IntroductionThe skeleton's principal role as a structure has predisposed bone to the unfortunate reputation of being an inert and static material. Given bone tissue's ability to adapt its mass and morphology to functional demands, its ability to repair itself without leaving a scar, and its capacity to rapidly mobilize mineral stores on metabolic demand, it is in fact the ultimate "smart" material [43] and a dynamic example of "form follows function" in biological systems [45]. Considering the ever-growing number of patients who suffer from devastating disorders of the skeleton and the ever-increasing opportunities inherent in the post-genomics era to treat diseases and injuries to bone [44], it is critical for both the physician and the scientist to more fully understand the biology of bone and how its ability to form and resorb tissue ultimately orchestrates the structural and metabolic successes of the skeleton.
CellsThree distinctly different cell types can be found within bone: the matrix-producing osteoblast, the tissue-resorbing osteoclast, and the osteocyte, which accounts for 90% of all cells in the adult skeleton. Osteocytes can be viewed as highly specialized and fully differentiated osteoblasts; Abstract The principal role of the skeleton is to provide structural support for the body. While the skeleton also serves as the body's mineral reservoir, the mineralized structure is the very basis of posture, opposes muscular contraction resulting in motion, withstands functional load bearing, and protects internal organs. Although the mass and morphology of the skeleton is defined, to some extent, by genetic determinants, it is the tissue's ability to remodel -the local resorption and formation of bone -which is responsible for achieving this intricate balance between competing responsibilities. The aim of this review is to address bone's form-function relationship, beginning with extensive research in the musculoskeletal disciplines, and focusing on several recent cellular and molecular discoveries which help understand the complex interdependence of bone cells, growth factors, physical stimuli, metabolic demands, and structural responsibilities. With a clinical and spine-oriented audience in mind, the principles of bone cell and molecular biology and physiology are presented, and an attempt has been made to incorporate epidemiologic data and therapeutic implications. Bone research remains interdisciplinary by nature, and a deeper understanding of bone biology will ultimately lead to advances in the treatment of diseases and injuries to bone itself.