Ultrastructure of the cortex in the human egg.

Two distinct types of cortical granules were discerned in the human oocyte. The first type, G1 granules, had a mean diameter of 350 nm, contained a uniformly compacted, electron-dense substance; these granules were probably synthesized even before the oocyte was aspirated from mature follicles and their contents were never found to be secreted in any of the preovulatory (immature), unfertilized and fertilized eggs examined. The G2 granules measured 450 nm (mean diameter), had a granular content and were found to be synthesized and secreted at all the stages of egg development studied. Endocytosis was also evident in the unfertilized and fertilized eggs. The secretion of the G1 cortical granules even before fertilization suggests that they may have an additional role, rather than merely contributing to the blockade of polyspermy.