Activation of aSMA expressing perivascular cells during reactionary dentinogenesis I. Vidovic-Zdrilic, A. Vijaykumar & M. Mina Departments of Craniofacial Sciences, Division of Pediatric Dentistry, School of Dental Medicine, University of Connecticut Health Center, Farmington, CT, USA Abstract Vidovic-Zdrilic I, Vijaykumar A, Mina M. Activation of aSMA expressing perivascular cells during reactionary dentinogenesis. International Endodontic Journal. Aim To examine the contribution of perivascular cells expressing aSMA to reactionary dentinogenesis. Methodology An inducible, Cre-loxP in vivo fate- mapping approach was used to examine the contribu- tion of the descendants of cells expressing the aSMA- CreERT2 transgene to reactionary dentinogenesis in mice molars. Reactionary dentinogenesis was induced by experimental mild injury to dentine without pulp exposure. The Student’s t test was used to determine statistical significance at *P ≤ 0.05. Results The lineage tracing experiments revealed that mild injury to dentine first led to activation of aSMA-tdTomato + cells in the entire pulp chamber. The percentage of areas occupied by aSMA-tdTo- mato + in injured (7.5 Æ 0.7%) teeth were signifi- cantly higher than in teeth without injury (2 Æ 0.5%). After their activation, aSMA-tdTomato + cells migrated towards the site of injury, gave rise to pulp cells and a few odontoblasts that became inte- grated into the existing odontoblast layer expressing Col2.3-GFP and Dspp. Conclusion Mild insult to dentine activated perivascular aSMA-tdTomato + cells giving rise to pulp cells as well as a few odontoblasts that were inte- grated into the pre-existing odontoblast layer. Keywords: odontoblasts, perivascular cells, pulp biology, reactionary dentine, aSMA. Received 5 January 2018; accepted 6 July 2018 Introduction Odontoblasts are exclusively dentine-producing cells. These highly specialized tall columnar cells are located at the periphery of the dental pulp and differ- entiate from neural crest-derived dental papilla cells at the late bell stage of tooth development (Kawa- shima & Okiji 2016). The differentiation of the dental papilla into odontoblasts is dependent on signals and growth factors derived from the inner dental epithe- lium (Balic & Thesleff 2015). Available evidence suggests the similar signalling pathways including the local release of signalling molecules from the damaged/demineralized dentine are involved in the initiation of reparative and reac- tionary dentinogenesis (Smith et al. 1995, 2016, Couve et al. 2013, Bleicher 2014, Kawashima & Okiji 2016). It is well established that in reparative dentinogenesis, these signalling molecules regulate the generation of odontoblasts-like cells from residents MSCs. On the other hand, in reactionary dentinogene- sis, it is thought that increase in the synthetic and secretory activity of odontoblasts results from interac- tions between these signalling molecules with pre-existing odontoblasts without the involvement of resident MSCs (Smith et al. 1995, 2016). After differentiation, odontoblasts secrete unminer- alized predentine, a type I collagen-rich matrix consid- ered to be similar to the osteoid in bone (Bleicher 2014). Predentine mineralizes at the mineralization Correspondence: Mina Mina, Division of Pediatric Dentistry, Department of Craniofacial Sciences, School of Dental Medi- cine, University of Connecticut Health Center, Farmington, 06030 CT, USA (Tel.: (860) 679-4081; Fax: (860) 679- 4078; e-mail: Mina@uchc.edu). International Endodontic Journal © 2018 International Endodontic Journal. Published by John Wiley & Sons Ltd doi:10.1111/iej.12983 1