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= Vitelline Membrane = From Wikipedia, the free encyclopedia The vitelline membrane, also referred to as the vitelline envelope, is the structure surrounding the outer surface of the plasma membrane of an ovum. It is composed mostly of protein fibers, with protein receptors needed for sperm binding which, in turn, are bound to sperm plasma membrane receptors. The species-specificity between these receptors contributes to prevention of breeding between different species.

The vitelline membrane is called the zona pellucida in mammals.

Sea urchins and mammalian eggs have layers to them. The layer outside of the vitelline membrane is called the jelly layer in sea urchins. Once the sperm hits the jelly layer there will be species-specific recognition, furthermore causing a reaction from the sperm to the sugars in the jelly. The sugars are going to trigger an acrosomal reaction. The proteins that are released from the acrosome of the spermatozoan in this reaction are going to degrade the jelly layer in order for the spermatozoan to make it to the vitelline membrane of the ovum.

As soon as the spermatozoan fuses with the ovum, signal transduction occurs, resulting in an increase of cytoplasmic calcium ions. This itself triggers the cortical reaction, which results in depositing several substances onto the vitelline membrane through exocytosis of the cortical granules, transforming it into a hard layer called the “fertilization membrane”, which serves as a barrier inaccessible to other spermatozoa. This phenomenon is known as the slow block to polyspermy.

Fertilization Envelope
The fertilization envelope is the structure surrounding the inner surface of the plasma membrane of the ovum, formed by the hardening of the vitelline envelope, after entry by a spermatozoan. The fertilization envelope forms post fertilization acting as a barrier to prevent any further penetration or fusion of other spermatozoa. Bindin is a protein found on the surface of the acrosomal process of sperm which helps the sperm bind to the egg membrane. Bindin receptors are found on the egg and function as receivers the sperm. Specific bindin proteins will only bind to the receptors of its species, otherwise known as species-specific binding. This is done to ensure that only one sperm is recognized and fertilizes the egg.

Upon fertilization, the cortical granules sitting in the cytoplasm of the egg fuse to the membrane of the egg. The cortical granules exocytose, or break open, and secrete proteases which cleave bindin receptors on the plasma membrane of the egg. By cleaving the bindin receptors, sperm can no longer bind to the egg, blocking polyspermy. This is one of the initial steps necessary to the formation of the fertilization envelope causing the sperm to have nothing to bind to.

Additionally, these proteases are going to cleave other proteins that are holding the vitelline envelope to the plasma membrane of the egg. As a result, the vitelline envelope will be released from the plasma membrane and will begin to be elevated from the surface of the egg and float away. Water comes in and further pushes the vitelline membrane from the egg cell surface. This process is aided by secreted mucopolysaccharides and hyalin, hydrophillic molecules both of which absorb water and provide pressure and support. The vitelline membrane then hardens to become the fertilization envelope. The resulting hard shell will become impervious to more sperm coming in to prevent additional fusion of sperm to the egg membrane.

Echinoderms
Sea urchins, more specifically sea urchin eggs, are used as a model organism in developmental biology when studying vitelline membranes and fertilization envelopes. In sea urchins, the layer outside of the vitelline membrane is called the jelly layer. The jelly layer is composed of various sugars and is what comes into contact with the spermatozoan first, before the vitelline membrane.

Mammals
The formation of the fertilization envelope in mammals is very similar to sea urchins and other echinoderms. However, in mammals, the vitelline envelope is referred to as the Zona pellicuda. Once the zona pellucida hardens, it is still called the fertilization envelope. However, the outer layer of the ovum referred to as the jelly layer in a sea urchin, is called the cumulus layer in mammalian organisms.

Complications
Irregular formation of the vitelline membrane or the fertilization envelope can lead to complications in fertilization and furthermore, embryology. Each ovum is surrounded by hundreds of sperm. A single spermatozoan is going to fertilize the egg, triggering the cortical reaction which results in the change from the vitelline membrane to the fertilization envelope preventing any further sperm from fertilizing the egg. If the fertilization envelope fails to form the halo around the egg's plasma membrane, there may be entry by more than one sperm to the egg. This complication in fertilization is known as triploidy, meaning that while normal embryos have two sets of each chromosome, these embryos will develop 3 sets of each chromosome, which is often incompatible with life.