Ultrastructure of the labyrinth, interlobium and parietal yolk sac endoderm. (A-B) The interhaemal barrier. Only a single layer of syncytiotrophoblast is interposed between the blood flowing in the maternal blood spaces (mbs) and the endothelium of the fetal capillary (fc). The apical membrane contains invaginations (arrowheads) and the cytoplasm includes small coated vesicles (cv) and larger vacuoles. There are recesses in the basal membrane (arrows). (C) Interlobium. Most of the trophoblast is syncytial (sy tr) with abundant rough endoplasmatic reticulum. There are many microvilli where it is in contact with the maternal blood space. Cytotrophoblast cells with large nuclei are found in some regions of the interlobium. Desmosomes are found between the lateral membranes of adjacent cells (arrowheads). (D) Parietal yolk sac endoderm. These columnar cells have numerous microvilli at the apical surface. The supranuclear cytoplasm contains vacuoles and vesicles, tubular mitochondria and rough endoplasmic reticulum. Desmosomes are found between the lateral membranes of adjacent cells (arrowheads). Scale bars: 1 μm (A-B); 5 μm (C); 2 μm (D).