[Morphological and functional features of endodermal cells in rat yolk sac, with special reference to the fetal...

Morphological and functional features of the yolk sac endodermal cells with special reference to the fetal macrophage differentiation were investigated morphologically under the light and electron microscopes and immunologically with the antigen phenotypic analysis and the phagocytic activity-test, using the syngeneic DA rat-embryos from 8 to 16 days of gestation. Based on the staining property with toluidin blue and the ultrastructural features, the endodermal layer from day 8 to 16-yolk sacs has been known to consist of two kinds of cell type; 10% "clear" cells with clear cytoplasm and 90% "dark" cells with dark cytoplasm. Numerous primary lysosomes, phagolysosomes, lipid droplets and coated vesicles distributed preferentially in the supranuclear portion of endodermal cells. A broad intercellular space was found between "clear" cells and "dark" cells, indicating the loose intercellular binding. It was often found that "clear" cells tend to migrate from the endodermal layer into the mesenchymal layer, where the poor development of basement membrane was seen between them. Cells phagocytosing red blood cells, that resemble morphologically "clear" cells, were also observed in the fetal liver. At ten hours after latex-injection into the yolk sac cavity of 14 days embryos, some cells which phagocytosed latex beads in their cytoplasm were found in the endodermal layer, and also in the liver tissue and loose connective tissue of fetus. These cells were stained positively with monoclonal antibody Mar3 which recognizes preferentially rat-mononuclear phagocyte system. In vitro-latex uptake of separated endodermal cells was also demonstrated by the culture-study of endodermal cell suspension. The present findings indicate that the yolk sac-endodermal layer derived from the proximal endoderm consists of at least two kinds of cell-population with a great similarity to tissue macrophages in morphological and functional senses, and support the concept that some cell-populations of endodermal layer may migrate into fetal tissue and are closely related to the differentiation of fetal macrophages and their precursors.