yolk sac

Yolk sac
Human embryo of 2.6 mm.
Human embryo from thirty-one to thirty-four days
Gray's	subject #11 54
Carnegie stage	5b
Days	9
Precursor	endoderm
MeSH	Yolk+Sac

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The yolk sac is a membranous sac attached to an embryo, providing early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and primitive mammals. It functions as the developmental circulatory system of the human embryo, before internal circulation begins.

In mice

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

In humans

It 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 human pregnancy using ultrasound.

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.

Histogenesis

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.

Modifications

• 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.^[1]

• 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.

Additional images

Surface view of embryo of Hylobates concolor.	Human embryo—length, 2 mm. Dorsal view, with the amnion laid open. X 30.	Dorsum of human embryo, 2.11 mm. in length.	Section through the embryo.
Diagram showing earliest observed stage of human ovum.	Diagram illustrating early formation of allantois and differentiation of body-stalk.	Diagram showing later stage of allantoic development with commencing constriction of the yolk-sac.	Diagram illustrating a later stage in the development of the umbilical cord.
Fetus of about eight weeks, enclosed in the amnion. Magnified a little over two diameters.	Model of human embryo 1.3 mm. long.	Section through ovum imbedded in the uterine decidua	Human embryo about fifteen days old. Brain and heart represented from right side. Digestive tube and yolk sac in median section.

See also

• Yolk sac tumor

References

1. ^ "Text for first three lectures". http://isc.temple.edu/marino/embryology/lecture1-3.htm. Retrieved 2007-10-13. 

yolk sac: - Is the first hemopoietic organ - primordial germ cell: derived from endoderm of yolk sac