Osteoblasts produce osteoid, the collagen matrix that makes up bone . Osteoblasts are derived from mesenchymal stem cells that differentiate into osteochondro progenitor cells (Figure ). These are often classified as uncommitted osteoblasts. Osteochondro progenitor cells later differentiate into committed preosteoblast cells (also known as precursor osteoblasts). During bone remodeling, signaling factors activate the proliferation and migration of preosteoblasts to the resorption site, where they differentiate into osteoblast cells. When osteoblasts become trapped in the osteoid collagen matrix, they differentiate into osteocytes. The osteoblasts that remain after bone formation become bone lining cells or undergo apoptosis.