The site of the acrosome reaction during in vivo penetration of the sheep oocyte

In vivo fertilization of sheep eggs has been studied by electron microscopy. Remnants of the acrosome reaction were present at the zona surface of every penetrated egg, indicating that the acrosome reaction in sheep occurs at the surface of the zona pellucida. To determine whether follicular oocytes could specifically bind spermatozoa, oocytes isolated from different size classes of antral follicles were transferred into the oviducts of mated ewes, recovered 4 hr 30 min later, and analyzed by electron microscopy. Oocytes from follicles up to 1 mm in diameter failed to bind spermatozoa and were not penetrated. In contrast, the zona of oocytes from follicles ⩾ 2 mm in diameter induced the acrosome reaction. These oocytes were penetrated but failed to achieve cortical granule exocytosis and so to mount a block to polyspermy. Moreover, sperm nuclei incorporated into the ooplasm did not decondense although the sperm nuclear envelope was dispersed.