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Encyclopedia > Yolk sac
Yolk sac
Human embryo of 2.6 mm.
Human embryo from thirty-one to thirty-four days
Gray's subject #11 54
MeSH Yolk+Sac
Dorlands/Elsevier s_01/12716805

The yolk sac is the first element seen in the gestational sac during pregnancy, usually at 5 weeks gestation. Image File history File links Gray22. ... For other uses, see Embryo (disambiguation). ... Image File history File links No higher resolution available. ... Medical Subject Headings (MeSH) is a huge controlled vocabulary (or metadata system) for the purpose of indexing journal articles and books in the life sciences. ... Elseviers logo. ... The gestational sac is the only available intrauterine structure that can be used to determine if an intrauterine pregnancy (IUP) exists, until the embryo is identified. ... A pregnant woman near the end of her term Pregnancy is the carrying of one or more embryos or fetuses by female mammals, including humans, inside their bodies. ... Gestation is the carrying of an embryo or fetus inside a female viviparous animal. ...

It is a critical landmark, identifying a true gestation sac.

It is quite echogenic (light) to ultrasound, and reliably seen early. Ultrasound is a form of cyclic sound pressure with a frequency greater than the upper limit of human hearing, this limit being approximately 20 kilohertz (20,000 hertz). ...


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 endoderm, outside of which is a layer of mesoderm. For other uses, see Embryo (disambiguation). ... The endoderm, sometimes refered to as entoderm, is one of the three germ layers of the developing embryo, the other two being the ectoderm and the mesoderm. ... The mesoderm is one of the three germ layers in the early developing embryo, the other two layers being the ectoderm and the endoderm. ...

It is filled with fluid, the vitelline fluid, which possibly may be utilized for the nourishment of the embryo during the earlier stages of its existence.

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. The aorta (generally pronounced or ay-orta) is the largest artery in the human body, originating from the left ventricle of the heart and bringing oxygenated blood to all parts of the body in the systemic circulation. ... The Vitelline Veins run upward at first in front, and subsequently on either side of the intestinal canal. ... Human embryo of 2. ...

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. 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. For the alien race in Stephen Donaldsons The Gap Cycle, see Amnion (Gap Cycle). ... For the entertainment company see Chorion (company) The chorion surrounds the embryo and other membranes. ... The placenta is an ephemeral (temporary) organ present in female placental vertebrates during gestation (pregnancy), but a placenta has evolved independently also in other animals as well, for instance scorpions and velvet worms. ...

As a rule the duct undergoes complete obliteration during the seventh week, but in about three percent of cases its proximal part persists as a diverticulum from the small intestine, Meckel's diverticulum, which is situated about three or four feet above the ileocecal valve, and may be attached by a fibrous cord to the abdominal wall at the umbilicus. A Meckels diverticulum is a true congenital diverticulum. ... This page is a candidate for speedy deletion, because: lack of content If you disagree with its speedy deletion, please explain why on its talk page or at Wikipedia:Speedy deletions. ... This article or section does not cite its references or sources. ...

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


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

In the meantime the Heuser membrane, located on the opposite pole of the developing vesicle, starts his upwards proliferation and meets the ipoblast. The last one keeps on descending for a little bit more.

We have arrived to the constitution of the Yolk sac.


  • Primary yolk sac: it is the vesicle constituted in the second week, its floor is represented by the Heuser membrane and its ceiling by the ipoblast.
  • 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 stricure. 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 ipoblst.
  • The final yolk sac: during the fourth week of development, during which we can see the shaping of the embryonic areas, the yolk sac undergoes externally the pression of the fore folding, of the posterior one and of the lateral ones. 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.

  Results from FactBites:
immunology - system organs I (1366 words)
In birds and other vertebrates that lay eggs the yolk sac is a very large structure compared to the size of the embryo because of course the embryo is isolated from the mother and unlike humans cannot obtain further nutrients from her.
The yolk sac of a chicken lasts from day zero until after hatching and continues to be a source of blood cells throughout although its activity diminishes as the yolk reduces in size and the bone marrow develops to take over production of blood cells.
While blood cell development in the yolk sac membrane was actually inside the blood vessels themselves, the blood cell development in the fetal liver occurs outside of the blood vessel walls.
Tiger: Raggin' & Rantin': Making Kang A Roo (359 words)
When the foetal membranes, such as the embryonic yolk sac or allantois, are applied to the uterine wall the structure formed is referred to as the placenta.
The vascular part of the yolk sac placenta seems to be largely involved in respiratory gas exchange and it expands quickly in the later stages of gestation when the metabolic rate of the foetus also rises.
The non-vascular part of the yolk sac is concerned with the transfer of nutrients and the building blocks for growth, such as proteins.
  More results at FactBites »



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