Mammalian Sperm Cannot Penetrate the Zona Pellucida Solely by Force

The zona pellucida of the mammalian egg is a barrier which sperm must penetrate if fertilization is to proceed. The physical properties of zonae remain unknown. The response of intact hamster zonae to defined force and shear stress has been established by using quartz fibres as 'pokers' mounted on tungsten beams of known stiffness. Fibres with cross-sectional areas as small as 2.8 micron 2 fail to penetrate zonae even when subjected to forces of 150 nN, i.e. stresses greater than 50 kN/m2. Moreover, although a force of 150 nN produces extensive strain, zonae recover essentially elastically, and show little or no creep over 1 200 sec. These forces and stresses are in excess of those which are currently thought to be produced by sperm, and it follows, therefore, that sperm cannot penetrate zonae solely by force.