In the 1950s it was demonstrated that the sperm of many mammalian species, including humans, must undergo some kind of modification in the female reproductive tract in order to be capable of fertilizing an ovum. This process is termed capacitation and the length of time required for it varies with species, but is usually several hours. The meaning of the term capacitation has undergone some change over the past decades as more has been learned about the process. It is now generally understood to mean the removal or destabilization of proteins in and around the acrosomal region of the sperm head to enhance its penetration ability, as well as effects on the flagellum to increase motility. The recognition of the existence of this process and the ensuing success in replicating it outside the female reproductive tract have had a profound effect on the development of assisted reproductive technology in humans as well as on artificial insemination in domesticated species.

The steps in mammalian fertilization are shown in Figure 1. The initial binding of the sperm to the zona pellucida surrounding the ovum involves a species-specific interaction between the sperm membrane and one or more of the glycoproteins, ZP1, ZP2, ZP3, and (in humans) ZP4, that make up the zona pellucida. This interaction, through an increase in intracellular Ca2+, cyclic AMP, and pH, triggers the acrosome reaction in which hydrolytic enzymes, including hyaluronidase and a protease known as acrosin, are released from the acrosome. The digestive action of these enzymes forms a tunnel in the zona pellucida around the head of the sperm which, along with the propulsive force of the flagellum, enables the sperm to traverse it and bind to the outer surface of the ovum. The plasma membranes of the sperm and egg then fuse, allowing the sperm’s pronucleus to enter the ovum, and move towards the ovum’s pro nucleus, formed from the completion of the ovum’s final meiotic division. The two pronuclei join, forming the zygote nucleus. During this time, the fusion of the two plasma membranes initiates the cortical reaction in which the enzymes contained in the cortical granules lying just inside the ovum’s plasma membrane are released into the perivitelline space. This renders the zona pellucida refractory to sperm penetration, which, along with the destruction of ZP proteins, blocks the entrance of any other sperm. IP3 (inositol triphosphate) and DAG (diacylglycerol) signaling pathways are involved in this important step of blocking polyspermy.

Fig1. Fertilization. The outer layer of the ovum in the fallopian tube is the zona pellucida, consisting of the glycoproteins ZP1, ZP2, ZP3, and ZP4. When the sperm binds to one or more of these proteins (1), the proteolytic enzymes in the acrosome are released (2) and the sperm penetrates this coating. The penetration of the zona pellucida is completed (3), leading to the fusion of plasma membranes of the sperm and the egg (4). The sperm pronucleus is released into the egg (5); it and the ovum pronucleus move toward each other and fuse, forming the zygote nucleus (7). Meanwhile, the contents of the cortical granules lining the intracellular face of the ovum’s plasma membrane are released to form the cortical granule membrane (6), which prevents the entry of other sperm.

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