Figure 3.Human Sperm-Oocyte InteractionStages of human fertilization. Spermatozoa swim through the surrounding medium and cumulus mass (not shown) and bind to the surface of the zona pellucida. The acrosome reaction is stimulated by zona proteins and the acrosome reacted sperm penetrates the zona, enters the perivitelline space and binds to the oolemma via the equatorial segment. Oocyte processes surround the sperm head and it enters the ooplasm and decondenses. Infertility could result from defects of any of these processes. For example, abnormal sperm particularly with defective head morphology bind poorly to the zona. (From Baker, H.W.G., Male Infertility. Chapter 141 In Endocrinology, 6th edition, Jameson J. L. and DeGroot L.J. (Chief Eds.), Saunders Elsevier Philadelphia PA. pp 2556-2579, 2010)