Find and fuse: Unsolved mysteries in sperm-egg recognition.

Enrica Bianchi,Gavin J Wright

doi:10.1371/journal.pbio.3000953

Enrica Bianchi, Gavin J Wright

Open Access

https://doi.org/10.1371/journal.pbio.3000953

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Abstract

Sexual reproduction is such a successful way of creating progeny with subtle genetic variations that the vast majority of eukaryotic species use it. In mammals, it involves the formation of highly specialised cells: the sperm in males and the egg in females, each carrying the genetic inheritance of an individual. The interaction of sperm and egg culminates with the fusion of their cell membranes, triggering the molecular events that result in the formation of a new genetically distinct organism. Although we have a good cellular description of fertilisation in mammals, many of the molecules involved remain unknown, and especially the identity and role of cell surface proteins that are responsible for sperm–egg recognition, binding, and fusion. Here, we will highlight and discuss these gaps in our knowledge and how the role of some recently discovered sperm cell surface and secreted proteins contribute to our understanding of this fundamental process.

Highlights

Fertilisation is the union of 2 haploid cells—the egg and sperm—to create a new diploid organism that ensures the propagation of genetic information from one generation to the
IZUMO1, named after a Japanese marriage shrine, is the only example of a sperm cell surface protein that was initially demonstrated to be essential for fertilisation by adding a monoclonal antibody recognising this protein in in vitro fertilisation (IVF) assays [19] and whose role was unequivocally confirmed in gene-deficient mice [20]
In just a few months, 4 new genes that encode sperm cell surface or secreted proteins have been reported that are essential for male fertility: SPerm ACrosome membrane-Associated protein 6 (Spaca6), Fertilisation Influencing Membrane Protein (Fimp), Sperm–Oocyte Fusion required 1 (Sof1), and TransMEMbrane protein 95 (Tmem95) [25,26,27,28]

Summary

Introduction

Fertilisation is the union of 2 haploid cells—the egg and sperm—to create a new diploid organism that ensures the propagation of genetic information from one generation to the next. The acrosome-reacted sperm must recognise and adhere to the oolemma, which is followed by the fusion of the sperm and egg membranes so that two cells become one; a new diploid organism has been created, and fertilisation has been achieved [3].

Results

Conclusion