It 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 reliably seen early 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 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
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.