The yolk sac is a membranous sac attached to an

embryo An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sp ...

, formed by cells of the

hypoblast In amniote embryology, the hypoblast is one of two distinct layers arising from the inner cell mass in the mammalian blastocyst, or from the blastodisc in reptiles and birds. The hypoblast gives rise to the yolk sac. The hypoblast is a layer of ...

layer of the

bilaminar embryonic disc The bilaminar embryonic disc, bilaminar blastoderm or embryonic disc is the distinct two-layered structure of cells formed in an embryo. In the development of the human embryo this takes place by day eight. It is formed when the inner cell mass, a ...

. This is alternatively called the umbilical vesicle by the

Terminologia Embryologica The ''Terminologia Embryologica'' (''TE'') is a standardized list of words used in the description of human embryologic and fetal structures. It was produced by the Federative International Committee on Anatomical Terminology on behalf of the In ...

(TE), though ''yolk sac'' is far more widely used. The yolk sac is one of the

fetal membranes The fetal membranes are the four extraembryonic membranes, associated with the developing embryo, and fetus in humans and other mammals. They are the amnion, chorion, allantois, and yolk sac. The amnion and the chorion are the chorioamniotic ...

and is important in early embryonic blood supply. In humans much of it is incorporated into the primordial gut during the fourth week of

embryonic development In developmental biology, animal embryonic development, also known as animal embryogenesis, is the developmental stage of an animal embryo. Embryonic development starts with the fertilization of an egg cell (ovum) by a sperm, sperm cell (spermat ...

In humans

The yolk sac is the first element seen within the

gestational sac The gestational sac is the large cavity of fluid surrounding the embryo. During early embryogenesis, it consists of the extraembryonic coelom, also called the chorionic cavity. The gestational sac is normally contained within the uterus. It is the ...

during

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

, usually at three days

gestation Gestation is the period of development during the carrying of an embryo, and later fetus, inside viviparous animals (the embryo develops within the parent). It is typical for mammals, but also occurs for some non-mammals. Mammals during pregn ...

. The yolk sac is situated on the front (

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

) part of the

; it is lined by extra-embryonic

endoderm Endoderm is the innermost of the three primary germ layers in the very early embryo. The other two layers are the ectoderm (outside layer) and mesoderm (middle layer). Cells migrating inward along the archenteron form the inner layer of the gastr ...

, outside of which is a layer of extra-embryonic

mesenchyme Mesenchyme () is a type of loosely organized animal embryonic connective tissue of undifferentiated cells that give rise to most tissues, such as skin, blood, or bone. The interactions between mesenchyme and epithelium help to form nearly ever ...

, derived from the epiblast. Blood is conveyed to the wall of the yolk sac by the primitive

aorta The aorta ( ; : aortas or aortae) is the main and largest artery in the human body, originating from the Ventricle (heart), left ventricle of the heart, branching upwards immediately after, and extending down to the abdomen, where it splits at ...

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 Vitelline circulation refers to the system of blood flowing from the embryo to the yolk sac and back again. 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. It is ...

, which in humans serves as a location of

haematopoiesis Haematopoiesis (; ; also hematopoiesis in American English, sometimes h(a)emopoiesis) is the formation of blood cellular components. All cellular blood components are derived from haematopoietic stem cells. In a healthy adult human, roughly ten ...

. 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 The amnion (: amnions or amnia) is a membrane that closely covers human and various other embryos when they first form. It fills with amniotic fluid, which causes the amnion to expand and become the amniotic sac that provides a protective envir ...

and the

chorion The chorion is the outermost fetal membrane around the embryo in mammals, birds and reptiles (amniotes). It is also present around the embryo of other animals, like insects and molluscs. Structure In humans and other therian mammals, the cho ...

and may lie on or at a varying distance from the

placenta The placenta (: placentas or placentae) is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas, and waste exchange between ...

. 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 vitelline 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 vitelline duct. It is the most common malformation of the Human gastrointestinal tract, gastrointesti ...

, which is situated about 60 cm proximal to the

ileocecal valve In many Animalia, including humans, an ileocolic structure or problem is something that concerns the region of the gastrointestinal tract from the ileum to the large intestine, colon. In Animalia that have cecum, ceca, the ileocecal region is a sub ...

, and may be attached by a fibrous cord to the abdominal wall at the

navel The navel (clinically known as the umbilicus; : umbilici or umbilicuses; also known as the belly button or tummy button) is a protruding, flat, or hollowed area on the abdomen at the attachment site of the umbilical cord. Structure The u ...

. 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 may ...

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

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

. It is also known as the exocoelomic cavity. *Secondary yolk sac: this structure is formed when the extraembryonic

mesoderm The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical ...

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

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 (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.

Other animals

The yolk sac is an ancient structure. It is found in fish and probably the last common ancestor of vertebrates. The fish structure was innovated upon by the

amniote Amniotes are tetrapod vertebrate animals belonging to the clade Amniota, a large group that comprises the vast majority of living terrestrial animal, terrestrial and semiaquatic vertebrates. Amniotes evolution, evolved from amphibious Stem tet ...

s (reptiles, birds, and later mammals), which evolved bigger yolks and tougher eggs for life on land. The placental mammalian yolk sac is a further elaboration of the amniote structure.

References

{{DEFAULTSORT:Yolk Sac Embryology of cardiovascular system