Q.29 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
Correct Answer: (A) Formation of fertilization envelope
A rise in cytosolic calcium ion concentration immediately after fertilization in sea urchin eggs triggers the cortical granule exocytosis, which modifies the vitelline envelope into a hardened fertilization envelope to block polyspermy.
Option Analysis
Acrosomal Reaction (B)
The acrosomal reaction occurs in the sperm before fusion with the egg, triggered by egg jelly contact that raises sperm calcium levels for acrosome exocytosis and process extension. Cytosolic calcium rise happens in the egg post-fusion, so this option is incorrect.
Fertilization Envelope Formation (A)
Post-fusion, sperm entry generates IP3, releasing calcium from egg intracellular stores in a propagating wave within seconds. This calcium surge causes cortical granules to fuse with the egg plasma membrane, releasing enzymes and proteins that harden and lift the vitelline envelope into the fertilization envelope within 1-5 minutes.
Vegetal Pole Formation (C)
The vegetal pole emerges during cleavage stages (after 1-2 hours), defined by unequal fourth cleavage producing micromeres from maternal factors like Dishevelled and beta-catenin, not immediate calcium signaling.
Animal Pole Formation (D)
The animal pole forms concurrently with vegetal structures during early embryo patterning (post-fertilization but hours later), unrelated to the acute calcium transient that activates the egg cortex.
The rise in cytosolic calcium fertilization sea urchin egg process is a cornerstone of developmental biology, especially for CSIR NET Life Sciences aspirants studying fertilization mechanisms. This calcium wave activates essential egg responses post-sperm fusion.
Calcium Wave Trigger
Sperm-egg fusion produces IP3, opening ER calcium channels for a rapid cytosolic spike (~1 μM) propagating across the egg in 10-30 seconds. Experiments with ionophores like A23187 mimic this, confirming intracellular release without external calcium.
Role in Envelope Formation
Elevated calcium prompts cortical granule exocytosis, releasing proteases that digest vitelline envelope links to the plasma membrane and peroxidases that cross-link proteins for hardening. The envelope lifts within minutes, ensuring monospermy.
Exam Relevance
This event distinguishes egg activation from sperm processes; calcium chelators like BAPTA block envelope formation but not fusion. Master this for CSIR NET questions on signaling and blocks to polyspermy.
