Ultrastructural studies of the porcine zona pellucida during the solubilization process by Li-3,5-diiodosalicylate

Isolated porcine zonae pellucidae were investigated using transmission electron microscopy. It was found that the fine structure of the zona is not homogenous. The region near the oocyte consists of a more tightly packed micellar structure than the structure near the external surface. However, no clear boundary between the two structural features could be detected. The assumption that the molecular structure of the external and internal surface of the zona is not the same is substantiated by the finding that antibodies against the whole zona structure react apparently only with the more loosely bound structure on the external surface. This effect is attributed to differences in the spacial arrangements of the micellar structures rather than to differences in chemical composition. Solubilization of the zona results in a reorientation of the otherwise randomly interconnected structure. At lower Li-3,5-diiodosalicylate concentration (0.05 M) the fibrils seems to expand so that the individual fibers lie almost parallel to each other. At a higher Li-3,5-diiodosalicylate concentration (0.2 M) the individual micelles begin to break up from the zona surface, while at a still higher concentration (0.3 M) the rigid structure of the zona is completely solubilized. In the latter case no residual material could be detected in the sediment following high speed centrifugation of the mixture. These electron microscopic results correlated with the protein concentrations in the supernatant indicating that the maximal protein content (35 ng/zona) is obtained at 0.3 M or higher Li-3,5-diiodosalicylate concentrations.