Fig. 1.

Schematic illustration of bone remodeling. 1 Bone marrow-derived mononuclear cells (BMMs, osteoclast precursors) migrate on osteoblast. 2 In healthy bone, osteoclast precursors stay on the osteoblast layer and are named osteal macrophages (OsteoMacs) or cell cycle-arrested quiescent osteoclast precursors (QuOP). 3 A microcrack in the bone induces osteocyte apoptosis following which, osteal (or QuOP) and BM-derived macrophages migrate under the osteoblast layer. The macrophages may differentiate into preosteoclast (pre-OC). 4 The pre-OCs fuse with each other under the osteoblast layer. 5 Multinuclear osteoclasts erode the bone matrix containing dead osteocytes. 6 Osteogenic osteoblasts proliferate and cover the bone resorption area. 7 Bone is synthesized. 8 Some osteoblasts differentiate into osteocytes