Mammalian fertilization comprises sperm migration through the female reproductive tract, biochemical and morphological changes to sperm, and sperm-egg interaction in the oviduct. Recent gene knockout approaches in mice have revealed that many factors previously considered important for fertilization are largely dispensable, or if they are essential, they have an unexpected function. These results indicate that what has been observed in in vitro fertilization (IVF) differs significantly from what occurs during “physiological” fertilization. This Review focuses on the advantages of studying fertilization using gene-manipulated animals and highlights an emerging molecular mechanism of mammalian fertilization.

Gene disruption experiments have proven that the acrosomal protein IZUMO1 is essential for sperm–egg fusion in the mouse. However, despite its predicted function, it is not expressed on the surface of ejaculated spermatozoa. Here, we report the dynamics of diffusion of IZUMO1 from the acrosomal membrane to the sperm surface at the time of the acrosome reaction, visualized using a fluorescent protein tag. IZUMO1 showed a tendency to localize in the equatorial segment of the sperm surface after the acrosome reaction. This region is considered to initiate fusion with the oolemma. The moment of sperm–egg fusion and the dynamic movements of proteins during fusion were also imaged live. Translocation of IZUMO1 during the fertilization process was clarified and a fundamental mechanism in mammalian fertilization is postulated.

SPACA1 is a membrane protein that localizes in the equatorial segment of spermatozoa in mammals and is reported to function in sperm-egg fusion. We produced a Spaca1 gene-disrupted mouse line and found that the male mice were infertile. The cause of this sterility was abnormal shaping of the sperm head reminiscent of globozoospermia in humans. Disruption of Spaca1 led to the disappearance of the nuclear plate, a dense lining of the nuclear envelope facing the inner acrosomal membrane. This coincided with the failure of acrosomal expansion during spermiogenesis and resulted in the degeneration and disappearance of the acrosome in mature spermatozoa. Thus, these findings clarify part of the cascade leading to globozoospermia.