Dear Colleagues,

Sperm–oocyte adhesion and fusion are critical steps in mammalian fertilization. The first proteins recognized as essential to these stages were discovered in the last two decades. First, it was the oocyte tetraspanin CD9 that proved essential for fertilization, and its expression is ubiquitous. Next, the first essential sperm player, the immunoglobulin IZUMO1, was identified, and finally its oocyte receptor JUNO, a GPI-AP. Three decades were needed to discover these three players, mainly thanks to knock-out mouse models. More recently, their structures have been resolved. In the last few weeks, several articles were published describing four new sperm proteins essential to adhesion/fusion (SPACA6, FIMP, SOF1 and TMEM95). Despite the recent extension of this list of proteins that are a priori involved in adhesion, the adhesion/fusion mechanism at the origin of the procreation of a new being remains a mystery. In addition to a fusogenic element that remains to be discovered, it will certainly be necessary to understand how these proteins interact together and also to find an articulation between these proteins and membrane lipids and mechanical parameters. Indeed, sperm phosphatidylserine and its oocyte receptors have also been identified as key elements in fusion, and it was revealed that a key element of the adhesion phase to initiate fusion is the oscillatory motion of the sperm head on the oocyte plasma membrane, generated by a specific flagellum-beating mode. Beyond these so-called essential actors, other protein and lipid elements participate in the optimization of fertilization and even if they are not individually essential, their combined contributions participate in the success of fertilization. 

Dr. Ahmed Ziyyat
Guest Editor

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• fertilization
• adhesion fusion
• protein actors
• lipid components
• mechanical parameters