Reparative dentinogenesis is a regenerative process that leads to dentin bridge formation and the maintenance of the dental pulp vitality. Reparative dentinogenesis occurs after intense injuries that lead to odontoblast death and involves the recruitment and proliferation of progenitor cells to the site of injury and their differentiation ito a second-generation of odontoblasts or odontoblast-like cells. At the present time the origin/identity of the progenitor cells and signaling pathways involved in reparative dentinogenesis remain elusive. The goal of this proposal is to examine the roles of perivascular cells in the pulp expressing alpha smooth muscle actin-GFP (SMA) in reparative dentinogenesis and examine the effects of FGF signaling on this population. We will use an inducible Cre-loxP recombination system (SMACreERT2 x Ai9 that is Rosa26- tdTomato) for cell lineage-tracing experiments in vivo and in vitro. Two specific aims have been proposed. Experiments in Aim 1 are designed to test the hypothesis that SMACreERT2;Ai9 expressing cells in the dental pulp of adult teeth give rise to odontoblast-like cells during reparative dentinogenesis. The contribution of SMACreERT2;Ai9 expressing cells in dental pulp to reparative dentinogenesis will be examined in vivo after experimental pulp exposure by cell lineage-tracing experiments in SMACreERT2;Ai9 double transgenic mice. The dentinogenic potential of the SMACreERT2;Ai9+ population will be confirmed in primary pulp cultures. Experiments in Aim2 are designed to test the hypothesis that FGF signaling has stimulatory effects on reparative dentinogenesis that are mediated by increased numbers of SMACreERT2;Ai9 expressing progenitors in dental pulp progressing into the dentinogenic lineage. The effects of FGF2 on proliferation and differentiation of SMACreERT2;Ai9 expressing cells will be examined in vivo after experimental pulp exposure and application of FGF2-soaked beads. The effects of exogenous FGF2 on proliferation and odontoblast and osteoblast differentiation of FACS purified SMACreERT2;Ai9+ and SMACreERT2;Ai9- populations will be examined in vitro. Ultimately, the results of this study will provide a better understanding of the identity of the progenitors involved in reparative dentinogenesis and the roles of FGF signaling in this process. These findings will be critical for the development of improved treatments for vital pulp therapy and improved dentin regeneration.