vitellogenesis

thumb|right|Diagram of vitellogenesis in the digenean [[Crepidostomum metoecus. GER: granular endoplasmic reticulum; L: lipid droplet; M: mitochondrion; N: nucleus; Nl: nucleolus; SG: shell globule; SGC: shell globule cluster.]]

Vitellogenesis is the process of yolk protein formation in the oocytes during sexual maturation. The term vitellogenesis comes from the Latin vitellus ("egg yolk"). Yolk proteins, such as lipovitellin and phosvitin, provides maturing oocytes with the metabolic energy required for development. Vitellogenins are the precursor proteins that lead to yolk protein accumulation in the oocyte. In vertebrates, estrogen 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, 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 mother 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.