INTRODUCTION: Dental mesenchymal stem cells (dMSCs) might differentiate into 
odontoblast-like cells and form mineralized nodules. In the current study, we 
investigated the effects of senescence on odontogenic differentiation of dMSCs.
METHODS: dMSCs were serially subcultured until senescence. Telomere lengths and 
telomerase activities were determined by quantitative polymerase chain reaction. 
Expression of genes involved in cell proliferation and differentiation, eg, 
Bmi-1, p16(INK4A), osteocalcin (OC), dentin sialoprotein (DSP), bone 
sialoprotein (BSP), and dentin matrix protein-1 (DMP-1) were assayed by Western 
blotting and quantitative reverse transcription polymerase chain reaction. 
Exogenous Bmi-1 was expressed in dMSCs by using retroviral vectors. Odontogenic 
differentiation was assayed by alkaline phosphatase activity.
RESULTS: Subculture-induced replicative senescence of dMSCs led to reduced 
expression of Bmi-1, OC, DSP, and BSP compared with rapidly proliferating cells, 
whereas p16(INK4A) level increased. The cells exhibited progressive loss of 
telomeric DNA during subculture, presumably as a result of lack of telomerase 
activity. Bmi-1 transduction did not affect proliferation of cells but enhanced 
the expression of OC and DSP in the late passage cultures. Bmi-1-transduced 
cells also demonstrated enhanced alkaline phosphatase activity and mineralized 
nodule formation.
CONCLUSIONS: These results indicate that dMSCs lose their odontogenic 
differentiation potential during senescence, in part by reduced Bmi-1 
expression.