*2.2.3 Regulation of osteocytes*

Osteocytes represent the highest amount of bone cells in the body (90–95%) [4]. They are the result of the last differentiation of the osteogenic lineage. Indeed, during bone gap filling, some osteoblasts get trapped into the osteoid and differentiate into osteocytes [27]. Some authors have distinguished between several stages of osteocytogenesis. Based on [28], it has been mentioned that the transition from osteoblasts into osteocytes is governed by seven stages: (i) osteoblast, (iii) osteoblastic osteocyte (Type 1 preosteocyte), (vi) osteoid osteocyte (Type 2 preosteocyte), (v) Type 3 preosteocyte, (vi) young osteocyte, and (vii) old osteocyte. Throughout the transition, a multitude of actions took place. Actually, during all these differential stages, the cell undergoes many morphological changes to become a cell with dendritic extensions [29, 30]. In contrary to MSC to osteoblast transition, the signaling mechanisms during the process governing osteoblast to osteocyte transition is poorly understood. However, some authors have recently found some regulating factors impacting the process such as PTH and insulin-like growth factor type 1 (IGF-1) as, according to [31], they enhance osteoblast-osteocyte differentiation. The osteocytes surround the blood vessels in a cylindrical way and are organized in parallel to the bone surface. They are interconnected between each other by means of their dendritic extensions that occupy tiny canals called canaliculi. These canaliculi permit the transduction of biochemical signals to other osteocytes and bone cells located in the surface [32, 33]. Thanks to their characteristics, osteocytes are the best known for their ability to sense the mechanical loadings and transmit the information to the other bone cells in such a way the bone remodeling process can start.
