Texture of the zona pellucida of the mature pig oocyte. The mammalian egg envelope revisited

The zona pellucida (ZP) of mature pig oocytes is believed to consist of a dense filamentous meshwork, less compact on the inner and outer faces. The uneven surface of the ZP is made of unordered and stretched fibrils surrounding deep funnels which are the openings of the radial canaliculi. The topography of the ZP surface may contribute to the initial interplay between male and female gametes. Using cytochemical techniques for transmission electron microscopy (TEM), such as tannic acid and ruthenium red treatments, we found that the ZP of pig oocytes was essentially made of bundles of fibrils distributed in concentric layers (except in the innermost and outer parts). A correlation appears between the dense structure of the core layer of the ZP and its texture: it is constituted of superposed layers of fibril bundles, whereas only a random meshwork is found in a very thin innermost and in the outer layer. The fascicular configuration may control the permeability of the ZP, giving its semi-rigidity and elasticity, and may facilitate sperm penetration. The liquid crystal-like design of the core layer of the ZP is similar to textures found in the the vitelline envelope (zona radiata) of other vertebrates and possibly of all the deuterostomes. Such texture is probably related to the unique ZP protein composition and to a coordinated synthesis