The yolk sac is known mainly as the yolk in bird eggs. In fact, a yolk sac accompanies the placenta in humans as well and performs important functions in embryonic development.
What is the yolk sac?
A yolk sac is an organ that serves exclusively to nourish an embryo. It first appeared in vertebrate evolution in reptiles and continues in birds. To this day, every egg-laying animal forms the dorsal sac around the embryo contained in the egg. However, it still occurs in mammals and is more than an evolutionary remnant in them. Until the formation of the placenta, the yolk sac also serves to nourish the embryo in mammals and thus also in humans during this early phase of development. Furthermore, it reaches a size of up to 5 mm and serves as a substitute for the liver during this time until it is developed. In the human fetus, the yolk sac takes over vital metabolic functions until then. In some mammals, the yolk sac remains until birth and they are even born with a yolk sac placenta. Humans, however, shed the yolk sac once the intestine has developed.
Anatomy and structure
The yolk sac in humans is very simple in its anatomy, consisting of an outer membrane and a nutrient-rich filling. It remains with the midgut during the early stages of embryonic development via what is known as the yolk duct. It is visible in early ultrasound examinations. Later on, the intestinal tube of the embryo is cut off from the yolk sac, from now on it is called secondary yolk sac. Before that, it is lined with hypoblasts, which are involved in the formation of blood. These are stem cells, which are also interesting for many other purposes in research. In humans, unlike in horses, for example, the yolk sac does not remain intact until birth.
Function and tasks
The yolk sac in reptiles and birds is designed to nourish the embryo for as long as it must remain in its egg. In humans, the mass of the fertilized egg lasts just long enough for it to implant in the lining of the uterus – after which its reserves are depleted. The placenta forms very quickly and the egg is also immediately absorbed by the mucous membrane of the uterus, so that the possible nutritional bottleneck is well bridged. The yolk sac has merely taken over other functions than in reptiles and birds – in humans it can replace the liver in its metabolic function until the embryo has developed it. Liver function is also vital for an embryo in early development. Equally crucial in the yolk sac are the stem cells that make up the membrane of the primary yolk sac. The germ cells and stem cells for blood formation emerge from this. Once these two processes have been completed, the embryo has been connected to the maternal circulation via the placenta for some time and has developed all its organs to such an extent that it can manage without the yolk sac. In humans, a separate yolk sac placenta no longer forms alongside the placenta, as is still the case in some other mammals. Instead, the yolk sac disappears from this point on and is also no longer visible in the ultrasound image. The embryo now has only the placenta.
Diseases
The yolk sac is a comparatively unproblematic component of early embryonic development. It must develop because otherwise the embryo would not be able to replace the function of the liver and also would not form blood. Under these conditions, it would not be viable at all and would die and be expelled shortly after fertilization of the egg. However, it is very rare for a fertilized egg to develop into an embryo without a yolk sac – if the egg is rejected by the woman’s body at this early stage, it is more often for other reasons. Until about the ninth week of pregnancy, when the yolk sac is needed to replace the liver, it is important that it remains undamaged and can continue to perform this function. If its function were to fail before then, for example due to external injuries to the mother such as severe falls or exposure to violence, the embryo would no longer be viable and would be rejected. By the ninth week of pregnancy, the stem cells on the membrane of the yolk sac have also fulfilled their most important function and triggered blood formation.Whether the stem cells found in the yolk sac can give rise to all types of blood cells is as yet unknown. It is also not clear to what extent blood formation under the influence of the yolk sac is responsible for the later development of leukemia. However, yolk sac tumors, which belong to the group of germ cell tumors, are already possible. Depending on the location, such tumors can be surgically removed before the birth of the child, but these are individual decisions and it must also be weighed between the benefit of surgery and the risk to mother and child. Often, such tumors lead to the death of the embryo before birth and it is rejected by the mother’s body or must be removed by curettage, depending on the stage of development.
