[Electron microscopic study of the human yolk sac epithelium in the I trimester of pregnancy].

The ultrastructural organization of the endodermal epithelium of the human yolk sac was investigated during weeks 6 to 12 of intrauterine development. In the beginning of the period studied, the epithelium had features of the tissue specialized both in the active protein synthesis and in the degradation of organic substances transported by endocytosis. Subcellular organization of the yolk sac epitheliocytes is indicative of their functional homology with both the epithelial cells of the small intestine and hepatocytes. By the end of the I trimester of pregnancy, the involutive changes in the human yolk sac took place. The probable mechanisms controlling the organ tissue remodeling are autolysis and apoptosis. A significant ultrastructural remodeling of the endodermal epithelium within the I trimester of pregnancy reflects the short period of human yolk sac activity and is another corroboration of the fast and asynchronous development of extraembryonic organ tissues.