Yolk sac

Yolk sac

Infobox Embryology
Name = Yolk sac
Latin =
GraySubject = 11
GrayPage = 54

Caption = Human embryo of 2.6 mm.

Caption2 = Human embryo from thirty-one to thirty-four days
System =
CarnegieStage = 5b
Days = 9
Precursor = endoderm
GivesRiseTo =
MeshName = Yolk+Sac
MeshNumber = A10.615.284.981
DorlandsPre = s_01
DorlandsSuf = 12716805
The yolk sac is the first element seen in the gestational sac during pregnancy, usually at 5 weeks gestation.

It is a critical landmark, identifying a true gestation sac

It is reliably seen early in pregnancy using ultrasound.

In mice

In the mouse, the yolk sac is the first site of blood formation, generating primitive macrophages and erythrocytes.

In humans

The yolk-sac is situated on the ventral aspect of the embryo; it is lined by extra-embyronic endoderm, outside of which is a layer of extra-embryonic mesenchyme, derived from the mesoderm.

Blood is conveyed to the wall of the sac by the primitive aorta, and after circulating through a wide-meshed capillary plexus, is returned by the vitelline veins to the tubular heart of the embryo. This constitutes the vitelline circulation, and by means of it nutritive material is absorbed from the yolk-sac and conveyed to the embryo.

At the end of the fourth week the yolk-sac presents the appearance of a small pear-shaped vesicle (umbilical vesicle) opening into the digestive tube by a long narrow tube, the vitelline duct.

The vesicle can be seen in the after-birth as a small, somewhat oval-shaped body whose diameter varies from 1 mm. to 5 mm.; it is situated between the amnion and the chorion and may lie on or at a varying distance from the placenta.

As a rule the duct undergoes complete obliteration during the seventh week, but in about two percent of cases its proximal part persists as a diverticulum from the small intestine, Meckel's diverticulum, which is situated about 60cm proximal to the ileocecal valve, and may be attached by a fibrous cord to the abdominal wall at the umbilicus.

Sometimes a narrowing of the lumen of the ileum is seen opposite the site of attachment of the duct.


The Yolk sac starts forming itself during the second week of the embryonic development, at the same time of the shaping of the amniotic sac. The hypoblast starts proliferating laterally and descending.

In the meantime the Heuser membrane, located on the opposite pole of the developing vesicle, starts its upward proliferation and meets the hypoblast.


*Primary yolk sac/primitive yolk sac: it is the vesicle constituted in the second week, its floor is represented by the Heuser membrane and its ceiling by the hypoblast. It is also known as the exocoelomic cavity. [cite web |url=http://isc.temple.edu/marino/embryology/lecture1-3.htm |title=Text for first three lectures |accessdate=2007-10-13 |format= |work=]

*Secondary yolk sac: this first transformation is determined by the modification of its cover, in the connection zone between the ipoblast and the Heuser membrane. We can observe a structure. The two parts detach and the inferior one, which is smaller, forms a cyst destined to be eliminated. The upper one is now covered only by the ipoblast.

*The final yolk sac: during the fourth week of development, during which we can see the shaping of the embryonic areas. A little portion of the sac, in the upper part, constitutes the intestinal tube. On the other side, the distal part forms a little vesicle that is what remains of the yolk sac.


ee also

*Yolk sac tumor


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Look at other dictionaries:

  • yolk sac — n. Zool. 1. a saclike vascular membrane containing yolk, an outgrowth from the ventral surface of very yolky vertebrate embryos, as those of fishes, birds, or reptiles, that supplies nourishment to the embryo 2. a homologous organ in most… …   English World dictionary

  • yolk sac — yolk′ sac n. dvl an extraembryonic membrane that encloses the yolk of eggs • Etymology: 1860–65 …   From formal English to slang

  • yolk sac — yolk sac. См. желточный мешок. (Источник: «Англо русский толковый словарь генетических терминов». Арефьев В.А., Лисовенко Л.А., Москва: Изд во ВНИРО, 1995 г.) …   Молекулярная биология и генетика. Толковый словарь.

  • Yolk sac — Not all yolk has to do with birds’ eggs. Human embryos have a yolk sac, a membrane outside the embryo but connected by a tube (the yolk stalk) though the umbilical opening to the embryo’s midgut. The yolk sac serves as an early site for …   Medical dictionary

  • yolk sac — noun 1. membranous structure that functions as the circulatory system in mammalian embryos until the heart becomes functional • Syn: ↑vitelline sac, ↑umbilical vesicle, ↑vesicula umbilicus • Hypernyms: ↑sac 2. membranous structure enclosing the… …   Useful english dictionary

  • yolk sac — thin membrane which surrounds the yolk of an egg and supplies nutrients to the embryo (in mammals, the yolk sac ceases to function when the placenta forms) …   English contemporary dictionary

  • yolk sac — noun Date: 1861 a membranous sac of most vertebrates that encloses the yolk, is attached in most forms (as in humans) through the yolk stalk with the intestinal cavity of the embryo, and is supplied with blood vessels that transport nutritive… …   New Collegiate Dictionary

  • yolk sac — a sac containing yolk used for nourishment in larval fish …   Dictionary of ichthyology

  • yolk sac — noun A membranous sac attached to an embryo, providing early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and primitive mammals …   Wiktionary

  • yolk sac — noun Zoology a membranous sac containing yolk attached to the embryos of reptiles and birds and the larvae of some fishes …   English new terms dictionary

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