Introduction

Bone represents the largest proportion of the connective tissues in the human body where it functions in the protection of internal organs, as a framework for muscle attachment to generate locomotion, and as an ion reservoir for calcium and other mineral elements. The unique biophysical properties of bone reflect the macromolecular composition and organization of the mineralized extracellular matrix (ECM), which is largely produced and regulated by specialized cells of the osteoblastic lineage. The ECM is formed from a scaffold of collagen fibrils within and between which are found uniform-sized crystals of carbonate-substituted hydroxyapatite. Other proteins, including proteoglycans, sialoproteins and various acidic glycoproteins regulate the formation of the collagen fibrils and apatite crystals and mediate interactions with the osteoblastic cells that generate and maintain the matrix. Characteristically, bone is remodeled continuously during postnatal growth and skeletal maintenance, thus permitting continuous adaptivity for changes in size and structure. Bone mass is maintained through a carefully regulated balance between synthesis and resorption which can be monitored by analysis of metabolized matrix components in body fluids — typically in serum and urine. Loss of regulation, observed in metabolic bone diseases, can lead to increased (osteopetrosis) or decreased (osteoporosis) bone mass, primarily reflecting changes in the amount of mineralized ECM.