IntroductionSyncytial nuclear aggregates (SNAs) are increased in pregnancy complications; however, little is known about their origin or function. This study aimed to characterise SNAs in more detail than has been reported previously.MethodsImmunohistochemistry and morphological examination at the light and ultrastructural level were used to determine the nature and structure of SNAs.ResultsSNAs comprising bridges and syncytial knots had similar frequency with 974 per mm3 of villous tissue (IQR 717–1193) and 833 per mm3 (IQR 766–1190), respectively while there were approximately four times as many sectioning artefacts than knots and bridges combined. SNAs had increased proportions of condensed nuclei compared to the remaining syncytiotrophoblast (33.3% vs. 8.9%) and decreased proportions of euchromatic nuclei (0.0% vs. 16.2%), as assessed by examination of an electron micrograph archive. SNAs showed little evidence of apoptosis, with weak positivity for the apoptosis markers M30-neoepitope at 16.6% and TUNEL at 10.0%; strong staining was rarely seen for either marker. Immunofluorescence demonstrated rare association of actin (α, β or γ) with SNAs, whereas tubulin was in close proximity to SNAs and cytokeratin was seen within and surrounding SNAs.DiscussionM30-positive SNAs traced through serial sections were significantly more likely to be syncytial knots or sectioning artefacts than bridges. Nuclei within SNAs showed signs consistent with degeneration; however, this is unlikely to be an apoptotic process. There are few changes in configuration of cytoskeletal proteins around SNAs.ConclusionsThese data suggest that the biogenesis and functional significance of SNAs still require resolution.

Background Aggregations of nuclei within the syncytiotrophoblast cell layer of the placenta are increased in pre-eclampsia and intrauterine growth restriction (IUGR). Syncytial nuclear aggregates (SNAs) are described using various terms including: knots, sprouts and intervillous bridges. The authors aimed to investigate the structure of SNAs. Methods Placental villous tissue was fi xed and wax-embedded (n=7). Serial sections were stained with H&E. SNAs were defi ned as close groups of 10+ nuclei which protrude from, but do not touch another villus and intervillous bridges as groups of 10+ nuclei which touched another villus. The authors investigated the relative proportion of each. Apoptosis was assessed by staining for cytokeratin M30 and TUNEL. Cytoskeletal proteins including cytokeratin, α-, β-and γ-actin, α-and β-tubulin were detected by immunofl uorescence. Results 52.3% SNAs were intervillous bridges (IQR 48.8-54.4%), 18.6% were syncytial knots (14.1-35.1%) and 21.2% were other structures (12.6-33.0%). Between 73.5% and 80% of SNAs do not contain apoptotic nuclei, as tested by M30 and TUNEL respectively. Electron microscopy showed cytoskeletal proteins are found close to and sometimes encircling SNAs. Immunofl uorescence showed cytokeratin to be associated with SNAs. α-tubulin, β-tubulin, β-actin and γ-actin were found in syncytiotrophoblast but not SNAs. Conclusion The majority of SNAs are intervillous bridges. Cytoskeletal proteins were found in and close to SNAs suggesting a role in nuclear aggregation. Fewer SNAs than expected contained dying fragments of trophoblast. Further research is needed to understand how nuclei move into SNAs and why they are increased in placental pathologies such as IUGR.