The role of cytokines in the regulation of local bone resorption.

The skeleton is a complex organ that has a variety of functions. It provides a supportive framework for the body, it is the site of hematopoiesis, and it is the principal storehouse for calcium reserves. Recently, it has become clear that the multiple cell types within bone may interact by producing paracrine factors. These substances were originally recognized as either the products of activated immune cells (cytokines) or as local growth factors. However, they appear to have potent effects on both the cells responsible for the structural integrity of the skeleton and the cells involved in hematopoiesis. In humans the skeleton is constantly remodeling. The cells involved in maintaining the skeleton fall into two broad categories: those responsible for the removal of bone (bone resorption) and those responsible for bone formation. This review concentrates on defining the effects that locally produced factors have on bone-resorbing cells and on the interactions between hematopoietic cells that reside in the skeleton and the cells responsible for maintaining skeletal integrity.