Zona Pellucida: Definition & Concept

Instructor: Christine Morgan

Christine has taught college Biology and Anatomy, and has a Master's degree in Anatomy.

The zona pellucida (ZP) is an important, integral structure for the egg both before and after fertilization. Learn how the ZP works to make reproduction possible and protects the new life it surrounds.

The ZP and the Amazing Race to Fertilization

Have you ever wondered how, with all the millions of sperm that try to fertilize one egg, one lucky guy is able to be the only one? Is it speed, luck, or just good looks? Well, part of it is the speed and luck thing, but a big part has to do with a structure that surrounded the unfertilized egg way back when its owner was still a fetus herself -- long before she was old enough for kids of her own.

The zona pellucida (ZP) is a name given to a clear (lucid) zone, or region that surrounds the unfertilized egg and works to keep out those pesky stray sperm that hang around after the new couple meets up. It even stays close to the developing cells after fertilization to help out along the road to implantation in the uterus, or womb.

The ZP Forms

The ZP is formed by protective secretions that come from the early egg, or oocyte, and 'helper' cells called follicle cells that surround the oocyte at its earliest stage. You can see it around the egg as a thick clear line surrounded by the other cells.

Zona Pellucida surrounding Oocyte
Zona Pellucida around Oocyte

You may be surprised to know the entire trip to the uterus from the ovary where the oocyte was released takes only a few days -- fertilization has to happen soon for the egg to survive. The ZP has several types of special chemicals called glycoproteins to help with this. The word part glyco means sugar -- combine that with a protein and you have a super power snack. Some of these chemicals not only help to make fertilization possible, but also keep out all the other sperm immediately after one of them has penetrated the egg.

The Lucky Bachelor is the First Sperm to Arrive

Millions of sperm surround one egg, and a combination of their movements and enzymes (or substances that make chemical reactions easier and faster) allow a few of them to get through the outer layer of helper cells by dissolving some of the glue that holds the cells around the egg.

When one sperm gets through it meets receptors, or 'docking' sites in the ZP that react when it arrives, binding with the sperm and setting off a cascade of events that will single him out as the winner of the game. This process for the ZP is species specific, meaning it will prevent cross-species fertilization -- each species can only reproduce with its own kind.

Zona Pellucida Mediation of Fertilization
Zona Pellucida mediating Fertilization

The events happen like this: after the sperm binds with the receptors of the ZP, one of the zona glycoproteins is released, which, in turn, causes the now prospective father sperm to release more enzymes. So now the ZP will be dissolved in that region and the sperm can finally get all the way through to the actual egg cell.

When our two contestants meet, the exchange works something like a key in a lock -- again, receptors in the egg membrane recognize receptors in the sperm cell membrane. Yep, you guessed it, that causes another change -- substances are released from the egg that immediately harden the zona and destroy all the other sperm receptors. Talk about a way to get rid of all the other competitors!

The ZP Cell Protection Lasts and Lasts

So, our contestants now fade out of the picture to make way for a new life, which at the 2 cell stage is called a zygote. The ZP remains in the hardened state to provide continued protection for the cells of the new organism as it grows and divides, kind of like a Super Nanny. You can see it in high powered images from the zygote stage and beyond, up to about the sixth day after fertilization.

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