Vitelline membrane

The vitelline membrane or vitelline envelope is a structure surrounding the outer surface of the plasma membrane of an ovum (the oolemma) or, in some animals (e. g., birds), the extracellular yolk and the oolemma. 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. It is called zona pellucida in mammals. Between the vitelline membrane and zona pellucida is a fluid filled perivitelline space. As soon as the spermatozoon 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 the slow block to polyspermy. In insects, the vitelline membrane is called the vitelline envelope and is the inner lining of the chorion.

The vitelline membrane of the hen is made of two main protein layers that provide support for the yolk and separation from the albumen. The inner layer is known as the perivitelline lamina. Before ovulation occurs, the inner layer forms from follicular cells that surround the oocyte. After ovulation, fertilization of the egg proceeds with the formation of the outer layer that is secreted by infundibulum glands located along the first parts of the oviduct.

After the sperm digests its way through the jelly layer, the acrosomal process of the sperm makes contact with the vitelline envelope. The vitelline envelope has glycoproteins and peptides that allow for species specific sperm binding and recognition. For example, in the sea urchin species, red sea urchin and purple sea urchin, the vitelline membrane has bindin receptors for the bindin protein present on the sperm head. In the African clawed frog, it was found that the gp69/gp64 glycoprotein pair is involved in sperm recognition and binding.

The vitelline membrane serves a different purpose in chickens. In the chicken egg, the yolk is separated from the albumen by the vitelline membrane which acts as a barrier to microbial infection. Apart from the 13 proteins identified to make up the membrane, the proteins that are key to providing antimicrobial properties to the membrane are the vitelline outer membrane proteins (VMO) 1 and 2. The innermost membrane of all animal eggs except some cnidarians is called the vitelline membrane. Some invertebrates and some lower chordate eggs are covered by this membrane only, while most have other membranes. Frog and bird eggs have a very thin vitelline membrane which are surrounded by either a jelly layer (frogs) or other membranes (birds). In mammals, the structure is called the zona pellucida and is surrounded by a layer of support cells, called the corona radiata.