Ultrastructural observations on polyspermic penetration of zona pellucida-free human oocytes inseminated in vitro.

An ultrastructural analysis of polyspermic human ova, obtained by insemination in vitro of zona pellucida-free oocytes, has demonstrated the following: 1. Sperm with an intact or reacted acrosome fuse equally well with the oocyte, and the sperm head is incorporated first. Sperm flagellum entry into the ooplasm is accomplished by sliding, while the sperm plasma membrane is being incorporated into the vitelline membrane. 2. Dismantling of the sperm flagellum within the ooplasm rapidly occurs by dispersion and disintegration successively of the mitochondria, the outer dense fibers of the midpiece, and the fibrous sheath of the principal piece. The axial filament complex persists much longer, and can be demonstrated by electron microscopy for 24 hours or more postinsemination. 3. The mode of assembly of the male pronucleus membrane and the incorporation of maternal chromosomes into the female pronucleus. 4. The vital role of the zona pellucida, which appears to be the essential, if not the unique, site of the block to polyspermy.