January 13, 2010 – How does PZP work? (Part 2, PZP’s Function)

(Note: Please read yesterday’s post (How does PZP work? (Part 1, Reproduction) to have background on today’s topic.)

Yesterday, I discussed how pregnancy results from the release of different hormones that culminate in the release of a mature egg that is fertilized. I also discussed how the different steps leading up to the release of a mature egg can be disrupted to prevent pregnancy from occurring. Let’s focus today on how the last step, fertilization, can be disrupted.

Mammal eggs are all similar in that they share basic structures. This is a simply drawing of a mammal egg. It has a nucleus within cytoplasm that is surrounded by a noncellular layer (zona pellucida) and a cellular layer (corona radiata). In order for the egg to be fertilized, sperm must get past these layers and into the egg.

Now we’ll focus on that part of the egg called the zona pellucida. Zona pellucida is a glycoprotein. This means it is a noncellular substance made from proteins combined with carbohydrates. It is a membrane surrounding the egg that acts as a barrier that interacts with sperm on a molecular level in order to permit or deny the sperm access to the egg. Think of the zona pellucida as having holes around it that are of a certain shape. The shape of these holes matches the shape of sperm from the same species.

If the egg encounters sperm from its same species, the zona pellucida allows it to enter the egg so that fertilization can occur.

If the egg encounters sperm from a different species, the zona pellucida will not allow the sperm to enter the egg; and fertilization will not occur.

As you can see, the zona pellucida plays an important role in fertilization. If the function of the zona pellucida was disrupted, then fertilization obviously could be prevented.

Disrupting the function of the zona pellucida is just how PZP works. The zona pellucida of pigs (porzine zona pellucida, hence PZP) can be isolated and prepared into a vaccine. (This is done through a very complicated process that we’ll touch on tomorrow.) To get a handle on how PZP does work, let’s think about how a vaccine works. A vaccine consists of a substance that causes the body’s immune system to respond and create structures (antibodies) whose task it is to act against the introduced substance. For example, a flu vaccine consists of dead or weakened flu viruses. When injected into the body, the immune system is able to easily develop antibodies to combat these particular viruses. These antibodies will continue to remain in the body in preparation for future attacks. Let’s now go back to a PZP vaccine. If PZP is injected into a mare, her immune system will develop antibodies against this substance, which it perceives to be invasive. Thus, these antibodies are built to bind to the PZP.

These defenses also remain in the body, and they will soon encounter the mare’s own eggs. Remember, that mammal eggs are very similar. The zona pellucida of a pig isn’t all that different from the zona pellucida of a horse. Similarly, if a mare produces antibodies to act against the zona pellucida of a pig, these antibodies will also act against the mare’s own zona pellucida. There is a fit that is just close enough for the antibodies to attach. When this happens, the mare’s own zona pellucida is effectively blocked. As a result, sperm is unable to interact with the egg; and so fertilization cannot occur.

To summarize, PZP is simply a glycoprotein that surrounds the eggs of pigs. When used as a vaccine, it leads to an immune response that can eventually result in the prevention of fertilization. This is why PZP is referred to as an immunocontraceptive; it is a vaccine that works with the immune system in preventing pregnancy. It is not a hormone; and it does not act against any hormones, including those that are involved in reproduction. It again prevents pregnancy at the last possible step before conception, which is fertilization.

When I was first taught about the way PZP works, I remember being surprised at its simplicity but a little confused about the reason that the zona pellucida of pigs is used as an immunocontraceptive. Why is the zona pellucida of pigs used? How is PZP produced from pig eggs? These are some of the questions that will be answered in tomorrow’s post.

Published in: on January 13, 2010 at 7:05 pm  Comments (3)  

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3 CommentsLeave a comment

  1. Thank you for explaining it in layman terms. It certainly makes it easier to understand.

    I’m looking forward to more!!

  2. What a clear and informative explanation!


  3. wooow, what a great simple explanation i really understood it well. thanks and god bless you

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