Haemopoiesis in the human yolk sac.

Haemopoiesis in human yolk sacs was examined using tissues obtained from a total of 27 cases in various stages of development from the fourth to eleventh week of pregnancy. In the early stages of development, the yolk sac was observed to be connected to the midgut by the vitelline duct, which became slender with later growth of the embryo. In the early stages of pregnancy, endodermal tissues were found to be a predominant component, whereas in the later stages, the mesenchymal tissues increased. The most immature blood cells and their mitotic figures were observed in the endodermal tissue. Haemopoiesis was found in endodermal tissue before mesenchymal tissue had developed. Electron microscopy revealed that maturation of the blood cells proceeded as the cells were formed in mesenchymal tissue and in blood vessels.