Vitellogenesis

Diagram of vitellogenesis in the digenean ^[1]

Vitellogenesis (also known as yolk deposition) is the process of yolk formation via nutrients being deposited in the oocyte, or female germ cell involved in reproduction of lecithotrophic organisms. In insects, it starts when the fat body stimulates the release of juvenile hormones and produces vitellogenin protein. It occurs in all animal groups other than the mammals. In cockroaches, for example, vitellogenesis can be stimulated by injection of juvenile hormone into immature females and mature males. Chemically yolk is 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.

In mosquitoes infected with Plasmodium, vitellogenesis may be manipulated by the parasites to reduce fecundity.^[2]

In mammalian vitellogenesis, vitellogenin is the major protein, produced by the Vit gene and regulated by oestrogen. The yolk consists of lipids (triglycerides, cholesterol etc.) and proteins, mainly vitellogenin.

The term vitellogenesis comes from the Latin vitellus ("egg yolk").

**Summary of the main patterns of yolk accumulation and cleavage in animal embryology** (after ^[3] and ^[4]).
I. Holoblastic (complete) cleavage	II. Meroblastic (incomplete) cleavage
A. Isolecithal (sparse, evenly distributed yolk) 1. Radial cleavage (echinoderms, hemichordates, amphioxus) 2. Spiral cleavage (annelids, most mollusks, flatworms) 3. Bilateral cleavage (tunicates) 4. Rotational cleavage (placental mammals, nematodes, marsupials [?]) B. Mesolecithal (moderate vegetal yolk disposition) Displaced radial cleavage (amphibians, some fish [the lampreys, gars and bowfins)	A. Telolecithal (dense yolk throughout most of cell) 1. Bilateral cleavage (cephalopod molluscs) 2. Discoidal cleavage (some fish [the hagfishes, chondrichthyans and most teleosts], sauropsids [reptiles and birds], monotremes) B. Centrolecithal (yolk in center of egg) Superficial cleavage (most insects)

References[]

^ Greani, Samuel; Quilichini, Yann; Marchand, Bernard (2016). "Ultrastructural study of vitellogenesis and oogenesis of Crepidostomum metoecus (Digenea, Allocreadiidae), intestinal parasite of Salmo trutta (Pisces, Teleostei)". Parasite. 23: 47. doi:10.1051/parasite/2016057. ISSN 1776-1042. PMC 5112763. PMID 27845028.
^ Hurd, Hilary (2003). "Manipulation of medically important insect vectors by their parasites". Annual Review of Entomology. 48 (1): 141–161. doi:10.1146/annurev.ento.48.091801.112722. ISSN 0066-4170.
^ Gilbert SF (2003). Developmental biology (7th ed.). Sinauer. p. 214. ISBN 0-87893-258-5.
^ Kardong, Kenneth V. (2006). Vertebrates: Comparative Anatomy, Function, Evolution (4th ed.). McGraw-Hill. pp. 158–64.

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[GreaniQuilichini2016-1] Greani, Samuel; Quilichini, Yann; Marchand, Bernard (2016). "Ultrastructural study of vitellogenesis and oogenesis of Crepidostomum metoecus (Digenea, Allocreadiidae), intestinal parasite of Salmo trutta (Pisces, Teleostei)". Parasite. 23: 47. doi:10.1051/parasite/2016057. ISSN 1776-1042. PMC 5112763. PMID 27845028.

[Hurd2003-2] Hurd, Hilary (2003). "Manipulation of medically important insect vectors by their parasites". Annual Review of Entomology. 48 (1): 141–161. doi:10.1146/annurev.ento.48.091801.112722. ISSN 0066-4170.

[3] Gilbert SF (2003). Developmental biology (7th ed.). Sinauer. p. 214. ISBN 0-87893-258-5.

[4] Kardong, Kenneth V. (2006). Vertebrates: Comparative Anatomy, Function, Evolution (4th ed.). McGraw-Hill. pp. 158–64.

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