The 1980s and 1990s have seen major advances in the understanding of regulation of bone metabolism. The section on cellular and hormonal environment of bone reviews the basics of biochemistry and cell biology of bone. With the elucidation of the synthesis and post-translational modification of bone collagen, has come the ability to measure markers of both collagen synthesis and its breakdown. In the formation of new bone, and the breakdown or resorption of old bone, components of the non-collagen matrix of bone are also released by the cells that are synthesizing these products or remodeling the bone matrix. Because these components are released from bone into the circulation, their measurement may provide a window for clinical assessment of the process of bone resorption and formation.

The normal adult human skeleton is constantly remodeling. This is illustrated diagrammatically in Fig. 17.1. The skeleton may be regarded as being made up of millions of basic multicellular units (BMUs) or bone remodeling units (abbreviated in some publications as BRU). At any given time, most BMUs are in a resting stage. In a response to a variety of stimuli (mechanical stress, parathyroid hormone, withdrawal of estrogen, local release of growth factors and cytokines, etc.) a resting BMU can be stimulated into activity.