Mechanics of in vitro fertilization in the hamster

The mechanics of sperm entry into the ovum in vitro were studied with the scanning electron microscope. The morphological aspects of fertilization are different in ova with intact zonae pellucidae compared with ova that have had the zona mechanically removed. In monospermic fertilized ova with intact cumuli and zonae pellucidae, association first occurs between the anterior tip of the sperm head and oocyte microvilli. With time the sperm head is incorporated tip first, the postacrosomal region being the last portion of the sperm head to be incorporated into the ooplasm. These observations are in marked contrast to the in vivo situation in hamsters where the equatorial segment of the acrosome first associates with oocyte microvilli and the anterior tip of the sperm head is the last portion to be incorporated. In fertilization in vitro of zona-free ova, we observe variation in the way that spermatozoa associate with ova. It is concluded that the morphological events which we observe suggest physiological differences both between in vivo and in vitro fertilization as well as in fertilization among different in vitro systems.