Osteopontin deficiency disturbs dentin bridge formation after direct pulp capping with mineral trioxide aggregate

After direct pulp capping with mineral trioxide aggregate (MTA), osteopontin (OPN) deposition occurs beneath the exposed pulp before the differentiation of odontoblast-like cells. This study aims to determine whether OPN is directly required for dentin bridge formation following direct pulp capping.

Pulp exposures were made on the occlusal surface of maxillary first molars of five to six week-old, wild-type (WT) and Opn knockout (KO) mice. The cavity was filled with MTA and then with glass ionomer cement. One to 28 days (PODs 1-28), specimens were subjected to immunohistochemistry for evaluating nestin, OPN, Ki67, dentin matrix protein (DMP)-1, F4/80, and CD206; and quantitative real-time polymerase-chain reaction was performed for evaluating nestin (Nes), dentin sialophosphoprotein (Dspp), Dmp-1, and Opn mRNA.

In WT mice, OPN was deposited just beneath the exposed pulp after odontoblast degeneration on POD 3, followed by an increase in Nes mRNA expression and CD206 immunoreactivity on POD 5. Nestin-positive odontoblast-like cells aligned on POD 14, resulting in dentin bridge formation on POD 28. Conversely, the arrangement of nestin-positive odontoblast-like cells was disrupted in Opn KO mice, with decreased expression levels of Nes mRNA, Dspp mRNA, and CD206-immunoreactivity on PODs 3-7. Eventually, dentin bridge formation was suppressed, resulting in pulp necrosis on POD 28.

Following direct pulp capping with MTA, Opn KO mice exhibited impaired arrangement of nestin-positive odontoblast-like cells and failed to form a dentin bridge, indicating that OPN contributes remarkably to these responses.