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The process by which yolk is formed in the ooplasm of an oocyte.



the synthesis and storage of yolk in the developing female sexual cells of animals (oocytes) during oogenesis. Vitellogenesis starts in the relatively late stages of oogenesis, toward the end of which the size of the oocyte can increase tens of thousands of times. The growing oocyte acquires nucleic acids, amino acids, carbohydrates, fats, and proteins, which are used for the synthesis and storage of yolk. Vitellogenesis leads to the formation of a rich egg yolk in those animals whose embryos are incapable of feeding independently. In cases in which the embryo begins at an early stage to actively obtain food (larval development) or can obtain food from the maternal organism, only a small amount of yolk forms in the egg. In most animals the oocytes are surrounded by auxiliary cells—follicle and nutritive cells (trophocytes). In a number of animals (echinoderms, some mollusks and worms) the oocyte develops without the participation of auxiliary cells.

The chemical composition of the yolk varies in different animals. Yolk granules usually consist of proteins (including enzymes), lipoproteins, carbohydrates, and ribonucleic acid. In addition to the yolk granules, the cytoplasm of the oocyte contains drops of oil, which are also yolk inclusions. The degree of participation of the specific cell structures of the oocyte in vitellogenesis depends upon the presence and activity of auxiliary cells, the chemical composition of the yolk, and the amount of yolk in the cell. In many animals the yolk granules are formed in the area of the Golgi apparatus. In some crustaceans the yolk is synthesized and stored in cavities of the endoplasm network. In a number of mollusks and amphibians the crystalline yolk granules are formed inside the mitochondria. In insects, fish, and amphibians the surface of the oocyte has active pinocytosis and pinocytotic bubbles that contain proteins synthesized in other organs of the animal’s body (in the liver, for example) and provide the beginning for part of the yolk granule. In some invertebrates (for example, in a number of worms), special yolk cells develop outside the ovary.


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