Osteoblast migration in vertebrate bone.

Bone formation, for example during bone remodelling or fracture repair, requires mature osteoblasts to deposit bone with remarkable spatial precision. As osteoblast precursors derive either from circulation or resident stem cell pools, they and their progeny are required to migrate within the three-dimensional bone space and to navigate to their destination, i.e. to the site of bone formation. An understanding of this process is emerging based on in vitro and in vivo studies of several vertebrate species. Receptors on the osteoblast surface mediate cell adhesion and polarization, which induces osteoblast migration. Osteoblast migration is then facilitated along gradients of chemoattractants. The latter are secreted or released proteolytically by several cell types interacting with osteoblasts, including osteoclasts and vascular endothelial cells. The positions of these cellular sources of chemoattractants in relation to the position of the osteoblasts provide the migrating osteoblasts with tracks to their destination, and osteoblasts possess the means to follow a track marked by multiple chemoattractant gradients. In addition to chemotactic cues, osteoblasts sense other classes of signals and utilize them as landmarks for navigation. The composition of the osseous surface guides adhesion and hence migration efficiency and can also provide steering through haptotaxis. Further, it is likely that signals received from surface interactions modulate chemotaxis. Besides the nature of the surface, mechanical signals such as fluid flow may also serve as navigation signals for osteoblasts. Alterations in osteoblast migration and navigation might play a role in metabolic bone diseases such as osteoporosis.