7. Which one of the following statements is INCORRECT?
(1) Transient rise in Ca2+ is necessary for egg activation in mammals.
(2) Sperm induces egg activation and does not involve Ca2+
(3) In many organisms, eggs secrete diffusible molecules that attract and activate sperm.
(4) Capacitated mammalian sperm can penetrate the cumulus and bind the zona pellucida

Introduction

Fertilization in mammals is a complex biological process involving sophisticated cellular communication and molecular signaling events. Crucial among these is the role of calcium (Ca2+) ions, which act as intracellular messengers to activate the egg upon sperm entry. Capacitation prepares sperm for successful fertilization, while eggs actively influence sperm through secretions.

Understanding these events allows identification of misinformation or inaccuracies commonly associated with fertilization biology.

Role of Calcium in Egg Activation

A transient rise in intracellular calcium concentration is essential for egg activation in mammals (Statement 1). When the sperm fuses with the egg:

• The sperm introduces a factor—phospholipase C zeta (PLCζ)—that initiates calcium release from intracellular stores.

• This calcium oscillation triggers fundamental changes such as cortical granule exocytosis, resumption of meiosis, and metabolic activation of the egg.

• Calcium waves are pivotal for preventing polyspermy and initiating embryonic development.

This is well-established by extensive research showing that without calcium, egg activation and subsequent embryogenesis fail to occur.

Sperm’s Involvement in Calcium-Mediated Egg Activation

Contrary to Statement 2, sperm-induced egg activation definitely involves calcium signaling. The idea that sperm induce egg activation independent of calcium is incorrect.

• The sperm delivers PLCζ, which catalyzes production of IP3 causing release of calcium from the egg’s endoplasmic reticulum.

• This calcium signaling cascade is the intrinsic and indispensable trigger for the egg’s transition from a dormant to active state.

Therefore, the claim that egg activation by sperm does not involve Ca2+ is inaccurate.

Egg-Secreting Diffusible Molecules to Attract and Activate Sperm

Statement 3 is accurate as in many species, including sea urchins and mammals, eggs or their surrounding tissues release diffusible molecules (e.g., chemotactic peptides, progesterone) that:

• Chemotactically attract sperm toward the egg.

• Induce capacitation-like changes and increase sperm motility.

• Facilitate sperm readiness for fertilization.

This biochemical communication enhances the probability of successful fertilization.

Capacitated Sperm Penetrating the Cumulus and Binding the Zona Pellucida

Statement 4 reflects a key facet of mammalian fertilization biology:

• Capacitation is essential for sperm to penetrate the cumulus oophorus (a layer of cells surrounding the egg).

• Only capacitated spermatozoa can bind to the zona pellucida, the glycoprotein layer around the egg.

• This binding initiates further processes like the acrosome reaction, setting the stage for membrane fusion and fertilization.

This statement aligns with established reproductive biology.

Summary of Correct vs Incorrect Statements

Conclusion

The incorrect statement is:

(2) Sperm induces egg activation and does not involve Ca2+.

Calcium signaling is central to egg activation triggered by sperm entry and is a well-documented feature of mammalian fertilization. The other statements accurately reflect the biology of fertilization and capacitation processes.