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 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.
|I. Holoblastic (complete) cleavage||II. Meroblastic (incomplete) cleavage|
A. Isolecithal (sparse, evenly distributed yolk)
B. Mesolecithal (moderate vegetal yolk disposition)
A. Telolecithal (dense yolk throughout most of cell)
B. Centrolecithal (yolk in center of egg)
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