Figure 3.

Schematic diagram of the osteoblastic stem cell system. This conceptual drawing illustrates the primary candidate populations of stem cells and transit cells thought to be associated with bone formation and remodeling. Vascular pericytes (green), Westen-Bainton cells (orange), type I or pre-osteoblasts (pink), secretory osteoblasts (maroon), osteocytes (brown), lining cells (purple), and adipocytes (yellow). Vascular pericytes may give rise to the Westen-Bainton cells. Pericytes and Westen-Bainton cells may contribute to the formation of pre-osteoblasts and also adipocytes. New osteoblast are added in the region immediately behind the advancing front of osteoclastic resorption. Secretory osteoblasts produce new bone matrix until they become quiescent on the surface of bone as a lining cells (purple) or become embedded in the matrix as osteocytes (brown), or die via apoptosis. Osteoclast formation is also illustrated. A fraction of the monocytes population in systemic circulation (blue) will become resident in the bone marrow space. Osteoclasts are formed by fusion of monocytes resident in bone marrow to form multinucleated functional units. The nuclei in active osteoclasts continue to be turned over as a result of nuclear loss and ongoing fusion events with new marrow-derived monocytes [103]. The black arrow indicates the direction of bone resorption by the osteoclastic front, followed by bone formation.