The Human Yolk Sac During Physiological Pregnancy: Its Histological Structure and Relationship with the Primitive Gut

The aim of the study is to investigate the structural organization of the human yolk sac compartments during the first trimester of physiological pregnancy and the potential connection of this organ to the primitive gut. Material and methods . 27 human yolk sacs and 23 fetal small intestine fragments obtained from voluntary termination of physiological pregnancy (gestational age: 5–12 weeks) were examined. The morphological features of the organs were studied on serial paraffin sections stained with hematoxylin and eosin, as well as with Mallory's stain. Results . In the early stages of human development, the wall of the yolk sac is primarily composed of an endodermal-type epithelium. A unique structure of this organ is the “glomus-like body”, which has a folded form and contains a developed capillary network. The yolk sac is connected to the primitive gut via the vitelline duct, through which the contents of the organ are transported into the intestinal lumen until the 6th week of development. During this period, the formation of all its layers and structural features continues. The presence of microvilli on the surface of the enterocytes suggests their involvement in the absorption process. Fibrosis and thinning of the yolk sac wall, reduction in the size of the glomus body, and simultaneous obliteration of the vitelline duct indicate the completion of the organ's primary functions by the end of the first trimester of pregnancy. Conclusion . The human yolk sac is a provisional structure with a unique organization, which functions in close interaction with embryonic endodermal derivatives. Its life cycle follows the principle of accelerated development, which is characteristic of the extraembryonic organ system as a whole.