Question

A rise in cytosolic calcium ion concentration just after fertilization in a sea urchin egg leads to

• A. Formation of fertilization envelope
• B. Acrosomal reaction
• C. Formation of vegetal pole
• D. Formation of animal pole

Answer 1

The Correct Option is A

The question is related to the events following the fertilization of a sea urchin egg. Specifically, it asks what a rise in cytosolic calcium ion concentration leads to immediately after fertilization.

Let's break down the process step by step:

1. When a sperm enters the egg, it triggers a release of calcium ions (Ca²⁺) in the cytosol of the egg. This increase in calcium ion concentration is crucial for subsequent cellular changes.
2. This influx of Ca²⁺ acts as a signaling mechanism that initiates several downstream processes. The primary event triggered by this calcium wave is the formation of a fertilization envelope.
3. The fertilization envelope is formed through the exocytosis of cortical granules. These granules release their contents outside the plasma membrane of the egg, leading to a modification of the vitelline layer, which hardens into a protective envelope.

The formation of the fertilization envelope serves a critical role in protecting the fertilized egg from being penetrated by additional sperm, thus preventing polyspermy.

Examining the options provided:

• Formation of fertilization envelope: This is the correct answer, as discussed above. The rise in cytosolic calcium triggers the cortical reaction leading to its formation.
• Acrosomal reaction: This occurs in the sperm, not the egg, and involves the release of hydrolytic enzymes needed for penetrating the egg membrane.
• Formation of vegetal pole: This refers to a part of the egg containing the yolk, which is determined before fertilization and is not a result of calcium concentration changes.
• Formation of animal pole: Similarly, the animal pole is pre-determined and does not form as a result of post-fertilization calcium changes.

Thus, the correct answer is clearly the Formation of fertilization envelope.