Animal Fertilization: Reactions & Activation

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  • 0:02 Animal Fertilization
  • 0:53 The Acrosomal Reaction
  • 1:54 The Cortical Reaction
  • 3:05 Egg Activation
  • 4:30 Lesson Summary
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Lesson Transcript
Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

The fertilization of an animal egg cell is not a straight-forward process; it's a back and forth between two gamete cells. In this lesson, we'll explore the major steps involved in this process and see how two cells manage to create something new.

Animal Fertilization

Dancing is a complex art. It requires two people who react to each other's steps. One person steps, the other person steps. It's a complicated back and forth, but the result is the creation of something pretty cool. Well the romance of dancing is not lost on the natural world.

In the process of fertilization within animals, the fusion of gametes to create a new individual, an egg cell and sperm must join together and combine their genetic information. How this happens is a delicate back and forth in exchange of chemical reactions necessary to create a single genetically unique individual.

It's not as simple as a sperm cell finding an egg. Both cells have a part to play. After all, it takes two to tango!

The Acrosomal Reaction

The process of creating a fertilized cell with two sets of genetic information called a zygote is an intricate dance between gametes. So, let's get to know the steps.

Let's start with the acrosomal reaction, a release of chemicals by the sperm cell to break down the egg's outer wall. The egg cell is thick, designed to be resistant. So getting through it is tough work. This is especially true in animals, which have egg cells covered by a dense layer of proteins called the Zona Pellucida. But the sperm cell has a trick.

As it comes into contact with the egg's protective shell, the cap-like structure at the head of the sperm called the acrosome fuses with the sperm's plasma membrane.This releases the contents of the acrosome, enzymes that are designed to break down the egg's shell. These enzymes pick away at the shell, creating a path for the sperm to enter the egg.

The Cortical Reaction

The acrosomal reaction is the sperm's part of this process. So now it's time for the egg's. Once a single sperm manages to break through the shell and begin fusing with the egg, the egg starts its own cortical reaction, the creation of a permanent, impermeable barrier. The point is to prevent polyspermy, or the fusing of multiple sperm with one egg.

Here's how it works, just below the egg's plasma membrane are vesicles called cortical granules. When a single sperm binds to the egg, it sets off a chemical reaction and these cortical granules fuse with the egg's plasma membrane releasing the enzymes contained inside them. It's essentially the same sort of process that occurs with the sperm's acrosomes.

A part of the cell containing special molecules fuses with the plasma membrane releasing proteins that create a specific reaction. Only in this case the proteins aren't breaking down a wall, they are building one up. The proteins released from the cortical granules bind with the existing cell matrix to create an impenetrable shell around the egg blocking any additional sperm from getting in.

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