s sophisticated work which first described JUNO, the oolemma receptor to the IZUMO1. A series of experiments down the line showed that mouse eggs lacking the JUNO receptor were unable to fuse with sperm and confirmed the paramount role of the interaction in fertilisation. They also observed that JUNO, which normally takes hours to establish, vanishes seconds after conventional IVF but remains on the egg surface after assisted fertilisation via ICSI, suggesting a partial contribution of JUNO-IZUMO1 to the block of polyspermy. Bianchi hypothesised that JUNO may undergo modifications when it diffuses onto the zona pellucida and possibly acts as a decoy.
Further elaboration on the JUNO-IZUMO1 interaction is limited by the knowledge gap in the events surrounding gamete fusion, with ‘calcium oscillations unlikely to be involved’. Inspired by her seminal work published in Nature, other groups scrutinised JUNO in humans and two important studies emerged last year. A publication by Jean et al in Human Reproduction documented the importance of JUNO in human fertilisation and a study of Yu et al found a mutation on the gene encoding JUNO in Chinese women to be associated with fertilisation failure. Bianchi inspired the audience with suggestions of novel trajectories for investigation, such as the molecules HAP2 and Bouncer, with proven fusion roles in other species.
Up next was Professor Markus Montag, an established embryologist, academic and entrepreneur, to discuss the events occurring during the first 24 hours of human development. He presented several investigational approaches mainly extracted from the clinical setting to shape our current understanding of events following the fusion of the gametes. Molecular experimentation with oocyte activation, resumption of meiosis and sperm remodeling, he said, had enabled embryologists to consolidate their knowledge of the downstream cellular events. Additionally, the various and ever evolving imaging strategies, including conventional microscopy, video microscopy, time-lapse monitoring and polarisation microscopy have all allowed description of the course of physiological events associated with very early development in vitro. At the same time, deviations from that normal course were recorded. Such deviations, he added, may be a result of in-patient variation, but also an effect of suboptimal laboratory conditions.
Indeed, embryologists routinely face fertilisation failure, with the most common explanation being failed egg activation (which accounts for 15-66% of all cases). Montag detailed key elements observed in the fertilised egg in vitro during the first 24 hours and highlighted those warranting regular assessment. For example, the association of embryological outcomes with the incidence of cytoplasmic wave, pronuclei formation and positioning features, localisation of DNA and arrangement of chromosomes, first mitotic division, presence of perivitelline threads and incidence of fragmentation could potentiate the development of novel markers predicting developmental fate. Montag cited the findings of Coticchio et al published last year in Human Reproduction whereby the morphokinetic impact of early development on embryo competence was made clear.
A thought-provoking point concluded the session: most of our knowledge is based on in vitro discoveries - which may thus be laboratory artifacts or indeed representative of the in vivo events. The textbooks on fertilisation are likely to be revised in the foreseeable future as imaging gains its momentum in IVF.