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yolk sac
yolk sacn. A membranous sac attached to the embryo and enclosing the yolk in egg-laying vertebrates. In humans and other placental mammals, it functions as the circulatory system for the embryo before internal circulation begins.yolk sac n 1. (Zoology) the membranous sac that is attached to the ventral surface of the embryos of birds, reptiles, and some fishes and contains yolk 2. (Zoology) the corresponding part in the embryo of mammals, which contains no yolk yolk′ sac`
n. an extraembryonic membrane that encloses the yolk of eggs. [1860–65] yolk sac A sac attached to the gut of an embryo that encloses the yolk in bony fish, sharks, reptiles, mammals, and birds. In most mammals, the yolk sac functions as part of the embryo's circulatory system before the placenta develops.Thesaurus| Noun | 1. |  yolk sac - membranous structure that functions as the circulatory system in mammalian embryos until the heart becomes functionalumbilical vesicle, vesicula umbilicus, vitelline sacsac - a structure resembling a bag in an animal | | 2. | yolk sac - membranous structure enclosing the yolk of eggs in birds, reptiles, marsupials, and some fishes; circulates nutrients to the developing embryosac - a structure resembling a bag in an animal |
Yolk Sac
Yolk sac An extraembryonic membrane which extends through the umbilicus in vertebrates. In some elasmobranchs, birds, and reptiles, it is laden with yolk which serves as the nutritive source of embryonic development. In mammals, as in birds, the yolk sac generally develops from extraembryonic splanchnopleure, and extends beneath the developing embryo. A blood vessel network develops in the mammalian yolk sac lining. Though these blood vessels are empty, they play an important role in absorbing nourishing food and oxygen from the mother. Thus, although the yolk sac in higher mammals may be considered an evolutionary vestige from its yolky-egged ancestors, it still serves important functions in the young embryo. As the embryo ages, the yolk sac shrinks in size, and the allantois takes over the role of nutrition. See Allantois Yolk Sac the organ of nutrition and respiration in the embryos of cephalopod mollusks, cartilaginous and bony fish, reptiles, birds, mammals, and man. The yolk sac arises in the early stages of embryonic development, usually by means of the overgrowing of the yolk with endoderm and with the visceral layer of the lateral plates, and consists of an enlarged outgrowth of the midguts, the cavity of which in the majority of animals (except higher mammals and man) is filled with unbroken yolk. In the wall of the yolk sac blood cells and blood vessels are formed, which provide for transport of nutritive substances to the embryo and for its respiration. As the embryo develops, the size of the yolk sac decreases, its cavity shrinks, and it is either gradually drawn into the body cavity and resorbed or is cast off. yolk sac[′yōk ‚sak] (embryology) A distended extraembryonic extension, heavy-laden with yolk, through the umbilicus of the midgut of the vertebrate embryo. yolk sac
sac [sak] a baglike organ or structure; see also bag, pocket, and pouch.air s's (alveolar s's) the spaces into which the alveolar ducts open distally, and with which the alveoli communicate; see also lung" >lung.amniotic sac the sac formed by the amnion, enclosing the fetus suspended in amniotic fluid; popularly known as the bag of waters.conjunctival sac the potential space, lined by conjunctiva, between the eyelids and the eyeball.endolymphatic sac the blind, flattened cerebral end of the endolymphatic duct.hernial sac the peritoneal pouch that encloses protruding intestine.lacrimal sac the dilated upper end of the nasolacrimal duct; see also lacrimal apparatus.yolk sac the extraembryonic membrane connected with the midgut; in vertebrates below true mammals, it contains a yolk mass. Yolk sac in a developing embryo. From Applegate, 2000.yolk sac1. in vertebrates with telolecithal eggs, the highly vascular layer of splanchnopleure surrounding the yolk of an embryo; 2. in humans and other mammals, the sac of extraembryonic membrane that is located ventral to the embryonic disk and, after formation of the gut tube, is connected to the midgut; by the second month of development, this connection has become the narrow yolk stalk; the yolk sac is the first hematopoietic organ of the embryo, and its vitelline circulation plays an important role in the early embryonic circulation; the sac is also the site of origin of the primordial germ cells. Synonym(s): umbilical vesicle, vesicula umbilicalis, vitelline sacyolk sacn. A membranous sac attached to the embryo and enclosing the yolk in egg-laying vertebrates. In humans and other placental mammals, it functions as the circulatory system for the embryo before internal circulation begins.um·bil·i·cal ves·i·cle (ŭm-bil'i-kăl ves'i-kĕl) A saclike structure formed from the exocelomic cavity of a blastocyst.
Synonym(s): yolk sac. yolk sac A tiny bag attached to the embryo that provides early nourishment before the PLACENTA is formed.yolk sac the sac-like structure that contains YOLK and is in direct contact with the gut of embryos in fish, reptiles and birds. Though present in mammalian embryos, it does not contain yolk but absorbs uterine secretions until the PLACENTA (1) becomes functional.yolk sac
Related to yolk sac: Gestational sacSynonyms for yolk sacnoun membranous structure that functions as the circulatory system in mammalian embryos until the heart becomes functionalSynonyms- umbilical vesicle
- vesicula umbilicus
- vitelline sac
Related Wordsnoun membranous structure enclosing the yolk of eggs in birds, reptiles, marsupials, and some fishesRelated Words |