Mechanical properties of zona pellucida hardening

We have investigated the changes in the mechanical properties of the zona pellucida (ZP), a multilayer glycoprotein coat that surrounds mammalian eggs, that occur after the maturation and fertilization process of the bovine oocyte by using atomic force spectroscopy. The response of the ZP to mechanical stress has been recovered according to a modified Hertz model. After oocyte's maturation a transition from a purely elastic behaviour, which occurs when low stress forces are applied, towards a plastic behaviour has been observed. The high critical force necessary to induce deformations, that well supports the non-covalent long interactions lifetimes of polymers, increase after the cortical reaction. From a mechanical point of view, the transition of the mature ZP membrane toward fertilized ZP, through the hardening process, consists in the recovery of the elasticity of the immature ZP, while maintaining a plastic transition that, however, occurs with a much higher force with respect to that required in mature ZP.