The process of fertilization takes place long after other steps in reproduction like spermatogenesis, sperm transport, and sperm swimming through the reproductive tract. Once the sperm have reached the egg, or oocyte, they begin the process of binding to the egg via the acrosome reaction.
The acrosome, a structure at the head of sperm, contains enzymes that are required to bind and break through the coating of the egg. The acrosome reaction begins as the sperm approaches the zona pellucida (ZP). The ZP is a mixture of sugars and proteins that surround the outer membrane of an egg. When the ZP binds sperm, enzymatic interactions provide almost a “lock and key” recognition that allows the process to continue.
A human ovum. The zona pellucida is seen as the thick exterior halo
The enzymes stored in the acrosome play a number of functions, all required to penetrate the egg. Additionally, ion channels are involved in a process called sperm hyperactivation, which is crucial to fertilization. The whipping of the tail of sperm provides a mechanical boost to the sperm, thus allowing further penetration.
Once the sperm has made it’s way past the ZP, the cortical reaction occurs, where new enzymes stored in granules are expelled. These enzymes harden the ZP, making it impermeable to other sperm.
Sperm and egg in the process of fusion.
At this point, the membranes of the sperm and egg are prepared to fuse. Once they do, their genetic material combines to make a mature ovum.
All of these processes are tightly regulated, complicated orchestrations with multiple failure points. Some of them have been identified as possible targets for male contraception. MCI is funding work investigating the TSSK and CRISP families of proteins, as well as the interaction between the sperm / egg recognition targets Juno and Izumo.
A drug that prevents fertilization has some exciting potential, as theoretically they could be used by both men and women, providing true non-hormonal equity to the contraceptive world. Additionally, it would have the benefit of a short onset time, and could be taken possibly a few hours before sexual intercourse.
