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=225 kPa, toward plastic behavior is observed. The critical indentation necessary to induce plastic deformations occurs at yield=55050 nm. This high critical value, corresponding to two ZP layers, well supports the noncovalent long lifetimes of interactions that take place between constituent glycoproteins. © 2009 American Institute of Physics. DOI: 10.1063/1.3107265 The zona pellucida ZP is a multifunctional porous glycoprotein coat that surrounds mammalian eggs. Penetration of this spherical glycoprotein shell by spermatozoa plays a crucial role in mammalian fertilization, and any incapacity of spermatozoa to penetrate the ZP inevitably leads to infertility. The ZP is a three-dimensional network of sulfated glycoproteins ZP2, ZP3, and ZP4 in bovine arranged to form fibrils. 1‐3 Electron microscopy observation shows that several fibrils are arranged in cylindrical bundles distributed in concentric layers measuring about 250 nm in diameter oriented in strata parallel to the oocyte’s surface. Bundles, randomly arranged in the inner and outermost areas, are organized in closely opposed parallel ranks in the core stratified layers 4 see Fig. 1. The physical properties of the ZP have not been studied extensively. Nevertheless, when oocytes with intact ZP are subjected to vigorous pipetting, the ZP can be made to rupture and consequently fragmentate. The shearing forces that cause rupture must involve considerable temporary distension of the ZP, but it is apparently elastic enough to recover its original shape. This suggests that the ZP behaves as an elastic solid over periods of time extending to minutes, and possibly much longer. 5 When the ZP is exposed to sodium dodecyl sulfate denaturation it separates into its constituent glycoproteins; however, this dissociation apparently leaves no residual scaffolding. Therefore it can be stated that the ZP is, by all appearances, a wholly noncovalent gel. Hence it follows that the noncovalent interactions that hold the ZP together probably have relatively long half-lives, and that the said interactions are of high affinity. 6