Syncytial nuclear aggregates are not universally apoptotic in nature and are closely associated with cytokeratin, -actin and -tubulin.

Introduction: Syncytial nuclear aggregates (SNA) are clusters of nuclei found in the syncytiotrophoblast of the human placenta. The origin and fate of SNAs is unknown and contrasting theories exist. SNAs may have a role in disease as their number is increased in common pregnancy pathologies. We hypothesise that SNAs are formed by cytoskeletal protein interactions with nuclei, and that SNAs are not exclusively apoptotic. Methods: Fresh placental villous tissue was fixed and wax-embedded (n=7). Apoptosis was assessed on single sections by staining for M30 and TUNEL. a-tubulin, b-tubulin, b-actin, g-actin, cytokeratin-7, dynein inter- mediate-chain-1 and kinesin 5B were localised using immunofluorescence. An archive of electron micrographs was reviewed to investigate nuclear morphology and surrounding cytoskeletal proteins. Results: At term, 20% to 26.5% of SNAs contained apoptotic material as tested by TUNEL and M30 respectively (Figure 1). Immunofluorescence showed cytokeratin-7 in syncytiotrophoblast associated with SNAs. b- tubulin and b-actin were found throughout the trophoblast and underlying some SNAs. a-tubulin and g-actin were observed throughout the trophoblast but were not associated with SNAs. Dynein intermediate chain 1 was found sparsely at localised regions in the trophoblast but was not associ- ated with SNAs. Transmission electron microscopy confirmed that inter- mediate filament arrays often surround SNAs. It also showed various morphologically distinct stages of apoptosis in nuclei within the same SNA. Discussion: Only a minority of SNAs contain apoptotic nuclei, suggesting they may serve different functions, rather than just the extrusion of senescent nuclei as has previously been suggested. Cytoskeletal proteins, and especially cytokeratin, were found within and surrounding SNAs and may have a role in forming or maintaining these nuclear clusters. Further research is required to determine how cytoskeletal proteins are involved in SNA formation, and how apoptosis is activated in specific SNA nuclei.