Live imaging captures the events immediately following fertilization

Area of Science:

• Reproductive Biology
• Cell Biology
• Developmental Biology

Background:

• Fertilization research traditionally relies on static samples, limiting understanding of dynamic sperm-egg interactions.
• The spatiotemporal dynamics of sperm remodeling and movement during fertilization remain incompletely understood.

Purpose of the Study:

• To visualize and characterize the dynamic process of mammalian fertilization using high-resolution live imaging.
• To identify and describe distinct phases of sperm remodeling and movement from sperm binding to fusion.
• To provide a continuous spatiotemporal framework for mammalian fertilization, including initial observations in human oocytes.

Main Methods:

• High-resolution live imaging of zona-intact mouse oocytes from sperm binding.
• In vivo manipulations in mice to investigate sperm movement requirements.
• Live imaging of human oocytes to capture sperm-egg fusion and post-fusion events.

Main Results:

• Identified two phases of sperm remodeling: a static phase (DNA decondensation, histone loading) and a mobile phase (stereotyped movement).
• Sperm movement is initiated by cytoplasmic streaming and requires chromatin decondensation and oocyte polarization.
• Convergent cortical flows, driven by polar body cytokinesis, guide sperm toward the female pronucleus.
• Captured sperm-egg fusion in human oocytes, detailing meiotic resumption and sperm movement post-fusion.

Conclusions:

• Established a continuous spatiotemporal framework for mammalian fertilization dynamics.
• Demonstrated the crucial roles of cytoplasmic streaming, chromatin decondensation, oocyte polarization, and cortical flows in sperm transport.
• Provided novel live-imaging insights into human fertilization dynamics and post-fusion events.