###### Abstract  
This study aimed to evaluate the effect of leukocyte and platelet-rich plasma (L-PRP) on the pulpal healing process following immediate and intentionally delayed tooth replantation in mice. After the maxillary first molars of 3-week-old mice were extracted, the teeth were immersed for 1 min [immediate reimplantation (IR)] or 30 min [intentionally delayed reimplantation (IDR)] in phosphate-buffered saline (PBS) solution. The alveolar socket was filled with or without 1.5 uL of L-PRP [experimental or control groups (EG or CG)] followed by tooth replantation. Samples were collected from day 1 to week 4 after the operation, processed for histology, and evaluated by immunohistochemistry for Nestin and Ki-67 expression. Quantitative analysis revealed positive Nestin staining during pulpal healing in the EG at week 1 following IR and week 2 following IDR. Hard tissue deposition was significantly increased in the EG after IR at week 2. Cell proliferation was higher in the EG compared with that in the CG at week 1 and significantly decreased in the coronal pulp of the EG after the IDR at week 2. Our data suggest that treatment with L-PRP may have a positive effect on pulpal healing, even in teeth replanted after an extended extra-oral period. cel proliferation; dental pulp; immunohistochemistry; platelet-rich plasma; tooth replantation; mice 20241411358 10390/  
properties and high platelet concentration above physiological levels [5; 6]. In addition, PRP has been used in dentistry to promote bone augmentation following third molar surgeries, the management of endo-perio lesions, placement of dental implants, treatment of temporomandibular joint lesions, and as a scaffold for regenerative endodontic treatments (RETs) [7; 8; 9; 10; 11]. PRP may be divided into leukocyte-PRP (L-PRP) and pure-PRP (P-PRP) depending on the number of leukocytes relative to autologous blood [12]. Recently, the concentration and composition of leukocytes have been analyzed because they influence the biological properties of PRP through their immunological and antibacterial properties. They are also required to promote wound healing and regulate local factors [13]. Furthermore, the antibacterial properties of L-PRP induced by the presence of leukocytes are important for its application in tissue healing in areas of high bacterial concentrations, such as the oral cavity. Although there is compelling evidence for the use of L-PRP for bone and periodontal tissue regeneration, there are no reports on the effect of leukocytes on PRP under pulpal pathological conditions [14; 15]. Thus, the effect of L-PRP after application to the injured pulpal tissue must be carefully evaluated. Oral trauma encompasses a series of injuries, including crown and/or root fractures, luxation, subluxation, concussion, and avulsion of either primary or permanent dentition. In the case of tooth avulsions, the nerve and vascular supply are completely disrupted, leading to ischemia in the pulpal tissue. The outcome of such an injury is either total pulp revascularization or the development of partial or total pulpal necrosis, depending on the presence or absence of microorganisms at the injury site [16; 17]. Several factors may inhibit pulpal healing in avulsed permanent teeth after replantation, such as bacterial invasion of ischemic pulp segments. Previous studies have shown that the size of the apical foramen should be considered in the outcome of replantation, transplantation, or RETs [18; 19; 20]. Similarly, the effect of occlusal forces following the replantation of avulsed teeth and the extra-oral time before replantation in the alveolar socket can significantly affect pulpal prognosis [21; 22; 23]. The latter is particularly important because a longer extra-oral period worsens the prognosis of the pulpal tissue and periodontal ligament. To date, clinical interventions for avulsed teeth have primarily focused on the viability of the periodontal ligament, with the dental pulp being removed by endodontic treatments to limit bacterial contamination [24].