Electron microscopy of syncytiotrophoblast knot in tertiary villus (human placenta, almost full-term)

Electron microscopy. The mature villus, surrounded by intervillous space (1), contains capillaries (2) with erythrocytes and pericytes (3) embedded in fetal connective tissue elements (4). Two capillaries are localised close to the covering multinucleated syncytiotrophoblast cell (STC, 5) (vasculosyncytial membranes). The so-called syncytiotroblast knot comprises distinct nuclear accumulation in the STC (compare to the light microscopy in POJA-L1282). Quite commonly aggregated nuclei show signs of pre-apoptosis and after being sequestered in these knots they are extruded as trophoblast fragments via the intervillous space into the maternal blood vessels and hence eliminated by phagocytosis. At (6) a dense-stained cytotrophoblast cell (CTC) is shown in retraction due to progressive degeneration.