3. Which of the following is the last to occur after the binding of a sea urchin sperm to an egg?
(1) Initiation of mRNA synthesis
(2) Increase in cytosolic pH

(3) Increase in intracellular calcium concentration
(4) Exocytosis of cortical granules

Introduction

Fertilization in sea urchins is a classic and extensively studied example of external fertilization. This process triggers a well-orchestrated cascade of physiological and biochemical changes in the egg, setting the stage for successful embryonic development. Among the earliest responses to sperm binding are changes in ion concentrations, pH shifts, release of cortical granules, and later genetic activations such as mRNA synthesis. Understanding the precise sequence of these events reveals vital insights into reproductive biology.

Early Events Following Sperm-Egg Binding in Sea Urchins

Immediately after a sperm binds to the egg surface, several rapid events occur almost simultaneously:

• Increase in Cytosolic pH: Within seconds of sperm binding, the egg’s cytoplasmic pH rises. This alkalinization primes metabolic pathways and other activation mechanisms essential for subsequent developmental steps.

• Increase in Intracellular Calcium Concentration: One of the most critical early signals is a rapid, widespread increase in cytosolic calcium ions (Ca2+). This calcium wave is triggered by sperm-egg interaction and is essential to begin activation of the egg’s cellular machinery.

Exocytosis of Cortical Granules: The Final Immediate Block to Polyspermy

Following the calcium increase, exocytosis of cortical granules occurs. These granules, found beneath the egg’s plasma membrane, discharge their contents to the egg exterior in a process known as the cortical reaction.

The released enzymes and molecules modify the egg’s vitelline layer to form the fertilization envelope, a hardened structure that physically blocks additional sperm from entering the egg. This is termed the slow block to polyspermy, which complements the rapid membrane potential change (fast block) occurring immediately after sperm contact.

The cortical granule exocytosis typically occurs several seconds after the calcium surge and represents the last of the immediate physiological changes that establish fertilization and prevent multiple sperm from fertilizing the egg.

Initiation of mRNA Synthesis: A Later Event

Activation of the egg’s genetic program, including initiation of mRNA synthesis, occurs after these early membrane and ionic changes. This process involves transcriptional activation and increased protein synthesis essential for further embryonic development but happens later in the timeline compared to the immediate events triggered by sperm binding.

Timeline Summary of Key Events

Why Exocytosis of Cortical Granules Is the Last Immediate Event

The cortical granule exocytosis depends directly on the calcium wave triggered by sperm binding. Since calcium increase follows the pH rise, and the cortical reaction follows calcium influx, exocytosis is naturally the last among these rapid fertilization events.

Initiation of mRNA synthesis, though crucial, occurs well after these physical and biochemical fertilization responses. It is part of the metabolic activation phase as the zygote prepares for its first cell divisions.

Biological Significance of the Sequence

• Cytosolic pH increase helps activate various enzymes and metabolic pathways crucial for embryogenesis.

• Calcium rise acts as a universal activation signal, triggering the cortical reaction and subsequent developmental events.

• Cortical granule exocytosis ensures monospermy, a vital safeguard against abnormal chromosomal numbers.

• mRNA synthesis initiation guarantees the embryo’s ability to produce proteins necessary for growth and differentiation post-fertilization.

Understanding this sequenced activation is crucial for developmental biology and has applications in reproductive medicine and aquaculture.

Conclusion

In summary, after sperm binds to a sea urchin egg, the last immediate event among the options given is the exocytosis of cortical granules. This process is essential for the slow block to polyspermy, preventing fertilization by multiple sperm and ensuring normal embryonic development. The earlier increases in cytosolic pH and intracellular calcium trigger this exocytosis, while initiation of mRNA synthesis occurs later during embryonic activation.

Therefore, the correct answer to the question is:

(4) Exocytosis of cortical granules