Vitellogenesis
Vitellogenesisis the process of yolk protein formation in theoocytesduring sexual maturation.[2]The termvitellogenesiscomes from theLatinvitellus( "egg yolk" ).Yolkproteins, such as Lipovitellin andPhosvitin,provides maturing oocytes with the metabolic energy required for development.Vitellogeninsare the precursor proteins that lead to yolk protein accumulation in the oocyte. In vertebrates,estrogenand vitellogenin production have a positive correlation. When estrogen production in the ovary is increased via the activation of thehypothalmo-pituitaryaxis it leads to heightened vitellogenin production in the liver.[3]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 sesquiterpenoidjuvenile hormone(JH) and ecdysteroid 20-hydroxyecdysone (E20). More recent studies are showing the importance ofmiRNAin 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 thefat body.JH uses a JHMethoprenetolerant /Taiman receptor complex that is regulated by JH to synthesis vitellogenins in the fat body.[4]
In cockroaches, for example, vitellogenesis can be stimulated by injection ofjuvenile hormoneinto immature females and mature males. In mosquitoes infected withPlasmodium,vitellogenesis may be manipulated by the parasites to reducefecundity,thereby preserving nutrition in the infected individual.[5]
| I. Holoblastic (complete) cleavage | II. Meroblastic (incomplete) cleavage |
|---|---|
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A.Isolecithal(sparse, evenly distributed yolk)
B.Mesolecithal(moderate vegetal yolk disposition)
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A.Telolecithal(dense yolk throughout most of cell)
B.Centrolecithal(yolk in center of egg)
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References
[edit]- ^Greani S, Quilichini Y, Marchand B (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.PMC5112763.PMID27845028.
- ^Wallace RA (1985). "Vitellogenesis and Oocyte Growth in Nonmammalian Vertebrates". In Browder LW (ed.).Oogenesis.Vol. 1. Boston, MA: Springer US. pp. 127–177.doi:10.1007/978-1-4615-6814-8_3.ISBN978-1-4615-6816-2.PMID3917200.
{{cite book}}:|journal=ignored (help) - ^Ho SM (1987). "Endocrinology of Vitellogenesis". In Norris DG, Jones RE (eds.).Hormones and Reproduction in Fishes, Amphibians, and Reptiles.Boston, MA: Springer US. pp. 145–169.doi:10.1007/978-1-4613-1869-9_6.ISBN978-1-4612-9042-1.
- ^Wu Z, Yang L, He Q, Zhou S (2021-01-28)."Regulatory Mechanisms of Vitellogenesis in Insects".Frontiers in Cell and Developmental Biology.8:593613.doi:10.3389/fcell.2020.593613.PMC7901893.PMID33634094.
- ^Hurd H (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.PMID12414739.
- ^Gilbert SF (2003).Developmental biology(7th ed.). Sinauer. p. 214.ISBN0-87893-258-5.
- ^Kardong KV (2006).Vertebrates: Comparative Anatomy, Function, Evolution(4th ed.). McGraw-Hill. pp. 158–64.