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Direct pulp capping is an effective treatment for preserving dental pulp against carious or traumatic pulp exposure via the formation of protective reparative dentin by odontoblast-like cells. Reparative dentin formation can be stimulated by several signaling molecules; therefore, we investigated the effects of secreted frizzled-related protein (SFRP) 1 that was reported to be strongly expressed in odontoblasts of newborn molar tooth germs on odontoblastic differentiation and reparative dentin formation. In developing rat incisors, cells in the dental pulp, cervical loop, and inner enamel epithelium, as well as ameloblasts and preodontoblasts, weakly expressed Sfrp1; however, Sfrp1 was strongly expressed in mature odontoblasts. Human dental pulp cells (hDPCs) showed stronger expression of SFRP1 compared with periodontal ligament cells and gingival cells. SFRP1 knockdown in hDPCs abolished calcium chloride-induced mineralized nodule formation and odontoblast-related gene expression and decreased BMP-2 gene expression. Conversely, SFRP1 stimulation enhanced nodule formation and expression of BMP-2. Direct pulp capping treatment with SFRP1 induced the formation of a considerable amount of reparative dentin that has a structure similar to primary dentin. Our results indicate that SFRP1 is crucial for dentinogenesis and is important in promoting reparative dentin formation in response to injury.

We aimed to generate periodontal ligament (PDL) tissue-like structures from a multipotent human PDL cell line using three-dimensional (3D) bioprinting technology and to incorporate these structures with bioactive core materials to develop a new biohybrid implant system. After 3D bioprinting, single-cell spheroids were able to form 3D tubular structures (3DTBs). We established three types of complexes using 3DTBs and different core materials: 3DTB-titanium core (TIC), 3DTB-hydroxyapatite core (HAC), and 3DTB without a core material (WOC). The expressions of PDL-, angiogenesis-, cementum-, and bone-related genes were significantly increased in the three complexes compared with monolayer-cultured cells. Abundant collagen fibers and cells positive for the above markers were confirmed in the three complexes. However, more positive cells were detected in HAC than in WOC or TIC. The present results suggest that 3D-bioprinted structures and hydroxyapatite core materials can function similarly to the PDL and may be useful for the development of a new biohybrid implant system.