Fig. 1.

Stages of osteoblast differentiation. Osteoblasts are derived from mesenchymal stem cells. Wingless (WNT) and bone morphogenic protein (BMP) pathways promote their differentiation to mesenchymal stromal cells, in conjuction with Runx2, insulin-like growth factor (IGF) and parathyroid hormone (PTH). Activation of BMP signaling promotes Runx2 expression in stromal cells, leading to further differentiation to osteoprogenitors. Runx2 subsequently induces expression of the transcription factor osterix, with further differentiation to a mature osteoblast. These ultimately become embedded in bone matrix as terminally differentiated osteocytes