Vitellogenesis is the process of yolk protein formation in 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 ...

s during sexual maturation. The term ''vitellogenesis'' comes from the

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

''vitellus'' ("egg yolk").

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 ...

proteins, such as lipovitellin and

phosvitin Phosvitin is one of the Egg (biology), egg (commonly hen's egg) Egg yolk, yolk phosphoproteins known for being the most Phosphorylation, phosphorylated protein found in nature. Phosvitin isolation was first described by Mecham and Olcott in the yea ...

, provides maturing oocytes with the metabolic energy required for development.

Vitellogenin Vitellogenin (VTG or less popularly known as VG) (from Latin ''vitellus'', yolk, and ''genero'', I produce) is a precursor of the egg yolk proteins. It is synthesized and secreted by the liver of vertebrates, fat body of insects and the hepatopa ...

s are the precursor proteins that lead to yolk protein accumulation in the oocyte. In vertebrates,

estrogen Estrogen (also spelled oestrogen in British English; see spelling differences) is a category of sex hormone responsible for the development and regulation of the female reproductive system and secondary sex characteristics. There are three ...

and vitellogenin production have a positive correlation. When estrogen production in the ovary is increased via the activation of the hypothalmo-pituitary axis it leads to heightened vitellogenin production in the liver. Vitellogenin production in the liver is the first step of vitellogenesis. Once Vitellogenins are released into the blood stream where they are then transported to the growing oocyte where they lead to yolk protein production. The transport of vitellogenins into the maturing oocyte is done via endocytosis mediated by a receptor which is a low-density lipoprotein receptor (LDLR). Yolk is a lipoprotein composed of proteins, phospholipids and neutral fats along with a small amount of glycogen. The yolk is synthesised in the liver of the female parent in soluble form. Through circulation it is transported to the follicle cells that surround the maturing ovum, and is deposited in the form of yolk platelets and granules in the ooplasm. The mitochondria and Golgi complex are said to bring about the conversion of the soluble form of yolk into insoluble granules or platelets. The two hormones responsible for vitellogenesis stimulation in insects are sesquiterpenoid

juvenile hormone Juvenile hormones (JHs) are a group of acyclic sesquiterpenoids that regulate many aspects of insect physiology. The first discovery of a JH was by Vincent Wigglesworth. JHs regulate development, reproduction, diapause, and polyphenisms. In ...

(JH) and ecdysteroid 20-hydroxyecdysone (E20). More recent studies are showing the importance of

miRNA 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-transcri ...

in vitellogenesis stimulation as well. The pathways that these hormones regulate is largely dependent on the evolutionary growth of the insect species. Together, JH, E20, and miRNA help synthesize vitellogenins within the

fat body 300px, Stained cells of an insect fat body Fat body is a highly dynamic insect tissue composed primarily of storage cells. It is distributed throughout the insect's internal body cavity (the haemocoel), in close proximity to the hemolymph as ...

. JH uses a JH

Methoprene Methoprene is a juvenile hormone (JH) analog and insect growth regulator (IGR) used widely in pest control. Classified under Insecticide Resistance Action Committee (IRAC) group 7A, methoprene is an amber-colored liquid with a faint fruity odor. ...

tolerant /Taiman receptor complex that is regulated by JH to synthesis vitellogenins in the fat body. In cockroaches, for example, vitellogenesis can be stimulated by injection of

into immature females and mature males. In mosquitoes infected with ''

Plasmodium ''Plasmodium'' is a genus of unicellular eukaryotes that are obligate parasites of vertebrates and insects. The life cycles of ''Plasmodium'' species involve development in a Hematophagy, blood-feeding insect host (biology), host which then inj ...

'', vitellogenesis may be manipulated by the parasites to reduce

fecundity Fecundity is defined in two ways; in human demography, it is the potential for reproduction of a recorded population as opposed to a sole organism, while in population biology, it is considered similar to fertility, the capability to produc ...

, thereby preserving nutrition in the infected individual.

References

{{reflist Germ cells