Questions: Fertilization and Early Cleavage

Question 1

Why does the cortical reaction serve as a slow block to polyspermy, and why is a fast block also needed?

• A. The fast block is electrical (membrane depolarization) and acts within seconds but is temporary; the cortical reaction takes minutes to complete but creates a permanent physical barrier (the fertilization envelope) — together they ensure complete polyspermy prevention
• B. The fast block kills extra sperm with enzymes; the cortical reaction blocks the egg's nucleus
• C. The fast block only works in mammals; the cortical reaction only works in sea urchins
• D. Both blocks are redundant — either one alone is sufficient

Answer: (A)

In sea urchins (the classic model), sperm contact triggers immediate membrane depolarization (fast block) that prevents additional sperm binding within 1-3 seconds. But this electrical block is transient. The cortical reaction — calcium wave-triggered exocytosis of cortical granules that modify the extracellular matrix into a hardened fertilization envelope — takes 30-60 seconds but creates a permanent physical and chemical barrier. The two mechanisms cover different time windows: the fast block provides immediate protection while the permanent slow block is being assembled.

Question 2

During cleavage, cells divide but the embryo does not grow. The total cytoplasmic volume remains approximately constant while cell number increases.

• T. True
• F. False

Answer: True

Cleavage divisions are unusual: they lack G1 and G2 phases, so cells divide without growing between divisions. The large egg cytoplasm is simply partitioned into progressively smaller cells (blastomeres). A frog egg might go from one cell to thousands of cells without any increase in total volume. This rapid subdivision distributes maternal mRNAs and proteins (deposited during oogenesis) into distinct cellular compartments, creating the first differences between cells based on which cytoplasmic determinants each blastomere inherits.

Question 3

How does yolk distribution in the egg affect cleavage pattern?

Model answer: Yolk is dense and impedes the cleavage furrow. In isolecithal eggs (even yolk distribution, as in sea urchins), cleavage is equal and symmetric. In mesolecithal eggs (moderate yolk concentrated at the vegetal pole, as in frogs), cleavage is unequal — vegetal blastomeres are larger because yolk slows furrow progression. In telolecithal eggs (massive yolk, as in birds and reptiles), cleavage is restricted to a small disc of cytoplasm (meroblastic discoidal cleavage) because the cleavage furrow cannot penetrate the dense yolk mass. Yolk distribution thus determines whether cleavage is holoblastic (complete) or meroblastic (partial) and influences the geometry of cell arrangement.

This relationship between yolk and cleavage pattern is one of the oldest observations in developmental biology. It demonstrates how a simple physical constraint (yolk density impeding cytokinesis) shapes the spatial organization of the early embryo, which in turn influences later developmental events.