- Structure and Formation
- Yolk sac is a membranous sac formed by hypoblast cells in bilaminar embryonic disc
- First seen in gestational sac at three days gestation
- Located on ventral part of embryo, lined by extra-embryonic endoderm
- Forms during second week alongside amniotic sac development
- Function and Development
- Provides early embryonic blood supply and nutrition
- Forms vitelline circulation for hematopoiesis
- Most of yolk sac incorporated into primordial gut by fourth week
- Vitelline duct usually obliterates by 20th week
- Types and Evolution
- Primary yolk sac develops in second week with Heuser's membrane floor
- Secondary yolk sac forms when mesoderm separates
- Final yolk sac develops during fourth week as gut
- Ancient structure found in fish, evolved by amniotes for land life
- Clinical Significance
- Can be seen as small oval-shaped body in afterbirth
- About two percent of cases show Meckel's diverticulum
- External yolk sac has no clinical significance