Expression of α-Smooth Muscle Actin in Dental Pulp Wound Healing

Objectives: Alpha-smooth muscle actin (α-SMA) is a cytoskeletal protein that is localized in pericytes and smooth muscle cells in blood vessels, as well as some stem cells and precursor cells. This protein is also known as a marker of myofibroblasts, which are found in various tissues during tissue repair and regeneration. In this work, spatial and temporal expression of α-SMA was explored during reparative process of human dental pulps capped with Mineral trioxide aggregate (MTA) or Ca(OH)2. Methods: Clinically healthy pulps were mechanically exposed and capped with MTA (ProRoot White MTA, Dentsply) or Ca(OH)2. After 7-42 days, the teeth were extracted and processed for immunohistochemical analysis using antibodies against α-SMA and nestin (a neurogenic cytoskeletal protein expressed in odontoblasts). Results: In normal pulp, α-SMA-positive cells were localized along blood vessels, and nestin was detected in odontoblasts. There were marked changes in the distribution of α-SMA-positive cells during the cyto-differentiation and the formation of initial reparative dentin. Numerous α-SMA-positive fibroblasts (myofibroblasts) emerged at the exposure site in the cyto-differentiation stage, and later, the initial reparative dentin was lined with myofibroblasts, which were also reactive to nestin. In addition to myofibroblasts, the immunoreaction of α-SMA was detected in monocytic-like cells beneath the exposure site. In the advanced stage of reparative dentin formation of 42 days, α-SMA immunoreactivity was confined again along blood vessels. The reparative odontoblasts were strongly stained with nestin, and no staining of α-SMA was detected in those cells. Conclusion: The reparative odontoblasts induced by the direct pulp capping temporarily express α-SMA, a marker of smooth muscle differentiation. Supported by Grants-in-Aid for Scientific Research from JSPS.