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.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.

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.
Farlex Partner Medical Dictionary © Farlex 2012

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.
The American Heritage® Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.

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.
Medical Dictionary for the Health Professions and Nursing © Farlex 2012

yolk sac

A tiny bag attached to the embryo that provides early nourishment before the PLACENTA is formed.
Collins Dictionary of Medicine © Robert M. Youngson 2004, 2005

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.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005
References in periodicals archive ?
On the basis of the presence of yolk sacs and size of the embryos, we determined that none of them were considered near-term embryos.
In addition to the chorioallantoic placenta, rodents also have a well-developed visceral yolk sac (VYS) that acts as an active region for metabolic exchange and nutrition uptake (4,5).
The yolk sac is the only source of nutrients for the embryo developing inside the egg.
Pigmentation of larvae after hatch consisted of melanophores concentrated on the ventral surface of the yolk sac, as well as posterior to the anus both on ventral and dorsal areas along the myomeres.
The sublethal effects were indicative of stress and may have been causative factors in reduced larval survival; they included precocious hatch, smaller larval size with larger yolk sac at hatch, and reduced growth rate.
Studies of salmonid fishes have shown that embryos cannot produce the required developmental hormones until well after hatching and that the necessary hormones are aliquoted into the nutritive yolk sac during oogenesis in quantities that reflect female plasma levels (Schreck et al.
Evidence for the etiologic role of the yolk sac isolate in the epidemic has been obtained by indirect fluorescent antibody stains carried out by methods that are the same as those in regular use in the diagnosis of rickettsial diseases, except that the microdrops fixed to the slide were prepared from yolk sacs infected with isolate 1 and isolate 2 of the Philadelphia agent.
Like northern rock sole, late-hatching larvae in other marine species are generally larger and have smaller yolk sacs (e.g., capelin [Mallotus villosus], Chambers et al., 1989; wolffish [Anarhichas lupus], Ringo et al., 1987).
Newly hatched larvae were transparent and had a large, elongate yolk sac and single oil globule.
These were empty capsules, capsules containing one large ovum without a visible embryo (single ovum capsules), capsules with one developing embryo and attached yolk sac, capsules with 22-45 blastodisc-stage ova, and capsules containing 8-100 nonblastodisc-stage ova (nutritive capsules), some appearing atretic and in which individual ova were difficult to detect.
There are several theories regarding the histogenesis of extragonadal yolk sac tumor, including (1) arrested migration of or misplaced germ cells during embryogenesis, (2) reverse migration of germ cells, (3) abnormal differentiation of somatic cells, (4) derivation from pluripotential stem cells within a somatic tumor, (5) origination from residual fetal tissue following incomplete abortion (for primary endometrial yolk sac tumor), and (6) metastasis from an occult gonadal primary.
For successful development in a hatchery, knowledge of the following ontogenetic events is essential: size and time at hatching, flexion of the notochord, duration of the yolk sac, the presence or absence of an adhesive organ, retinal pigmentation, opening of the mouth and intestinal lumen, development of fins, gas-bladder filling, and cutaneous pigmentation pattern (Santos & Godinho, 1994; 1996a; 2002).