Which of the following blocks the entry of additional sperm, once a single sperm cell encounters ova?

Options

A. Corpus luteum
B. Plasma membrane
C. Corona radiata
D. Zona pellucida

Answer

To solve the question "Which of the following blocks the entry of additional sperm, once a single sperm cell encounters ova?", we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Process of Fertilization**:
- When a sperm cell encounters an ovum (egg), it can initiate the process of fertilization. However, it is crucial that only one sperm fertilizes the egg to ensure proper development.

2. **Identifying the Problem of Polyspermy**:
- Polyspermy is the condition where more than one sperm enters the egg. This can lead to abnormal development and is detrimental to the embryo.

3. **Cortical Reaction**:
- To prevent polyspermy, the egg employs a mechanism known as the cortical reaction. This reaction occurs immediately after the first sperm penetrates the egg.

4. **Role of Cortical Granules**:
- The egg contains cortical granules located just beneath its plasma membrane. When the first sperm enters, these granules release their contents into the space between the egg's plasma membrane and the zona pellucida (the outer protective layer of the egg).

5. **Changes in the Zona Pellucida**:
- The release of enzymes from the cortical granules causes the zona pellucida to undergo changes. It hardens and alters its structure, effectively blocking any additional sperm from entering.

6. **Conclusion**:
- Therefore, the correct answer to the question is the zona pellucida, as it is the structure that prevents the entry of additional sperm after the first sperm has successfully fertilized the egg.

### Final Answer:
The zona pellucida blocks the entry of additional sperm once a single sperm cell encounters the ova.

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