The vitelline membrane or vitelline envelope is a structure surrounding the outer surface of the

plasma membrane The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...

of an

ovum The egg cell or ovum (: ova) is the female reproductive cell, or gamete, in most anisogamous organisms (organisms that reproduce sexually with a larger, female gamete and a smaller, male one). The term is used when the female gamete is not capa ...

(the oolemma) or, in some animals (e.g., birds), the extracellular

yolk Among animals which produce eggs, the yolk (; also known as the vitellus) is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some types of egg contain no yolk, for example bec ...

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 The ''zona pellucida'' (Latin meaning "transparent zone") is the specialized area surrounding mammalian oocytes (eggs). It is also known as an egg coat. The ''zona pellucida'' is essential for oocyte growth and fertilization. The ''zona pelluc ...

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

s. Between the vitelline membrane and the oolemma (ovum cell membrane) is a fluid-filled

perivitelline space The perivitelline space is the space between the zona pellucida and the cell membrane of an oocyte or fertilized ovum. In the slow block to polyspermy, the cortical granules released from the ovum are deposited in the perivitelline space. Poly ...

. As soon as the

spermatozoon A spermatozoon (; also spelled spermatozoön; : spermatozoa; ) is a motile sperm cell (biology), cell produced by male animals relying on internal fertilization. A spermatozoon is a moving form of the ploidy, haploid cell (biology), cell that is ...

fuses with the

signal transduction Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a biochemical cascade, series of molecular events. Proteins responsible for detecting stimuli are generally termed receptor (biology), rece ...

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 Exocytosis is a term for the active transport process that transports large molecules from cell to the extracellular area. Hormones, proteins and neurotransmitters are examples of large molecules that can be transported out of the cell. Exocytosis ...

of the

cortical granules Cortex or cortical may refer to: Biology * Cortex (anatomy), the outermost layer of an organ ** Cerebral cortex, the outer layer of the vertebrate cerebrum, part of which is the ''forebrain'' *** Motor cortex, the regions of the cerebral cortex i ...

, 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 The insect vitelline envelope is the outer proteinaceous layer outside the oocyte and egg. The vitelline envelope, not being a cellular structure, is commonly referred to as a membrane. However, this is a technical misnomer as the structure is com ...

and is the inner lining of the

chorion The chorion is the outermost fetal membrane around the embryo in mammals, birds and reptiles (amniotes). It is also present around the embryo of other animals, like insects and molluscs. Structure In humans and other therian mammals, the cho ...

Structure and function

Birds

The vitelline membrane of the hen is made of two main protein layers that provide support for the yolk and separation from the

albumen Egg white is the clear liquid (also called the albumen or the glair/glaire) contained within an egg. In chickens, it is formed from the layers of secretions of the anterior section of the hen's oviduct during the passage of the egg. It forms aro ...

. The inner layer is known as the perivitelline lamina. It is a single layer that measures roughly 1 μm to 3.5 μm thick and is mainly composed of five

glycoproteins Glycoproteins are proteins which contain oligosaccharide (sugar) chains covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known a ...

that have been discovered to resemble glycoproteins of the

in mammals involved in maintaining structure ( ZP domain proteins). The outer layer, known as the extravitelline lamina, has multiple sublayers which results in thickness that ranges from 0.3 μm to 9 μm. It is primarily composed of proteins, such as

lysozyme Lysozyme (, muramidase, ''N''-acetylmuramide glycanhydrolase; systematic name peptidoglycan ''N''-acetylmuramoylhydrolase) is an antimicrobial enzyme produced by animals that forms part of the innate immune system. It is a glycoside hydrolase ...

ovomucin Ovomucin is a glycoprotein found mainly in egg whites, as well as in the chalaza and vitelline membrane. The protein makes up around 2-4% of the protein content of egg whites; like other members of the mucin protein family A protein fami ...

and vitelline outer membrane proteins that are responsible for constructing the network of dense, thin protein fibres that establish the foundation for further growth of the outer layer during embryonic development. Taking a wider view, ZP domain proteins are found in the vitelline membrane of

chordates A chordate ( ) is a bilaterian animal belonging to the phylum Chordata ( ). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics ( synapomorphies) that distinguish them from ot ...

(which contains the vertebrates) in general. The vitelline membrane is known to function as a barrier that allows for diffusion of water and selective nutrients between the albumen and the yolk.

Teleost fish

Teleost Teleostei (; Ancient Greek, Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts (), is, by far, the largest group of ray-finned fishes (class Actinopterygii), with 96% of all neontology, extant species of f ...

fish have no acrosome on their sperm cells. Their eggs coats have a small opening that lets a single sperm through.

Molluscs

Although

molluscs Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...

are not closely related to chordates, they too have ZP domain proteins in the vitelline envelope. Those recognize

sperm lysin Sperm (: sperm or sperms) is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail know ...

Insects

Insect egg coat is made of structural proteins, but these proteins are not related to the ZP proteins.

Formation

In the adult hen, liver cells express the proteins required for initial formation of the inner layer. These proteins travel via the blood from the liver to the site of assembly in the

ovary The ovary () is a gonad in the female reproductive system that produces ova; when released, an ovum travels through the fallopian tube/ oviduct into the uterus. There is an ovary on the left and the right side of the body. The ovaries are end ...

. Before

ovulation Ovulation is an important part of the menstrual cycle in female vertebrates where the egg cells are released from the ovaries as part of the ovarian cycle. In female humans ovulation typically occurs near the midpoint in the menstrual cycle and ...

occurs, the inner layer forms from follicular cells that surround the

oocyte An oocyte (, oöcyte, or ovocyte) is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in a female fetus in the ovary during female gametogenesis. The female ger ...

. 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 The oviduct in vertebrates is the passageway from an ovary. In human females, this is more usually known as the fallopian tube. The eggs travel along the oviduct. These eggs will either be fertilized by spermatozoa to become a zygote, or will dege ...

Sperm recognition and fertilization

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 The red sea urchin (''Mesocentrotus franciscanus'') is a sea urchin found in the northeastern Pacific Ocean from Alaska to Baja California. It lives in shallow waters from the low-tide line to greater than deep, and is typically found on rocky ...

and purple sea urchin, the vitelline membrane has bindin receptors for the bindin protein present on the sperm head. In the

African clawed frog The African clawed frog (''Xenopus laevis''), also known as simply xenopus, African clawed toad, African claw-toed frog or the ''platanna'') is a species of African Aquatic animal, aquatic frog of the family Pipidae. Its name is derived from the ...

, it was found that the gp69/gp64 glycoprotein pair is involved in sperm recognition and binding.

Infections and diseases

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. A recent study reports that VMO 1 can be a potential diagnostic marker for

ovarian cancer Ovarian cancer is a cancerous tumor of an ovary. It may originate from the ovary itself or more commonly from communicating nearby structures such as fallopian tubes or the inner lining of the abdomen. The ovary is made up of three different ...

in hens due to its ability to regulate estrogen and target

microRNA Micro ribonucleic acid (microRNA, miRNA, μRNA) are small, single-stranded, non-coding RNA molecules containing 21–23 nucleotides. Found in plants, animals, and even some viruses, miRNAs are involved in RNA silencing and post-transcr ...

s in the chickens' oviduct. Another difference is that the vitelline membrane has two major layers: the inner layer that faces the yolk, intermediary and external outer layer that contacts the albumen.

Other animals

sea urchin Sea urchins or urchins () are echinoderms in the class (biology), class Echinoidea. About 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal zone to deep seas of . They typically have a globular body cove ...

s, the formation of the vitelline membrane comes directly after fertilization and later thickens to form the fertilization membrane. This process is completed in about a minute. The innermost membrane of all animal eggs except some

cnidaria Cnidaria ( ) is a phylum under kingdom Animalia containing over 11,000 species of aquatic invertebrates found both in fresh water, freshwater and marine environments (predominantly the latter), including jellyfish, hydroid (zoology), hydroids, ...

ns is called the vitelline membrane. Some

invertebrates Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordate subphylum ...

and some lower

chordate A chordate ( ) is a bilaterian animal belonging to the phylum Chordata ( ). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics ( synapomorphies) that distinguish them from ot ...

eggs are covered by this membrane only, while most have other membranes.

Frog A frog is any member of a diverse and largely semiaquatic group of short-bodied, tailless amphibian vertebrates composing the order (biology), order Anura (coming from the Ancient Greek , literally 'without tail'). Frog species with rough ski ...

and

bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...

eggs have a very thin vitelline membrane which are surrounded by either a jelly layer (frogs) or other membranes (birds). In

s, the structure is called the

and is surrounded by a layer of support cells, called the

corona radiata In neuroanatomy, the corona radiata is a white matter sheet that continues inferiorly as the internal capsule and superiorly as the centrum semiovale. This sheet of both ascending and descending axons carries most of the neural traffic from and ...

Additional images

File:Anatomy of an egg.svg, Vitelline membrane in a bird egg (7) File:Amphibian Egg Diagram.svg, Vitelline membrane in an amphibian egg (2) File:Fish Egg Diagram (1).jpg, Vitelline membrane in a fish egg (A) File:Fertilization envelope.png, Formation of Fertilization envelope from the Vitelline envelope File:Parasite160001-fig2 - Vitellogenesis in Crepidostomum metoecus (Digenea) TEM.png, Parasitism and the Vitelline Membrane

References

{{Portal bar, Anatomy Embryology Germ cell structures