0. Explain the mechanisms underlying the blocks to poly polyspermy? Why spermatozoa normally arrive at the site of fer tion before the ove? (1 fertilization ks)

The mechanisms underlying the blocks to polyspermy are as follows:<br /><br />1. **Fast Block to Polyspermy**: This is a rapid, temporary block that occurs within seconds of sperm binding to the egg. It is mediated by the cortical reaction, where cortical granules release their contents into the perivitelline space (the space between the egg's plasma membrane and the zona pellucida). This causes changes in the egg's membrane and the zona pellucida, making it impermeable to additional sperm.<br /><br />2. **Slow Block to Polyspermy**: This is a slower, more permanent block that takes several minutes to hours to complete. It involves the zona pellucida reaction, where sperm-binding triggers the release of enzymes that harden and modify the zona pellucida, preventing further sperm from penetrating.<br /><br />Regarding why spermatozoa normally arrive at the site of fertilization before the ovum:<br /><br />1. **Sperm Capacitation**: Sperm undergo a process called capacitation in the female reproductive tract, which enhances their motility and prepares them for the acrosome reaction. This process requires time, ensuring that sperm reach the ovum when it is released.<br /><br />2. **Ovum Release Timing**: The ovum is released from the ovary at ovulation, and its journey through the fallopian tube to the site of fertilization takes time. By the time the ovum is ready for fertilization, sperm have already had the opportunity to travel and capacitate.<br /><br />3. **Sperm Competition**: Multiple sperm may reach the site of fertilization around the same time, but only one will fertilize the ovum. The first sperm to reach and successfully penetrate the zona pellucida will fertilize the ovum, a process known as the "race" or "competition" among sperm.<br /><br />These mechanisms ensure that fertilization occurs with a single sperm, preventing polyspermy and ensuring the correct genetic contribution to the zygote.