Our study revealed the distribution of pulp stem cells in chronic pulpitis for the first time. During the pathological processes, pulp reactivity occurs in spontaneous attempts at protection and/or repair . Tissue breakdown elicited migration of an important number of stem cells to the lesion site. These cells were concentrated in several specific niches, being activated and utilized in the repair mechanism against dental damage . The balance between the inflammation and reparative processes would determine the extent of pulp inflammation and the viability of the affected tooth .
To date, the precise identity of pulp stem cells remains a challenge because of the lack of a single specific stem cell marker. Different immunoselection protocols to characterize these cells were based primarily on their high expression of the STRO-1 and CD146 antigen [1, 3, 8]. Based on these markers, we found that in normal pulp, STRO-1- and CD146-positive cells were restricted to the cell-rich zone and pulp core around blood vessels. This is consistent with previous studies, which provided strong evidence that the perivascular tissue represents the main stem cell niche in the adult pulp. They inferred that the vasculature could provide support for cell survival and differentiation, and thus long-term dentinogenesis in dentin-pulp complex [3, 11]. In addition, real-time PCR analysis revealed that the expression of OCT4 gene was lower in the inflamed pulp; however, the expressions of other embryonic and dentinogenic genes in the inflamed pulp were similar to those in the normal pulp. The results supported the fact that MSCs derived from inflamed human pulps preserve the full capability of proliferation and multipotent differentiation as compared with those from healthy pulps [6, 7].
Then we revealed that in chronic pulpitis, STRO-1- and CD146-positive cells mostly gathered in two specific niches, both adjacent to inflammatory lesions: one in the pulp core, and another in odontoblastic area. This was in agreement with the well-known participation of inflammation-induced chemoattraction in progenitor cell migration in animal models after dental injury [12, 13]. By the tritiated thymidine labeling method, Fitzgerald et al found that odontoblast replacement in the exposed pulp of the monkey’s tooth involved multiple DNA replications and migration of pulpal cells from the deeper pulp . Harichane et al further demonstrated that 48 hours after pulp exposure the PCNA-positive cells mobilized near the exposure site of the rat’s molar . These observations lend support to the idea that the inflammatory process could induce rapid recruitment of progenitor cells resident in the pulp stroma to the vicinity of the lesion.
In reparative dentinogenesis, a cascade of events involving cell division, cell migration, and cytodifferentiation must occur before secretion of the matrix can take place . During chronic pulpitis, STRO-1 labeling was present throughout the odontoblast layer. It means that the chronic inflammation leads not only to the migration of stem cells to the lesion but also accelerates their differentiation and maturation. On the one hand, it causes an increase of the number of active odontoblasts and initiation of reparative dentinogenesis. On the other hand, the stem cells exhaustion may lead to tissue senescence at later stage .
Predictably, we found little movement of STRO-1- and CD146-positive cells in the root apex, suggesting that it’s the supportive microenvironment; especially specific molecules that regulate how pulp stem cell populations participate in tissue maintenance and regeneration. The specific molecules that promote or inhibit pulp stem cell migration are yet not clear. Nevertheless, the presence of certain molecules is well established, and several functional studies suggest important regulatory pathways. Recently, much attention has been paid to SDF-1α and its unique receptor CXCR4 for the migration of adult stem cells to the site of injury, which contributes to the impaired tissue remodeling [18, 19]. In the present study, intense staining of SDF-1α and CXCR4 was observed in zone I and II, adjacent to inflammatory sites. The explanation for this result is that the inflammation responding to tissue damage could produce many factors, such as interleukin-1, tumor- necrosis factor-α, hypoxia-inducible factor-1, etc., which stimulate the expression of SDF-1α. And the increased numbers of CXCR4-positive cells in the inflamed pulp could be due to chemotactic attraction towards the sources of the chemokine [12, 20]. SDF-1 interacting with its G-protein coupled receptor CXCR4 to induce SDF-1/CXCR4 signaling has already been well documented . Furthermore, in our data immunofluorescent co-localization of STRO-1 and SDF-1α/CXCR4-positive cells confirmed the presence of stem cells in the inflammatory progression frontier, providing direct evidence of close relation between stem cells and SDF-1/CXCR4 signaling. Li et al further revealed that SDF-1/CXCR4 axis induced human dental pulp stem cell migration in vitro through FAK/PI3K/Akt and GSK3β/β-catenin pathways .
In conclusion, our findings illustrated the distribution of pulp stem cells in chronic pulpitis and indicated a positive correlation between the SDF-1α-CXCR4 axis and stem cells. Further understanding of functional activity of the stem cell niches and specific signaling in pathologic conditions may lead to the development of a new and less invasive strategy for treatment of pulpitis.