## 1 Introduction  
Recent advances in dentistry have placed increasing emphasis on the transition from the use of conventional inert materials to regenerative technologies [1]. Common dental conditions, such as adult caries and pulp trauma, often require endodontic treatment, in which the damaged pulp is replaced with a composite material. However, these artificial substitutes are often insufficient to restore the full function of the tissue, mainly due to the lack of vascular and neural elements [2]. The rapidly growing field of regenerative medicine is exploring the potential of dental stem cells, known for their multifaceted differentiation capacity, including osteogenesis, contingogenesis, adipogenesis, and chondrogenesis [3; 4; 5; 6; 7]. Biomaterials acting as scaffolds create a cellular microenvironment that attracts exogenous or endogenous stem cells, promoting innate healing mechanisms that lead to cell proliferation and tissue repair. Although introducing exogenously cultured stem cells using these biomaterials carries a risk of contamination, utilizing endogenous stem cells reduces immune response complications and avoids pitfalls associated with cell transplantation [8]. Nonetheless, achieving the necessary angiogenesis and minimizing off-target effects remains a significant challenge.  
To address these challenges, our research focuses on photobiomodulation (PBM) using near-infrared (NIR), recognized for its tissue regenerative capabilities [9]. NIR light achieves deeper tissue penetration due to its unique properties of reduced scattering and absorption by water molecules, thereby affecting key biological processes such as stem cell proliferation and differentiation [10; 11; 12; 13; 14; 15]. However, the direct impact of NIR-mediated PBM on dentin and fibrous connective tissue generation, especially through innate stem cell stimulation, has not yet been fully investigated. Resident stem cells are in specific niches in dental tissues. These niches are highly dynamic and composed of multiple cell types. A stem cell niche typically contains solid components, including cells and extracellular matrix (ECM), and soluble factors, such as growth factors [16]. Among the possible dental niches, we focused on the dental pulp niche with vascularization through the blood supply from the apical foramen, containing pulp stem cells that differentiate into odontoblasts and bioactive molecules that release to initiate cellular events, leading to the regeneration of pulp and dentin, including the fibrous connective tissue of periodontal ligaments [17]. Such stem cells can be physiologically activated by external cues (physical and chemical cues), and NIR PBM can be one of them.  
To establish a fundamental understanding of these effects, our study initially focused on applying NIR light to normal, healthy dental tissue in a beagle in vivo model. This approach is an important step for establishing foundational knowledge before introducing the complexities associated with diseased tissue in the future. This systematic approach allows us to carefully evaluate the efficacy and safety of NIR PBM in a controlled environment before applying these findings to more complex scenarios involving diseased or damaged dental tissues. Therefore, the purpose of this study is to reveal the potential of NIR PBM in dental tissue regeneration, focusing on whether it can activate regeneration of dentin or periodontal ligament. We utilized the optimized PBM condition found in previous in vitro experiments and analyzed the changes in tissue structures using micro-computed tomography, hematoxylin and eosin staining, and scanning electron microscopy. This fundamental study can provide basic information for the use of PBM in dentistry.