acrosome reaction

Meaning: The acrosome is a vesicle derived from the Golgi apparatus that caps the sperm head. It holds hydrolytic enzymes such as hyaluronidase and acrosin. When the sperm binds species-specific receptors on the zona pellucida (the glycoprotein layer around the egg), the acrosome fuses with the sperm plasma membrane and releases its contents by exocytosis. The enzymes digest the zona pellucida, so the sperm can reach the egg plasma membrane. The sperm and egg membranes then fuse.

On the MCAT: Questions place the acrosome reaction in the order of fertilization. The order is: sperm pass through the corona radiata, bind the zona pellucida, undergo the acrosome reaction, and fuse with the egg membrane. Fusion triggers the cortical reaction. In the cortical reaction, cortical granules in the egg release their contents, the zona pellucida hardens, and additional sperm are blocked (the slow block to polyspermy). A passage might describe a sperm with a defective acrosome, or an inhibitor of acrosomal enzymes, and ask what fails. The expected answer is that the sperm cannot penetrate the zona pellucida, so fertilization does not occur.

Common confusion: The acrosome reaction occurs in the sperm. The cortical reaction occurs in the egg. Students mix up which cell does which, and which one prevents polyspermy. The acrosome reaction helps the sperm get in, and the cortical reaction keeps other sperm out. Also, the acrosome is not the mitochondria-rich midpiece. The mitochondria in the midpiece supply ATP for the flagellum.