Yolk Sac: Essential Embryonic Structure in Vertebrate Development

The yolk sac is an extraembryonic membrane critical for early development in all vertebrates—reptiles, birds, and mammals.

Key Functions

• Nutrition: Yolk reserves nourish the embryo in oviparous species (reptiles, birds).
• Hematopoiesis: First site of blood cell formation in all vertebrates (blood islands).
• Germ cell origin: Primordial germ cells arise in the yolk sac wall before migrating to gonads.
• Gut formation: Incorporates into the midgut as the embryonic disc folds.

Mammalian Specifics

• No yolk reserves (placental nutrition takes over).
• Forms the connection for the umbilical cord (vitelline duct).
• Vitelline circulation → portal venous system.
• Degenerates by late first trimester (~week 10–12).

Clinical Relevance

Yolk sac visualization on early ultrasound confirms intrauterine pregnancy. Absence of yolk sac with a gestational sac >25 mm suggests abnormal pregnancy (anembryonic or ectopic).