Yolk sac

From Wikipedia, the free encyclopedia

Yolk sac

Human embryo of 2.6 mm.

Human embryo from thirty-one to thirty-four days
Gray's	subject #11 54
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.

1 In mice
2 In humans
3 Additional images
4 See also
5 External links

[edit] In mice

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

[edit] In humans

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 aortæ, 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 three percent of cases its proximal part persists as a diverticulum from the small intestine, Meckel's diverticulum, which is situated about three or four feet above 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.

[edit] 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.
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)

This article was originally based on an entry from a public domain edition of Gray's Anatomy. As such, some of the information contained herein may be outdated. Please edit the article if this is the case, and feel free to remove this notice when it is no longer relevant.