Elaboration of Yolk- Reproduction
Elaboration of nutritive material, vitellin or the yolk, for the early states of embryonic development is one of the important functions of the non-mammalian ovary. You are aware that mammals except monotremes and marsupials are viviparous. The development of the embryos in the viviparous animals takes place inside the mothers" body. Nutrition and other required things are provided to the embryos through maternal circulation. Most of the non-mammalian vertebrates are oviparous forms in which development of the embryos takes place outside the mothers' body. Therefore they have to be provided with nutrient material enough for the development till they reach a stage at which they manage to procure food by themselves.
The nutritive material in the oviparous animals is stored in the egg cytoplasm in the form of yolk. The constituents of the yolk are proteins, lipids, glycogen, nucleic acids and some minerals like phosphors. The process of yolk synthesis is called vitellogenesis. Vitellogenesis is under the control of gonadotropins. Gonadotropins induce the synthesis and secretion of estrogens in the ovary, which in turn stimulate the synthesis of vitellogenin, a yolk protein in the liver secreted into the blood stream. Vitellogenin reaches the ovary through blood and enters the egg cell. In the cytoplasm it is used in the yolk synthesis. Apart from egg production, synthesis of hormones and yolk, ovaries also play a vital role in the maintenance of pregnancy.