MKI67 and Alzheimer disease: Next, as senescent cells are characterized by a reduced capacity of proliferation and the emergence of markers involved in cell cycle arrest as p21 [41], we evaluated by immunofluorescence the presence of Ki67+ proliferating cells (Fig. 5D d1 and d3) and p21+ cells (Fig. 5D d2 and d4) in both groups of astrocytes (AD and controls).