28. Fertilization in sea urchin eggs involves Ca²⁺ release from the endoplasmic reticulum for cortical granule reactivation. The major molecule responsible for releasing Ca²⁺ from intracellular stores is
    (1) zonapellucida glycoproteins
    (2) protamines
    (3) inositol 1, 4, 5-trisphosphate
    (4) N-acetylglucosaminidasev

Introduction

Sea urchin fertilization is initiated by a rapid and transient release of calcium ions (Ca2+) from intracellular stores, specifically the endoplasmic reticulum (ER). This calcium release is essential to trigger cortical granule exocytosis, which forms a block to polyspermy and activates the egg for embryonic development. The primary signaling molecule responsible for mobilizing Ca2+ in this process is inositol 1,4,5-trisphosphate (IP3).

Mechanism of Calcium Release by IP3

• IP3 is generated when phospholipase C (PLC) is activated by sperm-egg interaction through G-protein coupled receptors on the egg surface.

• IP3 diffuses rapidly through the cytoplasm to bind IP3 receptors on the ER membrane.

• Binding opens Ca2+ channels, causing release of Ca2+ into the cytoplasm, elevating intracellular calcium levels.

• This Ca2+ surge triggers cortical granule exocytosis, resulting in modifications to the egg’s extracellular matrix to prevent fertilization by multiple sperms.

Supporting Evidence

• Experiments have demonstrated that calcium release and fertilization responses persist even in calcium-free external media, indicating the reliance on intracellular stores and IP3 signaling.

• Microinjection of IP3 or calcium ionophores into sea urchin eggs initiates cortical granule exocytosis comparable to sperm-induced fertilization.

• Phosphoinositide signaling activation and IP3 production have been confirmed in sea urchin eggs upon fertilization.

Why Other Options Are Incorrect

• Zona pellucida glycoproteins (option 1) are components of the egg coat involved in sperm recognition, not calcium release.

• Protamines (option 2) are sperm nuclear proteins involved in DNA packaging, unrelated to calcium signaling.

• N-acetylglucosaminidase (option 4) is an enzyme possibly released during cortical granule reaction but does not mediate calcium release.

Conclusion

Inositol 1,4,5-trisphosphate (IP3) is the crucial intracellular messenger mediating Ca2+ release from the ER in sea urchin eggs during fertilization. This calcium surge triggers essential downstream events like cortical granule exocytosis, ensuring a successful and monospermic fertilization process.

This article highlights the fundamental role of IP3 in calcium signaling pathways at fertilization in sea urchins, a model vital for understanding early developmental biology.

Final Answer:
(3) inositol 1, 4, 5-trisphosphate