Evidence of elastic to plastic transition in the zona pellucida of oocytes using atomic force spectroscopy

We have investigated the mechanical properties of the zona pellucida (ZP), a multilayer glycoprotein coat that surrounds mammalian eggs, using atomic force spectroscopy. The response of the membrane to mechanical stress has been reconstructed using a modified Hertz model. The transition from elastic behavior, which occurs when low stress forces are applied (characterized by a Young's modulus E=(22 +/- 5) kPa), toward plastic behavior is observed. The critical indentation necessary to induce plastic deformations occurs at delta(yield)=(550 +/- 50) nm. This high critical value, corresponding to two ZP layers, well supports the noncovalent long lifetimes of interactions that take place between constituent glycoproteins.