yolk sac

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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.
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 sac

1. 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.

yolk sac

n.
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

a structure that develops in the inner cell mass of the embryo and expands into a vesicle with a thick part that becomes the primitive gut and a thin part that grows into the cavity of the chorion. The cells of the extraembryonic mesoderm differentiate to develop endothelium, primitive blood plasma, and hemoglobin. The yolk sac usually disappears during the seventh week of pregnancy. See also allantois, Meckel's diverticulum.

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

one of the extraembryonic fetal membranes that balloons out from the fetal midgut. It helps to form a primitive placenta and promotes the development of the vitelline circulation. The vestigial yolk sac can be found about halfway along the small intestine of birds. The yolk sac membrane produces lymphoid stem cells that subsequently colonize the thymus and bursa of Fabricius. The stalk of the yolk sac is sometimes retained as Meckel's diverticulum of the small intestine.

yolk sac infection
see avian omphalitis.
inverted yolk sac
a form of placentation in which fetal splanchnic mesoderm in direct contact with maternal uterine tissue; occurs in laboratory rodents. Called also yolk sac placenta.
yolk sac placenta
see inverted yolk sac (above).
yolk sac tumor
shows varying stages of differentiation into tissues of multiple germ layers. Called also endodermal sinus tumor.
References in periodicals archive ?
Yolk sac tumours revisited: a review of their many faces and names.
A diagnostic immunohistochemical panel for yolk sac (primitive endodermal) tumors based on an immunohistochemical comparison with the human yolk sac.
In some chickens form various reasons it happened that, the yolk sac does not undergo absorption and remains unabsorbed in the abdominal cavity.
Anjum (1997), on the other hand, reported on retention and bacterial infection of the yolk sac being the most common cause of death in chickens in Pakistan.
The lower lateral and ventral yolk sac is lightly pigmented with dendritic melanophores.
Cortisol levels in eggs produced by these females are then related to features of the larvae which are important in determining survival in the early life stages (larval size, yolk sac size, and eye development).
Standard length (SL), head depth (through the eye), eye diameter (maximum), and yolk sac area of 25-50 larvae from each clutch were measured.
Results with indirect fluorescent antibody stains of the agent cultivated in yolk sacs.
Yolksac length--The distance, parallel to longitudinal axis of the body, from anteriormost to posteriormost points of the yolk sac.
Upon hatching from the single-ovum capsules and after absorption of the external yolk sac, embryos begin orally feeding on yolk-filled nutritive ova capsules.
Larvae had a very heavy yolk sac and many were collected via gravel siphoning, suggesting that they were not yet swimming.