Human placental syncytiotrophoblast cells express markers of cellular senescence, p21. Sections of normal human post-partum, third trimester, placenta were evaluated by immunofluorescence co-staining for senescence regulators (B) p21. Green label: the syncytiotrophoblast marker βHCG, red label: p21. Blue label: DAPI nuclear stain. Arrowheads indicate positively stained syncytiotrophoblast cells , βHCG+/p21+ (B). Scale bar, 50 µm. 