yolk

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Related to vitelline: Vitelline duct, Vitelline vein

yolk

1. the substance in an animal ovum consisting of protein and fat that nourishes the developing embryo
2. a greasy substance secreted by the skin of a sheep and present in the fleece
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Yolk

 

(deutoplasm), a nutritive substance that accumulates in the ovum of animals and man in the form of granules or membranes that sometimes merge into a solid yolk mass (in insects, bony fish, and birds).

Yolk has been discovered in the ova of all animals and man, but its quantity and distribution vary considerably. In ova with a small amount of yolk, the yolk particles are distributed evenly throughout the cytoplasm (isolecithal ova). In ova with a large amount of yolk, the yolk particles are concentrated either in the vegetative part of the ovum (telolecithal ova) or in the central part of the cytoplasm, around the nucleus (centrolecithal ova). The type of egg cleavage depends on the quantity and distribution of the yolk. Chemically there are three principal varieties of yolk: protein, fat, and carbohydrate. However, in most animals the yolk granules have a complex chemical composition and contain proteins, fats, carbohydrates, ribonucleic acid, pigments, and mineral substances. For example, in the mature hen’s egg, the yolk contains 23 percent neutral fat, 16 percent protein, 11 percent phospholipids, 1.5 percent cholesterol, and 3 percent minerals. Various organelles of the ovum participate in the synthesis and accumulation of the yolk: the Golgi apparatus, endoplasmic reticulum, and mitochondria. In many animals the protein component of the yolk is synthesized outside the ovary and enters the growing ovum by means of pinocytosis. In some invertebrate animals the yolk may also accumulate in special cells of the ovary—the yolk cells, on which the developing embryo is nourished.

T. B. AIZENSHTADT

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

yolk

[yōk]
(biochemistry)
Nutritive material stored in an ovum.
The yellow spherical mass of food material that makes up the central portion of the egg of a bird or reptile.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
During embryonic development, the omphalomesenteric duct (Omphaloenteric duct, vitelline duct or yolk stalk) normally connects the embryonic midgut to the yolk sac ventrally, providing nutrients to the midgut during embryonic development.
A rapid method for evaluating the strength of the vitelline membrane of the hen's egg yolk.
It was demonstrated in the abalone that the sperm acrosomal lysin binds to the vitelline envelop receptor and creates a hole through which sperm could pass and fuse with the egg cell membrane, and this occurs by a nonenzymatic species-specific mechanism (Lewis et al.
The superior and the inferior anastomosis atrophy, followed by the proximal part of the left and the distal parts of the right vitelline vein.
As has been established by Selman and Wallace (1983), vitellogenin circulates in the blood and displaces among the endothelial cells of the capillaries, and subsequently among follicle cells, reaching the region where microvilli are extending through the pores of the vitelline membrane (Selman and Wallace 1983, 1986, Smythe 2003).
Degeneration of the vitelline membrane was evident at all treatment levels; severity was dose-related and greater in the outer vitelline membrane than the inner vitelline membrane.
1997; Perez-Coll and Herkovits 1996), enhanced in free-living embryos by protective barriers such as the vitelline membrane and jelly coats, provides support for the argument that this stage reflects very aggressive environmental conditions during the evolution of early multicellular organisms.
Thus, although the mammalian embryo has vessels analogous to the vitelline veins, the main supply of food and oxygen comes from the umbilical vein, which unites the embryo with the placenta.
Meckel diverticulum, 3-6 cm in length is a true diverticulum as it contains all three layers of intestinal wall supplied by a persistent vitelline artery located on the antimesenteric border of the ileum 50-100 cm form the ileocaecal valve.
The renal segment develops from the anastomosis between the subcardinal and supracardinal veins while the hepatic segment derives from the right vitelline vein [6, 7].