## 4 Discussion  
Our image analysis revealed that NIR-PBM induced structural changes in the pulp-dentin interface. We could observe a PBM-activated population of odontoblasts making a stack of odontoblasts (Figure 3). The odontoblast becomes increasingly crowded at the pulp-dentin interface when the dentin layer forms [20]. The dentin deposition after tooth structural completion is called the deposition of secondary dentin, which is in many cases related to tooth eruption or irritation. This secondary dentin deposition is the process of recovery from the diseased states, but the process is too slow (\(\sim\)0.4 \(\upmu\)m/day) to reach full recovery when necessary [21]. Therefore, if secondary dentin formation is accelerated by external physical cues, the modality may help protect teeth from deteriorating. External biophysical cues may be kinds of irritation. Such external stimulation can induce the enhancement of the activity of odontoblasts by up to 50% or more, higher than that  
Figure 5: H&E staining images of C-plane sectioned teeth, as shown in Figure 2. (**a–d**) are for Control, and (**e–h**) are for PBM-treated teeth. (**b–d**) are enlargements of the inserted square areas in (**a**), and (**f–h**) are enlargements of the inserted square areas in (**e**). The locations in images (**b,c,f,g**) represent the oblique PDL, and locations in images (**d,h**) represent the apical PDL. Photobiomodulation resulted in the microscopic appearance of a higher density of fibrous connective tissue within the periodontal ligament compared to the Control. of odontoblasts of fully matured, healthy teeth [22]. Dentinal hypersensitivity [23, 24], commonly associated with dental procedures such as gingival recession, caries removal, and prosthetic placement, is a prevalent concern which underscores the need for advanced treatment strategies in dentistry [25]. Despite maintaining normal tooth morphology, patients frequently experience discomfort due to exposed tooth roots and changes in dentin [24]. Traditional treatments, ranging from non-invasive desensitizers to invasive procedures like root canal treatment, often do not fully resolve these symptoms. Our results suggest that NIR PBM is one of the regenerative technologies which can be used over conventional inert materials [26, 27, 28]. Our findings also highlight PBM's effect on the fibrous connective tissue of the periodontal ligament and demonstrate active proliferation and densification, indicating the comprehensive benefits of PBM on dental health. This is consistent with the broader goal of regenerative dentistry to harness the potential of stem cells and biomaterials to create an environment conducive to innate healing and tissue repair. The impact of PBM goes beyond hypersensitivity and plays a pivotal role in the overall health of dental tissues, including dentin and periodontal ligament [29, 30, 31]. The continuous remodeling of dentin, characterized by an organic matrix within a permeable tubular structure and an apatite crystalline structure, was reported to be greatly enhanced by PBM [29, 32, 33]. An activated odontoblast might be from the dental pulp stem cell differentiation induced by PBM. The activation of stem cells was also confirmed by the reduction in the pulp chamber volume ratio (Figure 2) and by the increased vasculature in the odontoblast layer (Figure 4). The differentiation pathway into odontoblasts is mediated by the activation of transforming growth factor-beta 1 (TGF-b1) in pulp tissue. This activation cascade may be initiated by reactive oxygen species (ROS), which originate from the mitochondrial electron transport chain system triggered by PBM [34]. The underlying mechanism of PBM-mediated dentinogenesis through stem cell differentiation was further demonstrated in studies using PBM pulsed wave [18, 34, 35], which demonstrated superior ROS generation compared to their continuous counterparts. The exact mechanism of pulp stem cell differentiation by PBM in vivo should be elucidated in the future.