19. During fertilization in amphibians, the fusion of egg and sperm plasma is preceeded by
(A) release of enzymatic contents from the acrosomal vesicle through exocytosis
(B) binding and interaction of the sperm to vitelline membrane
(C) chemoattraction of the sperm to the egg by soluble factors secreted by egg
(D) passage of sperm through extracellular envelope
Which of the following is the correct sequence?
(1) (A) → (B) → (C) → (D)
(2) (B) → (A) → (C) → (D)
(3) (C) → (A) → (B) → (D)
(4) (C) → (B) → (A) → (D)

 

The correct sequence of events during fertilization in amphibians is:

(4) (C) → (B) → (A) → (D)

This means:

1. (C) Chemoattraction of the sperm to the egg by soluble factors secreted by the egg: The egg releases chemical cues that attract sperm toward it, guiding sperm movement in the aquatic environment.

2. (B) Binding and interaction of the sperm to the vitelline membrane: After arrival, sperm bind specifically to the egg’s outer extracellular envelope called the vitelline membrane.

3. (A) Release of enzymatic contents from the acrosomal vesicle through exocytosis: Following binding, the sperm undergoes the acrosome reaction, releasing enzymes that help digest the protective envelope surrounding the egg.

4. (D) Passage of sperm through extracellular envelope: The enzymes allow sperm to penetrate through the vitelline envelope and reach the egg plasma membrane for fusion.

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Introduction

Fertilization in amphibians involves a precise and sequential set of biological events to ensure successful union of sperm and egg. Understanding these events sheds light on reproductive adaptations in external fertilizers such as frogs and newts, where gametes meet in aquatic environments.

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Step 1: Chemoattraction of Sperm by Egg Secretions

Amphibian eggs secrete chemoattractant molecules that create gradients in their environment. These soluble factors help steer motile sperm towards the egg, enhancing fertilization efficiency in a vast milieu of water and other gametes.

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Step 2: Binding of Sperm to Vitelline Membrane

Once sperm reach the egg vicinity, they encounter the vitelline membrane, an extracellular envelope surrounding amphibian eggs. Sperm bind specifically to molecules on this membrane, a crucial step before penetration.

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Step 3: Acrosome Reaction – Release of Enzymes

Following binding, sperm undergo the acrosome reaction, an exocytotic release of hydrolytic enzymes from the acrosomal vesicle. These enzymes digest components of the vitelline membrane, creating a pathway for sperm progression.

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Step 4: Passage Through Extracellular Envelope

Enzymatic digestion facilitates the sperm’s penetration through the vitelline envelope. After passage, sperm can fuse their plasma membrane with the egg’s membrane, initiating fertilization.

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Conclusion

Amphibian fertilization is orchestrated through:

• Chemoattraction by egg-released factors (C)

• Sperm binding to the vitelline membrane (B)

• Enzymatic acrosome reaction (A)

• Sperm passage through the extracellular envelope (D)

This sequential process ensures specificity and efficiency in fertilization.