Answer
The term for fertilization of an egg by two or more sperm is "polyspermy." Polyspermy occurs when multiple sperm successfully penetrate the protective layers of the egg and attempt to fertilize it. However, in most cases, polyspermy is prevented by various mechanisms that the egg has evolved to maintain the integrity of the fertilization process. These mechanisms collectively constitute the "polyspermy block."
The egg normally prevents polyspermy through the following mechanisms:
1. **Zona Reaction:** Upon the binding of the first sperm to the egg's zona pellucida, a rapid change in the zona's structure occurs, making it less receptive to additional sperm. This change in the zona is known as the "zona reaction," and it physically blocks other sperm from binding.
2. **Cortical Reaction:** Once the first sperm successfully penetrates the zona pellucida and fuses with the egg's plasma membrane, the egg undergoes a cortical reaction. In this reaction, cortical granules within the egg release enzymes into the zona pellucida, causing it to harden and become impenetrable to other sperm. This process, known as the "cortical reaction," ensures that only one sperm can successfully fuse with the egg's plasma membrane.
3. **Fast Block to Polyspermy:** When a sperm fuses with the egg, it triggers a rapid change in the egg's membrane potential, known as the "fast block to polyspermy." This change in electrical potential temporarily repels other sperm, preventing their fusion with the egg's membrane.
4. **Slow Block to Polyspermy:** The cortical reaction also leads to the release of calcium ions into the egg's cytoplasm. These calcium ions play a role in the "slow block to polyspermy," a series of biochemical changes that occur inside the egg and further inhibit sperm penetration. These changes include modifications to the zona pellucida and the prevention of sperm binding.
Collectively, these mechanisms create a series of barriers that prevent additional sperm from fertilizing the egg after the successful fusion of the first sperm. The polyspermy block ensures the successful and controlled fertilization of the egg by a single sperm, which is essential for the development of a viable embryo.
Work Step by Step
The term for fertilization of an egg by two or more sperm is "polyspermy." Polyspermy occurs when multiple sperm successfully penetrate the protective layers of the egg and attempt to fertilize it. However, in most cases, polyspermy is prevented by various mechanisms that the egg has evolved to maintain the integrity of the fertilization process. These mechanisms collectively constitute the "polyspermy block."
The egg normally prevents polyspermy through the following mechanisms:
1. **Zona Reaction:** Upon the binding of the first sperm to the egg's zona pellucida, a rapid change in the zona's structure occurs, making it less receptive to additional sperm. This change in the zona is known as the "zona reaction," and it physically blocks other sperm from binding.
2. **Cortical Reaction:** Once the first sperm successfully penetrates the zona pellucida and fuses with the egg's plasma membrane, the egg undergoes a cortical reaction. In this reaction, cortical granules within the egg release enzymes into the zona pellucida, causing it to harden and become impenetrable to other sperm. This process, known as the "cortical reaction," ensures that only one sperm can successfully fuse with the egg's plasma membrane.
3. **Fast Block to Polyspermy:** When a sperm fuses with the egg, it triggers a rapid change in the egg's membrane potential, known as the "fast block to polyspermy." This change in electrical potential temporarily repels other sperm, preventing their fusion with the egg's membrane.
4. **Slow Block to Polyspermy:** The cortical reaction also leads to the release of calcium ions into the egg's cytoplasm. These calcium ions play a role in the "slow block to polyspermy," a series of biochemical changes that occur inside the egg and further inhibit sperm penetration. These changes include modifications to the zona pellucida and the prevention of sperm binding.
Collectively, these mechanisms create a series of barriers that prevent additional sperm from fertilizing the egg after the successful fusion of the first sperm. The polyspermy block ensures the successful and controlled fertilization of the egg by a single sperm, which is essential for the development of a viable embryo.
