Mammalian sperm cannot penetrate the zona pellucida solely by force

Area of Science:

• Reproductive Biology
• Biophysics
• Cell Biology

Background:

• The zona pellucida (ZP) is a vital glycoprotein layer surrounding mammalian oocytes.
• Successful fertilization requires sperm to penetrate the ZP barrier.
• The physical properties and mechanical resistance of the ZP are not well understood.

Purpose of the Study:

• To investigate the physical properties of the mammalian zona pellucida.
• To determine the mechanical forces and stresses the ZP can withstand.
• To elucidate the mechanism by which sperm penetrate the ZP.

Main Methods:

• Utilized quartz fibers as micro-manipulators to apply defined forces and shear stress to intact hamster zonae.
• Employed tungsten beams of known stiffness to precisely control applied forces.
• Measured the response of zona pellucida to forces up to 150 nN and stresses exceeding 50 kN/m².

Main Results:

• Zona pellucida resisted penetration by microfibers even at forces of 150 nN (stresses > 50 kN/m²).
• Zona pellucida exhibited significant elastic recovery after deformation, with minimal creep over 1200 seconds.
• Applied forces and stresses exceeded those currently estimated for sperm penetration.

Conclusions:

• The zona pellucida possesses substantial mechanical strength and elastic properties.
• Sperm likely utilize mechanisms beyond simple force to penetrate the zona pellucida.
• These findings challenge the prevailing understanding of sperm-zona interaction during fertilization.