Biological Systems MCAT Practice Exam

What triggers the cortical reaction during fertilization, preventing polyspermy?

1. Influx of potassium ions

2. Influx of sodium ions

3. Influx of calcium ions

4. Release of hCG

The correct answer is: Influx of calcium ions

The cortical reaction during fertilization is triggered by the influx of calcium ions. When a sperm successfully enters the egg, it causes a rapid increase in intracellular calcium concentration. This calcium spike initiates a series of reactions that leads to the cortical granules within the egg moving toward the plasma membrane and releasing their contents into the space surrounding the egg. This exocytosis of cortical granules results in a modification of the zona pellucida, the protective glycoprotein layer surrounding the egg. The released enzymes from the cortical granules help to create a physical barrier that prevents additional sperm from binding to and entering the egg, effectively blocking polyspermy. This is critical for ensuring that only one sperm fertilizes the egg and that the genetic material remains intact for proper embryonic development. In contrast, potassium and sodium ion fluxes primarily contribute to membrane potential changes and depolarization of the egg membrane, which are part of the initial response but do not directly initiate the cortical reaction. The release of hCG is related to signaling in pregnancy and is not involved in the fertilization process itself. Therefore, the specific and crucial role of calcium ions in triggering the cortical reaction distinguishes it as the correct answer.