Correct option is D
The cortical reaction in sea urchins is a key process during fertilization that prevents polyspermy (fertilization by more than one sperm). Let's review the options:
The entry of Ca²⁺ ions into the egg initiates development.
- Partially correct, but it is misleading because Ca²⁺ ions trigger the cortical reaction and other subsequent events, but they do not directly initiate development.
The exocytosed cortical granules during egg maturation contain the components of the zona pellucida.
- This is incorrect. The zona pellucida is a glycoprotein layer found in mammals, not sea urchins. In sea urchins, the cortical granules contain enzymes and other materials that help form the fertilization membrane, not the zona pellucida.
The depolarization of the plasma membrane after sperm entry helps to block polyspermy.
- This is partially true, but the depolarization of the egg’s plasma membrane, which is caused by sperm entry, does contribute to blocking polyspermy. However, depolarization alone is not enough. The primary mechanism to block polyspermy is the formation of the fertilization membrane after the release of cortical granules.
The release of the cortical granules after sperm entry converts the vitelline membrane into the fertilization membrane which blocks polyspermy.
- Correct. Upon sperm entry into the egg, the cortical granules release their contents, which cause changes to the vitelline membrane. This results in the formation of the fertilization membrane, which acts as a physical barrier to block additional sperm from entering the egg (polyspermy).
Information Booster:
The cortical reaction in sea urchins is essential for ensuring that only one sperm fertilizes the egg. It involves several steps:
Sperm Entry: When a sperm fuses with the egg plasma membrane, it triggers an influx of Ca²⁺ ions into the egg. This rise in calcium concentration plays a crucial role in activating the egg and initiating the cortical reaction.
Cortical Granule Exocytosis: The cortical granules are vesicles located just beneath the egg’s plasma membrane. Upon sperm entry, they undergo exocytosis, releasing their contents into the space between the plasma membrane and the vitelline membrane.
Formation of the Fertilization Membrane: The enzymes released from the cortical granules cause the vitelline membrane (a membrane that surrounds the egg) to harden and elevate. This transforms the vitelline membrane into the fertilization membrane, effectively sealing the egg from additional sperm.
Block to Polyspermy: The fertilization membrane serves as a barrier, preventing other sperm from entering the egg, thus ensuring only one sperm fertilizes the egg and maintaining normal development.
Additional Knowledge:
Fast Block: The depolarization of the egg membrane immediately after sperm entry serves as the fast block to polyspermy. This electrical change prevents other sperm from fusing with the membrane.
Slow Block: The cortical reaction forms the fertilization membrane, which is a slow block mechanism that reinforces the fast block and provides a permanent barrier to polyspermy.
