**2.2 Bone cells regulation**

As shown above, osteoblast, osteoclast, and osteocytes are predominantly controlling the bone turnover. Thus, it is essential to understand the different steps of these cells differentiation and how biological factors are regulating their activity.

## *2.2.1 Regulation of osteoblasts*

Osteoblasts are mononucleated cells that derive from mesenchymal stem cells (MSCs), which originate in the bone marrow. Those cells are multipotent stromal cells that have the ability to differentiate into a multitude of different cells thanks to their gene expression program. Among these cells, we can find osteoblasts, fibroblasts, adipocytes, and chondrocytes [16, 17]. The first step of osteoblastogenesis is regulated by Wingless-int (Wnt), specifically the Wnt10b protein coding gene. In [18], the authors proved that Wnt plays a key role in the stimulation of MSCs differentiation into osteo/chondro progenitors and the inhibition of preadipocyte commitment. Several other biological factors are involved in MSC differentiation into active osteoblast. Among which we can cite Runt-related transcriptor factor (Runx2), Osterix (Osc), Alkaline Phosphatase (ALP), type 1 parathyroid receptor (PTH1R), Osteopontin, and sialoprotein 2 (BSP II) (**Figure 1**). The process is described in details in [19].

Based on these information, we can deduct that osteoblastogenesis is a couple of complex biological interactions involving various factors where a simple alteration could destroy the whole system. For instance, Runx2, plays an important role in skeletal development. However, high amount of this transcription factor could inhibit the osteoblast maturation process and lead consequently to osteopenia disease [20]. Concurrently, other factors such as bone morphogenetic proteins (BMPs), transforming growth factor beta (TGFβ), parathyroid hormone (PTH), and fibroblast growth factor (FGF) have been judged as critical regulator factors of Runx2 [21]. Aside from Runx2, Wnt/β-catenin and BMPs have also a huge impact on bone formation and precisely on osteoblastogenesis regulation [22–24].

#### *2.2.2 Regulation of osteoclasts*

Osteoclasts are the only cells able to degrade the bone [2, 3]. They derive from hematopoietic stem cells, which are produced in the bone marrow. By dint of the macrophage colony stimulating factor (M-CSF), these stem cells are differentiated into macrophages and osteoclast progenitors. M-CSF is known as one of the main stimulator factors of osteoclastogenesis as it stimulates preosteoclasts proliferation and their expression of receptor activator of NFkB (RANK), that represents a key

**Figure 1.** *Schematic representation of oteoblastogenesis process.*

factor in osteoclasts maturation. Indeed, additionally to its principal function of bone synthesize, osteoblasts have been reported to regulate osteoclastogenesis through its secretion of the M-CSF and the receptor activator of NFkB Ligand (RANKL). According to the literature, RANKL is also secreted by osteocytes [25]. This ligand expression is stimulated by various types of hormones and factors such as PTH, prostaglandin E2 (PGE2) and 1.25-dihydroxyvitamin D3 (1*:*25ð Þ *OH* <sup>2</sup>*D*3). Indeed, RANKL's interaction with its receptor RANK is mandatory for osteoclasts' differentiation, maturation and activation as it induces the recruitment of many hormones inside the preosteoclasts. RANK/RANKL binding simulates the recruitment of the tumor necrosis factor receptor associated factor 6 (TRAF6), which triggers a succession of interactions leading to osteoclastogenesis transcription. During osteoclastogenesis the osteoclast progenitors get differentiated into preosteoclasts. Then as a result of preosteoclasts fusion, mature multinucleated osteoclasts are created, where the nuclei's number can variate between four to twenty nuclei [26].
