The yolk sac is a membranous sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though ''yolk sac'' is far more widely used. In humans, the yolk sac is important in early embryonic blood supply, and much of it is incorporated into the primordial gut during the fourth week of embryonic development.

In humans

Ultrasound of embryo at 5 weeks, colored

The yolk sac is the first element seen within the gestational sac during

pregnancy Pregnancy is the time during which one or more offspring develops (gestation, gestates) inside a woman, woman's uterus (womb). A multiple birth, multiple pregnancy involves more than one offspring, such as with twins. Pregnancy usually occur ...

, usually at 3 days gestation. The yolk sac is situated on the front (

ventral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

) part of the embryo; it is lined by extra-embryonic endoderm, outside of which is a layer of extra-embryonic mesenchyme, derived from the epiblast. Blood is conveyed to the wall of the yolk sac by the primitive

aorta The aorta ( ) is the main and largest artery in the human body, originating from the left ventricle of the heart and extending down to the abdomen, where it splits into two smaller arteries (the common iliac arteries). The aorta distributes o ...

and after circulating through a wide-meshed capillary plexus, is returned by the

vitelline veins The vitelline veins are veins that drain blood from the yolk sac and the gut tube during gestation. Path They run upward at first in front, and subsequently on either side of the intestinal canal. They unite on the ventral aspect of the canal. ...

to the tubular heart of the embryo. This constitutes the vitelline circulation, which in humans serves as a location of

haematopoiesis Haematopoiesis (, from Greek , 'blood' and 'to make'; also hematopoiesis in American English; sometimes also h(a)emopoiesis) is the formation of blood cellular components. All cellular blood components are derived from haematopoietic stem cell ...

. Before the placenta is formed and can take over, the yolk sac provides nutrition and gas exchange between the mother and the developing embryo. At the end of the fourth week, the yolk sac presents the appearance of a small pear-shaped opening (traditionally called the ''umbilical vesicle''), into the digestive tube by a long narrow tube, the

vitelline duct In the human embryo, the vitelline duct, also known as the vitellointestinal duct, the yolk stalk, the omphaloenteric duct, or the omphalomesenteric duct, is a long narrow tube that joins the yolk sac to the midgut lumen of the developing fetus. ...

. Rarely, the yolk sac can be seen in the afterbirth 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 The chorion is the outermost fetal membrane around the embryo in mammals, birds and reptiles (amniotes). It develops from an outer fold on the surface of the yolk sac, which lies outside the zona pellucida (in mammals), known as the vitell ...

and may lie on or at a varying distance from the placenta. There is no clinical significance to a residual external yolk sac. File:Gray24.svg, Diagram showing earliest observed stage of human ovum.
1 - Amniotic cavity
2 - Yolk-sac
3 - Chorion File:Gray25.svg, Diagram illustrating early formation of allantois and differentiation of body-stalk.
1 Amniotic cavity
2 Body-stalk
3 Allantois
4 Yolk-sac
5 Chorion File:Gray26.svg, Diagram showing later stage of allantoic development with commencing constriction of the yolk-sac.
1 Heart
2 Amniotic cavity
3 Embryo
4 Body-stalk
5 Placental villi
6 Allantois
7 Yolk-sac
8 Chorion File:Gray28.svg, Diagram illustrating a later stage in the development of the umbilical cord.
1 Placental villi
2 Yolk-sac
3 Umbilical cord
4 Allantois
5 Heart
6 Digestive tube
7 Embryo
8 Amniotic cavity As a rule the duct undergoes complete obliteration by the 20th week as most of the yolk sac is incorporated into the developing gastrointestinal tract, but in about two percent of cases its proximal part persists as a diverticulum from the small intestine,

Meckel's diverticulum A Meckel's diverticulum, a true congenital diverticulum, is a slight bulge in the small intestine present at birth and a vestigial remnant of the omphalomesenteric duct (also called the vitelline duct or yolk stalk). It is the most common malformat ...

, which is situated about 60 cm proximal to the

ileocecal valve The ileocecal valve (ileal papilla, ileocaecal valve, Tulp's valve, Tulpius valve, Bauhin's valve, ileocecal eminence, valve of Varolius or colic valve) is a sphincter muscle valve that separates the small intestine and the large intestine. Its ...

, and may be attached by a fibrous cord to the abdominal wall at the umbilicus. Sometimes a narrowing of the lumen of the

ileum The ileum () is the final section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms posterior intestine or distal intestine m ...

is seen opposite the site of attachment of the duct.

Histogenesis

The yolk sac starts forming during the second week of the embryonic development, at the same time as the shaping of the amniotic sac. The hypoblast starts proliferating laterally and descending. In the meantime Heuser's membrane, located on the opposite pole of the developing vesicle, starts its upward proliferation and meets the hypoblast.

Modifications

*Primary yolk sac: it is the vesicle which develops in the second week, its floor is represented by Heuser's membrane and its ceiling by the hypoblast. It is also known as the exocoelomic cavity. *Secondary yolk sac: this structure is formed when the extraembryonic mesoderm separates to form the extraembryonic coelom; cells from the mesoderm pinch off an area of the yolk sac, and what remains is the secondary yolk sac. *The final yolk sac: during the fourth week of development, during

organogenesis Organogenesis is the phase of embryonic development that starts at the end of gastrulation and continues until birth. During organogenesis, the three germ layers formed from gastrulation (the ectoderm, endoderm, and mesoderm) form the internal org ...

, part of the yolk sac is surrounded by endoderm and incorporated into the embryo as the gut. The remaining part of the yolk sac is the final yolk sac.

Additional images

File:Gray14.png, Surface view of embryo of

Hylobates concolor The black crested gibbon (''Nomascus concolor'') is a Critically Endangered species of gibbon found in China, Laos, and northern Vietnam, with four subspecies. Taxonomy The taxonomy of the species is confused. Previously grouped in the genus ...

(a gibbon). File:Gray17.png, Human embryo—length, 2 mm. Dorsal view, with the amnion laid open. X 30. File:Gray20.png, Dorsum of human embryo, 2.11 mm in length. File:Gray21.png, Section through the embryo. File:Gray30.png, Fetus of about eight weeks, enclosed in the amnion. Magnified a little over two diameters. File:Gray31.png, Model of human embryo 1.3 mm long. File:Gray32.png, Section through ovum imbedded in the uterine decidua File:Gray977.png, Human embryo about fifteen days old. Brain and heart represented from right side. Digestive tube and yolk sac in median section.

References

{{DEFAULTSORT:Yolk Sac Embryology of cardiovascular system

In humans

Histogenesis

Modifications

Additional images

See also

References