Yolk sac

# Overview
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.

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

# 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 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 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.

# Histogenesis
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. The last one keeps on descending for a little bit more.
We have arrived to the constitution of the Yolk sac.

# Modifications
- Primary 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.
- 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 stricure. 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 ipoblst.
- The final yolk sac: during the fourth week of development, during which we can see the shaping of the embryonic areas, the yolk sac undergoes externally the pression of the fore folding, of the posterior one and of the lateral ones. 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.