Are human placental bed giant cells merely aggregates of small mononuclear trophoblast cells? An ultrastructural and immunocytochemical study

The ultrastructure of placental bed giant cells in early human pregnancies of 7-12 weeks gestational age is described. Their nature and function was further characterized by confocal immunofluorescence microscopy of paraffin sections labelled for cytokeratin, gap junction connexins (CX) 32 or 43, and placental hormones, alpha-human chorionic gonadotrophin (alpha-HCG) and human placental lactogen (HPL). Placental bed giant cells were observed with two phenotypes; as single large trophoblast cells containing one or more nuclear profiles in a voluminous cytoplasm, and as cell aggregates comprising mononuclear trophoblast cells in close apposition separated by narrow intercellular spaces. Cells within the aggregates are attached to one another by desmosomes, and also possess gap junctions as shown by immunolabelling for CX32 and CX43. By contrast, gap junctions were absent in the true multinucleated giant cells. Organelles present within the cytoplasm of the giant cells and their immunoreactivity for HPL and alpha-HCG suggest protein synthesis.