## 4 Discussion  
This study for the first time evaluated the effectiveness of a topical treatment of L-PRP on the pulpal healing process using a mouse model for tooth replantation. The treatment with L-PRP induced a better healing of the pulpal tissue, showing a significantly positive effect in the IR model and a positive trend in the IDR model regarding the rate of newly formed hard tissue. Considering the lack of studies addressing the effects of L-PRP in the pulpal healing following exogenous tooth injuries, the results become important for understanding the implications of PCs as adjuvants in endodontic therapies, particularly in RETs. For example, the use of PRP as a scaffold in RET has been previously evaluated using a dog model, in which the teeth sustained apical periodontitis. PRP was placed at the enamel-cement junction after filling the canal space with blood induced from the periapical tissues. The histological evaluation showed no statistical differences between the PRP group and the conventional blood clot in terms of apical closure and newly formed tissues, including pulp-like tissue [31]. In this present study, the direct exposure of the still immature apical papilla to L-PRP did not significantly accelerate odontoblast-like cell differentiation between the groups at weeks 1 or 2 following IR or IDR. However, L-PRP induced a faster deposition of tertiary dentin, bone-like tissue, or a combination of both in the repaired pulp compared with the untreated replanted teeth at week 2 after the operation. This observation is supported by our findings regarding the proliferative activity in the dental pulp during the same observation period, when the number of proliferative cells decreased in the L-PRP group, particularly after IDR. We also found a positive trend in the number of proliferating cells in the L-PRP group at the early phases of the pulpal healing process after IDR. Because cell proliferation precedes cell differentiation, the presence of endogenous collagen and thrombin in the apical pulp tissue may facilitate the sustained release of growth factors available in L-PRP, thereby promoting an ideal environment for cell proliferation and differentiation [32]. To date, few studies have examined the mechanisms underlying the beneficial effects of PRP; however, an in vitro study demonstrated that treatment of human dental pulp cells (hDPCs) with PRP at various concentrations (5-20%) promoted cell migration, proliferation, and differentiation. Furthermore, PRP induced autophagic activity, thus promoting tissue regeneration [33]; however, further in vitro mechanistic studies are needed to understand the effects of PRP on tissue repair and regeneration. The presence of leukocytes remains controversial in PRP-related therapies. Although leukocytes may improve antibacterial properties to create a favorable environment for faster tissue repair/regeneration, a systematic review found insufficient evidence for this favorable effect. PRP preparations with or without leukocytes showed bacteriostatic effects, suggesting that L-PRP preparations may be useful as a prophylactic measure to reduce bacterial load and promote healing, rather than solely as a treatment [13]. Similarly, the concentration of leukocytes is another factor to consider for understanding the effects of L-PRP for clinical applications [12]. In this present study, white blood cell concentrations in the L-PRP preparations ranged from \(52\times 10^{2}/\mu\)L to \(138\times 10^{2}/\mu\)L. The variations in the concentration of these cells did not appear to be significantly related to the improvement of the pulpal healing process. In this context, a recent study from our group showed that L-PRP containing white blood cell concentrations ranging from \(21\times 10^{2}/\mu\)L to \(172\times 10^{2}/\mu\)L positively enhanced osseointegration after implant placement in mice [27].