Human embryo of 2.6 mm.
Human embryo from thirty-one to thirty-four days
|Latin||vesicula umbilicalis; saccus vitellinus|
The umbilical vesicle is a membranous sac attached to an embryo, formed by cells of the hypoblast adjacent to the embryonic disk. Formerly known as the 'yolk sac', this structure was renamed because it contains no yolk, and performs a different function than the similar structure found in other animals (see yolk). The umbilical vesicle is important in early embryonic blood supply, and is incorporated into the primordial gut during the fourth week of development.
The umbilical vesicle 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.
Blood is conveyed to the wall of the vesicle 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 umbilical vesicle presents the appearance of a small pear-shaped 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.
The umbilical vesicle 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.
- Primary umbilical vesicle: 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 umbilical vesicle: this structure is formed when the extraembryonic mesoderm separates to form the extraembryonic coelom; cells from the mesoderm pinch of an area of the vesicle, and what remains is the secondary umbilical vesicle.
- The final umbilical vesicle: during the fourth week of development, during organogenesis, part of the umbilical vesicle is surrounded by endoderm and incorporated into the embryo as the gut. What remains of the umbilical vesicle at this point is the final umbilical vesicle.
- The Developing Human: Clinically Oriented Anatomy: Chapter 7