2. Nuclear 3D architecture: syncytial knots have distinct 3D organisations in PE
Distinct nuclear aggregate shapes on villi were identified (Figures 1E - H). The term knots is often used to describe all nuclear aggregates seen in villi. However, nuclear aggregates can be subdivided in (apoptotic) knots, bridges and sprouts. Knots, mostly found towards term, are condensed nuclear aggregates protruding slightly from the villous surface36. Bridges are aggregated nuclei that connect two villi and are thought to provide structural support. Sprouts, mainly common in young placentas, are the initiating points for new developing villi37. Sprouts (Figure 1F), bridges (Figure 1G), and knots (Figure1E and 1H, Figure S3) could be easily distinguished from one another with 3D information. Prominent nuclear accumulations in term placenta were frequently bridges. Although bridges can be distinguished on 2D slices (Figure 1A), their interpretation is imaging-depth dependent and they are easily misinterpreted as knots without additional 3D information (Figure S3).
EO-PE placentas were characterized by presence of wave-like syncytial knots (WLKs) (Figure 1E). Although WLKs were most dominantly present on stem villi (Figure 1H), this pattern was also observed on intermediate villi (Figure 3A lower red arrow), and in villi from hypertensive pregnancy (Figure S1D). LO-PE placenta had numerous large, surface-extending knots (Figure 1H). Knots in term control placenta were frequently small, regional, mildly protruding (Figure 2E). Additionally, knots in term control placenta were observed in close relation with so-called vasculosyncytial membranes (Figure S3), which represent major areas of feto-maternal exchange38.