The human yolk sac is a temporary structure that forms early in embryonic development, playing a significant role in supporting the initial stages of life. Unlike in birds or reptiles, it does not contain yolk in humans, but serves several functions before the placenta fully develops. This transient structure is one of the first extra-embryonic membranes to appear. Its development is monitored during early pregnancy due to its importance.
Structure and Early Development
The yolk sac is a small, sac-like structure connected to the developing embryo by a stalk. It is located within the gestational sac, a fluid-filled cavity in the uterus that surrounds the early embryo. This structure is lined by extra-embryonic endoderm and surrounded by a layer of extra-embryonic mesenchyme.
Formation of the primary yolk sac begins during the second week of embryonic development, between days 8 and 14 after fertilization, from proliferating hypoblast cells. It can be visualized using transvaginal sonography around five weeks post-fertilization (seven gestational weeks). From about five to ten weeks post-fertilization, the yolk sac generally grows linearly in size. A healthy yolk sac appears round or pear-shaped on an ultrasound, with a diameter ranging from 3 to 5 millimeters.
Essential Roles in Embryonic Growth
The yolk sac performs several functions during early embryonic growth before other organs and the placenta take over. It acts as a temporary source of nourishment, facilitates blood cell production, and is the origin of cells that will develop into reproductive organs. Its multi-faceted role supports the rapid development of the embryo.
The yolk sac provides nutrients to the early embryo before the placenta becomes fully functional. It absorbs proteins, lipids, vitamins, and minerals from uterine secretions and maternal blood. Nutrients are then transported across the yolk sac’s epithelial cells and released into the embryonic bloodstream.
The yolk sac is also the primary site for the formation of blood cells, a process known as hematopoiesis. Primitive red blood cells are initially produced in blood islands within the yolk sac. These blood islands eventually merge to form an interconnected vascular network throughout the yolk sac, and these early blood cells circulate to meet the embryo’s metabolic needs.
The yolk sac is the origin of primordial germ cells (PGCs), which are the precursors to sperm and eggs. These cells are identifiable in the yolk sac wall around 3 to 4 weeks post-conception. PGCs then migrate from the yolk sac, through the hindgut epithelium and into the dorsal mesentery, eventually colonizing the developing gonads (ovaries or testes) by the beginning of the sixth week.
Its Gradual Disappearance
As pregnancy progresses and the placenta develops, the human yolk sac gradually diminishes in size and its functions are taken over by the placenta. This transition from yolk sac to placental nutrition is a gradual process that occurs between the 8th and 12th weeks of gestation. By the end of the first trimester, the placenta is fully established as the primary source of nutrients and gas exchange for the developing fetus.
The yolk sac begins to shrink around week 10 and is often no longer visible on ultrasound after 12 to 14 weeks of gestation, becoming undetectable between 14 to 20 weeks. It is either absorbed by the developing embryo or incorporated into other structures. In some cases, a tiny remnant of the yolk sac may persist, though this is generally insignificant.
Medical Relevance
The human yolk sac serves as an important indicator in early pregnancy, with its appearance, size, and presence on ultrasound providing insights into the health of the pregnancy. It is often the first structure identified within the gestational sac on an ultrasound, around five weeks of gestation. Observing the yolk sac helps clinicians confirm an intrauterine pregnancy and distinguish it from other conditions.
Abnormalities in the yolk sac’s appearance or size can suggest potential issues. For example, a yolk sac with a diameter larger than 6 millimeters may indicate a problem, such as an increased risk of miscarriage. A yolk sac that is too small or absent when it should be present can also raise concerns, though it might indicate a miscalculation of gestational age. In such cases, further ultrasound examinations are recommended to monitor the pregnancy.
While rare, remnants of yolk sac tissue can lead to the development of yolk sac tumors, also known as endodermal sinus tumors. These are a type of germ cell tumor that can occur in various parts of the body, including the ovaries or testes.