The yolk sac is a temporary structure that forms early in human embryonic development. Despite its name, in humans, it does not contain yolk but serves as a transient support system for the developing embryo. It appears around 8-9 days post-fertilization and is visible via ultrasound by five weeks of gestation. This membranous sac plays an initial role in providing nourishment and facilitating gas exchange before other organs become fully functional. The yolk sac is an extra-embryonic membrane during these formative weeks, ensuring the embryo receives resources as it grows.
The Yolk Sac’s Early Roles
Before the placenta fully develops, the yolk sac performs several important functions for the rapidly growing embryo. One primary role involves nutrient absorption and transfer. The yolk sac absorbs nutrients from uterine secretions and maternal blood, processing them within its epithelial cells and then transporting them to the embryonic circulation. These nutrients, including proteins, lipids, vitamins, and minerals, are important for early growth and tissue differentiation.
Beyond nutrition, the yolk sac is the initial site for primitive hematopoiesis, the process of blood cell formation. Beginning around the third week of embryogenesis, it produces the first red blood cells, white blood cells, and platelets. This occurs in specialized cell aggregates called blood islands within the yolk sac’s extra-embryonic mesoderm. The yolk sac also generates hematopoietic stem cells, which later migrate to the developing liver and, subsequently, the bone marrow, where definitive blood cell production takes over. It also contributes to the production of germ cells, precursors to sperm and egg cells, and plays a role in early immune function and metabolic regulation.
Its Transformation and Disappearance
As the embryo develops, the yolk sac undergoes transformation and eventual disappearance. Around the fourth week of embryonic development, as the embryo folds, a portion of the yolk sac becomes incorporated into the embryo itself, forming the primitive gut. This incorporation is important for the formation of the digestive system, as the endodermal lining of the yolk sac becomes continuous with the gut.
The yolk sac remains connected to the developing midgut via a narrow tube known as the vitelline duct. This duct serves as a temporary conduit for nutrients and facilitates communication between the yolk sac and the primitive midgut. However, as the placenta matures and assumes its role as the primary source of nutrient and gas exchange, the vitelline duct is no longer needed.
The vitelline duct begins to narrow and completely obliterates between the fifth and ninth weeks of gestation. This obliteration process starts at the umbilical end of the duct and extends towards the intestine. By approximately 10 to 14 weeks of gestation, the yolk sac itself has largely regressed and becomes undetectable by ultrasound. The regression and obliteration of the vitelline duct are important for normal fetal development, ensuring no persistent connections remain.
When Development Diverges
While the vitelline duct disappears completely, its failure to do so can lead to various congenital anomalies. These conditions arise when remnants of the duct persist, often causing symptoms later in life. The most common anomaly is Meckel’s diverticulum, a small pouch-like protrusion from the wall of the small intestine. This diverticulum represents the persistent proximal part of the vitelline duct. It is located within 60–100 cm of the ileocecal valve and can contain ectopic tissues, which may lead to complications like bleeding or inflammation.
Other, less frequent anomalies result from different patterns of incomplete vitelline duct obliteration. A vitelline cyst forms when the middle portion of the duct remains, with both ends obliterated, leading to a fluid-filled sac. If the entire duct remains open, it forms a vitelline fistula, creating an abnormal connection between the intestine and the umbilicus, potentially causing discharge. Fibrous bands, another remnant, can connect the intestine to the navel and may cause intestinal obstruction. These anomalies highlight the importance of the precise developmental process for the vitelline duct’s complete resolution.