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 the atomic force spectroscopy. The response of the ZP to mechanical stress has been recovered according to a modified Hertz model. ZP of immature oocytes shows a pure elastic behaviour. Matured and fertilized oocyte s ZPs evidence, instead, 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, increases after the cortical reaction. AFM images show that oocyte s ZP surface appear to be composed mainly of a dense, random meshwork of non-uniformly arranged fibril bundles. More wrinkled surface characterizes matured oocytes compared to immature and fertilized oocytes. From a mechanical point of view, the transition of the matured 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 compared to that required in matured ZP.