Yolk sac

Yolk sac
Human embryo of 3.6 mm.
Human embryo from thirty-one to thirty-four days
Details
Carnegie stage	5b
Days	9
Precursor	endoderm
Identifiers
Latin	vesicula umbilicalis; saccus vitellinus
MeSH	sac A10.615.284.981
TE	E5.7.1.0.0.0.4
FMA	87180
*Anatomical terminology* [edit on Wikidata]

The yolk sac is a membranous sac attached to an embryo, formed by cells of the hypoblast adjacent to the embryonic disk. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though yolk sac is far more widely used. The yolk sac is important in early embryonic blood supply, and is incorporated into the primordial gut during the fourth week of development.^[1]

In humans

Contents in the cavity of the uterus seen at approximately 5 weeks of gestational age by obstetric ultrasonography.

Artificially colored, showing gestational sac, yolk sac and embryo (measuring 3 mm as the distance between the + signs).

The yolk sac is the first element seen in the gestational sac during pregnancy, usually at 3 days gestation. Identification of the gestational sac is a critical landmark in monitoring an early pregnancy, and can be reliably seen in human pregnancy using ultrasound.

The yolk sac is situated on the ventral aspect of the embryo; it is lined by extra-embryonic endoderm, outside of which is a layer of extra-embryonic mesenchyme, derived from the mesoderm.

Blood is conveyed to the wall of the yolk 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 opening (traditionally called the umbilical vesicle), into the digestive tube by a long narrow tube, the vitelline duct.

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.

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 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 60 cm 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 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 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 constituted 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 of 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, 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

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.
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.

References

↑ The Developing Human: Clinically Oriented Anatomy: Chapter 7

Development of the reproductive system

Precursors	Mesoderm intermediate lateral plate Endoderm Cloaca Urogenital sinus Ectoderm Cloacal membrane

Internal	Development of the gonads Gonadal ridge Pronephric duct Mesonephric duct Paramesonephric duct Vaginal plate Definitive urogenital sinus

External	Genital tubercle Labioscrotal swelling Primordial phallus Gubernaculum Peritoneum Processus vaginalis Canal of Nuck

See also	List of related male and female reproductive organs Prenatal development Embryogenesis

Membranes of the fetus and embryo

Embryo	Trophoblast Cytotrophoblast Syncytiotrophoblast Intermediate trophoblast Allantois Decidua Decidual cells Chorionic villi/Intervillous space Amnion sac cavity

Fetus	Umbilical cord Umbilical artery Umbilical vein Wharton's jelly

Circulatory	Placenta Chorion

Other	Blastocoel Heuser's membrane Vitelline duct Gestational sac

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