Cellular senescence occurred widespread to multiple selective sites in the fetal tissues and organs of mice.

Cellular senescence protects multicellular organisms from tissue overgrowth including cancer, and contributes to tissue ageing. With stable cell cycle arrests, cellular senescence has been mostly studied in the adult tissues of mammals. In the present study, we report widespread cellular senescence within certain time windows of late-phase normal development of mouse embryos. Using in situ senescence-associated β-galactosidase (SA-β-gal) staining, we showed SA-β-gal activity in selected cell populations of the brain, stomach, interdigital webs, tail, ear, limbs and nasal mouth area on gestation day 14.5 of the mouse embryos. On day 18.5 of gestation, selected cells in the intestines and bone developmental areas showed SA-β-gal activity. The chondrocytes in ossification zones were significantly marked by the activities of SA-β-gal, p21, p15 and Hp1Y, suggesting activation of the cell cycle checkpoint by the p53 and Rb pathways, and development of senescence-associated heterochromatic foci. Throughout gestation days 14.5-18.5, the trophoblast cells in the labyrinth layer of the placentas also showed strong activities of SA-β-gal, p53 and p21. Increased expressions of p19, p16 and Rb of the p16/Rb pathway, and reduced expressions of Ki67 were also observed in the placentas. Taken together, the present findings suggest that cellular senescence represents an essential mechanism at multiple sites including the fetal bone forming zones and placenta during mammalian embryonic development, playing potential roles in the full embryonic development of tissue growth and organ formation.