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Among animals which produce , the yolk (; also known as the vitellus) is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some types of egg contain no yolk, for example because they are laid in situations where the food supply is sufficient (such as in the body of the host of a parasitoid) or because the embryo develops in the parent's body, which supplies the food, usually through a placenta. Reproductive systems in which the mother's body supplies the embryo directly are said to be matrotrophy; those in which the embryo is supplied by yolk are said to be lecithotrophy. In many species, such as all , and most and , the yolk takes the form of a special storage organ constructed in the reproductive tract of the mother. In many other animals, especially very small species such as some fish and , the yolk material is not in a special organ, but inside the egg cell.
As stored food, yolks are often rich in , dietary mineral, and . The proteins function partly as food in their own right, and partly in regulating the storage and supply of the other nutrients. For example, in some species the amount of yolk in an egg cell affects the developmental processes that follow fertilization.
The yolk is not living cell material like protoplasm, but largely passive material, that is to say deutoplasm. The food material and associated control structures are supplied during oogenesis. Some of the material is stored more or less in the form in which the maternal body supplied it, partly as processed by dedicated somatic cell, while part of the biosynthesis processing into its final form happens in the oocyte itself.Barnes, Richard Stephen Kent (2001). The Invertebrates: A Synthesis. Wiley-Blackwell, p. 347. .
Apart from animals, other organisms, like algae, especially in the oogamous, can also accumulate resources in their female gametes. In , the remains of the female gametophyte serve also as food supply, and in , the endosperm.
The yolk mass, together with the ovum proper (after fertilization, the embryo) are enclosed by the vitelline membrane, whose structure is different from a cell membrane.Bellairs, Ruth; Osmond, Mark (2005). Atlas of Chick Development (2 ed.). Academic Press. pp. 1–4. link.Bellairs, R., Harkness, M. & Harkness, R. D. (1963). The vitelline membrane of the hen's egg: a chemical and electron microscopical study. Journal of Ultrastructure Research, 8, 339–59. The yolk is mostly extracellular to the oolemma, being not accumulated inside the cytoplasm of the egg cell (as occurs in ),Landecker, Hannah (2007). Culturing life: how cells became technologies. Cambridge, MA: Harvard University Press. p. 49. link. contrary to the claim that the avian ovum (in strict sense) and its yolk are a single giant cell.Patten, B. M. (1951). Early Embryology of the Chick, 4th edition. McGraw-Hill, New York, p. 17.Callebaut, M. (2008) Historical evolution of preformistic versus neoformistic (epigenetic) thinking in embryology, Belgian Journal of Zoology, vol. 138 (1), pp. 20–35, 2008
After the fertilization, the cleavage of the embryo leads to the formation of the germinal disc.
As food, the chicken egg yolk is a major source of vitamins and minerals. It contains all of the egg's fat and cholesterol, and nearly half of the protein. If left intact when an egg is fried, the yellow yolk surrounded by a flat blob of egg white creates a distinctive fried egg form. Mixing the two components together before cooking results in a yellow (from pale yellow to almost orange, depending on the breed of hen) mass, as in and scrambled eggs.
All of the fat-soluble vitamins (vitamin A, vitamin D, vitamin E and vitamin K) are found in the egg yolk. Egg yolk is one of the few foods naturally containing vitamin D.
The composition (by weight) of the most prevalent fatty acids in egg yolk typically is:National Research Council, 1976, Fat Content and Composition of Animal Products, Printing and Publishing Office, National Academy of Science, Washington, D.C., ; p. 203, online edition
Egg yolk is a source of lecithin, as well as egg oil, for cosmetic and pharmaceutical applications. Based on weight, egg yolk contains about 9% lecithin.
The yellow color is due to lutein and zeaxanthin, which are yellow or orange carotenoids known as .
A single yolk from a large egg contains roughly 22 mg of calcium, 66 mg of phosphorus, 9.5 micrograms of selenium, and 19 mg of potassium, according to the USDA.U.S. Department of Agriculture, Agricultural Research Service, 2010. USDA National Nutrient Database for Standard Reference, Release 23, Nutrient Data Laboratory Home Page: http://www.ars.usda.gov/nutrientdata
Double-yolked eggs seldom lead to successful hatchlings without human intervention, as the chicks interfere with each other's hatching process and do not survive.
Higher-order yolks are rare, though hens are known to occasionally lay even triple-yolk eggs.
Since these eggs contain no yolk, and therefore cannot hatch, they were traditionally believed to have been laid by roosters. This type of egg occurs in many varieties of fowl and has been found in chickens, both standard and bantams, guineafowl, and coturnix quail.
Yolks, particularly from free-range eggs, can be of a wide range of colors, ranging from nearly white, through yellow and orange, to practically red, or even olive green, depending on the pigments in their feed. Feeding fowl large amounts of capsicum peppers, for example, tends to result in red or deep orange yolks. This has nothing to do with adding colors such as cochineal to eggs in cooking.
ApoCr is a large glycolipoprotein, approximately 2,600 amino acids long, with conserved structural domains characteristic of LLTPs. These domains include an N-terminal lipid transfer module, a DUF1081 domain exclusive to apoLp-II/I and apoB, and a von Willebrand factor type D domain at the C-terminal. Additionally, it features a subtilisin-like cleavage site, a trait shared with apoLp-II/I. Evolutionary analyses reveal that apoCr is phylogenetically closer to apoLp-II/I than to Vtg proteins, indicating a distinct lineage for crustacean yolk proteins. In decapods, apoCr is typically expressed in both the ovary and hepatopancreas, supporting its dual roles in lipid metabolism and yolk formation. In some species, gene duplication events have resulted in multiple apoCr variants with tissue-specific functions.
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