The periodontium consists of periodontal ligament, cementum, and alveolar bone. The tooth anchorage uses a fully specialized connective tissue named periodontal ligament (PDL) that connects the tooth root to the alveolar bone (Shimono et al., 2003). Heterogenous cells (fibroblasts, PDL stem/progenitor cells (PDLSCs), epithelial cell rests of Malassez, and endothelial cells), blood vessels, nerves, fibers, and extracellular matrix (ECM) form the PDL tissue (Beertsen et al., 1997, Nanci, 2017, Nanci and Bosshardt, 2006).

Periodontitis is a highly prevalent, chronic inflammatory disease of the oral cavity associated with dental plaque biofilm accumulation, which results in the destruction of periodontal compartments and eventually leads to tooth loss (Atarbashi-Moghadam et al., 2022, Kwon et al., 2021). Most periodontal treatment modalities result in gingival epithelial down-growth; therefore, true periodontal attachment is not achieved. To overcome this problem, the concept of guided tissue regeneration was stated (Karring et al., 1980). PDL cells are the origin cells of this technique, and these cells slowly migrate over the root (Nyman et al., 1980). Therefore, The process of PDL regeneration after its destruction can be extremely challenging, so researchers have focused on this field to enhance PDL regenerative therapies (H. Maeda, Sakabe, et al., 2011). Many efforts have been made to achieve the regeneration of periodontium using regenerative medicine and tissue engineering approaches (Liang et al., 2020). Interestingly, some researchers achieved this goal during different experiments with different aims; Palma et al. found intra-canal cellular cementum and periodontal ligament during the endodontic regeneration process (Palma et al., 2017). Martins et al. showed for the first time the histological evidence of a specialized remodeling compartment in dental hard tissues (bone and cementum) (Martins et al., 2021).

The key to PDL regeneration is to safeguard the PDLSCs, which are multipotential cells and can differentiate into osteoblasts, cementoblasts, and fibroblasts (H. Maeda, Sakabe, et al., 2011). PDLSCs can be divided into multiple subgroups based on the surface markers like CD51/CD140α+, STRO1/CD146+, CD271+, SSEA4+, CD105+CD45- CD34-, and CD106+ (Rad et al., 2022).

The regeneration process of alveolar bone (Nokhbatolfoghahaei et al., 2022), cementum (Wu et al., 2022), and PDL (Nagayasu-Tanaka et al., 2023) can be promoted using growth factors. Although the mechanisms of PDLSC differentiation are not fully understood yet, morphogens and growth factors can be beneficial and efficient for stimulating these cells (Kao et al., 2009). Fibroblast growth factor 2 (FGF-2, also known as basic FGF) can cause growth induction through the mitogen-activated protein kinase pathway. At the same time, it maintains the differentiation potential of mesenchymal stem cells (Yu et al., 2007). It has been shown that FGF-2 can promote the expression of early ligamentogenic markers (scleraxis) (Hyun et al., 2017). Moreover, FGF-2 can suppress the osteogenic differentiation markers (RUNX2, OSTERIX, OSF-2, COL1, COL3, BSP) and has an antagonist effect on the cementogenic and osteogenic stimulation caused by BMP-2 and BMP-4 (Hyun et al., 2017, Kono et al., 2013). Transforming growth factors β (TGF-β) is a family of growth factors whose effect on tenogenic differentiation is proven (Barsby & Guest, 2013; T. Maeda, Sakabe, et al., 2011; Zhang et al., 2018). The TGF-β signaling pathway is based on the carboxy-terminal phosphorylation of smad2 and smad3 molecules (Nakao et al., 1997). There are disagreements regarding the effect of TGF-β on the expression of scleraxis. TGF-β1 induces the gene expression of tenomodulin, alpha-smooth muscle actin (ACTA2), Collagen I (COL1), and fibrillin-1 (FNB1) and reduces the expression of scleraxis, which results in teno/ligamentogenic differentiation (Kono et al., 2013). On the other hand, many studies show the role of TGF-β3 in the chondrogenic and teno/ligamentogenic differentiation of the mesenchymal stem cells (MSCs) (Choi et al., 2013, Juneja et al., 2013, Melzer et al., 2021, Moshaverinia et al., 2014, Roth et al., 2019). TGF-β3 can promote the expression of important tenogenic markers like scleraxis, decorin, biglycan, and tenomodulin in PDLSCs after 4 weeks (Moshaverinia et al., 2014), while it has been shown that TGF-β3 can also downregulate the expression of decorin and COL3A1 after 5 days of application (Roth et al., 2019).