yolk sac(redirected from Primitive yolk sac)
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Related to Primitive yolk sac: yolk sac placenta
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.
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.
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.
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.
Synonym(s): yolk sac.
yolk sacA tiny bag attached to the embryo that provides early nourishment before the PLACENTA is formed.
yolk sacthe 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.
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.