Coupling: The Influences of Immune and Bone Cells

Abstract

The skeleton is constantly renewed by a process termed bone remodeling, in which osteoclasts remove small quantities of bone, that are replaced by the subsequent action of osteoblasts (bone forming cells). Maintenance of skeletal structure requires that these two processes are matched, or “coupled.” Mechanisms have been proposed by which this may occur, but many of these are challenged by the knowledge that the activities of osteoclasts and osteoblasts occur on the same surface, but at different points in time. This chapter summarizes the range of paracrine and juxtacrine influences identified to date that may play a role in this process, and proposes a model of multifactorial dependency, where a range of factors from multiple cell types contribute to each stage of osteoblast differentiation. These factors may promote any of the following stages: mesenchymal stem cell recruitment, expansion, commitment, progression through differentiation, migration to the bone surface, matrix production and matrix mineralization, each being dependent on prior stages, and the factors that promote each stage.