11. In case of sea urchin, which of the following is the correct sequence of events taking place during the interaction of sperm and egg?
    (1) Chemo-attraction of sperm to the egg by soluble molecules secreted by the egg → exocytosis of the sperm acrosomal vesicle to release its enzymes → binding of the sperm to the extracellular matrix of the egg → passage of sperm through this extracellular matrix → fusion of egg and sperm cell membranes.
    (2) Chemo-attraction of sperm to the egg by soluble molecules secreted by the egg → binding of the sperm to the extracellular matrix of the egg → exocytosis of the sperm acrosomal vesicle to release its enzymes → passage of sperm through cell membranes.
    (3) Chemo-attraction of sperm to the egg by soluble molecules secreted by the egg → binding of the sperm to the extracellular matrix of the egg →passage of sperm through this extracellular matrix →exocytosis of the sperm acrosomal vesicle to release its enzymes →fusion of egg and sperm cell membranes.
    (4) Chemo-attraction of sperm to the egg by soluble molecules secreted by the egg → passage of sperm through this extracellular matrix →binding of the sperm to the extracellular matrix of the eggs→ exocytosis of the sperm acrosomal vesicle to release its enzymes →fusion of egg and sperm cell membranes.

Introduction

Fertilization in sea urchins is a classic model for studying the intricate and coordinated events necessary for successful reproduction. Understanding the correct order of sperm-egg interaction is important for insights into cellular communication, species-specific fertilization, and early embryonic development.

Step 1: Chemotaxis – Chemo-attraction of Sperm to the Egg

Sea urchin eggs release diffusible molecules such as Resact, which act as chemoattractants. These molecules create a concentration gradient in the surrounding water, guiding motile sperm toward the egg. This step ensures sperm efficiently locate eggs in the external aquatic environment.

Step 2: Acrosome Reaction – Exocytosis of Sperm Acrosomal Vesicle

Upon contact with the egg jelly coat, sperm undergo the acrosome reaction, which is an exocytosis of the acrosomal vesicle. This releases hydrolytic enzymes that help digest the egg’s jelly coat, and extends the acrosomal process containing adhesive proteins such as Bindin.

Step 3: Binding of Sperm to the Egg’s Extracellular Matrix

The exposed Bindin protein on the acrosomal process specifically binds to complementary receptors on the egg’s vitelline layer (the extracellular matrix). This binding is species-specific and crucial for ensuring fertilization by sperm of the same species.

Step 4: Passage of Sperm Through the Extracellular Matrix

Following binding, sperm enzymes break down components of the extracellular matrix, allowing the sperm to penetrate through the vitelline layer toward the egg plasma membrane.

Step 5: Fusion of Egg and Sperm Cell Membranes

Finally, the sperm plasma membrane fuses with the egg plasma membrane, allowing the sperm nucleus and associated components to enter the egg cytoplasm. This fusion initiates egg activation, beginning the developmental processes that lead to the formation of a zygote.

Summary Table of Events

Conclusion

The fertilization of sea urchin eggs follows a well-defined and orderly sequence beginning with sperm chemo-attraction, followed by the acrosome reaction, binding, penetration, and finally membrane fusion. This ensures species specificity and successful formation of the zygote.

Thus, option (1) accurately represents the correct sequence of events in sea urchin fertilization.

This article clarifies the fertilization sequence, vital for students and researchers delving into reproductive biology and developmental processes.