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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
Dorlands/Elsevier	s_01/12716805

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 quite echogenic (light) to ultrasound, and reliably seen early.

Contents 1 In mice 2 In humans 3 Histogenesis 4 Modifications 5 Additional images 6 References

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

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 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.

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 down.

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

• 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 ipoblast. 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.

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.
Opened uterus with cat fetus in midgestation: 1 umbilicus, 2 amnion, 3 allantois, 4 Yolk sac, 5 developing marginal hematoma, 6 maternal part of placenta (endometrium)

1. ^ Text for first three lectures. Retrieved on 2007-10-13.

v • d • e Human development of embryo and development of fetus (some dates are approximate - see Carnegie stages and a timeline)
Week 1	Zygote - Morula - Blastula (Blastomere) - Blastocyst - Inner cell mass
Week 2	Bilaminar disc (Hypoblast, Epiblast)
Week 3 (Trilaminar embryo, germ layers)	Archenteron/Primitive streak (Primitive pit, Primitive knot/Blastopore, Primitive groove) - Gastrula/Gastrulation Ectoderm: Surface ectoderm - Neuroectoderm - Somatopleure Endoderm: Splanchnopleure Mesoderm: Chorda- - Paraxial (Somite/Somitomere/Sclerotome/Myotome/Dermatome) - Intermediate - Lateral plate (Intraembryonic coelom, Splanchnopleure/Somatopleure)
Extraembryonic/uterus	Trophoblast (Cytotrophoblast, Syncytiotrophoblast) Blastocoele - Yolk sack/exocoelomic cavity - Heuser's membrane - Extraembryonic coelom - Vitelline duct Umbilical cord (Umbilical artery, Umbilical vein, Wharton's jelly) - Allantois Placenta - Decidua (Decidual cells) - Chorionic villi/Intervillous space - Gestational sac (Amnion/Amniotic sac/Amniotic cavity, Chorion)
Histogenesis and organogenesis	Limb development: Limb bud - Apical ectodermal ridge/AER