The

insect Insects (from Latin ') are Hexapoda, hexapod invertebrates of the class (biology), class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (Insect morphology#Head, head, ...

vitelline envelope is the outer

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

aceous layer outside 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 ...

and

egg An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell (a zygote) and to incubate from it an embryo within the egg until the embryo has become an animal fetus that can survive on its own, at which point the ...

. The vitelline envelope, not being a

cell Cell most often refers to: * Cell (biology), the functional basic unit of life * Cellphone, a phone connected to a cellular network * Clandestine cell, a penetration-resistant form of a secret or outlawed organization * Electrochemical cell, a de ...

ular structure, is commonly referred to as a

membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. Bi ...

. However, this is a technical misnomer as the structure is composed of

and is not a

ular component. It varies in thickness between different insects and even varies at different parts of the egg. It lies inside the outer shell of the egg, which is commonly referred to as 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 ...

''.Chapman, R.F. (1998) "The insects: structure and function", Section ''The egg and embryology''. Previewed in Google Book

on 26 Sep 2009. The presence of the vitelline membrane defines the embryo's boundaries. It is a critical structural element required to resist the forces of morphogenesis and the mechanical pressures experienced during egg-laying. Before egg activation, the vitelline membrane is permeable to water, ions, and small molecules. Egg activation is stimulated by mechanical deformation associated with traversing through the narrow channel in the oviduct and requires the presence of Ca2+. During egg activation, the vitelline membrane proteins are crosslinked via disulfide remodeling; the structure rigidifies and becomes impermeable to water but remains gas permeable. This process is hypothesized to have been selected to prevent polyspermy. The vitelline membrane is composed primarily of four glycoproteins, collectively referred to as vitelline membrane proteins (VMPs). This class of proteins contains a conserved "VM domain": (CX7CX8C). VMPs are secreted during stages 9–10 of oogenesis and accumulate as vitelline bodies in the extracellular space; these bodies fuse to form a continuous layer at the end of stage 10. This layer thins as the oocyte grows to reach a final thickness of ~0.4 um. Upon egg activation, peroxidase-mediated crosslinking occurs in the vitelline membrane resulting in a disulfide-linked network. After crosslinking, the envelope is impermeable to additional sperm, water, and other large molecules but remains permeable to gas exchange. Spatial information and developmental patterning are encoded on the surface of the vitelline membrane. For example, in D. melanogaster, the dorsal-ventral body axis is determined by ventrally sulfated eggshell proteins that recruit and activated the Spätzle ligand within the perivitelline space, which, in turn, activate the Toll receptor upstream of morphogens such as Dorsal and Twist.MLA Stein, David S., and Leslie M. Stevens. "Maternal control of the Drosophila dorsal–ventral body axis." Wiley Interdisciplinary Reviews: Developmental Biology 3.5 (2014): 301-330.

Footnotes

References

* (1998) ''The insects: structure and function''. (4th Ed). Cambridge University Press. , {{ISBN, 978-0-521-57890-5. Zoology