Human placental syncytiotrophoblast cells express markers of cellular senescence, p16 Sections of normal human post-partum, third trimester, placenta were evaluated by immunofluorescence co-staining for senescence regulators (A) p16 Green label: the syncytiotrophoblast marker βHCG, red label: p16 Blue label: DAPI nuclear stain. Arrowheads indicate positively stained syncytiotrophoblast cells βHCG+/p16+ (A) Scale bar, 50 µm. 